The Production of ExtrudedSemifinished Productsfrom Metallic Materials*
THE HOT-WORKING PROCESS extrusionis, in contrast to other compressive deformationprocesses used to produce semifinished prod-ucts, a deformation process with pure compres-sive forces in all three force directions. Thesefavorable deformation conditions do not existin other production processes for semifinishedproducts. Even in rolling, which is the most im-portant compressive working process for pro-ducing semifinished products, tensile forces oc-cur in the acceleration zone of the roll gap aswell as in the cross rolling process used topierce blanks in the rolling of steel tubes. Thesetensile forces cause problems in the rolled prod-uct if the deformation conditions are not opti-mized. The benefits of this three-dimensionalcompression in terms of deformation technol-ogy, which have already been discussed in thisbook, can be clearly seen in Fig. 5.1 based onexperimental results for face-centred cubic (fcc)aluminum and zinc with its hexagonal latticestructure.
The extensive variations in the extrusion pro-cess enable a wide spectrum of materials to beextruded. After rolling, extrusion can be consid-
*Extrusion of Materials with Working Temperatures between 0 and 300 �C, Gunther SauerExtrusion of Semifinished Products in Magnesium Alloys, Gunther SauerExtrusion of Semifinished Products in Aluminum Alloys, Rudolf AkeretMaterials, Gunther ScharfExtrusion of Semifinished Products in Copper Alloys, Martin BauserExtrusion of Semifinished Products in Titanium Alloys, Martin BauserExtrusion of Semifinished Products in Ziroconium Alloys, Martin BauserExtrusion of Iron-Alloy Semifinished Products, Martin BauserExtrusion of Semifinished Products in Nickel Alloys (Including Superalloys), Martin BauserExtrusion of Semifinished Products in Exotic Alloys, Martin BauserExtrusion of Powder Metals, Martin BauserExtrusion of Semifinished Products from Metallic Composite Materials, Klaus Muller
ered to be the most important of the hot-workingprocesses.
Extrusion of Materials withWorking Temperaturesbetween 0 and 300 �C
Gunther Sauer*
5.1 Extrusion of SemifinishedProducts in Tin Alloys
Tin is a silver-white, very soft metal with astable tetragonal lattice in the temperature range20 to 161 �C. The pure metal has a density of7.28 g/cm3 and a melting point of 232 �C.
The material can be easily worked with itsrecrystallization temperature below room tem-
Extrusion: Second Edition M. Bauser, G. Sauer, K. Siegert, editors, p 195-321 DOI:10.1361/exse2006p195
Fig. 5.1 Improving the workability of aluminum and zincwith a hydraulically produced increasing mean
compressive stress p measured by the increase in the reductionin area at fracture u in tensile tests at room temperature
Fig. 5.2 Flow stress kf as a function of the logarithmic strainof pure lead and pure tin (Source: Rathjen)
perature. Consequently, the softest condition de-velops rapidly in tin materials after cold work-ing. Work hardening by cold working is notpossible.
Tin is very stable at room temperature and isnot poisonous. Unlike lead, it can therefore beused in contact with food. The metal is used asthe base material for soft solder, anode materialsas well as bearing materials and for tin platingsteel sheet (tin plate).
Soft solders are the main application for tin.Lead, silver, antimony, and copper are used asalloying elements. Special soft solders also con-tain cadmium and zinc. Aluminum is used as analloying component for soft solders used to joinaluminum alloys. Bearing metals based on tincontain antimony, copper, and lead as the alloy-ing elements as well as additions of cadmium,arsenic, and nickel.
The production of semifinished products in tinalloys by extrusion is limited today to the pro-duction of feedstock for the manufacture of softand special soft solders on transverse extrusionpresses and to the direct extrusion of feedstockfor the manufacture of anodes with solid andhollow cross-sectional geometries for electro-chemical plating. Tin is also used for the pro-duction of rolled strip, as the feedstock for themanufacture of high-capacity electrical con-densers, as well as for roll cladding of lead-basestrips for organ pipes, wine bottle caps, andChristmas ornaments (tinsel).
In general, tin-base materials have good bear-ing properties and are consequently used as thebase material for bearing metals. Very little fric-tion occurs in direct extrusion between the billet
surface and the container inner wall because ofthese good bearing properties. Tin-base extrudedalloys, therefore, tend to flow largely accordingto flow pattern A (Fig. 3.11) in direct extrusion.
The flow stress kf that has to be overcome forthe deformation of very soft pure tin is signifi-cantly higher than that of pure lead, as can beclearly seen in Fig. 5.2. It is, however, extremelylow at 20 to 32 N/mm2 for logarithmic strainsug up to 0.7 [Sac 34]. Therefore, a specific presspressure of maximum 400 N/mm2 is sufficientfor the direct extrusion of tin alloys when thelow friction between the billet and the containerliner surface is taken into account.
Tin-base alloys for the production of soft sol-ders are generally extruded transversely at billettemperatures of 50 to 60 �C and a container tem-perature of approximately 100 �C. It is often suf-ficient to heat the front third of the billet to thistemperature so that, on one hand, the billet up-sets from the front to the back to prevent airentrapment and, on the other hand, the optimalwelding conditions for the resultant transverseweld in the extruded product are obtained. Thistransverse weld in soft solders has to be capableof withstanding the drawing loads during the re-duction in diameter of the extruded wires onmultispindle drawing machines.
Semifinished products in tin materials includ-ing bar, wire, tubes, and sections are producedon direct extrusion presses. For productivity rea-sons, higher extrusion speeds are desired in thisprocess than those used in the transverse extru-sion of soft solders, which are typically 4 m/min.It is therefore advantageous to heat the billets to100 to 150 �C. Again, it is beneficial to have atemperature profile with the temperature de-creasing toward the back of the billet. In auto-matic fully integrated extrusion plants complete
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 197
Fig. 5.3 Modern integrated fully automatic 5000 kN extrusion plant for the production of bar, wire, tube, and sections in lead andtin alloys complete with casting oven, triple billet casting plant, hot-billet shear, and oil hydraulic long-stroke extrusion
press (Source: Collin)
with melting furnaces and mold casting ma-chines, as shown in Fig. 5.3, the cast billets re-tain the heat from the casting process to assistthe deformation process. Care has to be taken toensure that the heat from the casting process,when combined with the deformation heat, doesnot result in excessive temperatures at high ex-trusion speeds in the deformation zone of theextrusion tooling. The solidus of the lead-tin eu-tectic at 183 �C has to be taken into account inthe tin alloys containing lead. If melting occursin the solid tin matrix, transverse cracks will def-initely be produced in the extruded products.Controlled cooling of the extrusion tooling im-proves the situation. In addition, the billet sur-face is sometimes brushed with a lubricant toimprove the product surface.
Tin alloys will weld during the extrusion pro-cess under suitable deformation conditions in-cluding the extrusion load and temperature.They can, therefore, be used with porthole andbridge dies as well as for billet-on-billet extru-sion with feeder chamber dies. These extrusiontools are, in principle, similar to those used inaluminum extrusion. Their design is simpler indetail and the dies are nitrided.
Simple two-column horizontal oil-hydraulicextrusion presses are used. Figure 5.3 shows atypical two-column extrusion press for tin andtin alloys with a maximum press load of5000 kN.
5.2 Extrusion of SemifinishedProducts in Lead Materials
Lead is a soft metal, matte blue in appearancewith a fcc lattice structure. The pure metal hasa very high density of 11.34 g/cm3 and a meltingpoint of 327 �C. Lead alloys can also be easilyworked. Work hardening after cold working atroom temperature, for example, from 40 to 120N/mm2 disappears within a few minutes becausethe recrystallization temperature of the base ma-terial is about 0 �C. Cold working of lead-basealloys can be approximately compared in its ef-fect to hot working of other metals. Work hard-ening by cold working is not possible.
Pure lead is very soft and ductile. Soft leadhas a very low flow stress kf as shown in Fig.5.2. It is, therefore, not possible to draw pure
1719 Permitted impurities, max 0.01Permitted impurities, max 0.015
Tube and wire for the chemical industry
Copper fine lead Pb 99.9 Cu 1719 Cu, 0.04–0.08Permitted impurities, max 0.015Pb remainder
Pressure tube
Primary lead Pb 99.94Pb 99.9
17641 Sb 0.75–1.25.2006As 0.02–0.05Pb remainder
Pressure tube
Sb 0.2–0.3Pb remainder
Waste pipe
Cable lead Kb–PbKb–Pb (Sb)Kb–PbSb0.5Kb–PbSn2.5Kb–PbTe0.4
17640
. . .
Sb 0.5–1.0
Standard cable sheath
Cable mantle sheath resistant to fatiguefailure resulting from severe vibration
Sn � 2.5Te � 0.035
Hollow anodes PbSn10 . . . Sn 8–12 Anodes for electrochemical coating ofbores of bearing shells and bushes forbearing production
Anodes for corrosion protection ofbearing shells
Source: Laue/Stenger
lead to wire. However, lead alloys have higherflow stresses. Additions of antimony and tin pro-duce solid-solution hardening of the lead, in-creasing both the base strength and the work-hardening capability. This raises therecrystallization temperature. The loads neededin deformation processes also increase. Anti-mony and zinc are the main alloying elementsfor lead, although small amounts of arsenic andcadmium, as well as copper nickel and silver,are also used.
Lead antimony alloys are referred to as hardlead because the antimony significantly hardensthe soft lead. Alloys of this type with antimonycontents up to 3 wt% also age harden. For ex-ample, the Brinell hardness of an alloy with 2wt% antimony increases from 56 to 130 N/mm2
within 100 days after quenching from 240 �C.The toughness and the fatigue strength, as wellas the corrosion resistance of these alloys, canalso be improved by the addition of specificamounts of tin. This is important for the me-chanically fatigue loaded sheathed cables on, forexample, bridges. Lead forms a eutectic with11.1% antimony at 253 �C. This has to be takeninto account when determining the deformationtemperature for the extrusion of PbSb alloyswith eutectic, low-melting point phases on grainboundaries.
Lead-tin alloys with their low solidus and li-quidus temperatures are particularly suitable for
the production of soft solders. Lead forms a eu-tectic with 38.1% tin at 183 �C. These soft sol-ders can be used to solder steel and copper al-loys. Organ pipes are produced from lead-tinalloy strips with similar tin contents. Tin con-tents from 45 to 75 wt% are used depending onthe timbre. Lead-tin-antimony alloys with ad-ditives of arsenic, cadmium, and nickel are veryimportant as bearing metals.
Table 5.1 gives recommended lead alloys forthe extrusion of rod, wire, tubes, sections, andcable sheathing. Lead alloys are extruded withdeformation temperatures in the range 100 to260 �C, depending on the composition, to obtaineconomic extrusion speeds of typically 50 to 60m/min. On the other hand, lead-base soft soldersare extruded at significantly lower temperaturesof approximately 55 �C and slower speeds of 2to 4 m/min. When determining the billet andcontainer temperatures, care must be taken toensure that any eutectics on the grain boundariesdo not melt as a result of excessive deformationtemperatures in combination with the deforma-tion and friction heat during extrusion. A liquidmetal phase on the grain boundaries of a solidmetal matrix in extrusion will unavoidably pro-duce transverse cracks in the extruded product.Lubricant is added to the billet surface with acontrolled brush stroke.
Lead has even better bearing properties thantin. Lead alloys, therefore, also flow mainly ac-
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 199
Fig. 5.4 Specific extrusion pressure in N/mm2 as a functionof the stem displacement and the extrusion ratio V
� A0/AS in direct extrusion of soft lead (Source: Siebal/Fang-meier)
cording to flow type A in direct extrusion asshown in Fig. 5.4 where the variation of the ex-trusion load over the container cross-sectionalarea A0 is plotted as a function of the stem dis-placement and the extrusion ratio [Sac 34, Hof62].
Lead alloys will weld during extrusion at suit-able temperatures and extrusion pressures simi-lar to tin alloys and certain aluminum alloys.Tubes and hollow sections can, therefore, be ex-truded with porthole and bridge dies. Feederchamber dies have also proved successful for theextrusion of these materials and are required forbillet-on-billet extrusion. The tools used for theextrusion of tin and lead alloys are in principlesimilar to those used for aluminum extrusion,although the designs are simpler because of thesignificantly lower thermomechanical stresses.All extrusion dies are nitrided.
Extrusion plants consist of integrated auto-matic complete plants with horizontal, hydrau-lically operated two-column presses and maxi-mum extrusion loads of 5000 to 10000 kN, asshown in Fig. 5.3. A specific extrusion pressureof 400 N/mm2 is similar to tin-base extruded ma-terials, adequate for the extrusion of lead-basealloys.
Large tubes with internal diameters up to 300mm in lead alloys are sometimes extruded onold vertical indirect extrusion presses over man-drels and charged with liquid metal. These ex-trusion presses can have loads between 5000 and15,000 kN. Cable sheathing with lead alloys iscarried out on special cable sheathing presses as
shown in Table 5.1 (see the section on cablesheathing in chapter 3).
5.3 Extrusion of Tin- andLead-Base Soft Solders
The standard soft and special tin solders inDIN 1707, as well as nonstandardized specialsoft solders, are extruded as solid bars, solderthreads, solid wire, and hollow wire (tube sol-der) filled with flux mainly on small hydraulicautomatic extrusion presses. The maximum ex-trusion loads are usually 2500 kN with a maxi-mum specific pressure of 600 N/mm2, which isrequired because of the extreme metal flow. Thebillet-on-billet process is used on transversepresses. Extrusion is usually followed by draw-ing to the finished diameter of 5 to 0.5 mm onmultispindle drawing machines. The majority offinished diameters fall in the range 1 to 2 mm.Special soft alloys with low cold-working ca-pacities sometimes have to be extruded as mul-tiple strands to the finished diameter using thedirect extrusion process. Economic wire solderpresses operate completely automatically in thesame way as large extrusion plants with auto-matic billet feed called by the press during theextrusion cycle and automatic removal of the ex-truded semifinished product. Figure 5.5 shows afully automatic plant with an integrated meltingfurnace and a chill mold billet casting machine.
Cored solders are hollow wire filled with fluxby a special tool during extrusion as shown inFigures 5.6 and 5.7. The extrusion tool systemneeded for the extrusion of hollow wire with fluxfilling, which includes the die head with the hol-low mandrel, the extrusion die, and the weldingchamber, and, in particular, their relative ar-rangement is similar to that for cable sheathing.Figure 5.6 depicts the construction of a die head.The application of “transverse extrusion” com-bined with “billet-on-billet” extrusion is an es-sential requirement for the production of contin-uous cored solder. Cored solder has, similar tothe cable sheath, both longitudinal and trans-verse welds from this production method. Thebillets are usually produced by the chill castingprocess using casting machines and are normallyprocessed with the cast skin forming the billetsurface.
Typical widely used solders are the soft sol-ders L-PbSn40, L-Sn50Pb, L-Sn60Pb and L-Sn60PbCu2, the special soft solders L-SnCu3
200 / Extrusion, Second Edition
Fig. 5.5 Modern fully automatic extrusion plant for the production of soft solder with casting oven, chill mold billet casting plant,hot-billet shear, and oil hydraulic 2500 kN extrusion press. The extrusion press is fitted with a fixed dummy block on the
stem. (Source: Collin)
Fig. 5.6 Extrusion tooling for the extrusion of hollow solder.1, connection to the flux container; 2, hollow man-
drel; 3, extrusion die; 4, extrusion die holder; 5, pressure nut; 6,one-piece die head with the container; 7, front nut; 8, hollowmandrel screw adjustment; 9, extrusion stem direction during ex-trusion; 10, direction of extruded product (Source: Collin)
and L-SnAg5 to DIN 1707 and, as an exampleof a nonstandard solder, the special soft solderL-SnCd25Zn5.
The easy and moderately difficult to extrudesoft solders are usually worked with a tempera-ture of 55 to 60 �C. Heating of the front third ofthe billet is sufficient. To avoid air entrapmentthe billet should upset under the influence of theextrusion load and the temperature from thefront end to the back and readily weld to thepreviously extruded billet. The container tem-perature is set to 90 to 110 �C. With automati-cally operating integrated extrusion plants forsolder wire that include a melting oven and chillmold casting machine, as shown in Fig. 5.3, thecast billet must be transferred with sufficientheat for deformation retained from the castingheat. Equally, an automatic extrusion plant forsolder wire need not include a melting oven andchill mold casting machine. Plants with a largenumber of program changes and small produc-tion batches also operate economically withoutthese facilities. The extrusion plant is thenequipped with a billet magazine and an induc-tion furnace in line with a hot billet shear. Thisis used to heat either the entire billet volume orthe front third to the deformation temperature.
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 201
Fig. 5.7 Die holder, left, and hollow mandrel, right, as well as the front nut of the container with the pressure nut for adjusting thehollow mandrel (See Fig. 5.6) (Source: Collin, Alchacht)
Special soft solders need to some extent lowerthe more difficult-to-extrude alloys’ higher de-formation temperatures, which can be as high as400 �C. The container temperature has to be setcorrespondingly high to avoid excessive heattransfer between the billet and the container dur-ing extrusion.
The wires are usually extruded to a diameterof approximately 15 mm and then draw onmultispindle drawing machines to diameters be-tween 5 and 0.5 mm. The transverse and longi-tudinal welds formed in the welding chambersof the extrusion die are able to withstand thedrawing deformation loads without any prob-lem. Good extruded surfaces are achieved bylightly lubricating the surface of statistically uni-formly selected billets with a special lubricant.The quantity applied of this lubricant must becontrolled extremely accurately; otherwise, thelubricant can enter the extrusion welds and pre-vent perfect welding. The wires would thenbreak at the welds during drawing and have tobe scrapped.
The extrusion process for the production offeedstock for solder wire commences with thebillet call from the extrusion press. The extrudedwire passes continuously through a multispindledrawing machine in line with the press corre-sponding to its capacity. The drawing machinecontrols the extrusion press. The billet is trans-ferred either directly from the chill molding ma-chine as shown in Fig. 5.8 or from a billet mag-azine via an induction furnace, depending on the
plant specification. The billet end faces aresheared in a hot shear and thus cleaned of anyoxide [Lau 76]. This is necessary to ensure per-fect transverse welds. After this operation thebillets collect in a magazine in front of the con-tainer and fall one by one into the centerline ofthe press when the stem is fully withdrawn.When the stem moves forward, the billet entersthe container and is pushed against the previ-ously extruded billet and the contacting materialvolumes weld together. The extrusion emergingfrom the die is automatically fed into a multi-spindle drawing machine and heavily reduced indiameter in one operation using a large numberof drawing dies. The extruded wire usuallypasses through a water cooling system beforereaching the drawing machine.
The production plan given subsequently forthe manufacture of solder wire with a flux core(hollow solder) explains the solder wire manu-facturing process in more detail.
Production of 1000 kg hollow solder with adiameter of 0.8 mm from the soft solder L-Sn60Pb according to DIN 1707 filled with a fluxof type F SW32 according to DIN 8511 with2.5% by weight:
1. Casting in a chill mold casting machine orheating of 168 billets with dimensions: 72mm diam. � 175 mm long (initial billetweight: 6.05 Kg) using an induction rapidbillet heating furnace to 50 �C.
202 / Extrusion, Second Edition
Fig. 5.8 Oil-operated 2500 kN solder wire extrusion press asshown in Fig. 5.5 with extruded hollow solder
emerging transverse to the press longitudinal axis. The vessel onthe left-hand side is filled with flux and linked by a tube to con-nection 1 on the hollow mandrel 2 in the extrusion tool in Fig.5.6. The container is located above the hollow mandrel, and theusually very liquid flux heated to approximately 60 to 120 �Cflows under hydrostatic pressure to the hollow mandrel.
2. Shearing of the billet end faces using the dou-ble shear visible in Fig. 5.5 during the trans-fer to the extrusion press.
3. Extrusion into air with subsequent watercooling after the press—one extruded bar perextrusion with the dimensions 14.7 � 6.7diam. using the billet-on-billet process andtransverse extrusion with simultaneous ad-dition of flux using an extrusion tool systemshown in Fig. 5.6 from a container: 75 mmdiam. on a 2500 kN extrusion press—con-tainer temperature: 100 �C; exit speed: 3.02m/min; extrusion ram speed: 5.5 mm/s; ex-trusion ratio V � 32.86.
4. Drawing to: 6 mm diam. in one operationwith 14 drawing dies on a roughing multi-spindle drawing machine. This drawing ma-chine controls the extrusion press accordingto the wire requirement via a dancing roll.
5. Drawing to: 1.35 mm diam. in one operationwith 40 drawing dies on an intermediatemultispindle drawing machine
6. Drawing to: 0.8 mm diam. in one operationwith 40 drawing dies on a fine multispindledrawing machine.
7. Coiling the wire on bobbins. This operationis frequently combined with operation 6.Quality control of the wire surface is also car-ried out.
8. Control and labeling
Solder threads are produced by multipledrawing of extruded solid wire approximately15 mm diam., depending on the final dimensionson the multispindle drawing machines and au-tomatically cut transversely after drawing.
The work hardening of the solder materialproduced during the cold working is counteredby the continuous softening from recrystalliza-tion during the multispindle drawing process.The process is helped by the heat of the drawinglubricant in the sump of the drawing machine,in which the entire drawing process takes place.Soft solder usually recrystallizes at room tem-perature following work hardening. Therefore,intermediate annealing is not needed to achievethe soft, i.e., recrystallized material, state re-quired for further cold working.
5.4 Extrusion of Zinc AlloySemifinished Products
Pure zinc has a density of 7.13 g/cm3 and amelting point of 419.5 �C. The metal crystallizeswith a hexagonal lattice structure, which, atroom temperature only permits material slip onthe (0001) basal plane. Consequently, polycrys-talline zinc alloys at room temperature and be-low, in particular, are brittle. Deformation of thismaterial is possible only after heating to tem-peratures between 150 and 300 �C. Twin for-mation during the deformation improves theplasticity. Zinc alloys do not have any greatwork-hardening capacity because the recrystal-lization temperature of these alloys is at roomtemperature or just above. The main alloyingelements of zinc are aluminum and copper. Alu-minum improves the mechanical properties ofzinc alloys particularly when combined with hotworking by extrusion. Copper also improves themechanical properties of zinc-base alloys butnot to the same extent as aluminum. However,
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 203
Fig. 5.9 Deformation behavior of the magnesium alloy MgAl6Zn in hot-compression tests in the temperature range between 200and 220 �C (Source: Schmidt/Beck)
copper improves the fatigue strength and mach-inability of zinc alloys.
Typical zinc-base extrusion alloys areZnAl1Cu and ZnAl4Cu1. Bars, wires, tubes, andsections are produced from ZnAl4Cu1. The ma-terial can also be drawn. ZnCu is an alloy thatis used for the production of sections and hot-stamping components. Car tire valves are pro-duced from the alloy ZnAl15.
After hot rolling, extrusion is the most im-portant deformation process for zinc-base alloys,although the main application of zinc alloys ispressure die castings. The extrusion of these al-loys has lost a lot of its importance. Zinc alloysare extruded only to a limited extent mainly be-cause the limited workability associated with thehexagonal lattice structure also affects the hotworkability. Zinc alloys are extruded at defor-mation temperatures of approximately 150 to300 �C. Only very low extrusion speeds of ap-proximately 3 to 6 m/min can be achieved be-cause of the limited extrudability of the extrudedmaterials at high specific pressures [Sac 34, Lau76, Schi 77, Schu 69].
Extrusion of Materials withDeformation Temperaturesbetween 300 and 600 �C
Gunther Sauer*
5.5 Extrusion of SemifinishedProducts in Magnesium Alloys
Magnesium is a metal with the low density of1.74 g/cm3, which is approximately 40% belowthat of aluminum. The densities of magnesiumalloys fall in the range 1.76 to 1.83 g/cm3. The
*Extrusion of Semifinished Products in Magnesium Alloys, Gunther Sauer
modulus of elasticity of pure magnesium is ap-proximately 41,000 N/mm2 and that of magne-sium alloys between 45,000 and 47,000 N/mm2,i.e., on average about 66% of that of aluminumalloys. Pure magnesium has a liquidus tempera-ture of 650 �C and crystallizes with a hexagonallattice structure. Metals with the hexagonal lat-tice structure can only slip on the (0001) baseplane at room temperature. Consequently, mag-nesium alloys are very brittle at room tempera-ture. However, temperatures above 200 �C per-mit the activation of other slip planes as well asthe formation of deformation twins enablingthese materials to be hot workable. The narrowtemperature range between the brittle and theplastic deformation behavior of magnesium al-loys is of interest and is illustrated in Fig. 5.9for the alloy MgAl6Zn [Sac 34, Bec 39].Whereas this material clearly has a limitedworkability at 208 �C as can be seen by the shearstress cracks running at 45�, it can be readily hotworked at 220 �C; i.e., a temperature increase ofonly 12 �C produces significantly better defor-mation properties in this alloy. The formation oftwin lamellae is associated with the improve-ment in the plastic workability.
Only magnesium alloys are used as structuralmaterials and in structural applications, the highnotch sensitivity has to be taken into account.This also has to be allowed for in the cross-sec-tional geometry of extruded sections. In addi-tion, magnesium alloys have a very elastic be-havior in compression because of their lowelastic moduli between 45,000 and 47,000 N/mm2, but this also increases the sensitivity tobuckling.
Specific alloy additions can raise the staticand dynamic materials properties of magnesiumby a factor of two or three. The principle alloy-ing elements of the wrought alloys are aluminumwith contents up to 10 wt%, zinc up to 4 wt%,and manganese up to 2.3 wt%. Zirconium, ce-rium, and thorium are also added. More recently,lithium has been increasingly used as an alloyingelement.
204 / Extrusion, Second Edition
Fig. 5.10 The influence of aluminum on the mechanical properties of extruded magnesium alloys (Source: Spitaler)
Aluminum is the most important alloyingelement for magnesium, which forms a solidsolution with aluminum at low contents andthe intermetallic phase Al2Mg3 at higher con-tents. A eutectic occurs at 436 �C at an alu-minum content of 32.2 wt%. Crystal segrega-tions can occur in wrought alloys at aluminumcontents above 6 wt%. They can be dissolvedand thus removed by homogenization of thecast in the temperature range of 400 to 450 �Cfor correspondingly long times. The solid-so-lution formation at aluminum contents of 2 toapproximately 6 wt% increase the fracturetoughness and hardness of magnesium alloysas shown in Fig. 5.10. At higher aluminumcontents, the fracture strength and, in particu-lar, the hardness can be further increased bythe formation of the hard c phase, but with areduction in the ductility of the material as canbe clearly seen in Fig. 5.10 [Schu 69]. Mag-nesium-base wrought alloys have aluminumcontents up to 9 wt%. Magnesium alloys canbe hot rolled at aluminum contents up to 7wt% and hot worked by extrusion at aluminumcontents up to 9 wt%. The magnesium alloybecomes increasingly more brittle at aluminumcontents � 9 wt% as shown by the elongationin Fig. 5.10. This also shows that magnesiumalloys with contents � 7 wt% can be age hard-ened after solution heat treatment. At the sametime, the alloy suffers a drastic loss in ductility[Schu 69].
Zinc readily dissolves, depending on the tem-perature, in magnesium by the formation of solid
solutions with the phase MgZn. An addition ofup to 3 wt% increases the fracture strength andthe fatigue strength of magnesium. Zinc contentsmore than 3 wt%, however, result in a drasticreduction in the elongation to fracture. Zinc con-taining magnesium alloys can be age hardened.This can increase the fracture strength of mag-nesium alloys by 30 to 40%.
Manganese dissolves in magnesium by, de-pending on temperature, up to 3.4% at 645 �C.At this temperature magnesium forms a eutecticwith manganese. Manganese is dissolved bymagnesium with the formation of solid solutionsand the phase MgMn. It increases the strengthof magnesium at contents � 1.5 wt%. Manga-nese containing magnesium alloys have bettercorrosion resistance than other magnesium-basealloys. Manganese contents of between 0 and0.5% are therefore added to all magnesium al-loys to improve the corrosion resistance.
Additions of zirconium form finely dispersedzirconium oxides that act as the nuclei for a fine-grain structure and thus increases the tensilestrength of magnesium alloys with no reductionin the elongation.
Cerium also increases the fine-grain structureand improves the hot strength.
Thorium improves the hot strength even morethan does cerium. This has resulted in the de-velopment of magnesium-thorium alloys withparticularly high resistance to softening. Tho-rium also significantly improves the fatiguestrength and specifically the creep strength [Tech
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 205
Fig. 5.11 Tensile strength RM and elongation to fracture A as a function of the extrusion ratio determined on single cavity extrudedround bars in the alloy MgAl10. Billets 175 mm diam were used for the experiments with a container diam of 180 mm
(Source: Beck)
96/97/98]. However, in England, alloys withmore than 2 wt% of thorium are classified asradioactive. In the United States, use of mag-nesium-thorium alloys is very restricted becausethorium-free magnesium alloys have been sub-stituted.
Magnesium alloys are very susceptible tocorrosion compared with aluminum alloys be-cause they are electrochemically less noble. Ad-ditions of nickel, iron, copper, and so forth pro-mote the corrosion of the material. Therefore,these additions are held in the range ppm byusing purer starting materials as well as im-proved melt refining processes. This enables thecorrosion sensitivity to be significantly reduced.High-purity-based magnesium alloys todayhave corrosion resistances comparable to alu-minum alloys.
This has been one of the factors that has in-creased the interest of the automobile industryin magnesium-base alloys and provided the in-
centive to improve the existing alloys and to de-velop new ones with the intention of:
● Improving the static and dynamic materialsproperties, including elongation and tough-ness
● Improving the temperature resistance● Further reducing the density● The development of alloys with self-healing
surface passive films
The coarse cast structure is transformed intoa fine grain elongated structure by hot workingin the extrusion process. This structural trans-formation is associated with a significant im-provement in the mechanical properties of themagnesium alloys in the same way as aluminumalloys. The mechanical properties that can beachieved improve with increasing extrusion ra-tio, i.e., with increasing working during extru-sion. Figure 5.11 illustrates this process, whichis of considerable importance for the supply of
206 / Extrusion, Second Edition
Table 5.2 Extruded magnesium-base alloys
Material designationProperties
Symbol No. (DIN standard) ASTM Composition, wt% Billet, �C Shape and conditionRp0.2
300–420 Bar and sectionExtruded and straightenedAge hardened if required
200–230 280–320 6–10 (A5)
MgZn5Zn0.6 3.5161(DIN 1729)
. . . Zn: 4.80–6.20Zr: 0.45–0.80
300–400 Bar, tube, sectionExtruded and straightenedAge hardened if required
200–250 280–320 4–5 (A5)
MgTh3Mn2(a) . . . . . . Th: 2.50–3.50Mn: �1.2
. . . Bar, tube, sectionExtruded and straightened
180 270 4 (A5)
(a) Experimental material Fuchs Metallwerke. Source: Schimpke, Schropp, and Konig
extruded semifinished products [Bec 39]. Theextrusion ratio V � A0/AS should not fall below5, and it is better to specify the minimum at 7.A fine-grained extruded structure with good me-chanical properties can be achieved with extru-sion ratios of 25 [Har 47].
Magnesium alloys are mainly extruded usingthe direct extrusion process. Indirect extrusionis possible and is used. Magnesium alloys flowmore uniformly in direct extrusion than do alu-minum alloys. The discards can be reduced inthickness because of the lower distortion in theregion of the billet surface and the resultant flat-ter dead metal zones in the deformation zone ofthe container. Magnesium alloys can be weldedin the extrusion process. Therefore, portholedies and bridge dies can be used to producetubes and hollow sections. Similar to aluminumalloys, large-diameter tubes are extruded overmandrels. It is also possible to use billet-on-billet extrusion with magnesium alloys. How-ever, the weldability is reduced by increasingaluminum contents.
Typical magnesium alloys for hot working byextrusion are given in Table 5.2. There are other
wrought magnesium-base alloys in addition tothose listed, particularly abroad, for example al-loys of the MgAl family. The working tempera-tures for extrusion are at 250 to 450 �C, depend-ing on the alloy and are higher than those usedfor hot rolling the same magnesium alloys. Ex-trusion ratios V � A0/AS similar to those for alu-minum alloys can be achieved. The extrudablemagnesium alloys belong to the category of dif-ficult-to-extrude alloys mainly because of theirhexagonal lattice structure. The extrusion speedsfall in the range of moderate- to difficult-to-ex-trude aluminum alloys. According to [Har 47],the extrudability of magnesium alloys decreaseswith increasing aluminum content as do the ex-trusion speeds. Only the alloys MgMn andMgMn2, as well as MgAl1Zn, can be extrudedat speeds up to a maximum of 30 m/min [Zol67]. The predominantly slow extrusion speedsnaturally result in long extrusion cycles. There-fore, the extrusion container temperature has tobe set so that the billet does not lose any heat tothe container during the extrusion cycle; other-wise, the billet will freeze in the container. In
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 207
Table 5.3 Dependence of the specific extrusion pressure for the extrusion of tubes 44 � 1.5 mm inthe alloy MgAl3 on the homogenization treatment and the heating conditions of the billet 48 � 150mm
Thermal treatment of the cast Heating before extrusion
before heating prior to extrusion Temperature, �C Time, h Specific extrusion pressure, N/mm2
Not carried out 380–350 1 250–300Not carried out 380–350 3 210–260Not carried out 380–350 6 180–220Not carried out 340–300 1 320–400Not carried out 340–300 3 250–300Not carried out 340–300 6 200–250350 �C–12 h 340–300 1 190–250350 �C–12 h 340–300 3 170–200350 �C–12 h 340–300 6 150–180350 �C–12 h 340–300 1 170–220350 �C–12 h 340–300 3 160–200350 �C–12 h 340–300 6 130–170
Source: Zubolob and Zwerev
addition, good soaking of the billet during heat-ing is required. Magnesium alloy extrusion bil-lets are normally extruded with the cast skin.
Heat treatment such as homogenizing im-proves the hot workability of the magnesium-base extrusion alloys. The effect of homogeni-zation on the extrusion loads needed for hotworking of the alloy MgAl3 is shown in Table5.3 [Zol 67, Lau 76].
Extruded magnesium alloy semifinishedproducts exhibit extreme anisotropy because ofthe hexagonal lattice structure of the metal. Forexample, tensile tests on extruded semifinishedmagnesium alloys in the longitudinal direction,i.e., the extrusion direction, have significantlyhigher values for the 0.2% proof stress, tensilestrength, and elongation than in the transversedirection.
If the ratio of the tensile strength transverse/longitudinal is taken as a measure of the aniso-tropy of a material, magnesium alloys have val-ues in the range 0.6 to 0.7.
The nitrided dies used for the hot working ofmagnesium extruded alloys have, in principle,the same design features as those used for theextrusion of aluminum alloys. Only the shape-forming regions of the die are matched to thespecific properties of magnesium alloys by hav-ing longer bearings and larger radii. The typicalhollow section die used for aluminum alloys canin principle be used for these alloys because oftheir ability to weld during extrusion. In addi-tion, in contrast to aluminum alloys, magnesiumalloys have a significantly lower affinity for ironalloys [Tech 96/97/98]. This property results ina slower buildup of an intermediate layer of theextruded material on the die bearing surfaces
compared with aluminum alloys, even thoughthe formation of a layer cannot be avoided withmagnesium alloys even when nitrided dies areused.
Cold working by drawing to improve the me-chanical properties is practically impossible forwrought magnesium alloys because of their lim-ited cold workability associated with their hex-agonal lattice structure.
Drawing in the form of a calibration draw is,however, of interest, as a means of improvingtolerances.
Extrusion of SemifinishedProducts in AluminumAlloys
Rudolf Akeret*
5.6 General
Of all the materials processed by extrusion,aluminum occupies the predominant role interms of both production volume and value.Based on the annual volume of production ofprimary and secondary metal, aluminum isplaced directly after iron. The majority are al-
*Extrusion of Semifinished Products in Aluminum Alloys,Rudolf Akeret
208 / Extrusion, Second Edition
Fig. 5.12 Increase in the flow stress of aluminum in hotworking with different alloying additions [Ake 70]
loyed to produce wrought alloys and formed intosemifinished products of which 25% are ex-truded. The majority of all extrusion plants,three quarters in Germany, for example, processaluminum alloys [Bau 82], 465,000 t in 1995.
The physical metallurgical properties of alu-minum alloys make them particularly suitablefor the extrusion of products very close to thefinished shape and with attractive properties.The face-centered cubic (fcc) structure with 12slip systems combined with a high stacking faultenergy is a requirement for good cold and hotworkability. Corresponding to the melting pointof 660 �C, the hot-working temperature of alu-minum alloys falls in the range of 350 to 550�C, which can be easily withstood by tools madefrom suitable hot-working steels. In this tem-perature range the flow stress is reduced to val-ues that require relatively low specific presspressures for processing. The natural oxide skingives aluminum an attractive appearance and agood corrosion resistance in the natural state. In-creased surface protection is given by anodic ox-idation. Aluminum forms age-hardening alloyswith low-alloying additions that combine goodhot workability with a high strength after a sim-ple heat treatment.
Sections with an extensive range of functionspecific cross sections can be extruded withinnarrow tolerances from aluminum alloys. Hol-low sections in the form of rectangular tubes andhollow engineering plates offer a high bendingand torsional stiffness. The extensive variety ofaluminum sections used today in a wide rangeof technologies is described in detail inChapter 2.
The selection of the extrusion process islargely determined by the physical metallurgicalproperties of the aluminum alloys. The high af-finity to steel and thus the tendency to adhere toall extrusion tools has to be included in the ma-terial properties in addition to low extrusiontemperature and the good extrusion weldability.
Direct hot extrusion without lubrication andwithout a shell is used for the majority of ex-truded products, including solid and hollow sec-tions from the easily and moderately difficult al-loys. Direct extrusion can be used for practicallythe entire spectrum of products, from the simpleround bar to complicated sections with a circum-scribing circle close to the container cross sec-tion. Flat dies are used for solid sections andporthole dies for hollow sections.
Indirect extrusion comes into considerationfor compact cross sections in hard-to-extrude al-
loys. Cold and hot extrusion with lubrication ofthe container and the die is also used for barsand tubes [Ake 73].
5.7 Extrusion Behavior ofAluminum Alloys
5.7.1 Flow Stress
The extrusion temperature range, the flowstress variations, and the friction across the tool-ing determine the extrusion behavior of alumi-num alloys. Alloy and quality requirements de-termine the necessary exit temperature for theextruded product and the temperature range forthe deformation. In the range 350 to 550 �C, theflow stress of aluminum alloys is very dependenton the temperature and the composition [Ake 70,DGM 78]. The increase in the flow stress withincreasing content of the most common alloyingelements is shown in Fig. 5.12 [Ake 70].
The dependence of the flow stress on tem-perature and speed is described in the context ofthe basics of metallurgy in Chapter 4. Figure5.13 shows for some non-heat-treatable alloysthat reducing the temperature by approximately100 �C results in an almost doubling of the flowstress providing the alloy additions stay in so-lution [Ake 70].
With age-hardening alloys similar to Al-MgSi1 the variation of the flow stress is dis-
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 209
Fig. 13 Flow stress of some non-age-hardening aluminum alloys as a function of the deformation temperature (maximum of theflow curve in torsion tests with � 0.655 s�1) [Ake 70]ug
Fig. 5.14 Flow types B, B1, and C and the flow inward ofthe billet peripheral layer along the dead metal
zone (path 1) and from the dummy block edge into the interiorof the billet (path 2) [Val 96]
placed by solution and precipitation processesaccording to the content of alloying elements inthe matrix.
5.7.2 Flow Process
The aluminum alloys are almost always ex-truded in direct contact with the container anddie manufactured in hot-working steel. How-ever, aluminum exhibits a significant chemicalaffinity and adhesion tendency with iron [Czi72]. Even in the solid state it tends to adhere totool surfaces. At face pressures above the flowstress, Coulomb’s laws of friction lose their va-lidity if the shear distortion of the peripherallayer requires less force than the slip along thesurface of the harder frictional party [Wan 78].The face pressure at the inner face of the con-tainer is of the order of 10 times the flow stress.Therefore, aluminum alloys flow in direct extru-sion according to flow type B1 as shown in Fig.5.14 [Val 96].
5.7.2.1 The Dead Metal Zone
The billet surface is stationary relative to thecontainer inner wall, and the shear distortion is
at a maximum immediately below the surface(Fig. 5.15) [Gat 54].
A dead metal zone forms in front of the faceof the flat die. The surface of the extruded sec-tion is not formed from the surface of the billet
210 / Extrusion, Second Edition
Fig. 5.15 Adhesion of the billet surface to the container walland the shear deformation of the peripheral layer
[Gat 54]
but from the interior of the billet by shearingalong the dead metal zone [Ake 88, Lef 92]. Theoutermost layer of the billet surface initially ad-heres to the inner wall of the container and isshaved off by the advancing dummy block. Thematerial that is compressed together in front ofthe dummy block and that contains oxide andexudations from the billet surfaces ends up wellbelow the surface of the extrusion following thepath shown in Fig. 5.14, No. 2, and forms anincompletely bonded intermediate layer referredas the piping defect. Material located farther be-low the surface is compressed at the upper edgeof the dead metal zone and follows path 1 of thedead metal zone into the region of the surfaceof the extruded product. Adhesion of the mate-rial also occurs in feeder chambers and in theports of porthole dies where there is a high pres-sure [Ake 88].
5.7.2.2 In the Shape-Producing Aperture
The pressure conditions in the die aperture aredifferent. The axial pressure at the exit is zero,or there can be a small tensile stress—if a pulleris used.
The face pressure, which determines the fric-tion, cannot be easily calculated because it strad-dles two boundary cases. If the section can slideas a solid body through the die aperture, the facepressure, and thus the friction, is low. If the sec-tion undergoes a reduction in thickness througha narrowing die aperture, the face pressure is atleast equal to the flow stress. The large adhesionaffinity of aluminum increases the friction stressto the same order of magnitude as the shearstress [Ake 88, Ake 85].
The friction in the die opening is responsiblefor the increase in the axial pressure against theflow direction.
Slip with Coulomb friction is impossible inthe extrusion of tubes over a mandrel because ofthe high radial pressure between the billet andthe mandrel. If the extrusion is carried out overa stationary mandrel, relative movement withshear friction takes place between the billet andthe mandrel from the start of extrusion over theentire billet length. The relative speed is there-fore of the order of magnitude of the stem speed.The billet material is only accelerated to the exitspeed of the tube as it crosses the deformationzone and at the same time the pressure on themandrel surface falls rapidly. The transition toslipping friction of the finish extruded tube overthe mandrel tip takes place in the region of thedie aperture.
When a moving mandrel is used, the zone inwhich relative movement occurs between thebillet and the moving mandrel is only the lengthof the deformation zone. The shear zone movestoward the back of the billet as the extrusionprogresses [Rup 82].
5.7.3 Thermal Balance andExtrusion Speed
The theoretical background to the thermalbalance in extrusion is discussed in chapter 3(see also Fig. 3.22).
In the direct extrusion of aluminum alloyswithout lubrication, two to three times the me-chanical work is needed than would be requiredfor an ideal loss-free deformation. The work car-ried out by the press on the material being ex-truded is practically completely transformed intoheat, which is partly transferred into the tooling
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 211
Fig. 5.16 Heating from deformation and shear deformationof the peripheral layer in the adiabatic limiting
case (example, AlMgSi0.5: flow stress 25 MPa, billet length: billetØ � 4, extrusion ratio 30). 1, temperature increase up to theentry into the deformation zone; On exit from the die: 2, tem-perature increase of the section core; 3, temperature increase ofthe section surface; 4, mean temperature increase of the section
Fig. 5.17 Limit diagram for maximum surface and minimummean section temperature
and partly removed in the emerging extrusion.In the adiabatic limiting case, each MPa of av-erage extrusion pressure corresponds to an av-erage increase in temperature of 0.3 K.
The magnitude involved is shown in Fig. 5.16for the case of an easily extruded material witha flow stress of 25 MPa in which a billet oflength l0 � 4D0 is extruded as quickly as pos-sible with an extrusion ratio of 30 to avoid anyheat losses. When a material volume corre-sponding approximately to the volume of the de-formation zone is extruded, there is initially asteep increase in temperature. With loss-free de-formation, which approximately occurs in thecore of the extrusion, the material is initiallyheated by 25 K.
The shearing of the material flowing throughthe deformation zone along the dead metal zonerequires approximately the same amount ofwork as the pure deformation [Ake 72, Sah 96].An additional contribution, which cannot be ig-nored, is the friction in the die aperture, the mag-nitude of which depends on the angle and thelength of the bearings [Ake 85, Moo 96, Mue96]. The shear takes place in a surface layer ap-proximately 10% of the diameter of the defor-
mation zone, and the friction heats an even thin-ner surface layer of the section. A peripherallayer approximately one-third of the cross-sec-tional area is therefore heated to a significantlyhigher temperature than the core of the section[Ake 72]. This temperature difference is veryshort lived and is practically completely equal-ized on the way from the die to the press platen.Experiments with sheathed thermocouples in thedie land have shown that the maximum surfacetemperature can be 60 to 100 K higher than themean section temperature [Joh 96].
With sections, the maximum temperature,which governs the onset of tearing, occurs dur-ing the passage through the die opening usuallyat an edge where the longitudinal stress is thehighest and the temperature probably at the max-imum.
The possible working range of the press, i.e.,the “window” in the temperature-speed field, islimited by the following thermal and mechanicalconditions:
● The surface temperature cannot exceed themaximum value that results in excessivescoring or even transverse cracks at anypoint (limit curve 1 in Fig. 5.17). The tem-perature equalizes to some extent during thecrossing of the deformation zone.
● The average section temperature has to behigh enough to ensure adequate solution heat
212 / Extrusion, Second Edition
Fig. 5.18 Flow stress and extrudability [Ake 71]
treatment with AlMgSi alloys and to reducethe tendency of AlCuMg and AlZnMgCu al-loys to recrystallization (limit curve 2).
● The press and its power source, on the otherhand, determine the maximum extrusionload and the ram speed. The latter decreaseswith increasing extrusion load (reduction inbillet temperature) because of the increasingslip loss (limit curve 3).
In the literature there are several different pro-posals for depicting the process limits [Joh 96,Hir 58, Ste 73, She 77].
The range of extrusion speeds that can bepractically obtained with aluminum alloys isshown in Fig. 5.18 [Ake 71] plotted against theflow stress at the extrusion temperatures used inpractice.
The exit speeds VA are scattered around a linewith a steep negative gradient that can be ap-proximated by the expression:
�2V � kA f
This empirical equation [Ake 68] can be ex-plained by the laws of heat penetration [Lag 72].
5.7.4 Section Surface and Surface Defects
In all stages in the production of aluminumextruded sections, the plant and process tech-
nologies are influenced mainly by the need toavoid defects that would impair the mechanicalproperties or the decorative appearance [Bry 71,Fin 92, Par 96]. The following defects are par-ticularly important:
● Uneven color tone (bands or flecks)● Coarse recrystallization● Excessive roughness (scoring or pickup)● Imperfectly bonded brittle contact areas be-
tween material streams flowing together(piping, defective welds)
● Material separation in the form of transversecracks, blisters, and flaking
● Subsequent mechanical damage or corrosion
The cause of these defects can be found in thestructure of the billets, the flow process, and theextrusion temperature, in a subsequent heattreatment or defects in the logistics (handling,transport, and storage of the finished sections).
To determine and eliminate the causes ofthese defects, correct diagnosis and an accurateknowledge of the process technological relation-ships are necessary. The results obtained frommore accurate analysis of the flow processes andthe temperature field contribute significantly tothe understanding of these relationships. Men-tion should also be made of the extrusion defectcatalog produced by the “extrusion” workingparty in conjunction with the Institute for Ma-terial Science of the RWTH Aachen University.
5.7.4.1 Section Surface andthe Peripheral Layer
Microgeometric surface defects and struc-tural-related tone variations (banding, flecks) arerelated to the method of formation of the sectionsurface and the additional through working andheating of the peripheral layer compared withthe section core. Using high resolution markingand evaluation processes, it is possible to local-ize the exact source of the peripheral layer in thebillet [Val 88, Val 92, Val 92a]. Only a smallmaterial volume located between the dead metalzone and the deformation zone is involved in theformation of a 50 lm-thick peripheral layer, andthis can be extended by a factor of more than1000 [Val 92a]. This extreme working of the pe-ripheral layer deforms a cast grain to a strip thatis thinner than the subgrain size correspondingto the deformation conditions. The subgrainsthen grow by recovery through the original grainboundaries, as shown in Chapter 4 in the sectionon metallurgical basics.
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 213
The peripheral layer therefore has a subgrainstructure without defined grain boundaries, pro-viding the heat from extrusion or subsequent so-lution heat treatment does not result in static re-crystallization. This high deformation of theperipheral layer does not produce any higherhardness in the as-extruded condition [Moe 75].
An additional feature of the highly deformedperipheral layer is its �001� {211} shear texture,which differs from the �111� � �100� doublefiber structure of the core zone [Moe 75,Auk 96].
Rows of precipitates are pulled apart by theextreme deformation and moved closer togetherin the transverse direction so that the particlesappear to be irregularly arranged. The recrystal-lization retarding effect of layers heavily deco-rated with particles is then lost. This results inundesired coarse grain recrystallization of theAlCuMg and AlZnMg alloys, particularly at theend of extrusion where the peripheral layerforms up to 90% of the cross section.
5.7.4.2 Cast Structure andAnodizing Capability
Knowledge of the relationship between the lo-cation of the volume element in the billet and inthe section is the key to explaining and over-coming linear and banded tone variations in an-odized sections [Ake 88, Bau 71, Lyn 71,Sta 90].
The formation of the cast structure of the bil-let is discussed in Chapter 4, in the section onmetallurgical basics. From the point of view ofthe uniformity of the anodized surface, the fol-lowing regions of the billet surface are impor-tant:
● The oxide skin (10–100nm)● The segregation zone (up to �600 lm)● The impoverished peripheral zone● The periodic cold shut (up to several mm)● The columnar solidified crust (up to 15 mm)● The primary solidified regions scattered over
the entire billet cross section
Differences in the brightness of the oxidelayer are primarily caused by differences in thenumber of depressions from the initial alkalineetching. The etch attack does not occur uni-formly but preferentially at all types of particlesat grain boundaries. The brightness is affected,in particular, by the depressions that form at allAlFeSi particles. Particularly deep craters de-velop at the locations where the Mg2Si equilib-
rium particles are nucleated on AlFeSi. Experi-ments on the etching kinetics have shown that itis these craters that have a strong effect on thematte finish [Sta 90].The number of particles perunit area depends on the average dendrite spac-ing (cell size) in the cast structure and also onwhether a region is normally elongated or highlydeformed during extrusion. The impoverishedperipheral zones and the primary solidified re-gions appear bright after anodizing, whereas theoxide skin and the segregation zones matte. Thecontrolling factors for the frequency of nuclea-tion of Mg2Si and AlFeSi are the temperaturechanges during billet heat treatment and duringcooling or quenching after extrusion.
Pits form during etching along the grainboundaries. The shape of the pits depends on theformation of the age-hardening b-MgSi phase.In general, different cooling conditions (influ-encing the pit width) as well as different grainsize (influencing the pit length) can result in sig-nificant variations in brightness.
An orientation-dependent face etching occurson the surface during etching and this consistsof small cube faces with an edge length less than0.5 lm. The deformation texture of the sectionwith the {011} �211� principle components canform to different extents in different parts of theextrusion die. This results in an inhomogeneousdistribution of the cube faces and thus in differ-ent reflection properties. This is how some ofthe undesired leg marks can occur.
Investigations on a hollow section [War 95]have shown that different die designs for thesame profile cross section can result in com-pletely different decorative appearances. Feederchambers and splitting can affect the magnitudeand direction of the local strain of the surfacelayer. The die design therefore has a strong in-fluence on the texture, precipitate distribution,and grain size.
The three mechanisms just described for theroughening of the surface (depressions, pits, tex-ture) have approximately equal importance forthe decorative appearance of the anodized sec-tion. In addition, precipitates of the base metal,which can neither be dissolved nor removed dur-ing etching, end up in the oxide layer, giving ita cloudy or colored appearance.
The external oxide skin in single-cavity ex-trusion follows path 2 (Fig. 5.14) and finishes inthe interior of the sections forming a brittle layer(piping defect). In multicavity extrusion, sec-tions with reentrant angles, and hollow sections,the side adjacent to the center of the die consists
214 / Extrusion, Second Edition
Fig. 5.19 Highly deformed peripheral zone material (light) and normally elongated core material (dark) at the end of extrusion[Ake 88a]
of normally elongated material, whereas the ex-ternal side, particularly at the end of extrusion,can consist of highly deformed material [Ake88a] (Fig. 5.19).
If the distance between two die openings islarge, a dead metal zone can form so that thesection surfaces extend into highly deformedmaterial. Within a short distance the central deadmetal zone disappears and the section surfaceextends partly into the normally wrought fibrousmaterial [Ake 88, Val 96a]. In between the oxideand segregation layer of the billet appears on thesurface.
Cold shuts, which can extend several milli-meters into the billet peripheral zone, can finishup in the peripheral layer of the section follow-ing path 1 (Fig. 5.14). If the defects are deep,and if they occur in the front part of the billet,the entire section can be unusable [Bag 81].
In addition, other variations in the materialflow and in the through working with subse-quent static recrystallization can result in varia-tions in the size and orientation of the recrystal-lized grains and thus to undesired contrasts inbrightness on anodized section surfaces. Knownexamples include the side opposite to a branchin the profile cross section and the highly de-formed material in the area of a weld (see alsosection 5.7.6.5). In the extreme case variationsin the billet structure and in the through workingoccur as regions with different tones on the sameflat polished surface of a section.
5.7.4.3 Surface Roughness
In contrast to most deformed surfaces, ex-truded section surfaces of aluminum alloys inparticular are not formed by the extension anddissipation of the surface of the starting material[Kie 65, Mie 62]. Instead, the section surface isnewly formed from the interior of the materialby a cutting process between the deformationzones and the dead metal zones.
Two shear zones meet during this cutting pro-cess. In one, the flowing material is shearedalong the dead metal zone; in the other, it ismoved into the exit direction.
The extremely severe deformation of the out-ermost surface layer in the region of the entryedge can result in cavitation defects in hetero-geneous materials. Hollow spaces form by theseparation of nonplastic particles from the ma-trix or by the fracture of these particles withoutthe matrix metal filling the space between thefracture pieces. If the hollow spaces bond to-gether in the direction of extrusion and the hardparticles separate from the matrix, microgroovesform on the surface [She 88, Clo 86].
The surface formed in the region of the entryedge undergoes a change by adhesion wear as itpasses through the die aperture. The roughnessof the section is therefore usually higher thanthat of the die bearing.
In the same way as the friction load as anintegral value is strongly dependent on the
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 215
Fig. 5.20 Velocity distribution transverse across the die ap-erture
length and the angle of the bearing surface, thelocally effective mechanisms of friction andwear of the surfaces of the section and die varyalong the die bearing.
The following possibilities have to be consid-ered, as shown in Fig. 5.20:● Slip of the section as a solid body with a
sudden speed increase at the steel surface● Formation of a coherent layer of aluminum
adhering to the bearing surface and slip ofthe solid section with a sudden speed in-crease between the section and the bondedlayer
● Adhesion of the section peripheral layer onthe bearing surface, parabolic speed distri-bution across the section thickness
Slip exactly corresponding to “the first point” isnot seen in hot extrusion of aluminum withoutlubrication.
A friction “a�b” with adhesion of discretealuminum particles on the bearing surfaces cor-responds most closely to the experience of diecorrectors and with the nitriding of bearing sur-faces. Microscopic investigations on split die in-serts confirm that aluminum particles bond to thebearing surfaces to an increasing extent and sizein the extrusion direction and determine the mi-crogeometry of the section surface. The alumi-
num nodules that adhere to the bearing surfacesform grooves on the section surface. The for-mation of layers of aluminum, which can be in-terspersed with oxide, increases the number ofadhesion points opposing the extrusion direction[The 93]. The nodules are finally torn off withthe section and appear as particles (“pickup”) onthe surface [Tok 88, She 88, The 93, Abt 96].
In the majority of cases, die bearing lengthsare less than the section thickness. If the bearingsurfaces are parallel, mixed friction occurs overthe entire length. The surface roughness of theextruded product is governed by the adheredlayer, i.e., by the tendency for the material toadhere to the die [Mue 96, Mie 62, The 93]. Thisadhesion tendency cannot be completely sup-pressed by nitriding the bearing surfaces, but thealuminum layer builds more slowly on nitrideddie bearings. In the absence of lubrication, theaim for a high surface quality is not the maxi-mum possible undistorted slip of aluminum oversteel (slip process (a) in Fig. 5.20) but the mostuniform buildup of a thin adhered layer of alu-minum on the die bearing surface. This aim isbest fulfilled by die bearings ground perpendic-ular to the extrusion direction. Die surfacesground or polished parallel to the extrusion di-rection or polished favor an irregular buildup ofthe adhered layer in the form of long elongatedand correspondingly wide and high ridges. Fig-ure 5.21 [Tok 88] shows the roughness variationover the length of a flat section on a small re-search press.
The surface roughness of the extruded prod-uct decreases as the bearing length increases be-tween lk � 0 and lk � 1 to 3 mm because withincreasing back pressure the adhesive layer re-gresses at the entry edge. As the bearing lengthincreases, the roughness increases again as aconsequence of higher temperatures [Wel 96]and possible mechanical activation processes[She 88] (Fig. 5.22). The optimum has beenfound to be bearing lengths of the order of halfthe thickness for flat sections [Mie 62, Tok 76,Tok 88, She 88, The 93].
As the number of extrusions increases, thefine aluminum nodules combine to form a layerthat ultimately completely covers the die bearingsurface. The relationship between the surfaceroughness of the extruded product and thebuildup of the adhesive layer on the slip surfaceis shown in Fig. 5.23 [The 93].
In the initial region 1, no aluminum adheresto the bearing surfaces so that section roughnessis governed by the surface finish of the die. In
216 / Extrusion, Second Edition
Fig. 5.21 Influence of the finish of the bearing surface on the quality of the section surface [Tok 88]
Fig. 5.22 Influence of the bearing length on the roughnessRa of the section surface [She 88]
region 2, the aluminum adhesive layer builds upand the section roughness increases rapidly.When the bearing has been completely covered,dynamic equilibrium develops in region 3 be-tween adhesion and tearing away of aluminumparticles so that the surface roughness remainsalmost constant. The section roughness is sig-nificantly lower than the roughness of the ad-hered layer. A reduction in the section roughnesswas periodically observed in trials involving 180extrusions, and this was attributed to a partial orcomplete detachment of the coating.
The angle of the die bearing has a significantinfluence on the friction force and results in adifferent formation of the coating [Mue 96].However, it has only a small influence on thesection roughness. Hand-filed bearing surfacesclose up at the entry and open up at the exit.Mixed friction dominates on bearings that arenot too long, yet the changes in the face pressureand the angle result in a different formation ofthe coating. In the opening exit area loose ad-hesions form and damage the section surface bydie lines and pickup [Clo 90].
As the billet temperature increases, the ten-dency for adhesion between the section surfaceand the bearing surfaces increases [Mie 62, Tok88, Clo 90].
For a given billet temperature, the roughness,the degree of pickup, and the scatter of the grayscale (as a measure of the degree of streaking)increase to a maximum with increasing extru-sion speed and then decrease at even fasterspeeds. This is explained by the hypothesis thatat higher speeds the section surface slides overa thin layer of almost liquid metal [Par 96].
Alloy type, alloy content, and differences inthe thermal pretreatment of the billets can havea significant influence on the extrusion speedthat can be achieved for given surface finish re-
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 217
Fig. 5.23 Relationship between adhesive layer formation,friction stress, and surface roughness [The 93]
quirements. Precipitates of alloying constituentshave the worst effect.
5.7.4.4 Transverse Cracks and Peeling
The tensile stresses produced in the peripherallayer by the friction on the bearing combinedwith the considerably higher temperature [Moo96, Joh 96] and the correspondingly lower flowstress result in defects, which can penetrate deepinto the section from the surface. Figure 5.24 (a–d) shows the qualitative variation in the velocityand the stress at the entry to the die aperture.
The material being formed flows from the de-formation zone into the die aperture. The profilecore flows faster than the surface section, whichis retarded by the friction at the surface. The core
remains in compression, but a tensile stress pre-vails in the peripheral zone. If this stress is toolow to produce plastic strain, the section canflow as a solid body through the die aperture(Fig. 5.24a). Higher tensile stresses can result inan elongation of the hotter and at the entryslower flowing peripheral layer. This is seen par-ticularly on thin ribs and internal legs of hollowsections (Fig. 5.24b). The elongation of the pe-ripheral layer after the removal of the dominat-ing high mean compressive stress in front of thedie results in various types of material defects,depending on the structure and the peripherallayer temperature. Lubricant residues that inad-vertently have reached the inner wall of the con-tainer can flow into the section following path 1in Fig. 5.14 and form an interface parallel to thesurface. The surface layer can break away dur-ing extrusion (peeling) (Fig. 5.24c). In othercases, the material of the interface can break upduring a subsequent heat treatment, and the sur-face layer bulges out in the form of a row ofblisters. Peeling is also caused by the melting ofisolated eutectic regions resulting from unfavor-able casting and homogenization conditions [Rei84, Lef 96].
A classic defect process is the formation,growth, and the amalgamation of voids resultingin transverse cracks that penetrate deeply intothe section (fir tree) (Fig. 5.24d), The propaga-tion and expansion of the cracks occur at thelocation of the plastic elongation of the periph-eral layer. At the same time the transfer of thetensile stresses into the billet is prevented so thatless material flows into the peripheral layer. Thevolume deficit in the peripheral zone is thereforeconsiderably larger than in the case of plasticelongation as shown in Fig. 5.24(b).
In the common aluminum wrought alloys themelting of individual structural regions, e.g.,along grain boundaries, is the defect initiatingprocess (hot shortness). The transient exceedingof the solidus temperature in the peripheral layeris the most common cause of transverse cracks.The average section temperature measured at theexit is naturally well below the solidus tempera-ture.
Another type of transverse crack that can oc-cur well below the solidus temperature occurs inmaterials with a high fraction (�15%) of non-plastic brittle structural components.
5.7.5 Procedures to Controlthe Thermal Balance
As a general rule, the optimal productivityand quality with aluminum alloys can only be
218 / Extrusion, Second Edition
Fig. 5.24 Influence of tensile stresses in the section peripheral layer. 1, appearance of the extrusion; 2, velocity variation acrossthe section; 3, variation of the longitudinal stresses across the section
achieved by taking the thermal balance of theextrusion process into consideration. The tem-perature control is optimized by measures thateither reduce the amount of heat that has to beremoved or accelerate the heat transfer [Ake 71,Ake 80]. These measures have been describedto some extent along with the extrusion process(Chapter 3). The technicalities of temperaturecontrol are described in more detail in Chapter 6.
5.7.5.1 Material
The heat produced during the extrusion pro-cess from the transformation of the mechanicalwork can be minimized by selecting the mosteasily extruded alloy from those that meet theproduct specification (see the section ”Materi-als”).
The additional fine adjustment covers the op-timization of the composition, the solidificationconditions, and the thermal treatment of the bil-let. Small changes appear to result in consider-able differences in the extrusion speeds that canbe achieved [Ake 71, Sta 90, Rei 84, Sca 69,Lyn 71, Lan 82, Spe 84, Lan 84].
5.7.5.2 Billet Temperature
The exit temperature is higher than the billetpreheat temperature because of the deformationinduced heating of the material being deformed;therefore, the billet is usually heated to a tem-
perature below the desired average exit sectiontemperature. The material reaches the desiredvalue only as it passes through the deformationzone. This is described in detail in Chapter 3.
A uniform temperature gradient from thefront to the rear of the billet can compensateapproximately for the increasing heat from theshear deformation of the peripheral layer. Ad-ditional heating of the front of the billet reducesthe load needed at the start of the extrusion pro-cess and ensures that there is sufficient solutionheat treatment effect at the front of the section.This method of achieving “isothermal extru-sion,” however, requires rapid localized heatingor cooling because of the good thermal conduc-tivity of the aluminum. The billet then has to beextruded with minimum delay.
In general, the effectiveness of the proceduresin this first group increases as the billet cycletime decreases, for example the extrudability ofthe alloy improves.
5.7.5.3 Heat Removal
The opposite applies to the following proce-dures, which require heat flow into the toolingand which are most effective for slow extrusionspeeds and long cycle times.
Container. The most important heat sink isthe container, which plays an important role inthe productivity in the extrusion of aluminumalloys. The container is basically heated to a
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 219
temperature below that of the specified billetpreheat temperature. The temperature differenceis usually limited to 50 K in order to avoid thebillet sticking in the container because of inade-quate press power (“sticker”). The thermal bal-ance in the system container-billet is describedin Chapter 3.
The temperature set or recorded on the con-tainer heater controller applies only to the hottestlocation, where, from experience, overheating ofthe tool steel has to be avoided. The actual tem-perature field in the container is shown in theexample in Fig. 3.19. There are temperature dif-ferences of the order of 100 K in the axial andradial direction. The temperature maximummoves during 5 to 10 extrusions from the heat-ing zone to the liner inner face [Ake 71, Ake 72,Sch 79].
The cooling, i.e., the heat flow from the con-tainer bore through the wall and into the atmo-sphere, is determined largely by the thermal con-ductivity of the container wall. Cooling of themantle surface by an intensive air blast has onlyan insignificant effect on the cooling. Blowingonto the container end face has more effect.
Today, combined heating and cooling systemsare installed in containers for temperature con-trol (see the section on tooling in chapter 7).
Die. Removing excess deformation heatthrough the die is helped by the short thermalpath from the deformation zone to the locationof the cooling channels. Another advantage isthat the cooling takes place during the final stageof the deformation so that the load requirementis increased only during the final deformationstage. There is also the possibility of specificallycooling the critical regions, e.g., at the edgesprone to tearing.
Because a significant amount of heat has toflow through a relatively small die cross section,it is necessary from the cooling point of view tohave an intense heat transfer to a coolant withsufficiently large thermal capacity.
This requirement is more easily fulfilled withliquid coolants than gas. On this basis, the fol-lowing methods can be differentiated for diecooling:
● Water cooling● Cooling with liquid-supplied evaporating ni-
trogen● Flushing with gaseous nitrogen
Approximate calculations demonstrate thatthe cooling capacity through a plate 200 mm indiameter is in the range between 2 and 12 kW,
depending on the thickness and the coolant. Thiscan be compared with the drive power of a pressof up to 400 kW.
The approximate calculations show that withthe fast extrusion of alloys with a high extrud-ability, only a small amount of deformation heatcan be removed through a cooled die. With aplate thickness of 50 mm, an aluminum flow of1 kg/s can at best be cooled by approximately 6K with water and by only approximately 2.5 Kwith liquid nitrogen.
A significant reduction in the exit temperatureis achieved only with the slow extrusion of thedifficult-to-work alloys. In trials it has been pos-sible to increase the exit speed of an AlCuMgalloy by 30% using water cooling. Approxi-mately 15% of the deformation heat can be re-moved by the water cooling [Bat 79].
The limited resistance of the die materials totemperature fluctuations makes water cooling ofa die difficult.
Liquid nitrogen is usually fed through chan-nels machined either in the die itself [Sca 79] ormore commonly in the front face of the backer[Wag, Sel 84, Yam 92]. If a separate feeder plateis used, an additional network of cooling chan-nels can be machined in the rear face so that thedie is cooled from the front and the back [Ros].
The aim is for the nitrogen fed in as a liquidto evaporate extensively as it flows through thecooling channels. This gives an approximatelyconstant cooling capability, the effect of whichis distributed over a more or less large aluminummaterial flow, depending on the extrusion speed.
In experiments with nine different alloys, theextrusion speed of AlMgSi0.5 could be in-creased by approximately 8% by cooling, butwith the more slowly flowing alloy AlZn-MgCu1.5, by a significant 80% [Sca 79].
The dwell time of the deformation material inthe die region is short so that the peripheral zoneof the section is cooled the most. This reducesthe temperature peaks, which are largely respon-sible for the tearing of the section surface at highextrusion speeds (Fig. 5.25) [Sel 84].
This is the reason why even with limited cool-ing effect the speed can often also be increasedconsiderably with fast-flowing alloys. The evap-orated nitrogen emerges from behind the die ap-erture onto the surface of the newly extrudedsection to utilize the protective atmosphere ef-fect (see below).
Gaseous nitrogen is fed through channels inthe gap between the die and the backer into thespace behind the die exit. The mass flow of the
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Fig. 5.25 Temperature variation in the die with and withoutcooling with liquid nitrogen [Sel 84]
nitrogen stream is usually not more than 1% ofthat of the aluminum stream because of the lowdensity. The nitrogen flow is therefore not suf-ficient to cause any serious cooling.
The regularly observed improvement in thesection surface must therefore be more attribut-able to an influence on the dynamic equilibriumof adhesion and tearing off of the aluminum par-ticles on the die bearing by the reaction of thenewly formed aluminum surface with the at-mosphere.
Mandrel. In contrast to heavy metal extrusionwhere cooling of the mandrel is unavoidable be-cause of the high thermal loading, in aluminumextrusion, mandrel cooling is not required tocompensate for high temperatures. In practice,mandrel cooling is therefore not used. However,temperature measurements on the extruded pro-file have shown that the specific heat removedby cooled mandrels is at least equal to that as-sociated with die cooling with liquid nitrogen sothat it appears to be possible to achieve highermaximum extrusion speeds by extruding overcooled mandrels [Rup 82].
5.7.5.4 Optimization of theExtrusion Parameters
Following the discussion of the differentmeans of influencing the thermal balance their
application to the optimisation of the extrusionparameters will now be considered. Apart fromother limiting additional parameters the extru-sion parameters are at an optimum if the exittemperature over the entire cross-section fallswithin the permitted range for the alloy underconsideration, the extrusion speed is as high aspossible in the range limited by the equipment,and the extrusion load needed uses the maxi-mum press power of the press within a specificsafety factor [Rup 77, Rup 77a, Rup 83].
The parameters that can be set at the press arethe ram speed, the initial billet temperature, andthe container temperature. The billet tempera-ture and speed can be constant for a heating andextrusion cycle or be decreasing from the frontto the back and then be adjusted in subsequentextrusion cycles.
Of all the set parameters, only the extrusionspeed can be varied during a single extrusioncycle. The billet temperature and the tempera-ture profile can, in contrast, be set for each cyclebefore loading into the press. A change in theaverage temperature or the temperature profileis possible in experimental work and under pro-duction conditions in the next billet. The tem-perature profile in the container with the usualarrangement of heating elements in the containermantle follows a change in the set temperatureonly after a long delay of several hours. A rapidchange in the container temperature can beachieved with heating and cooling systems in-stalled in the liner [Har 94].
The parameter that has to be controlled is thesection exit temperature; however, there are nu-merous difficulties in its continuous measure-ment, particularly in the case of aluminumalloys:
● The critical maximum temperature occurs inan unknown location in the die aperture thatis practically impossible to access for mea-surements.
● The critical temperature occurs for only avery short time.
● The section surface is soft and easily dam-aged.
● The radiation is in the invisible infraredrange.
● The emissivity of bright aluminum surfacesis very low and also is strongly dependenton the roughness and the wavelength (it isnot a “gray” body).
● The measurement area is surrounded bystrong sources of additional radiation.
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 221
Fig. 5.26 Laminar material flow [Ake 92]. 1, adhesive fric-tion; 2, dead metal zone
● The display of all temperature sensors is as-sociated with definite delays and with sig-nificant measurement scatter.
The original concept of isothermal extrusionassumed a continuous control of the speed basedon the measured exit temperature [Lau 60]. Thisconcept is difficult to achieve because of the dif-ficulties mentioned previously, as well as the de-layed response of the exit temperature to achange in the speed associated with the heat con-tent of the material in the press. Changing thespeed with a rapid reacting control system wouldresult in an unstable reaction in the control loop.
The control systems used or proposed andtested today attempt to cope with these problemsassociated with exit temperature measurement invarious ways. All these control systems arebased on optimization concepts with the aim ofmaintaining the exit temperature within the de-sired range by suitable combinations of theavailable parameters and simultaneously maxi-mizing the productivity of the press. One factorto take into account is that every change of oneparameter changes all heat sources and heatflows, i.e., the factors that combine to give theexit temperature [Ake 80]. This is described inmore detail in Chapter 6.
5.7.6 Joining by Extrusion
The production of complex sections with oneor more cavities plays a particularly importantrole in aluminum alloys. Approximately half thesections produced in the easy to moderately dif-ficult to extrude AlMgSi alloys are hollow sec-tions such as box beams and hollow engineeringplates. Compared with the functional corre-sponding solid sections (I-beams, plates with T-shaped reinforcements) the hollow sections havethe advantage of a much higher torsional andbending stiffness. With the exception of tubes inthe strictest sense, the hollow sections have lon-gitudinal welds where the metal streams split bythe supporting legs of the mandrel reunite in ametallic bond.
The joining of the metal from billets extrudedone after the other by transverse welds is usedfor products in long lengths and large coilweights. In addition, having a material residuein the die entry or a feeder chamber providesexact positioning of the front of the section,which is essential for automation of the guidingof the front end. The next billet has to join tothe residual material in the die.
Special applications of joining in extrusionare cladding or locally reinforced sections of twodifferent materials (two aluminum alloys) (alu-minum-copper, aluminum-steel), the compac-tion and deformation of powder and granularscrap, and the production of composite materialswith a metal matrix. This is described later inthis chapter.
5.7.6.1 Extrusion Welding
The general metallurgical principles of bond-ing by extrusion are discussed in Chapter 4. Thefollowing section describes how the extrusionprocess relevant properties of the aluminum al-loys influence the technology of extrusion weld-ing and the quality characteristics of the ex-truded welds.
Aluminum alloys can be extruded in a tem-perature range at which the tooling is not over-heated so that the stability and service life ofeven complex hollow section dies are adequate.The high adhesion affinity for steel results inadhesive friction in the die at areas with highsurface pressures and to the formation of deadmetal zones (Fig. 5.26).
After only a short time in air, newly formedaluminum surfaces are coated with an oxidelayer that cannot be reduced by any gas atmo-sphere or removed by diffusion of the oxygeninto the parent metal. Aluminum therefore be-longs to the group of metals that cannot bejoined without macroscopic shape change, i.e.,not by mere pressure and heat (diffusion weld-ing) [Jel 79, Bry 75].
222 / Extrusion, Second Edition
Fig. 5.27 Extrusion of hollow sections. (a) Straight into the ports. (b) At an angle under the legs. (c) At an angle between two mandrelheads. (d) Emergence of the section [Ake 92]
In order for two aluminum bodies to join inthe solid state, these brittle separating oxide lay-ers require a significant combined deformationto break up the oxide skin and to bring nonox-idized metal to the joint. The generic term de-formation welding includes roll cladding, com-posite impact extrusion, and powder extrusion,as well as extrusion welding. It is possible fromobservations of these processes using generalmetallurgical basic principles to draw conclu-sions on the processes (previously studied onlyto a limited extent) that occur during bonding byextrusion.
The influence of the initial unevenness of thesurfaces is also important [Ast 75]. The adhesionprocess is the result of the plastic flow of themetal in the contact zone, the deformation of therough tips, the formation of slip and shear bandsin the region of the contact surfaces, and themovement of the dislocation fields of the indi-vidual contacts. Uneven contact areas, however,rarely occur in longitudinal welds and then inconjunction with cavities under the support legsof the mandrel system.
5.7.6.2 Longitudinal Welds inHollow Sections
When applying these thoughts on deforma-tion welding to extrusion welding, it is assumedthat the latter consists of three sequential extru-sion processes and the associated transport and
deviation processes (Fig. 5.27). In the first stage(Fig. 5.27a), the billet is divided into two ormore streams that flow together under the legs(Fig. 5.27b, c). In the deformation final stage(Fig. 5.27d), the hollow section forms by themetallic bonding of the streams.
The processes at the contact areas are relatedto the material flow (Fig. 5.26). The conditionson the exit side of the leg are critical where ei-ther a dead metal zone or a cavity can form.
Normally, the pressure in the welding cham-ber is so high that the space under the legs iscompletely filled and a dead metal zone forms.The conditions for the formation of a dead metalzone and for a good weld have been expressedby several authors in the form of ratios that canbe deduced from the die geometry and providea reference point for the magnitude of the totaldeformation. As well as the size of the weldingchamber, the ratio of the welding chamber crosssection to the profile cross section is important[Gil 75].
The pressure applied at the point of formationof the longitudinal weld is composed of com-ponents for the reduction in cross-sectional areafrom the welding chamber to the section, for thedeviation in the flow of the metal stream and forovercoming the friction in the die aperture.
Oxide can only enter the welding chamberwith the first filling of the die when the free ox-ide coated front faces of the streams meet (Fig.5.28a) [Ake 92]. The fractured residues of these
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 223
Fig. 5.29 Formation of a hollow space (gas pocket) under the mandrel support leg at a low extrusion ratio (a) and a dead metalzone at a higher extrusion ratio (b). On the left, flow lines; right, lines of equal strain rate [Wel 95]
Fig. 5.28 Breakup of the surface layer in longitudinal welds. (a) Meeting under the leg. (b) Breaking apart. (c) Clean longitudinalweld [Ake 92]
oxide films are largely removed with the frontpart of the section (Fig. 5.28b). Further bondingoccurs between material regions that initiallyhave been upset in front of the legs and mandrelsand have then flown along the flanks of the legwith severe shear distortion to collect in the deadmetal zone under the legs. The streams that meettogether have no free surface and, therefore, noroughness and no oxide film. The welding pro-cess then merely consists of the initially smallcontact area being extended over the total lengthof the section and thus being enlarged by severalorders of magnitude, whereby the subgrains alsocontinuously reform over the original contact ar-eas. This repolygonization is associated with themovement of dislocations but does not requirediffusion of atoms.
Die geometries where the cross section of thematerial flow path is too narrow should beavoided because these result in inadequate feedinto the weld chamber and in an inadequatepressure buildup. This can result in cavities
forming on the exit side of the leg (gas pockets)[Val 95].
Two-dimensional numerical simulation andexperimental semiplanar extrusion both showedthe formation of cavity with a section thicknessof 25 mm and a dead metal zone with a sectionthickness of 12.5 mm for the same weldingchamber height (Fig. 5.29) [Wel 95].
The contact with the leg is lost under the con-ditions where a cavity is formed, and it is thefree more or less roughened surfaces of thestreams that meet in the welding chamber.
The bonding is then restricted to the roughcrests [Val 95, Val 96], has little ductility, and isabove all very brittle [Ake 92]. Voids in the jointare recognizable in the fracture as bands withoutfracture dimples and they are preferentiallyetched in the cross section by alkaline etchants.
Transverse cracks can also occur in the regionof the longitudinal welds. In principle these areno different to the surface and edge cracks thatoccur on solid sections when the peripheral layer
224 / Extrusion, Second Edition
Fig. 5.30 Extrusion of aluminum “billet on billet.” (a) Extrusion of a billet on the discard R in the container. (b) With feeder die. (c)In the ports of a porthole die. (d) Pulling off the discard in a bridge die to avoid transverse welds [Ake 92]
reaches the hot-shortness temperature [Val 96a].The fact that the transverse cracks occur pref-erentially next to the longitudinal welds can beattributed to the severe heating of the peripherallayers of the metal streams, particularly underthe legs [Ska 96].
5.7.6.3 Transverse Welds
Transverse welds are usually the severelycurved surfaces where the material of the nextbillet bonds with the residue of the previouslyextruded billet. This residue can be located inthe container, in the feeder chamber of a die fora solid section, or in the entry ports of a hollowdie (Fig. 5.30a–c).
The process where the residue is left as a thickdiscard in the container (Fig. 5.30a) is usuallyused for the extrusion of cable sheaths and elec-tric conductors in large coil weights. Specialmeasures have to be taken to avoid smearing ofthe contact surfaces.
The process shown in Fig. 5.30(b) and (c) inwhich the discard is removed in front of thefeeder chamber or in front of the entry ports iscommon for aluminum sections. Stripping of thediscard (Fig. 5.30d) is also possible with alu-minum but increases in difficulty with morecomplex sections.
The originally flat contact surface between theresidue and the next billet is deformed to atongue because of the adhesion of the aluminumto the tool surface. The outline of the tongueapproximates to the contour of the correspond-ing material stream. In the region of the tips ofthese tongues the contact surface can contract byas much as 50% so that the fractured residues ofthe oxide layer are more likely to slip over each
other than be pulled apart. In the most unfavor-able case, there is no bonding [Ake 72], particu-larly with the current practice considered to beunavoidable of lubricating the dummy block andthe discard shear [Joh 96a].
With highly stressed sections, a length longenough to contain the tongue of the transverseweld is removed as scrap from the stop mark.This section usually includes one to two timesthe content of the entry zones and the weldingchambers.
In most of the transverse weld the fracturedresidues of the oxide layer are extensively pulledapart. These profiles can be supplied with thetransverse weld for noncritical applications pro-viding the material surrounding the tongues,which originates from the previously extrudedbillet, can withstand the stresses from thestretching of the section.
When the method of formation is considered,it is easy to understand that there is a greater riskof transverse welds being impaired by surfacecoatings than longitudinal welds. When materialseparations apparently occur in longitudinalwelds, usually the transverse welds runningalong both sides are involved [Val 95].
The most important sources of defects in ex-trusion welds are [Wei 92]:
● Lubricant or corrosion on the dies● Impurities on the billet front surface or dirt
on the billet surface● Incorrect die design
Basically, all parts that come into contact withthe material being extruded must be clean and,in particular, be free of oil, grease, or graphite.The requirement for perfect separation of the
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 225
dummy block and the discard, however, resultsin practice in compromise solutions in whichspecific lubricants are considered to be indis-pensable but have to be used as little as possibleand accurately applied.
5.7.6.4 Testing of Extrusion Welds
The extrusion weld has to be considered as asurface of which a fraction fD is covered withfragments of the original surface coating [Ake92]. The remaining fraction f1 consists of me-tallic ligaments with the same flow stress as theparent material. These ligaments are very shortin relationship to their width and can thereforewithstand a high three-dimensional tensile stressunder tensile loading.
In a tensile test perpendicular to an extrusionweld the specimen can completely neck outsidethe weld. In the plastic region where the limit ofelastic deformation is exceeded many timesover, failure in the extrusion weld can still occur.The concentration of fragments of the surfacecoating on the contacting surfaces facilitates thedamage with increasing deformation, i.e., theformation, the growth, and the combination ofpores to form microcracks and, finally, failure.
The usual parameters of 0.2% proof stress andthe tensile strength from the tensile test havelimited relevance to the quality testing of extru-sion welds. The increased sensitivity to damagein the region of the extrusion weld comparedwith the parent material is best expressed by adecrease in the reduction in area at fracture. Thequicker the extrusion weld is damaged, thelower is the strain to fracture [Ake 92]. Withdecreasing extrusion weld quality, the elonga-tion and, in the extreme case, the tensilestrength, are also impaired as well as the reduc-tion in area.
Structural Investigation. Because the infor-mation from tensile tests is limited, numerousother testing procedures are used for testing thequality of extrusion welds. The location of thelongitudinal and transverse welds can be seen inthe macrostructure. If a section is cut from anextrusion at an externally visible interface be-tween two billets, it is possible to identify fromthe structure between neighboring transversewelds whether sufficient material has been cutout to achieve sound material. An extremely in-tensive etch attack will reveal the inclusion of alubricant or the peripheral layer.
Ultrasonic Testing, Fatigue Investigations.Ultrasound enables both hidden material sepa-
rations and laminar enrichments of noncoherentparticles to be detected. For example, thetongues of transverse welds can be localized. Infatigue studies the quality of the extrusion weldsdoes not have a significant influence on the totalnumber of cycles but does on the rate of growthof a crack in the extrusion weld at the end of theendurance.
Various Mechanical Tests. A critical me-chanical test for extrusion welds requires that thestrain is reached at which failure will occur inthe parent material or in the weld. This criticalstrain is strongly material dependent because theenergy released from the material separation(“the damage potential”) grows with the squareof the elastic limit. Various technological testingprocedures [Gil 75] enable the controlled trans-verse of strains that correspond in tensile testsapproximately to the region of uniform elonga-tion up to fracture necking:
● Bending tests on samples taken transverse tothe extrusion welds. The limiting bendingangle is measured at a section dependentspecified bending radius.
● Folding tests along and between the weldsup to an internal radius �0 and elongationof the outer surface of �100%
● Axial compression of a tube section to thepoint of folding
● Bending of tubes with a roll bending systemunder longitudinal loading of the extrusionweld
● Expansion with a conical mandrel and mea-surement of the strain in the circumferentialdirection
These technological testing procedures are suit-able for following quality variations along andbetween profiles with the same cross section andthe same material and for monitoring the main-tenance of tolerance limits that have been spec-ified for the individual application. However,they do not supply any characteristic values thatcould be used for numerical validation of thesafety of a design. The “calibration” of a tech-nological testing process from the point of viewof a fracture mechanics test value is still an un-resolved problem.
5.7.6.5 Influence of the Extrusion Weldon the Surface Appearance
Although it is only a few atomic spacingsthick, an extrusion weld can affect the appear-ance of the anodic film on a visible surface in
226 / Extrusion, Second Edition
the form of a band several times wider than thewall thickness [War 95]. Depending on the in-cidence angle of the light, the area of the sameextrusion weld can appear to be lighter or darkerthan the rest of the surface. The primary causeof this optical contrast between the region of theextrusion weld and the residual material is theheavier working of the peripheral zones of thesplit streams compared with the core zone. Inthe region of the weld the section consists ofheavily deformed material throughout its fullthickness, which results in variations in the sizeand orientation of the grains and thus the reflec-tion behavior.
5.7.6.6 Suitability of the Material forHollow Section Extrusion
The selection of the material and the extrusiontemperature as well as the die geometry and theavoidance of impurities play an important rolein determining the practical applicability of ex-trusion welding. As a basic rule, all wrought alu-minum alloys can be extrusion welded [Ake72a]. The extrusion pressure, which is alreadyincreased by the flow through narrow angledchannels, increases with increasing flow stressto values that overload the mandrel. However,hinge sections with a small core (the hole for thepivot) are extruded with bridge dies in the high-strength AlCuMg1.5 and AlZnMgCu alloys[Cre]. Engineering sections with large cores areusually produced in the alloys of the groupAlMgSi0.7, AlMgSi1, and AlZnMg.
Only the strength and toughness of the parentmetal in the thickness direction is achieved inthe transverse direction across the weld becauseof the fiber orientation in the region of the ex-trusion weld. In alloys that have a high manga-nese content to obtain the extrusion effect, trans-verse sections frequently exhibit lamellar tearsalong surfaces parallel to the extrusion weld,which are enriched with eutectically precipitatedparticles. The mechanical properties in the re-gion of the weld can also be impaired by coarse-grain recrystallization of heavily worked mate-rial regions. There are alloys available forhollow engineering sections that recrystallizecompletely and that include peritectically solid-ifying transition elements instead of manganese[Scw 84].
With the more easily extruded alloys, mainlyAlMgSi0.5, a complete fine-grain recrystalliza-tion is desired to give the decorative appearance.
Materials
Gunther Scharf*
The selection of the alloy for extrusion is usu-ally based on achieving a given property rangewith specific surface properties at the lowestpossible cost. These include the costs for the bil-let; the actual extrusion; the heat treatment; andthe additional operations including stretching,detwisting, and cross-sectional correction.
In contrast to rolling where the strength is in-creased significantly by cold working, extrudedproducts are hot worked and, therefore, the ma-terial condition is practically soft. Cold workingcan be considered for wires and tubes, which aredrawn to their final dimensions. However, alu-minum alloys are usually extruded to the finaldimensions and the optimal mechanical proper-ties are produced by a suitable heat treatment.
With the exception of the isolated use of sil-ver, the usual alloying additions to aluminumalloys have only a slight effect on the cost of thebillets. However, the influence on the cost of ex-trusion, heat treatment, and correction is muchhigher.
The influence of the most common alloyingadditions, magnesium, copper, silicon, and zinc,on the flow stress can be found in section 5.5and in Fig. 5.12 in section 5.7.
Magnesium increases the flow stress the mostfollowed by copper, whereas silicon only has asmall influence and zinc, practically no increase[Ake 70]. The influence of intermetallic phases,in particular of Mg2Si, has also been extensivelystudied. This has shown that it is not only theamount of the alloying constituents that in-creases the flow stress and thus reduces the ex-trusion speed, but also that the microstructure inwhich the alloying constituents occur plays animportant role [Sca 69, Gru 66, Sca 69b]. Su-persaturated dissolved or fine dispersions of pre-cipitated Mg2Si increase the flow stress and thusmake deformation more difficult, whereascoarse precipitates of Mg2Si phases reduce theflow stress.
The influence of the structure has a particu-larly significant effect on those alloying ele-ments that tend to supersaturation during solid-ification and later precipitate during subsequent
*Materials, Gunther Scharf
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 227
Fig. 5.31 0.2% proof stress of extruded aluminum alloys inthe standard as-supplied condition as a function
of the flow stress at the usual extrusion temperatures [Ake 71]
billet heat treatment as a reversible fine disper-sion. The influence of manganese, in particular,is strongly dependent on the precipitate state andon the number and fineness of the intermetallicparticles [Gru 66]. Intermetallic phases includ-ing Al6Mn, MnFeSi, Al3Zr, and FeAl3 reducethe extrudability significantly [Sca 67, Sca 69a].
Figure 5.31 shows the product strength of themost common aluminum alloys as a function ofthe flow stress at the extrusion temperature.
The majority of extruded products thereforeare produced in heat treatable alloys with thelowest possible alloy content. The desire to re-duce the magnesium content by a tenth or evena hundredth of a percent is due to the stronginfluence of the flow stress on the cost of extru-sion as well as the downstream processes. Theextrusion load required is proportional to theflow stress as is the deformation work convertedinto heat. Another important consideration isthat the time needed for this heat to penetrateinto the tooling increases with the square of theflow stress, and the extrusion speed decreasesaccordingly. Apart from the dead cycle time,which is largely independent of the material, themachine costs of the extrusion process alone in-crease to the cube of the flow stress. The heat
treatment costs and correction processes also in-crease with higher alloy contents.
The heat treatment of age-hardening alloysbasically consists of the processes (see alsoChapter 4):
● Dissolution of the alloying constituents● Quenching to retain in metastable solution● Age hardening, usually hot, in one or more
temperature stages
An important simplification is possible if theproduct leaves the press with a temperature inthe region of solution heat treatment and can beimmediately quenched to achieve the desired re-sult. This requires that the interval between thesolution temperature and the start of melting isgreater than the unavoidable temperature errorand differences along the length of and acrossthe section. In other cases an optimum solutionheat treatment can be achieved only by a sepa-rate operation in a very accurately controlledoven. In this case, the material cannot be “pressquenched.”
As the alloy content is reduced, the precipi-tation pressure decreases. The section does nothave to be quenched so quickly but can becooled using less drastic means. This has the ad-vantage that distortion resulting from excessivelocalized temperature differences is avoided.Thin AlMgSi0.5 sections can be adequatelycooled in static air or under fans. A more inten-sive heat transfer is necessary for thick wall sec-tions and higher alloy contents (e.g., Al-MgSi0.7). This can be obtained with high airvelocities (nozzles) or air water mixtures (mistnozzles) [Kra 93, Str 96]. The cooling require-ments provide an additional material-dependentcost factor partly related to the cost of the cool-ing system but mainly because of the correctioncosts that rapidly increase with increasing cool-ing rate.
Because the extrusion load depends mainly onthe flow stress kf, the wrought aluminum alloysare classified according to the flow stress intothe groups easy to extrude, moderately difficult,and hard to extrude. Thus
2Easy to extrude k � 30 N/mmf2Moderately difficult k 30 to 45 N/mmf
2Hard to extrude k 45–57 N/mmf
On top of this, there is the increasing investmentin equipment and the cost of heat treatment andcorrection as well as the quality control.
228 / Extrusion, Second Edition
Fig. 5.32 Torsion flow curves of AlMgSi0.5 [DGM 78]
The material behavior during plastic workingcan be derived from high-temperature torsionflow curves. These show the deformation load kfas a function of the logarithmic principal strainu. It should be emphasized here that the flow
curve is a material parameter for the ideal defor-mation and depends on the working temperatureand, in particular, the rate of deformation.
Figure 5.32 shows the torsion flow curves forthe extrusion alloy AlMgSi0.5 at 450 �C and a
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 229
Table 5.4 Flow stress of different aluminumalloysExperimental material: homogenized cast billets. Test: torsionflow curves; test temperature 450 �C. Logarithmic principalstrain ug � 2.7; deformation rate (log principal strain rate)
(a) The prefix “ENAW” has been omitted from the DIN EN 573-3 designations.A199.5 is ENAW-A199.5; 1050A is ENAW-1050A.
Table 5.5 Comparison of the aluminum alloysaccording to DIN and DIN EN
DIN 1712 T3/DIN 1725 T1
Symbol
DIN EN 573–3(a)
Symbol No.
Easy-to-extrude alloys
Al99.5 Al 99.5 1050AAl99.8 Al 99.85 1085Al99.98R Al 99.98 1098E-Al E Al 99.5 1350AlMg1 AlMg1 5005AAlMn1 AlMn1 3103AlMgSi0.5 AlMgSi 6060AlMgSi0.5 AlMg0.7Si 6063AlMgSi0.7 AlSiMg 6005A
(a) The prefix “ENAW” has been omitted from the DIN EN 573-3 designations.Al99.5 is ENAW-Al99.5; 1050A is ENAW-1050A.(b) Not standardized in DIN
logarithmic strain rate of du/dt � 0.1 s�1 as afunction of the logarithmic principal strain ugbetween 1 and 7.5.
The flow stress of the most important extru-sion alloys is shown in Table 5.4 at a logarithmicprincipal strain of ug � 2.7. This correspondsto an extrusion ratio of 15:1. The kf values listedillustrate the different deformation properties ofthe individual extruded alloys. These were ob-tained from torsion flow curves taken from boththe DGM Atlas of Hot-Working Properties, Vol-ume 1 [DGM 78], and internal investigations atVAW Aluminium, Bonn, Germany.
The aluminum alloys are designated below bythe chemical symbols, which was the custom un-til recently. Table 5.5 compares the material des-ignation of the old DIN 1712 T and 1725 T withthe new DIN EN 573-3.
5.8 Easily Extruded Alloys
Alloys with low kf values (kf � 30 N/mm2)are usually classified as easily extruded. To afirst approximation, the lower the required flowstress the better is the workability. Normally thepossible exit speed increases with the workabil-ity [Ake 68].
5.8.1 Aluminum AlloysIn addition to aluminum, the naturally hard
alloys AlMg1 and AlMn1, as well as AlMgSi0.5
and AlMgSi0.7, are considered to belong to theeasily extruded alloys. Because AlMg1 andAlMgSi0.5 are frequently used for anodizing orbright finish applications, care must be taken toensure that the Mg and Mg2Si components aredissolved in the solid solution in the as-extrudedstate. Also, none or, if necessary, very small ad-ditions of insoluble or supersaturated dissolvedelements such as manganese, chromium, zirco-nium, or iron may be used. For this reason,AlMg1 and AlMgSi0.5 are produced from abase metal of a higher purity for bright finishqualities. In DIN EN 573-3 the individual alloysare shown based on 99.85, 99.9 and 99.98 Al. Ifthis is not taken into account, secondary precip-itates can form during billet heat treatment andthese can impair the surface quality of the ex-truded sections by streaking.
The billet heat treatment—referred to ashomogenization—of AlMg1 is mainly to reducecrystal segregation and dendritic residual melt-
230 / Extrusion, Second Edition
ing and to produce an easily worked cast struc-ture. In contrast, in AlMn1 the manganese exitsin a supersaturated solid solution. This nonequi-librium state is activated by the thermal treat-ment and tries to attain equilibrium by the for-mation of secondary precipitates. The size of theprecipitated particles depends on the tempera-ture and the heat treatment time. The higher thetemperature and the longer the heat treatmenttime, the coarser are the particles. It is knownthat the degree of dispersion influences the re-crystallized grain structure and the workability.It is therefore important with AlMn1 to heat treatat a high temperature of 590–620 �C in order toproduce a fine grain during extrusion.
The most common easily extruded alloy isAlMgSi0.5. It differs from the materials de-scribed so far in that it achieves its properties byage hardening. AlMgSi0.5 is characterized bygood mechanical properties as well as by out-standing workability combined with an excellentsurface quality.
As already mentioned, the temperature rangefor the hot-working process coincides with thatfor solution heat treatment. In this case, separatesolution heat treatment and quenching of the ex-truded section—as required for the high-strengthmaterials—is not required.
AlMgSi0.5 extruded sections only need to becooled with moving air because of the lowquench sensitivity in order to retain the age-hardening phase Mg2Si in �-solid solution. Acharacteristic of this alloy is that Mg2Si can beretained in supersaturated solution with a cool-ing rate of only 2 K/s. The desired increase instrength can be obtained by subsequent cold orwarm age hardening.
The important requirements for AlMgSi0.5and AlMgSi0.7 type alloys are small fractionsof the principle alloying elements dissolved inthe �-solid solution at the press exit temperatureand the avoidance of large additions of the re-crystallization retarding elements of manganese,chromium, and zirconium because, in the formof fine secondary particles, these significantlyincrease the flow stress and thus reduce the ex-trudability. In addition, they act as foreign nucleifor the age-hardening phase. Mg2Si is depositedat the Al(Mn,Fe)Si containing crystals and inthis form can no longer contribute to the agehardening [Sca 64].
The quality of the continuously cast billets hasa not insignificant influence on meeting the highquality and productivity requirements of the ex-trusion plant. It is known that the formation of
a smooth billet surface goes hand in hand withthe improvement of the internal structure. Afine-cellular dendritic cast structure with littlevariation in the cell size across the billet diam-eter is desired [Sca 78] (see also the section“Melting and Casting Processes” in Chapter 4).
During the billet heat treatment (homogeniz-ing), the plasticity of the cast structure can besignificantly improved. This occurs by the re-duction in grain segregation and the formationof the intermetallic AlFeSi cast phases duringthe billet heat treatment just below the solidusline. These effects are the more marked the finerthe dendritic structure of the cast billet becausethe initial conditions are then more favorable forthe thermally activated processes.
The billet heat treatment cycle involves notonly the heat treatment temperature of 560–580�C and the holding time of 6 to 8 hours, but alsothe heating and cooling of the billets. The rateof heating is not limited for any metallurgicalreasons. With AlMgSi0.5, rapid cooling fromthe heat treatment temperature is recommendedbecause with slow cooling (� 50 K/h), theMg2Si is precipitated in a coarse format. Withrapid billet heating to the extrusion tempera-ture—in an induction furnace—the heat treat-ment time can be too short to redissolve thecoarse particles. This reduces the mechanicalproperties after age hardening. In addition,coarse precipitates reduce the surface quality,particularly in anodizing.
5.8.2 Extruded Products
Extruded products are classified as bar, tube,and extruded sections and are standardized inDIN 1746, 1747, and 1748 (See The AluminumAssociation, ANSI H35.1).
The range of profile cross sections is almostinexhaustible for the easily extruded aluminumalloys. Profiles with asymmetric cross sectionsand considerable wall thickness variations canbe produced. Reference should be made toChapter 2, which covers the wide range of ap-plications.
Aluminum and the easily extruded AlMgSitypes also have excellent extrusion weldingproperties. Hollow sections can therefore be pro-duced with extruded longitudinal welds usingspecial dies (bridge, porthole, and spider dies).The wall thickness that can be held with ex-truded sections is determined by the followingparameters:
● Material● Specific pressure
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 231
Table 5.6 Comparison of standards for extruded products
Product formatMaterial/material
conditionTechnical delivery
conditionsDimensions, permitted
deviations Other comments
Round tube DIN 1746, part 1Board 4.6
DIN 1746, Part 2 DIN 9107 . . .
Round-, rectangular-, square-,and hexagonal bar
DIN 1747, Part 1Board 4.6
DIN 1747, Part 2 DIN EN 755-3DIN EN 755-5DIN EN 755-4DIN EN 755-6
. . .
Extruded sectionRectangular tube
DIN 1748, Part 1Board 4.7
DIN 1748, Part 2DIN17615, Part 1
DIN 1748, Part 4DIN17615, Part 3Board 4.1
Cross-sectional shapeDIN 1748, Part 3Board 5.17
● Dimensions and type of section● Degree of difficulty
The problems and limits of hollow sectionproduction are covered in section 5.7.6.1.
Extruded AlMgSi0.5 sections are used for fa-cades in high-rise buildings because of theirgood surface quality, particularly after anodiz-ing. Particular attention has to be given to thedesign of the die and the location of the extru-sion welds. Texture and grain variations in thelocation of the welds can occur as a result of thematerial separation as it flows into the die andthe bonding of the metal streams in the weldingchamber by pressure welding resulting in a dif-ferent appearance to the other visible surfaces.This is covered in detail in section 5.7.6.5.
With high demands on the brightness of theextruded sections, for example, for trim onhousehold goods, furniture, or automobiles,chemical or electrochemical brightening has tobe carried out before anodizing. Because parti-cles of AlFeSi phases impair the brightness alloyvariations based on Al99.85, Al99.9 or Al99.99are used for both AlMg1 and AlMgSi0.5.
Large extruded sections are preferred for railand road vehicles. It is possible to produce pro-file cross sections with a circumscribing circleof a maximum of 540 mm and a weight per me-ter up to 80 kg on presses in the range 72 to 100MN installed for this application.
Round and flat bars as well as tubes and sec-tions are used as conductors in electrical engi-neering. The mechanical and electrical proper-ties are standardized in DIN 40501 parts 1 to 4for the product shapes mentioned. E-Al and E-AlMgSi0.5, which are mentioned in DIN EN573-3 are used (ASTM Internation Standards foraluminum bus conductors are B 236 and B 317).A special heat treatment in the range of over-aging increases the conductivity of E-AlMgSi0.5 without any loss in mechanical prop-erties [Ach 69].
Extruded products are standardized accordingto the product shape, material, and material con-dition, the technical delivery conditions, and thedimensional tolerances. Table 5.6 summarizesthe standards for bar, tube, and extruded sec-tions.
5.8.3 Extrusion and Materials Properties
The interaction of a suitable alloy, the qualityof the cast billet and the correct billet heat treat-ment, the optimal tool design, the extrusion con-ditions and the cooling of the extruded section,and the subsequent age hardening is needed toproduce a high-value product that completelyfulfills all the geometric, chemical, physical, andmetallurgical requirements of the extruded sec-tion.
The alloys AlMg1 and AlMn are naturallyhard alloys in which the mechanical propertiesare obtained mainly from the solid-solutionhardening. Therefore, the extrusion conditionsare determined by the material deformation pa-rameters. The extrusion is selected to give theoptimal hot-working parameters. This, however,depends on the cast quality and the billet heattreatment (homogenizing), which forms the ba-sis for improving the plasticity. The heating ofthe homogenized extrusion billets is usually car-ried out in induction or gas-fired continuousovens to a temperature that can be found inTable 5.7.
In general, the billet temperature should be aslow as possible in order to obtain a smooth sec-tion surface at a high extrusion speed. It alsodepends on the degree of difficulty of the sectionand the extrusion ratio. With the easily extrudedalloys, the extrusion ratio should be between V� 20 and V � 100, but, if possible, a value ofV � 40 to V � 60 is preferred.
The container temperature is usually approx-imately 50 �C lower than the billet preheat tem-perature.
232 / Extrusion, Second Edition
Table 5.7 Typical values for billet high-temperature heat treatment and extrusion conditions ofaluminum alloys
With alloys that do not age harden, the exittemperature and the rate of cooling of the ex-truded section are not critical for the mechanicalproperties. Nevertheless, cooling with fans isrecommended for economic reasons and to pre-vent precipitation and grain growth. The guide-lines for the extrusion parameters discussed forthe individual alloys are given in Table 5.7.
In the production of extruded sections in theage-hardening alloys AlMgSi0.5 and Al-MgSi0.7, the combination of special metallur-gical and process technological measures en-ables not only the economic production ofcomplicated profile cross sections but also si-multaneously the attainment of favorable me-chanical properties. This is possible becausewith these alloys the temperature range for hotworking and solution heat treatment largely co-incide. A further positive characteristic of thesealloys is that the age-hardening phase Mg2Si canbe retained supersaturated in solution even witha cooling rate of only about 2 K/s and the me-chanical properties subsequently improved byprecipitation during age hardening.
It is important to set the process parametersto meet both the metallurgical and productionrequirements. In many cases a compromise hasto be found. The exit temperature is determinedby two opposing criteria. The lowest possibleexit temperature is needed to obtain a smoothsurface finish. This is also desired from an eco-
nomic point of view because higher extrusionspeeds are achieved with lower extrusion tem-peratures. This, however, conflicts with the met-allurgical requirement of the exit temperature�500 �C in order for the age-hardening phase—in this alloy family approximately 1% Mg2Si—to dissolve in the �-solid solution. If this doesnot occur, the section cannot be age hardened tothe desired values.
The exit temperature depends on the opti-mized flow stress of the material produced bythe billet heat treatment, the billet temperatureand the heating conditions before extrusion, theextrusion ratio, and the rate of deformation.
The billet heating in an induction oven re-quires approximately 4 to 6 minutes. This is sig-nificantly faster than a gas-heated continuousoven where approximately 45 to 50 minutes areneeded. These differences influence the diffu-sion-controlled solution processes. It is easy tounderstand that rapid heating of the billet beforeextrusion is advantageous where rapid coolingfrom the billet heat treatment temperature pre-vents the formation of coarse Mg2Si particles. Ifpreheating is carried out in a gas-fired continu-ous furnace, then the opposite applies becausethe cooling from the billet heat treatment doesnot play the decisive role.
It should be pointed out here that in the UnitedStates in particular, one method of operation isto heat the billets to a temperature above the
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 233
Table 5.8 Typical values for heat treatment of age-hardening aluminum alloys
Material Temper(a)Quenching at the press
exit temperature, �CSeparate heattreatment, �C
Coolingmethod
Age-hardeningtemperature/time
AlMgSi0.5 T5 � 500 . . . Moving air 160–180 �C/12–6 hAlMgSi0.7 T5 � 510 . . . Moving air 160–180 �C/12–6 hAlMgSiCu T5 � 520 . . . Moving air 160–180 �C/12–6 hAlMgSiPb T5 � 520 . . . Moving air 160–180 �C/12–6 hAlMgSi1 T6 . . . 530–540 In water . . .AlCuMg2 T3 . . . 495–502 In water . . .AlCuMgPb T4 . . . 480–485 In water . . .AlCuSiMn T4 . . . 500–505 In water . . .AlCuMgFeNi T6 . . . 525–535 In water . . .AlZn4.5Mg1 T5 � 460 . . . Air 185–195 �C/18–20 hAlZnMgCu0.5 T6 . . . 475–480 In water 90 �C/10–12h�120 �C/18–20 hAlZnMgCu1.5 T6 . . . 475–480 In water 90 �C/10–12h�120 �C/18–20 hAlZn8MgCu T6 . . . 475–480 In water 120VC/20–24 h
(a) Material temper condition according to DIN EN515
subsequent extrusion temperature to achievecomplete dissolution of the Mg2Si phase. Thebillet is cooled outside the oven to the desiredlower production temperature before extrusion.Higher mechanical properties can then beachieved [Sca 64, Rei 88].
As mentioned previously, a wide range of ex-trusion ratios from V � 20 to V � 100 can beused for the easily extruded alloys. The selectionusually depends on the container diameter. Fromthe point of view of the grain formation, it canbe advantageous to change to higher extrusionratios because the recrystallized grain size de-pends on the degree of deformation as well asthe temperature.
Obviously, the deformation rate should be ashigh as possible for economic reasons. High de-formation rates are also desired for metallurgicalreasons because the deformation temperaturehas a significant influence on the exit tempera-ture, which in turn determines the hardening be-havior. Limiting factors are again the specificpressure (stem load over the container cross-sec-tional area) and the roughening or tearing of thesection surface. The effect of die cooling withnitrogen can be found in section 5.7.5.3.
As a guideline, exit speeds of 30 to 100 m/min can be expected with AlMgSi0.5, depend-ing on the degree of difficulty. With AlMgSi0.7,the values are approximately 15 to 20% lowerbecause of the higher amounts of magnesiumand silicon, as well as small manganese addi-tions.
Immediately after leaving the press, the sec-tions are cooled with air or an air water mistmixture to a temperature �200 �C. This proce-dure avoids deformation or twisting of the sec-tions. A cooling rate of 2 K/s is sufficient toretain the Mg2Si in solid solution.
The extruded sections are finally stretched by0.5 to 1.5% to achieve the straightness specified.
The final operation after cutting to the fin-ished length is age hardening. With the AlMgSialloys, the mechanical properties are usuallyreached after 4 to 12 hours at 160–180 �C, de-pending on the selected temperature (Table 5.8).
Preaging at room temperature has a favorableinfluence on the properties that can be achievedwith alloys with low contents of the main alloy-ing elements [Dor 73].
5.9 Moderately Difficult Alloys
Aluminum alloys with flow stresses �30 to45 N/mm2 are classified as moderately difficult.The relative extrudability of these alloys in thiscategory is only 40 to 60% of AlMgSi0.5 if theextrudability of this easily extruded alloy is setat 100% [Hon 68]. This reduction is due to theincrease in the main alloying elements, whichare added for alloy technical reasons to increasethe material properties as well as the addition ofmanganese to produce the desired hot-workingstructure.
5.9.1 Aluminum Alloys
The naturally hard materials include theAlMg alloys with higher magnesium contents aswell as the AlMgMn alloys. The magnesiumcontents vary between 1.8 and 3% and the man-ganese content between 0.3 and 0.8%. The billetheat treatment temperature has to be loweredcompared with the easily extruded alloys AlMg1and AlMn to avoid melting because the solidusline in the phase diagram has a strong tempera-ture dependence (Table 5.7).
234 / Extrusion, Second Edition
Fig. 5.33 Extruded section (extrusion effect with thin recrys-tallized peripheral zone) of an AlMgSi1 bar (cross
section, Barker anodized, micrograph in polarized light)
The heat treatable materials that belong to themoderately difficult alloys include the alloyAlMgSi1 that is frequently used in Germany andthe alloy AlMgSiCu preferred in the UnitedStates, the free-cutting alloy AlMgSiPb and thereadily weldable alloy AlZn4.5Mg1, as well asthe more recent lithium-containing alloysAlCuLi and AlCuMgLi.
AlMgSi1 does not only differ from theAlMgSi0.5 alloy in the higher Mg2Si content butalso in the addition of about 0.4 to 1.0% man-ganese. From the metallurgical point of view,this results in the recrystallization temperaturebeing increased to a significantly higher tem-perature and higher strain. It is important to notethat both the manganese dissolved in the �-solidsolution as well as the precipitated manganese-containing phases have a recrystallization re-tarding effect and increase the deformation re-sistance. The size of the manganese-containingphases can be controlled by the temperature ofthe billet heat treatment [Sca 69a] (see also thesection “Extrudability of Metallic Materials” inChapter 4). Coarse particles that form at a tem-perature of 560 to 580 �C have a lower recrys-tallization retarding effect than fine particles thatare produced at a heat treatment temperature of460 to 480 �C.
AlMgSi1 extruded sections do not have a re-crystallized grain structure like AlMgSi0.5 be-cause of the recrystallization retarding effect ofthe manganese additions but an elongated hot-worked structure with deformation texture com-ponents (Fig. 5.33). This formation of the grainstructure is referred to as the extrusion effect. Itis characterized by an increase in the mechanicalproperties over those of the recrystallized ex-
truded structure, particularly in the direction ofextrusion [Die 66]. The formation of a coarsegrain recrystallized peripheral layer and an un-recrystallized core is well known in these alloysafter age hardening.
The same observations are made in the cop-per-containing AlMgSi alloy where manganeseis partly replaced by chromium, which metal-lurgically functions in a similar way. The low-copper addition favorably affects the grain for-mation and the mechanical properties as well asthe general corrosion resistance [Sca 65].
Both the alloys AlMgSi1 and AlMgSiCu aresignificantly more quench sensitive than Al-MgSi0.5 because of the manganese addition.Sections in these alloys have to be rapidlycooled immediately after extrusion with a watermist or in a standing water wave. A separatesolution heat treatment with water quenching isrecommended with thick-walled tubes and barsto maintain the strength and the toughness. Themanganese-containing phases of the order of0.05 to 0.7 lm obtained from low-temperaturebillet heat treatment and which are uniformlyand densely distributed in the matrix have apositive effect on the toughness behavior [Sca82]. The particle arrangement described dis-places the fracture from the grain boundary intothe matrix by cavity formation. Manganese-con-taining AlMgSi alloys therefore are less suscep-tible to intercrystalline facture and thus have sig-nificantly higher toughness values.
Another moderately difficult-to-extrude alloythat should be mentioned is AlZn4.5Mg1. Me-chanical properties even higher than AlMgSi1can be achieved and a structure with the extru-sion effect is also needed for this alloy to achievehigh mechanical properties. Figure 5.34 showsthat only a small recrystallized fraction is to beexpected if the billet heat treatment is carried outat 460 to 480 �C and the extrusion ratio is nothigh. With this alloy a higher billet heat treat-ment temperature should not be used as other-wise recrystallized grains occur, and there is atendency toward stress-corrosion cracking(SCC), and the values in the standard are notreached [Sca 73].
AlZn4.5Mg1 is characterized by a signifi-cantly lower quench sensitivity than AlMgSi1.Maintaining a cooling rate of 0.5 K/s is not onlyrecommended for process technology reasonsbut has to be held for metallurgical reasons toavoid SCC.
Since the middle of the 1980s, intensive at-tempts have been made worldwide to develop
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 235
Fig. 5.34 Recrystallized structural fraction as a function ofthe billet heat treatment temperature and the ex-
trusion ratio (extrusion temperature, 550 �C) [Sca 73]
new aluminum alloys with improved materialsproperties. The specific aims for the aerospaceindustry are a higher component stiffness andweight reduction. Lithium was considered as analloying element because of its density of only0.534 g/cm3 and because it is one of the fewelements that increase the modulus of elasticityof aluminum alloys. In addition, the Al-Li phasediagram shows a temperature-dependent solu-bility so that the requirement for an increase instrength by age hardening is also fulfilled [Web89].
Age-hardening lithium-containing alloysbased on AlCu and AlCuMg have been devel-oped with a 10% lower density and a 15% in-creased modulus of elasticity. The lithium con-tent has to be approximately 3%. Specialmelting and casting technology is required be-cause of the high reactivity of lithium-contain-ing melts with oxygen and moisture and the ag-gressive attack of the lining of the meltingfurnaces.
The age hardening of the AlLiCuMg alloysis, in contrast to the conventional aluminum al-loys, characterized by the partly coherentAl2CuMg particles that occur in addition to thecoherent Al3Li precipitates during further hard-ening. These cannot be cut by dislocations andthere is an increase in cross slip, which has apositive effect on homogeneous slip distribu-
tion. With extended age-hardening, this resultsin an increase in the elongation to fracture witha simultaneous increase in the flow stress.
Mechanical property variations across thecross section are found in lithium-containing al-loys because of the nonuniform texture forma-tion [Tem 91].
5.9.2 Extruded Products
With their outstanding corrosion resistance toseawater, the naturally hard alloys are particu-larly suitable for industrial atmospheres and forpipe systems in the chemical industry. Theseamless tubes extruded over a mandrel are fur-ther processed by drawing.
The age-hardening materials with their favor-able mechanical properties are mainly used forsuperstructures in rolling stock and as hollowgirders for bedplates as well as the manufactureof processing plants.
The moderately difficult alloys can be pro-duced as either solid sections or hollow sectionswhereby the wall thickness has to be set 20 to40% higher than the easily extruded alloys, de-pending on the alloy, for the same circumscrib-ing circle because of the higher deformationstress. The AlZn4.5Mg1 alloy is often used forhighly stressed welded structures because of itsgood weldability and the rehardening in theheat-affected zone. In the “age-hardened” con-dition (2-stage aging), this alloy has a goodresistance to general corrosion and SCC (Ta-ble 5.7).
Lead-containing alloys such as AlMgSiPb areavailable for chip forming machining. They areparticularly suitable for the machining on auto-matic drilling, milling machines, and lathes be-cause they enable fast cutting speeds to be used.
The alloy AlMgSiPb has good mechanicalproperties, is corrosion resistant, and can be dec-oratively anodized. The extruded or extrudedand drawn products are primarily supplied asround, hexagonal, and octagonal bars. The tol-erances are standardized in DIN1769–1799 andDIN 5000-59701 (Aluminum AssociationH35.2).
Extruded sections in the lithium-containingaluminum alloys are used as stringer sectionsand floor plate supports in the aerospace industrybecause of their lower density and increasedmodulus of elasticity compared with the con-ventional high-strength alloys. Further applica-tions are limited by the present high billet pro-duction costs.
236 / Extrusion, Second Edition
5.9.3 Extrusion and Materials Properties
The flow stress of AlMg and AlMgMn in-creases with increasing magnesium content, andthe melting point decreases. To compensate forthe lower extrudability of, for example, AlMg3,the preheat temperature can be set to 30 �Chigher than for AlMg1. However, the exit tem-perature should not significantly exceed 500 �Cto avoid the surface turning brown because ofincreased oxidation. As mentioned earlier withnaturally hard alloys, the cooling from the ex-trusion temperature to room temperature is notthat critical. The process parameters for the in-dividual alloys are summarized in Table 5.7.
Higher mechanical properties are obtainedwith AlMgSi1 and AlMgSiCu by age hardening.However, some compromises have to be madefor extrusion. The metallurgical procedures thatimprove the extrudability (e.g., higher billet heattreatment temperature) have an unfavorable ef-fect on the grain formation and thus on the me-chanical properties [Sca 67].
In the production of AlMgSi1 sections, thedecision has to be taken in the selection of themetallurgical conditions whether the solutionheat treatment and quenching are coupled withthe extrusion process or whether these should becarried out as separate operations. For economicreasons it is better if additional operations canbe avoided. From the metallurgical point ofview, there are advantages in the formation ofthe grain structure if no additional heat treatmentin the shape of solution heat treatment has to becarried out. The decisive factor is whether theage hardening producing Mg2Si is extensivelydissolved at the exit temperature and can be re-tained in solution during the cooling. AlMgSi1and AlMgSiCu sections have to be cooledquickly with water because they are quench sen-sitive due to the manganese content [Sca 64].Whether this requirement can be fulfilled de-pends on one hand on the section shape to beproduced, in particular, the wall thickness, andon the other hand, on the cooling system in-stalled on the extrusion press. As a general rule,the rate of cooling of AlMgSi and AlMgSiCu inthe temperature range 530 to 150 �C has to be afactor of 10 faster than with AlMgSi0.5; i.e., itshould be at least 10 to 20 K/s. The requiredcooling rate can only be achieved with water.Quenching with water after the die can, how-ever, result in uncontrolled distortion of the sec-tion that must be kept to a minimum by pullers.
If the solution heat treatment and quenchingare to be carried out simultaneously with the ex-
trusion process, care should be taken to ensurethat the exit temperature is at least 530 �C. Thepreheat temperature should, accordingly, be 500to 520 �C because there is no significant tem-perature increase from the heat of deformation.On the other hand, with a separate solution heattreatment a lower preheat temperature should beselected, and there is no demand on the exit tem-perature. The exit speed that can be achievedwith AlMgSi and AlMgSiCu is approximatelyonly half that of AlMgSi0.5. The extrusion ratioon the other hand does not differ significantlyfrom that with AlMgSi0.5.
The increase in the mechanical properties ofthe section is achieved by age hardening for 6to 8 hours at 160 �C or 4 to 6 hours at 180 �C.However, it should be pointed out that withAlMgSi1, the maximum achievable mechanicalproperties decrease with increasing room tem-perature intermediate storage. The damaging in-fluence of an intermediate room temperature ag-ing can be largely overcome by a preventativeheat treatment. The sections have to be heatedto 180 �C and held for 3 minutes, which elimi-nates the cold aging that would otherwise takeplace by approximately 1 day [Koe 61].
The alloy AlMgSi1 is suitable for sectionsthat have to be color anodized because the man-ganese content that is embedded in the metallicform in the oxide layer produces the brown tone.Attention must be paid to the grain structure be-cause different microstructures can result inbands on the surface after anodizing. In this caseit has proved advantageous to try and obtain acompletely fine-grain recrystallized structure.This can be achieved when the recrystallizationretarding influence of the manganese is largelyexcluded by the formation of coarse secondaryprecipitates and thus the rate of nucleation ac-celerated and recrystallization promoted [Sca69]. Billet heat treatment at 570 to 590 �C hasproved successful in this respect. The associatedsimultaneous reduction in the mechanical prop-erties is not important for applications in build-ings.
The copper-free age-hardening AlZn4.5Mg1alloys are very suitable for the production of hol-low sections and flat sections because with thismaterial the working temperature in extrusioncoincides with the solution temperature range,and this alloy has a low quench sensitivity sothat air cooling suffices.
Also, the mechanical properties that can beachieved are higher than AlMgSi1. However,with incorrect treatment, a tendency to SCC can-
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 237
not be completely excluded. Today it is gener-ally recognized that the danger of SCC of ex-truded sections can be excluded if the productionconditions are strictly maintained. These con-ditions are [Sca 73]:
● The billet heat treatment must be carried outat a low temperature (12 h at 460–480 �C)to suppress the recrystallization by fine man-ganese, chromium, or zirconium phases.
● The extrusion temperature should be as lowas possible (450–490 �C).
● The cooling with air in the temperature range350–200 �C should be in the range 0.5–1.5K. Water quenching should be avoided at allcosts.
● The age hardening—after an intermediateroom temperature aging of at least 3 days—has to be carried out in stages (12 h at 90 �Cplus 12–24 h at 120–130 �C) to produce anoveraged structure.
The extrusion behavior of the alloyAlZn4.5Mg1 from the point of view of the ex-trusion speed is comparable to the alloy Al-MgSi1. The advantages of AlZn4.5Mg1 are therelatively low solution heat treatment tempera-ture and the use of air even with thick-wall sec-tions to achieve the necessary slow cooling rate.
The extrudability of the alloy AlLiCuMg iscomparable with that of the moderately difficultalloys AlMgSi1 and AlZn4.5Mg1. The flowstress of AlCuMgSi is approximately 10 to 15%lower than that of AlMgSi1. The maximum ex-trusion speed that can be obtained is reduced toa value of only 60% of that of AlMgSi1 becauseof the low-melting-point copper phase. It is ben-eficial with the lithium-containing alloy as withthe other high-strength materials to pass the sec-tion through nitrogen as it leaves the die to avoidoxidation of the surface [Sca 79].
The age hardening of the lithium containingalloys is carried out by solution heat treatmentat 530 to 540 �C and quenching in water. Coldworking (stretching) of 1.5 to 2% before agingat 185 �C has proved to be beneficial to both themagnitude of the strength and the elongation[Tem 91].
5.10 Difficult-to-Extrude Alloys
The highest mechanical properties in thealuminum alloys can be achieved with the age-hardening alloys of the AlCuMg and, in partic-
ular, the AlZnMgCu systems. Alloy technolog-ical procedures have to be followed, however,that can be disadvantageous from other perspec-tives. The addition of up to a few percent copperresults in the formation of low-melting-point in-termetallic phases. They also have an unfavor-able effect on the general corrosion resistance.Recrystallization retarding elements, includingmanganese, chromium, and zirconium, alsohave to be added in order to obtain a hot-workedstructure in the extruded sections that containselongated grains in the longitudinal direction.The mechanical properties in the longitudinal di-rection are higher than in the transverse direc-tion. This is referred to as the extrusion effect[Sca 69c]. The alloy technological measures de-scribed do fulfill the mechanical property re-quirements but have serious hot-working dis-advantages. The additions of the main alloyingelements magnesium and copper and the addi-tions of manganese and chromium increase theflow stress significantly so that AlCuMg2 andAlZnMgCu1.5 have to be classified among thedifficult-to-extrude alloys. The difficulty is in-creased by the low-melting-point phases, whichseverely limit the extrusion temperature range,and if the latter is exceeded, hot cracking occurs.These alloys have a marked quench sensitivityand a severely limited solution range so that inmost cases they have to be solution heat treatedand quenched in a separate operation.
In addition to these alloys, the AlMg andAlMgMn alloys with 4 to 5% magnesium areincluded in the difficult-to-extrude alloys. Theflow stress of 47 to 56 N/mm2 is attributable tothe high magnesium content and the manganesesecondary precipitates, particularly if these oc-cur in a fine format and a high density in thematrix.
It should be pointed out here that new pro-duction processes are being used to develop newaluminum alloys for extruded sections with im-proved materials properties. These include al-loys produced by powder metallurgy and parti-cle reinforced conventional alloys [Web 89] (seethe section “Extrusion of Powder Metals” in thischapter).
5.10.1 Aluminum Alloys
There are various types of alloys with copperas the main alloying element. They differ in spe-cial material properties. These alloys are usuallycold age hardened to give the most favorableproperties. AlCuMg2 has the highest mechani-
238 / Extrusion, Second Edition
cal properties and a good toughness. It is veryimportant in the aerospace industry. The alloyAlCuMgFeNi has the best high-temperature me-chanical properties of all the conventional al-loys. The alloy AlCuSiMn is characterized byhigh mechanical properties at room temperatureand elevated temperatures. Because this alloyalso has excellent forging properties, preformsare extruded and then forged in dies. Bars fordrilling, milling, and turning are made from thealloys AlCuMgPb and AlCuBiPb.
The billet heat treatment of these alloys isusually carried out at 480 � 5 �C. Only Al-CuMgFeNi is heat treated at 525 � 5 �C be-cause of the additions of nickel and iron. Theheating should not significantly exceed 50 �C/hto avoid melting. After the heat treatment, slowcooling to approximately 360 �C is recom-mended, preferably in the homogenizing fur-nace. The cooling can then be carried out in air.These measures help avoid quenching stresses,which can produce cracks in the billets in thesealloys.
The following AlZnMgCu alloys are the mostcommon: AlZnMgCu0.5 and AlZnMgCu1.5,which are both standardized, as well as alloyswith higher zinc contents in which manganeseand chromium are replaced by zirconium. Thisreduces the quench sensitivity of these alloys.The highest mechanical properties measured onextruded sections are typically Rp0.2 580–610 N/mm2 and Rm 610–640 N/mm2 and approxi-mately 8–10% for A5.
The billet heat treatment of these alloys is par-ticularly important because of the complexity ofthe cast structure. Structural rearrangements oc-cur during heating, holding at temperature, andcooling, and these have a significant influenceon both the mechanical properties and the ex-trudability. Cast billets have a lower meltingpoint than homogenized billets. The meltingpoint is increased by 70 to 90 �C by reducingthe residual melt bands and segregations as wellas dissolution during the billet heat treatment.On the other hand, the supersaturated dissolvedfractions of the peritectic phases are precipitatedin a fine form in the matrix by thermal activa-tion. In the billet heat treatment—usually 12 hat 480 � 5 �C —the cooling of these alloys hasa decisive influence on the extrusion speed [Fin96]. If the cooling is not fast enough, coarse par-ticles can form during the cooling. These do notdissolve completely during heating and thenmelt in the extrusion process, resulting in sur-face cracks in the section.
The high-alloyed alloys AlMg5 andAlMg4.5Mn are particularly suitable for lowtemperature applications. The mechanical prop-erties increase as the temperature falls; this alsoapplies to the elongation to fracture. If the mag-nesium content exceeds 3%, some of the mag-nesium exists preferentially as intermetallicphases on the grain boundaries and some in thematrix. This applies in particular if the materialis held for a long period in the temperature range250 to 150 �C. Intercrystalline corrosion suscep-tibility can occur under unfavorable conditionsif the application temperature is �80 �C. Themagnesium phase can be precipitated by suitablethermal treatments during production so that nocoherent precipitates form and intercrystallinecorrosion can be practically eliminated.
5.10.2 Extruded Products
The high-strength materials are mainly usedin the aerospace industry, military applications,and machinery. The products include solid sec-tions, bars, and seamless extruded tubes.AlCuMg2 and AlZnMgCu1.5 are normally notsuitable for the production of hollow sectionswith extrusion welds because of the low-melt-ing-point phases and the high extrusion pres-sures required [Wei 78]. Simple, thick-wall tu-bular components are, however, occasionallyextruded with bridge or porthole dies.
Whereas AlCuMg2 is usually used in theroom temperature age-hardened condition forhigh-stress applications, AlCuSiMn andAlCuMgFeNi as well as AlZnMg alloys aremainly used after hot age hardening. AlCuSiMnis used in the as-extruded or annealed state forsubsequent processing to forged components.
Only the alloy AlMg4.5Mn of the difficult-to-extrude naturally hard alloys can be extruded asa hollow section with extruded welds. However,this is possible only with a large wall thicknessbecause of the high deformation loads. The ap-plications are primarily chemical equipment andmachinery.
5.10.3 Extrusion and Materials Properties
Only extrusion ratios of V � 10 to V � 40are used for the difficult-to-extrude alloys be-cause of the high flow stress of over 45 to 57 N/mm2. The extrudability is less than 10% of theeasily extruded alloys.
Indirect extrusion of high-strength alloys hasbeen used to an increasing extent in recent years.This has clearly confirmed the advantages of the
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 239
indirect extrusion process for the high-strengthand difficult-to-extrude alloys AlCuMgPb,AlCuSiMn, and AlZnMgCu1.5 [Eul 75]. Indi-rect extrusion has a higher productivity com-pared with direct extrusion. Significant advan-tages include the higher extrusion speed at lowertemperatures (340–360 �C) and the use of longerbillets (3–7 times the container diam). The dif-ferent material flow in indirect extrusion pro-duces a homogeneous structure and thus moreuniform properties. However, indirect extrusionplaces higher demands on the casting quality.Only defect-free turned or scalped billets give agood extruded surface finish. The distance be-tween the die and the cooling section at the pressexit is greater because of the design of indirectextrusion presses. This makes the quenching ofquench-sensitive alloys at the press more diffi-cult. In addition, quenching at the press requiresa sufficiently high exit temperature to achievecomplete age hardening. In direct extrusion, theextrusion speed of difficult-to-extrude alloys isincreased by 20 to 40% by die cooling with ni-trogen. This is demonstrated with the copper-containing alloys, which have a relatively lowmelting point. Die cooling enables some of thedeformation heat to be removed so that the criti-cal melting temperature, which results in sectioncracking, is reached at a higher exit speed[Sca 79].
The solution heat treatment of the high-strength alloy AlZnMgCu1.5 is usually carriedout separately after extrusion. The reason forthis has already been discussed. This is then fol-lowed by age hardening. Room temperature in-termediate aging for 3 to 5 days has proved ben-eficial. The age hardening is carried out in oneor two stages (Table 5.8) and depends on thecustomer specifications. If the maximum me-chanical properties are required, age hardeningis carried out at only 120 �C to the maximum ofthe age-hardening curve. However, a slightlyoveraged condition is recommended becausethis significantly improves the stress-corrosionresistance for only a slight loss in mechanicalproperties. A two-stage age hardening is used toachieve the T73 designated properties. The sec-tions are initially aged for 12 to 24 hours at 120�C and then for 3 to 5 hours at 170 �C [Dah 93].
The alloy AlCuMg2 is usually aged at roomtemperature after quenching in water as hot agehardening has a negative influence on the cor-rosion properties; a tendency toward intercrys-talline corrosion can be detected. Hot age hard-ening is the standard process for the AlCuSiMn
and the AlCuMgFeNi alloys to achieve the de-sired high mechanical properties. The age-hard-ening parameters are given in Table 5.8.
Finally, the difficult-to-extrude alloy AlMg4.5Mn is a naturally hard alloy where the in-crease in mechanical properties is not obtainedby heat treatment but by solid solution and workhardening. For this reason, the extrusion ratioshould not be too high; otherwise, the fractionof recrystallized structure increases to such anextent that the specified values are not reached.This occurs, in particular, when a higher extru-sion temperature has to be used because of thelimited specific pressure of the extrusion press.
Extrusion of Materials withDeformation Temperaturesof 600 to 1300 �C
5.11 Extrusion of SemifinishedProducts in Copper Alloys
Martin Bauser*
5.11.1 General5.11.1.1 Copper, Bronze, and Brass—
A Long History
The knowledge and application of copper andsome of its alloys extends back into prehistory(Bronze Age). They were used up to the begin-ning of our technical era mainly for jewelry andhousehold goods. The good workability of cop-per and copper-zinc alloys combined with theattractive appearance is responsible today fortheir use for metal wares, including containers,lamps, and trays as well as brass instruments.
terials only reduce the conductivity slightly butsignificantly improve the mechanical properties.The high thermal conductivity corresponds tothe high electrical conductivity. Copper is lo-cated close to the noble metals in the electricalchemical series and has a natural resistance tonumerous corrosive effects.
The good corrosion resistance and the ease ofworking make copper one of the most importantmaterials for water supply pipes. It is also par-ticularly suitable for heat exchangers because ofits good thermal conductivity. The property ofcopper used most is the excellent electrical con-ductivity, which secures its very wide applica-tion in electrical engineering and electronics—naturally also in the form of rolled products.
5.11.1.3 Importance of Extrusionin the Processing
Pure copper melts at 1083 �C. The meltingpoint is only increased by the addition of nickel(continuous solid solution). All other elementadditions lower the melting point, sometimes toless than 900 �C (Table 5.9). The recrystalliza-tion temperature falls in the range 350 to 650 �Cdepending on the composition. Copper alloysare extruded at temperatures between 550 and1000 �C corresponding to the melting tempera-ture. Along with aluminum alloys, they belongto the group of materials where long semifini-shed sections are mainly produced by extrusion.
According to the statistics in Europe in 1988,1.4 million tonnes (metric tons) of extruded
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 241
Table 5.10 Copper alloy groups and theassociated composition DIN standards
Alloy groups
No. of DINstandardized
alloysASTM
standard
DIN 1787: copper 6 B 133DIN 17666: copper wrought
alloys—low alloyed20 E 478
DIN 17660: copper-zinc alloys(brass, special brass)
31 B 371
DIN 17662: copper-tin alloys (tinbronzes)
4 B 505(a)
DIN 17665: copper-aluminumalloys (aluminum bronzes)
8 B 150, B 359
DIN 17664: copper-nickel alloys 6 E 75DIN 17663: copper-nickel-zinc
alloys (nickel-silver)6 B 151
(a) For continuous casting
Table 5.11 Euro standards for extruded copperalloy semifinished products
Standard Designation
EN 1057 Seamless round copper tubes for water and gassupplied for sanitary installations
EN 12735 Seamless round copper tubes for air conditioningEN 13348 Seamless round copper tubes for medicinal gasesEN 12449 Seamless round tubes for general applicationsEN 12451 Seamless round tubes for heat exchangersEN 12163 Bar for general applicationEN 12164 Bar for machiningEN 12165 Feedstock for forged componentsEN 12166 Wire for general applicationsEN 12167 Section and rectangular bar for general applicationsEN 12168 Hollow bar for machining
products were produced in copper alloys by 70companies and approximately 120 presses [Zei93]. In Germany there are 14 companies with 37presses in the range 10 to 50 MN press power.
Sixty percent of all extruded products are bar,wire, and section of which the majority are pro-duced in copper-zinc alloys (brass). The remain-ing 40% are tube—usually in copper.
5.12 The Groups of Extruded CopperAlloys—Their ImportantProperties and Applications
5.12.1 Alloy Groups
The DIN standards cover 81 wrought alloysin seven groups that can be extruded under spe-cific conditions (Table 5.10).
The conversion to European standards had notbeen completed at the time of publication. Onlyproduct standards and not individual alloygroups, as is the case in DIN, are included in theEuropean standards. Each EN then usually ap-plies to all alloy groups (Table 5.11). ASTM In-ternational Standards cover alloys, productshape, and specific applications.
In addition to these alloys produced from castbillets there are a few composite materials andpowder metals discussed in the sections “Extru-sion of Powder Metals” and “Extrusion of Sem-ifinished Products from Metallic Composite Ma-terials.”
5.12.2 Tooling Temperatures in Extrusion
With extrusion temperatures between 500 and1000 �C, the tooling is subjected to significantly
higher temperatures than with aluminum alloys.However, as copper alloys can usually be ex-truded at much higher speeds than aluminum,the contact time with the tooling is so short thatheating of the tools over the limit of 500 to 600�C can be avoided. The tooling wear is, however,naturally much higher than with aluminum al-loys (see the section “Tools for Copper AlloyExtrusion,” in Chapter 7).
5.12.3 Structure
Copper and numerous copper alloys, e.g.,copper-tin (up to 8% Sn) and copper–zinc (upto 37% Zn) have a pure face-centered cubic (fcc)� structure up to the melting point and thereforehave good cold workability but only moderatelygood hot workability. On the other hand, thebody centered cubic (bcc) b phase, e.g., copper-zinc over 40% zinc, has excellent hot workabil-ity but is difficult to cold work.
5.12.4 Typical Extruded SemifinishedProducts and Applications
The section “Copper Alloy Extruded Prod-ucts” in Chapter 2 describes typical extrudedsemifinished products and their applications.
Whereas bar and wire are produced over theentire alloy range, tubes are mainly in SF-Cu forwater supply, brass for plumbing fittings, and inspecial brasses and copper-nickel alloys for cor-rosive media. Large quantities of free-machiningbrass are machined to fittings and turned com-ponents, including bolts.
In contrast to aluminum, the production ofsections is no longer important. The higher ex-trusion temperature results in higher die tem-peratures and thus greater wear and more severetool defection than with aluminum alloys. The
242 / Extrusion, Second Edition
Fig. 5.35 Hot shortness cracking at excessive extrusion tem-perature. (a) CuSn8 extruded tube with coarse,
moderate, and fine hot shortness cracks. (b) Extruded round barin CuSn6 with gaping hot shortness cracks [Die 76]
extrusion tolerances of copper alloys are widerthan with aluminum and it is also not possibleto produce thin profile cross sections (see DIN17 674, page 3). With few exceptions the sec-tions have to be subsequently drawn. Sectionproduction is usually limited to copper and low-alloy copper materials as well as the easily ex-truded �-b brasses.
The most economic production of readilycold-worked alloys to tubes, bar, and section de-pends on the equipment available at the individ-ual companies. Depending on the type of press,the size, and the number and size of the drawingmachines, the most suitable extruded dimen-sions vary for the same finished product.
5.13 Extrusion Properties ofCopper Alloys
Numerous authors have covered the extrusionof copper alloys [Tus 80]. Only the work rele-vant to practical applications is included here.
5.13.1 Extrudability of Different Materials
Table 5.9 gives the extrusion temperatures,the maximum extrusion speeds, and the maxi-mum extrusion ratios. The data were obtainedfrom practical experience in various extrusioncompanies. These can differ from plant to plant.
5.13.2 Temperature and Speed—Structureof the Extrusion
The temperature dependence of the workabil-ity of different copper alloys is shown in thefollowing section in the form of hot-strengthcurves. Data obtained from tensile tests onlyhave limited application in calculating the forcesneeded for extrusion. However, if they have allbeen measured using the same method on softspecimens, they are suitable for comparing thematerials (see also Chapter 4). Values of the flowstress kf obtained from torsion tests, which areoften described in the literature, are more suitedto calculating the load. Strains comparable tothose in extrusion can be achieved. The Atlas ofHot-Working Properties of Non-Ferrous Metals,Volume 2, Copper Alloys [DGM 78] gives thesekf values as a function of the logarithmic prin-cipal strain ug and the logarithmic principalstrain rate . Unfortunately, not all the impor-ugtant alloys are included. In addition, as different
authors have used various methods of measure-ment these data are not suitable for comparison.
Up to the extrusion temperature, copper andlow-alloy copper materials have the face cen-tered �-structure, which does not have good hot-working properties.
Brass in the �-b range (free-machining brass)has very good hot workability. The extrusion ofbrass bars was the first large-scale application ofthis process (A. Dick, 1890, see the section “His-toric Development of Extrusion” in Chapter 1).
With difficult-to-extrude alloys, the flowstress, depending on the press size and the ex-trusion ratio, places a lower limit on the extru-sion temperature and an upper limit on the sus-ceptibility to hot shortness [Lau 76]. Aluminumbronzes and lead-containing nickel silvers areparticularly susceptible to hot shortness. Thecracks can range from light surface cracks to firtree defects (Fig. 5.35). It is not only on eco-
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 243
nomic grounds that the extrusion speed shouldbe as high as possible and the initial billet tem-perature as low as possible. The output is higherand the thermal stressing of the tooling duringextrusion shorter. The material itself also coolsless during extrusion by the conduction of heatinto the container, which is at a maximum of 500�C, which results in a more uniform section exittemperature and structure over the length of theextrusion.
The age-hardening copper-chromium andcopper-chromium-zirconium alloys represent aspecial case where it is possible to quench smallsections from the extrusion temperature, whichis also the solution heat treatment temperature.In this case, extrusion is carried out at the high-est temperature possible (900 to 1000 �C).
The maximum extrusion speed is frequentlylimited by the equipment, e.g., by the puller sys-tem for sections or by the speed of the downcoilers for wire.
The extrusion temperature and the deforma-tion are normally so high that the extruded sec-tion completely recrystallizes as it leaves the die,usually with a fine-grain structure. However, itis possible to have structural variations from thefront of the extrusion to the back. At the start ofextrusion the deformation is lower than in themiddle and at the end. Depending on the extru-sion ratio, the extrudability, and the extrusionspeed, the exit temperature can fall or even in-crease. With multiphase structures the quantityand distribution of the second phase can varyover the cross section and the length of the ex-trusion. Details are given in the material-specificsections 5.16 to 5.16.9.
5.13.3 Extrusion to Finished orClose-to-Finished Dimensions
Because the �-b brasses are difficult to coldwork, brass bar and sections in these alloygroups are extruded close to the final dimensionsand then brought to the desired finished dimen-sions and mechanical properties by a subsequentcold-working operation (usually by drawing). Incontrast, the alloys that have good cold worka-bility (copper, low-alloy copper materials, �-brasses) are extruded well above the finished di-mensions and then cold worked in severalstages. Cold working of sections can be difficultand requires considerable experience in decidingthe extruded dimensions, the tool design, and thedrawing parameters.
5.13.4 Lubrication
Lubricants based on graphite oil mixtures arewell suited for the temperature range in whichcopper alloys are extruded.
There have been many attempts to introduceother lubricants for the extrusion of copper al-loys. These have largely been disappointing.Usually, viscous oil blended with graphite flakesis used. If automatic lubrication systems areused, which is increasingly common on modernequipment, the lubricant has to have a low vis-cosity. “Lubricant sticks” of graphite containingwax are available for die lubrication. The con-tainer is usually unlubricated so that only the dieand the mandrel in the case of tubes have to beregularly lubricated. To reduce graphite inclu-sions and banding on the surface of the section,the minimum of lubricant should be accuratelyapplied. However, too little lubrication will re-sult in wear between the tooling and the ex-truded material.
5.13.5 Extrusion with a Shell
Materials that are oxide free or have a limitedamount of oxide usually bond to the tooling inthe absence of lubrication. Brasses, aluminumbronzes, and copper-nickel alloys have to be ex-truded with a stable shell. At the end of eachextrusion the dummy block is pushed out andthe shell removed with a cleaning disc. The shellthickness is usually about 1 mm. If it is too thickthere is the risk that the material will flow backbetween the dummy block and the containerover the stem. This can occur in particular withalloys that are easy to extrude. An incompleteshell can form if the shell is too thin. A fixeddummy block, which is used for the easily ex-truded aluminum alloys, cannot be used for cop-per alloys.
Copper and low-alloy copper materials, aswell as bronzes, tend to oxidize. The oxide layeradhering to the hot billet acts as a lubricant [Bla48, Bei 76, Vat 70]. However, because the billetsurface is generally not uniformly oxidized, par-tial adhesion to the container can occur and ex-trusion has to be carried out with a shell.
The thin shell in the extrusion of copper andlow-alloy copper materials can be easily re-moved from the container and squashed so thatit is possible to operate with a combinationdummy block and cleaning pad (see Chapter 7).The front ring of this block has the diameter ofthe dummy block and the rear the diameter ofthe cleaning block. In between there is a deep
244 / Extrusion, Second Edition
and wide circular recess into which the thin shellfolds. Extrusion with a shell is still carried outwith the combination dummy block and clean-ing pad but the additional cleaning operation iseliminated.
5.13.6 Different Material Flow Behavior
In the direct extrusion of copper alloys witha shell and unlubricated container, flat dies areused. Differences in the hot-working propertiesand the flow behaviors of different materialgroups result in different types of material flow.The material flow patterns can be found in Chap-ter 3.
5.13.6.1 Flow-Type C—Piping Defect
Aluminum bronzes and �-b brasses adhere tothe container because of the minimal oxide for-mation and flow according to type C because thelayer close to the surface cools during extrusionand does not flow as readily as the billet core.The danger of the piping defect (see Fig. 5.46)is countered by restricting the billet length.However, the end of the extrusion has to be reg-ularly tested using a fracture test.
It is advantageous with these alloys to changeto the indirect extrusion process where there isno friction between the billet and the containerand the risk of the piping defect is removed. Thematerial then flows according to type B. Longerbillets can then be used, providing the other con-ditions allow it, and the discard length is reduced[Zil 82].
5.13.6.2 Flow-Type B—Shell Defect
Copper and low-alloyed copper alloys similarto CuCrZr or CuNiSi but also CuNi, CuNiZn,�-brasses as well as CuSn are frequently ex-truded at temperatures so high that, dependingon the material, a more or less thick oxide layerforms that in turn can result in extrusion defectsincluding shell defects and thus blisters on theextruded section (see Fig. 5.39). The materialflows according to type B. The dead metal zoneswhen flat dies are used hold back the oxide. Nev-ertheless, the billet length has to be limited orthe discard thickness correspondingly increasedto avoid shell and blister defects. This is coveredin more detail in Fig. 5.38.
The risk of shell defect is somewhat less inindirect extrusion than in direct extrusion.
5.13.6.3 Back End Defect
If the extrusion ratio is too low (heavy sec-tions), there is the risk of the “back end defect”forming particularly with material that flows ac-cording to type C in direct extrusion. The ac-celerating material in the center of the billet canform a funnel at the end of extrusion. Short billetlengths and large discard lengths can be used toreduce this (see Fig. 5.47).
Back end defect is seen less in indirect extru-sion than in direct extrusion and only when theextrusion ratio is too small and the discard toothin.
5.13.7 Discard Length
Discard lengths between 20 and 40 mm areused for copper, �-brasses, and tin bronzes,which also flows according to flow pattern B indirect extrusion.
For �-b brasses and special brasses, whichflow following flow pattern C, discard lengthsare between 30 and 50 mm.
The lengths should be 40 to 70 mm with thehard-to-extrude aluminum bronzes—also flowpattern C.
These are not always valid. Sometimes—in-dependent of the material and the container—30to 50 mm discard lengths are used in direct ex-trusion and the possible defective section endsremoved by careful control of cross sections(fracture tests).
The discard length can be significantlysmaller in indirect extrusion.
5.13.8 Direct Extrusion withLubrication and without a Shell
Extrusion is rarely carried out with internallubrication of the container, which reduces theextrusion load because the friction between thebillet and the container is largely eliminated.This process was adopted for the vertical extru-sion presses that used to be used when high ex-trusion ratios were required from relatively low-powered presses. Even today thin-walled tubesand small hollow sections are produced on ver-tical presses with lubricated containers. On hor-izontal presses it can be necessary to processdifficult-to-extrude alloys, e.g., copper nickel, athigher extrusion ratios than would be possibletaking into account the friction between the bil-let and the container. Lubrication of the con-tainer can help in this case.
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 245
Blisters on the surface of the extrusion can beavoided on susceptible alloys by extruding witha lubricated container. It is necessary to use aconical entry in the die and to machine the billetsurface. The billet surface forms the surface ofthe section because of the laminar flow andevery defect on the billet surface appears on thesection.
5.13.9 Extrusion into Water or Air
Alloys that tend to oxidation are often ex-truded into a water bath or through a water wave.This ensures that the extruded product does nothave to be pickled before subsequent furtherprocessing. It is also possible with copper tubeto restrict the secondary recrystallization ob-served when extruding into air and thus a coarsegrain on the exit from the die [Gre 71]. A finegrain is often required for further processing.
Water cooling should, as a general rule, beavoided for �-b materials because otherwise, asthe material leaves the die, the structure con-sisting mainly of the b-phase is undercooled andit is possible for some of the �-phase to precip-itate as needles. Both reduce the workability andincrease the strength of the extrusions [Bro 73].If more rapid cooling than air cooling is required(e.g., with brass wire), the material can onlycome into contact with water after air cooling(to 500–300 �C, depending on the material) if itis to remain soft. Direct slower cooling with awater-air mixture is possible.
5.14 Extrusion Processes andSuitable Equipment
Whereas earlier extrusion plants were mainlymultipurpose plants capable of producing rodand sections as well as wire and tube, today ex-trusion plants are designed specifically for theproduction of large quantities of one product[Ste 91]. Specific design details are given inChapter 6.
5.14.1 Extrusion Presses forBrass Wire and Sections
There are some presses on which large quan-tities of brass are produced where only wire isextruded onto down coilers and others that areequipped with pullers (up to 4 sections) for theproduction of rod and sections. In contrast to theextrusion of aluminum sections where several
strands can be pulled with a single puller, withcopper alloys a specific puller device must beprovided for each strand. The exit speeds of theindividual sections are not exactly equal so thatthe slower section would be severely stretchedat the high exit temperature.
Indirect extrusion is often used for brass wireand rod because this avoids the piping defect andlong billets can be used.
A rod and section press—usually the directprocess—has a section cross-transfer system aswell as the puller system and frequently a watertrough in which the sections can be quicklybrought to room temperature after crossing thecritical temperature of approximately 300 �C.Wire is also cooled in water after a specific cool-ing time in air.
If a wide range of materials and dimensionshave to be processed, a multipurpose press isstill the correct choice.
5.14.2 Tube Extrusion
Tube presses today obviously have a piercersystem. Water-cooled mandrels are used thatusually move with the stem (moving mandrel).The mandrel that is stationary in the die duringextrusion is subjected continuously to a hightemperature. The “stationary mandrel” is there-fore used only for the extrusion of tubes withsmall internal diameters and suitable hollow sec-tions.
Copper sections are extruded under water (oroccasionally in a protective atmosphere).
In the 1950s and 1960s a large range of ver-tical tube presses were installed. They were builtbecause the vertical axis simplified the align-ment of the mandrel to the container and the diegiving a better tube concentricity. On today’shorizontal presses the tools can be so easily ad-justed and guided that this advantage no longerapplies. Horizontal presses need simpler foun-dations and enable larger section weights to beproduced and have therefore almost completelydisplaced the vertical tube presses.
5.14.3 Drive
Whereas water and accumulator driven extru-sion presses were previously exclusively usedfor copper alloys giving high ram speeds (up to150 m/s), these are used today only for coppertube and thick sections.
In other cases direct oil drives are used, al-though these allow only a maximum ram speedof approximately 50 mm/s at an economically
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acceptable investment; this is usually adequate.The advantages of oil drive (good speed controland regulation over the billet length, simplermaintenance, and smaller footprint) predomi-nate. Exact speed control is particularly advan-tageous for a linked puller and synchronouslyoperating wire coilers. Variable oil drive alsosimplifies the automation of the extrusion pro-cess.
5.14.4 Die Changing
It must be possible to easily change dies andto control them on copper alloy presses: the highthermomechanical stresses result in such severewear and deformation of the shape forming diesthat dressing can be required after only a fewextrusions. Chapter 6 describes suitable diechanging systems (rotating arm, slide).
Quick container changing is more importantthan in aluminum extrusion because of the wearof the liner.
5.14.5 Discard Separation
The discard is normally removed with thesaw, in contrast to aluminum extrusion where itis usually sheared. It is important that goodswarf extraction prevents damage of the productfrom swarf.
In indirect extrusion and direct extrusion ofsmall solid sections a powerful shear is, how-ever, preferred [KM 77].
5.15 Billet Production and Heating
5.15.1 Continuous Casting andHomogenizing
Chill cast molds were still used up to the1960s. They were completely replaced by thecontinuous casting method developed circa 1930(see the section “Systems for the Production ofCopper Billets” in Chapter 6). Horizontal or ver-tical casting is used depending on the billet crosssection and alloy and usually on continuousplants. Homogenization of the billets is not nor-mally required apart from tin bronze, which issusceptible to severe segregation, where the riskof cracking is reduced.
5.15.2 Billet Length
The length of the extrusion billet is usuallydetermined by the extruded length of the section
with rod, sections, and tubes. With coiled wirethis is obviously not necessary. In this case themaximum billet length is selected. Alloys thatare susceptible to piping are usually limited to2.5 to 3.5 times the diameter in direct extrusion.
For extruded tubes the traditional rule ofthumb is that the billet should not be longer than5 times the mandrel diameter because of the in-creasing risk of wall thickness eccentricity as thebillet length increases. The billet length also hasto be restricted because of the occurrence ofshell defects (e.g., with low-extrusion-ratio cop-per tubes).
5.15.3 Billet Processing
In the case of lubricated extrusion or if thebillet surface is defective (particularly with flowtype B), the billet has to be skimmed. This ex-pensive operation is, naturally, avoided as muchas possible by preferably extruding with an un-lubricated container and a thicker shell that col-lects the casting defects.
In indirect extrusion without a shell and a con-ical die, the billet surface forms the section sur-face similar to direct extrusion with a lubricatedcontainer. All billets then have to be skimmed.To avoid this, extrusion with a shell and a flatdie is also carried out with indirect extrusion sothat a dead metal zone forms. Both hinder theflow of the billet surface into the surface of thesection. The risk of defects is, however, signifi-cantly greater than in direct extrusion and par-ticular emphasis has to be placed on a good,smooth billet surface.
If large-format tubes are extruded from large-diameter billets, the piercing load of the pressmay not be large enough and the billet has to beprebored. The general rule is that the bore di-ameter should be approximately 5 mm largerthan the mandrel diameter so that the lubricantis not wiped off as the mandrel enters the billet.
5.15.4 Billet Quality Control
The billet quality must in any case be care-fully checked before extrusion. This includesporosity and crack monitoring of sensitive ma-terial as well as visual inspection of the billetsurface.
5.15.5 Billet Heating
Billet heating is described in detail in the sec-tion “Billet Heating Systems” in Chapter 6.
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 247
Fig. 5.36 Hot tensile strength of SF-Cu and low-alloy coppermaterials [Wie 86, Gra 89]
The most economic heating in a gas furnaceis adequate to heat materials with a relativelylow extrusion temperature (e.g., free-machiningbrasses), even though in terms of the final tem-perature it is less accurate and less reliable. Atemperature profile can be applied, if needed, bysubsequent heating in an inline induction fur-nace with several heating zones and, in any case,the billet temperature can be more accuratelycontrolled. There are also cases in which gas fur-naces are used even with higher billet heatingtemperatures (e.g., copper). In these cases in-duction furnaces are more common because therisk of overheating is reduced. High billet pre-heating temperatures are simultaneously close tothe solidus temperature and thus there is a riskof the billet melting.
To reduce costs with high billet heating tem-peratures, the first heating step can be carried outin a gas furnace and the second in an inductionoven.
In every case an inline electrically heatedequalization chamber can be used in which tem-perature variations between the billet front andback can be removed and in which billets canbe maintained at temperature during pressdowntime.
5.16 Copper Extrusion
5.16.1 General
Unalloyed copper is mainly extruded to tubesand, to a small degree, to bar and sections. Cop-per wire is, in contrast, usually cast, hot rolled,and drawn.
5.16.1.1 The Different Grades ofCopper, Their Propertiesand Applications
The different copper grades are standardizedin DIN 1787 (see Table 5.11 for the Euro stan-dards). Pure copper with a low oxygen content(less than 0.04%), which bonds the residual im-purities, giving the maximum electrical conduc-tivity, is used in electrical technology under thedesignation E-Cu. In North America a designa-tion beginning “OF” indicate oxygen free. Ex-truded and drawn sections as well as flat bar areused. Another high-conductivity copper gradethat is oxygen free by deoxidation is referred toas SE-Cu in Europe. In North America deoxi-dized grades, DLP and DHP, are used for no-
nelectrical purposes. The STP and ETP desig-nations are used for electric bus. The mostexpensive variation is OF-Cu that is oxygen freewithout a deoxidation agent.
Oxygen-containing copper is sensitive toheating in a hydrogen-containing atmosphere.The oxygen forms water vapor in the pores ofthe annealed material with the diffused hydro-gen, and this can burst open the structure. If thematerial has to be suitable for welding, brazing,or annealing, the oxygen has to be bonded,which is carried out by phosphorus in SF-Cu(0.015–0.04%). Water supply tubes are the mainapplication of this copper grade.
5.16.1.2 Hot Workability,Extrusion Temperature
The hot workability of copper is limited bythe fcc structure up to the melting point (see thehot-strength curve in Fig. 5.36). The extrusiontemperature is accordingly very high (800–950�C), and a high extrusion ratio is possible onlyon powerful extrusion presses.
5.16.2 Copper Tube
5.16.2.1 Application
SF-Cu is used exclusively for domestic waterpipe, the most common use of copper tubes, be-cause these need to be welded or soldered. How-ever, SF-Cu is also used for underfloor heatingor for industrial applications (heat-exchangertubes in air-conditioning units, for chillers, etc).
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Whereas water supply pipes are available ashard or half-hard pipes or annealed coiled tubes(DIN 1786/EN 1057), industrial tube is usuallysupplied in annealed multilayered coils (coilweight 60–120 kg). Internal and external finnedtubes for heat exchangers are also produced inSF-Cu because of the good thermal conductivityof copper and its excellent cold workability.
5.16.2.2 Production Methods
Numerous modern copper tube plants havebeen installed specifically for the production ofSF-Cu tubes. The combination of the very goodcold workability with the reduced hot workabil-ity enables hot working to be used to produce atube larger than the finished dimensions fol-lowed by cold working to the finished size bycold pilgering and drawing machines without in-termediate annealing. Soft tubes are given a finalanneal. Half-hard tubes are lightly drawn afterannealing.
Three processes compete for the productionof copper tubes [Tus 70]:
● Production using a piercer, cold pilger ma-chine (tube rolling), and drawing machines:Heated billets are rolled with inclined rollsover a mandrel to tube blanks. A straighttube up to 150 m long is then produced on amultistage cold pilger machine with a reduc-tion ratio of 10:1 followed by redrawing.
● Production on an extrusion press, cold pilgermachine, and drawing machines: The usualdimensions of the extruded tube are 80 �10 mm with a piece weight of more than 400kg. As in the first process, the extruded tubeis then further processed with cold pilger ma-chines and drawing machines. Schumag ma-chines and spinner blocks are used.
● Production on an extrusion press and draw-ing machines: A so-called thin tube, e.g., 73� 4 mm and a length of approximately 50m but a lower piece weight, is extruded.Without the need for a cold pilger operationthese tubes go directly to the drawing ma-chines.
In the first process (with no extrusion press)there is the risk of cracks and thus laps and oxideinclusions, which impair the quality of the tubesif the high demands of the casting process arenot fulfilled. In the production of gas and watersupply pipe with a minimum wall thickness of1 mm this does, however, play a minor role. Thisvery economic process is used almost exclu-
sively in the production of gas and water supplypipes.
In the production of industrial tubes with wallthickness down to 0.35 mm and finned tubes,the demands on the starting tube are very highso that only the second and third processes withextrusion are used.
5.16.2.3 Extrusion
The press and the tooling have to be accu-rately aligned so that a low tube eccentricity(�7% and under) can be achieved. In spite ofcareful alignment of the centerline of the con-tainer, stem, and mandrel as well as the die,movement of the mandrel during extrusion is al-most impossible to avoid. Therefore, to achievea low eccentricity it is usual to limit the lengthof the unmachined billet to 5 (up to 8) times themandrel diameter.
The tube is usually extruded at high speed un-der water within a few seconds, which reducesthe risk of mechanical damage and produces thefine grain needed for extensive cold working.
Only direct extrusion presses are used.To avoid water getting inside the tube, con-
trolled mandrel movement is used at the startand the end of extrusion to give a tube closed atboth ends (Fig. 5.37). The extruded tube is thenoxide free internally and externally and can bepassed to the cold pilger machine or drawingmachine without pickling.
5.16.2.4 Oxide on the Billet Surface,Shell and Blister Defect
As mentioned previously, there is the riskwith copper flowing according to type B that theoxide produced on billet heating acts as a lubri-cant between the billet surface and the containerand flows along the dead metal zone that formsin front of the flat die: extrusion shell defects arethe consequence (Fig. 5.38). They produce blis-ters when drawn tubes are annealed (Fig. 5.39).
Fast induction heating is recommended (atleast as the final stage) to ensure that the mini-mum of oxide forms on the billet surface. Theoxide on SF-Cu copper does not adhere stronglyto the surface—in contrast to the oxide on oxy-gen-rich qualities. It can be largely removed bya strong water spray at the furnace exit. At thesame time the billet hardly loses any heat. Hotscalping of the oxidized billet surface, which ismentioned in the literature, is now rarely used[Vat 70].
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 249
Fig. 5.37 Extrusion of copper tubes [Bau 93]
Fig. 5.38 Extrusion shell formation in the extrusion of copper tubes [Bau 93]
In conventional extrusion with a shell, the aimis for the oxide layer to be retained on the con-tainer liner. This does not occur completely. Be-cause, as shown in Fig. 5.38, the oxide first ar-rives at the die toward the end of extrusion, the
risk of lines of blisters is reduced by the selec-tion of a large extrusion ratio, which increasesthe distance between the billet surface and thesurface of the extrusion. If the billet is shortenough, the flow of the oxide is stopped in timein front of the die.
As already mentioned, the extruded shell is sothin and ductile that a combination dummyblock and cleaning block can be used in extru-sion.
5.16.3 Copper Rod and Section
5.16.3.1 Dimensions and Shape,Further Processing
The high extrusion temperature of copper pre-vents the production of thin-wall complex andsharp-edged sections. Extruded shapes thereforehave to be brought to the finished dimensions byone or more cold deformation steps—on drawbenches. The good cold workability of coppermakes this relatively simple. This also producesthe preferred hard state. Figure 5.40 shows ex-amples of extruded and drawn copper sections.
As described in section 5.16.2.3 for coppertubes, rods and sections in copper are also ex-truded underwater to achieve oxide-free, dam-age-free products with a fine grain. Small cross-sectional areas can be extruded in long lengthsthrough a water wave and then coiled.
5.16.3.2 Hollow Sections
Hollow copper symmetrical sections for in-ternally cooled bus bars in electrical engineering
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Fig. 5.40 Example of extruded and drawn copper sections. (a) Solid section. (b) Hollow section (Source: Kabelmetal Osnabruck)
Fig. 5.39 Blisters on the tube surface of a SF-Cu-tube after annealing. (a) Transverse section. (b) External surface with line of blisters[Die 76]
are, if possible, extruded from large billetspierced in the press and, for small openings, overthe mandrel tip (fixed mandrel). With difficultshapes, prebored billets can be necessary. Theshape of the mandrel tip and the exact locationin the die require considerable experience.
Hollow sections have to be extruded throughbridge dies if the openings are asymmetrical orif the section has several openings. This processis similar to that for aluminum alloys. However,with copper, deformation and cracking of thevery expensive dies can occur along with the
oxide flowing into the weld seam. This can beavoided only by good technical knowledge andexperience. The extrusion of copper throughbridge dies is therefore rarely used.
5.16.4 Extrusion of Low-AlloyCopper Materials
5.16.4.1 The Materials, Properties,and Applications
Low-alloy copper materials are covered bythe standard DIN 17666 (UNS C-10100–
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 251
C15815). The composition, properties, and ap-plications of the different alloys are described indetail in the DKI information sheet 8 [DKIa].
In the non-age-hardening alloys, additions ofsilver, cadmium, magnesium, and iron increasethe mechanical properties and, in particular, thesoftening temperature without reducing the con-ductivity significantly. Tellurium and lead im-prove the machinability. Additions of beryllium,nickel, silicon, chromium, and zirconium pro-duce age-hardening alloys that have high me-chanical properties and simultaneously highelectrical conductivity after solution heat treat-ment and age hardening.
Application examples of the low-alloy coppermaterials are found mainly in electrical technol-ogy and chemical plant construction, e.g., con-tacts and spring elements as well as (CuCr,CuCrZr) welding electrodes (see also the sectionon copper alloy extruded products in Chapter 2).
Alloy development in recent years has re-sulted in some nonstandard age-hardening ma-terials for elements of electrotechnology andelectronics. The majority are used only in theform of strip, and extruded and drawn productsare rarely used.
5.16.4.2 Extrusion
The low-alloy copper alloys have similar ex-trusion properties to unalloyed copper. The riskof flaking and blister formation by the inflow ofoxide into the funnel behind the dead metal zoneis usually even higher than with copper becauseof the greater tendency to oxidation [Moi 89].The oxide thickness also increases when ahigher extrusion temperature has to be used forthe same extrusion ratio because of the higherhot strength. It is advisable in such cases to blowoff the oxide from the hot billet using a waterspray.
Figure 5.36 shows some hot-strength curves.Extrusion data are given in Table 5.9.
5.16.4.3 Solution Treatment at the Press
The mechanical and electrical properties ofthe age-hardening alloys are obtained by solu-tion heat treatment followed by hot age hard-ening. If the cross section is not too high, thesolution heat treatment of CuCr and CuCrZr canbe carried out on the press. This process is wellknown from the low-alloy aluminum materials.Because, however, in this case the solution heattreatment has to be carried out at approximately1000 �C, the stresses in the container and the die
are extremely high as is, as described previously,the risk of flaking and blistering [Hes 82]. As ageneral rule, only relatively short billets can beextruded (length:diameter � 1 to 1.5:1). Thebillet heating time in the induction oven cansometimes be too short to completely dissolvethe second phase and to achieve the maximummechanical properties.
The section has to be cooled in water to freezethe solid-solution state. Above a specific sectionthickness (60–70 mm diam) water quenching atthe press is no longer sufficient. In this case thesection has to be solution heat treated after ex-trusion in a special oven and then quenched. Inthis case a lower extrusion temperature can beused because the extrusion process and the so-lution heat treatment are not combined.
The hot age hardening following solution heattreatment (e.g., 475 �C for CuCr) is carried outbefore or after the cold working, depending onthe properties required.
5.16.5 Extrusion of Copper-ZincAlloys (Brass and Tombac)
5.16.5.1 Binary Copper-Zinc Alloys
Properties, Structure. Brass alloys are themost commonly used of the copper alloys andlong sections are almost completely produced byextrusion. All copper-zinc alloys with a copperfraction more than 50% are referred to asbrasses. More than 72% copper, the copper-zincalloys are also referred to as “tombac.” Theseare alloys with a reddish color. As the coppercontent is reduced corresponding to an increasein the zinc content, the color changes more andmore to yellow, and at the same time the hard-ness increases.
The copper-zinc phase diagram is describedfirst because of the marked variations in me-chanical properties and structure with the zinccontent.
As shown in Fig. 5.41, three alloy groups canbe clearly differentiated:
● Single-phase �-alloys with a copper contentabove 61%
● Binary-phase �/b-alloys with a copper con-tent of 54 to 61%
● Single-phase b-alloys with a copper contentof 50 to 54%
The single-phase �-alloys have a fcc latticesimilar to pure copper. They can correspond-
ingly be easily cold worked (see structure inFig. 5.42).
The single-phase b-structure is body centeredcubic (bcc) and has very limited workability atroom temperature. Between these limits (shownby the lines BE and CF on the phase diagram)there is a region in which the �-phase and theb-phase can coexist. The greater the zinc con-tent, the lower is the �-content. The cold work-ability reduces in this region corresponding tothe increasing zinc content (see Fig. 5.43, �-b-structure).
The Materials, Properties, and Applica-tions. DIN 17660 covers the commercially usedcopper zinc alloys (See also ASTM B455 forCopper-Zinc-Lead [leaded brass] extrudedshapes). The DKI information sheets i.5 and i.15[DKIb, DKIc] cover these CuZn alloys in detail(See also Copper Development Association website www.copper.org).
The brasses with a pure �-structure as well asthe tombac (low-alloy CuZn) alloys are usedpredominantly in the form of sheets and strip,less as tubes, and rarely in the form of bar andsection. The producers of jewelery and metalware, brass instruments, lamp bodies, and light-bulbs use �-brasses. Tubes are used for air brakepipes in goods vehicles and for manometers.
The alloys in the �-b field with a b-fractionbetween 20 and 40% can be readily machinedand are therefore widely used as rod material.
Chip forming additions—usually between 1 and3% lead—that are embedded as droplets in thematrix further improve the machinability.
Materials with only a low b-content (CuZn36Pb1.5) can be readily machined and coldworked. They are used where stamping or bend-ing is required. The free-machining brasses(mainly CuZn39Pb3) are used in large quantitiesas hard drawn rods for processing on automaticlathes to turned components of all kinds (e.g.,bolts). They can be cold worked only to a limitedextent (see Table 5.11, EN 12164). Pure b-brass
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 253
Fig. 5.43 Brass with acicular �-b-structure, etched. Imagewidth is approximately 0.275mm [Wie 86]
Fig. 5.45 Some hot tensile strength curves for copper-zincalloys and SF-Cu [Wie 86]
(e.g., CuZ44Pb2) is almost brittle at room tem-perature but is used for extruded sections that donot have to be subsequently cold worked be-cause of its excellent hot workability. Sectionsin �-b brass can, on the other hand, be colddrawn to a limited extent and therefore be sup-plied to tight finished tolerances. Figure 5.44shows some examples of extruded and drawnbrass sections. They are covered by the DIN17674 standard (EN 12167).
Extrusion. At the normal extrusion tempera-ture of more than 600 �C, the b-structure has a
significantly lower flow stress than the �-struc-ture. The �-b brasses and especially pure b-brasses therefore have a high extrudability. Fig-ure 5.45 illustrates this by hot flow stress curves.
The workability of the CuZn alloys decreasesinitially in the normal extrusion temperaturerange of 600 to 800 �C with increasing zinc con-tent and then increases again from approxi-mately 30% zinc when b-phases occur duringextrusion.
The low flow stress of the b-phase at the ex-trusion temperature enables high extrusionspeeds and extrusion ratios to be attained. Withfree-machining brass extrusion ratios up to V �900 and exit speeds up to 8 m/s can be reached.
Table 5.9 gives extrusion data on copper-zincalloys.
Because the phase boundaries �/�-b and �-b/b are displaced to higher zinc contents on cool-ing, the �-phase fraction extends into the �-bfield on cooling (see Fig. 5.41). The �-precipi-tation can be partially suppressed by quenching(frozen) so that the b-fraction remains higherthan would be the case in the equilibrium state.If quenching is carried out after cooling to 500to 300 �C, depending on the composition, the �-precipitation has stabilized and the structure nolonger changes.
By quenching from the deformation heat andaging at temperatures approximately 200 �C, �-b-brasses can be given a substantial hardness in-crease by the fine precipitation of the secondphase [Bro 73]. This is rarely used in practicebecause it is difficult to maintain the extrusion
254 / Extrusion, Second Edition
Fig. 5.46 Extrusion defect (piping) in �-b-brass [Die 76]
conditions constant from billet to billet, and atthe same time the cold workability is restricted.
Possible Extrusion Defects and their Pre-vention. The high deformation resistance oflow-alloy �-brasses requires a high extrusiontemperature for a given extrusion press power.This increases the danger of coarse grain for-mation and in the extreme case, hot shortness.Low initial billet temperatures as well as lowextrusion speed reduce the risk of coarse grainformation and hot shortness, as with all materi-als. However, at the lower extrusion temperaturethe deformation resistance is higher and the pos-sible minimum extruded cross section for agiven press power larger.
With copper-zinc alloys containing more than80% copper, copper oxide forms on the billetsurface during heating in air. With alloys withless than 80% copper, zinc oxide mainly formsand this has no lubrication properties in contrastto copper oxide. Whereas the CuZn alloys withmore than 80% copper can still have shell andblister defects, this risk does not occur withhigher-zinc-containing materials because the ad-hesion between the billet and the container is solarge that a closed shell is formed when extrud-ing with a shell. On the other hand, the combi-nation dummy block and cleaning disc that canbe used in copper extrusion is not suitable be-cause of the large shell thickness. A separatecleaning cycle has to take place after extrusion.
As the zinc content increases, the thermalconductivity of brass decreases and the periph-eral cooling of the billet in the container is nolonger equalized. Because the b-rich hotter billetcore flows more easily than the b-impoverishedperipheral zone, the typical flow pattern type Cforms (see Fig. 3.11 in Chapter 3) with the riskof the piping defect. Figure 5.46 shows an ex-ample.
The billets should have a perfect surface qual-ity to avoid extrusion defects. The difference be-tween the container diameter and the billet di-ameter should also not be too large to avoid foldsforming as the billet is upset in the container.With a good quality liner surface, the shell fromthe previous extrusion can be completely re-moved with the cleaning disc [Lot 71].
In direct extrusion, the length of the billet islimited to reduce the risk of the piping defect.As a general rule the billet should not be longerthan 2.5 to 3 times the diameter. Routine fracturetesting of the end of the sections can remove anysections containing the extrusion defect and en-sure that the remaining length is defect free.
Indirect extrusion is recommended for �-b-brasses where there is the risk of the extrusionpiping defect forming in direct extrusion be-cause the different material flow excludes thisdefect. Significantly longer billet lengths canthen be used [Sie 78].
As mentioned elsewhere, in direct extrusion,if the extrusion ratio is too low, particularly withbrass, the acceleration of the center of the billetcan be so severe that cavities can occur towardthe end of the section (Fig. 5.47). Their forma-tion in the section can be prevented only by lim-iting the billet length and leaving a sufficientlylong discard.
A further defect, particularly with �-brasseswith a low zinc content, is the formation of zincflakes on the surface of the extruded product.Zinc vaporisation from the newly formed surfaceimmediately behind the die condenses ontocooler parts of the tooling in the form of finedrops, which can be picked up by the passingsection. The droplets form zones of high zinccontent, containing b phase, on the section sur-face. These are brittle and can result in defects.These zinc flakes can also be found on the inter-
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 255
Fig. 5.48 Schematic of a section and wire brass extrusion plant (SMS Hasenclever catalog)
Fig. 5.47 End cavities in a brass bar [Die 76]
nal surface of tubes. In order to prevent themforming it is necessary to ensure that the hot sec-tion does not contact the tooling behind the die.
Handling the Section. Because the �-b-brasses and the b-brasses are very soft at the exittemperature and sensitive to mechanical contact,good guiding out of the die (possibly lined withgraphite) is needed to avoid damage of the sec-tion surface combined with careful handling onthe runout table.
Whereas thick bars from single-cavity dies areextruded into air on graphite or steel-linedplates, or onto a freely rotating roller conveyorand then cross transferred until they can becooled—possibly in a water trough—and cut tolength, pullers are usually used for sections andthinner bar. Up to four sections can be simulta-neously pulled using independent jaws. The ma-
terial emerging from the die is very soft and itis necessary to use a defined low puller force toavoid undesired stretching of the sections.
If the cross-transfer conveyor is not largeenough, water cooling has to be used at the endbefore the sections can be cut to length. Figure5.48 shows the principle of the extrusion ofbrass sections.
Cu-Zn wires, usually in free-machining brass,are extruded from one or two cavity dies intodown coilers, the speed of which are synchro-nized with the ram speed. Periodic oscillationsof the rotation speed (wobble) ensure that theindividual layers on the drum are not directlyabove each other. To avoid damage, the strandsare usually extruded into pans, which at the endof extrusion are removed from the coilers andtransported on a roller conveyor until the brass
256 / Extrusion, Second Edition
Fig. 5.49 Displacement of the phase limits in the copper-zinc system using aluminum additions as an ex-
ample [Bar 32]
Table 5.12 Influence of element additions onthe b-phase fraction and the extrudability ofspecial brasses
Alloyingelement b-portion
Zincequivalent(a) Extrudability
Silicon Is improved 10 Is improvedAluminum Is improved 6 Is improvedTin Is improved 2 Is improvedManganese Is improved 0.5 Is improvedNickel Is improved �0.9–�1.5 Is improved
(a) 1% the alloying element corresponds to one in the effect on the structurecondition one as “zinc equivalent” of indicated multiples of the effect of zinc.Source: Lau 76
wire has cooled sufficiently and can be lifted out.Further processing is carried out after picklingby a combination of drawing, straightening, andcutting to length.
Straightness—b phase distribution—Fur-ther Processing of Bar. The straightness of thebar is very important for machining on auto-matic lathes with high cutting speeds. Bars thatare not clean or straight chatter in the bar feed.If the b-phase fraction and its distribution in thecross section vary over the length of the extru-sion, the rectification effect in the combinationdrawing, straightening, and cutting to length ma-chine changes from the first bar to the last of anextruded coil.
The indirect extrusion process again offers ad-vantages over the direct process. The variationin the size and distribution of the b-phase in the�-b mixed structure between the start of extru-sion to the end in indirect extrusion is less thanin direct because of the uniform material flowover the length of the extrusion.
In multicavity extrusion the billet no longerflows symmetrically in one strand because thedie apertures are arranged asymmetrically. Oneside of the section surface stems from the outerregion of the billet and the other side from theinner region. It is therefore particularly impor-tant to ensure a uniform cast structure and goodthrough heating; there are then no disadvantagesfor the quality of the section and, in particular,for the straightness of the finished bars.
In order to obtain uniform mechanical prop-erties and good straightness, care must be takento ensure that the degree of deformation in theindividual drawing operations is held within nar-row limits requiring that the dimensional toler-ances of the extruded bar are within tight limits.Suitable selection of the hot-working materialsfor the die and its design are discussed in detailin the section on tooling for the extrusion of cop-per alloys in Chapter 7.
5.16.5.2 Copper-Zinc Alloys with AlloyAdditions (Special Brasses)
The Different Materials and Their Prop-erties. If additional elements, including alumi-num and tin, are added to copper-zinc alloys,properties such as the b-phase fraction, the cor-rosion resistance, and the strength change sig-nificantly. These materials are referred to as spe-cial brasses. They are also covered by DIN17660 copper-zinc-aluminum alloys are knownas aluminum bronzes and copper-zinc-tin alloysare tin brasses.
Again, as with brass there are single-phasematerials with a fcc �-structure and two-phasematerials with an �-b structure. In both groupsadditional intermetallic phases can occur de-pending on the type and amount of the additions.
The b-phase fraction in the �-b-mixed struc-ture, which has a strong influence on the extrud-ability and the cold workability, can be con-trolled by these additions of alloying elements(see Table 5.12, Fig. 5.49).
Semifinished Products and Applications.The alloys CuZn20Al2 and CuZn28Sn1 are re-sistant to seawater and corrosion. They are usedin heat exchangers and condensers. Becausethey have a pure �-structure and consequentlyare difficult to extrude but have good cold work-ability, the tubes are extruded with large crosssections and brought to the finished dimensionson cold pilger machines and draw benches withintermediate annealing.
CuZn31Si1 is a material with an �-b-mixedstructure. Tubes for bearing bushes are produced
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 257
Fig. 5.50 Hot tensile strength curves of special brasses andSF-Cu [Wie 86] Fig. 5.51 Copper-tin phase diagram [Ray 49]
in this alloy. Other nonstandardized specialbrasses (with additions of Mn, Ni, Si, and Pb)have proved themselves as wear-resistant bear-ing materials in special cases.
High-zinc-containing special brasses are con-struction materials with moderate and high me-chanical properties used in the manufacture ofchemical plants.
Casting and Extrusion. Casting defects, in-cluding porosity and cracks, have to be assumedin special brasses, particularly with aluminumand manganese additions. Careful testing of thebillet quality (e.g., using a penetrant such as theMet-L check testing on the cut surface) is nec-essary. Billets with defects on the mantle surfacehave to be machined.
In the same way as with pure brasses, the ex-trudability depends on the amount of the b-phase present at the extrusion temperature. Thisis shown by the hot tensile test curves in Fig.5.50. The possible defects and their avoidancefollow those described in section 5.16.5.1.
5.16.6 Extrusion of Copper-TinAlloys (Tin Bronzes)
5.16.6.1 The Different Alloys,Their Structures, Properties,and Applications
The tin content of tin-bronzes can extend to20%. However, the higher-alloyed materials(more than 10% tin) are extremely difficult todeform and are, therefore, usually used only ascast materials. Complex tin-bronzes can containadditions of zinc and lead.
DIN 17662 describes the composition of thecommon alloys suitable for extrusion with amaximum tin content of 8%. The DKI infor-mation sheet i.15 [DKId] describes the copper-tin alloys in detail (See also Copper Develop-ment Center. These tin bronzes are also calledphosphor bronzes).
Tin-bronzes belong to the oldest known cop-per alloys. They are still today very important inthe chemical industry and ship construction be-cause of their good electrical and thermal con-ductivity with simultaneous high-strength andfavorable corrosion properties. In electrical tech-nology, significantly more tin-bronze strip isused than wire and sections.
CuSn4 and CuSn6 tubes are used for manom-eters or suction pipes. Highly stressed compo-nents, including gear wheels, bolts, and bear-ings, are made from CuSn8.
The phase diagram in Fig. 5.51 shows that thewrought materials in the equilibrium state havea pure fcc �-structure and therefore behave likecopper in hot and cold working.
258 / Extrusion, Second Edition
Fig. 5.52 Hot tensile strength curves of copper-tin alloys andSF-Cu [Wie 86]
5.16.6.2 Casting of Copper-Tin Alloys
Because the cast structure—particularly at thehigher tin contents—tends toward segregationbecause of the wide solidification interval andthe billets can exhibit tin sweating as a result ofinverse billet segregation, turning of the billetsis advised. Homogenization of the billets (630to 700 �C for several hours) reduces the risk ofextrusion defects in the form of longitudinal andtransverse cracks in the higher-alloyed materi-als. With more than 6% tin, the brittle �-b eu-tectoid that forms on solidification is partly re-tained in the cooled billet; it can be dissolvedonly by this homogenization. Some phospho-rous is added to the 8% tin-bronze to suppressthe tin-oxide formation in melts that have notbeen completely deoxidized. These tin-bronzesare referred to as phosphor-bronzes.
5.16.6.3 Extrusion of Copper-Tin Alloys
The flow stress of the tin-bronzes is higherthan that of copper as shown in the hot tensilestrength curves in Fig. 5.52 and increases withincreasing tin content. Tin-bronzes are bestworked between 700 and 750 �C. The extrusiondata can be found in Table 5.9.
Tin-bronzes containing up to 6% tin are mod-erately difficult and those with 8% difficult toextrude. High-alloyed tin-bronzes are very sus-ceptible to cracking and can be extruded onlywith slow speeds [Moe 80].
Bars smaller than 25 mm diameter are ex-truded as several strands and coiled. There is therisk of the billet “freezing” in the container be-
cause of the low speed and the large differencebetween the initial billet temperature and thecontainer temperature. Only short billets cantherefore be used.
With the good cold workability of the fcc lat-tice the section can be extruded above the finaldimensions and then worked to the final size inseveral stages—including where necessary in-termediate annealing—by section rolling anddrawing. This also produces the frequently re-quired high mechanical properties.
5.16.6.4 Flow Type and Shell Defects
Because tin-bronzes tend to oxidize and thisoxide exhibits good lubrication properties, flowtype A occurs in the direct extrusion of thesealloys, giving the risk of shell and blisterdefects—particularly with low extrusion ratios,i.e., thick bars [Vat 70]. With large extrusion ra-tios—thin bars—this risk is reduced. Extrusionis normally carried out without container lubri-cation and with a thin shell. Care has to be takenin the cleaning cycle to ensure that no residuesof the shell from the previous extrusion remain.
5.16.6.5 Competition from Wire Casting
As mentioned previously, only short billetscan be used in wire extrusion giving low coilweight. The extrusion process and subsequentdownstream processing is correspondingly ex-pensive. Direct continuous casting of wire inlarge coils has grown in competition to the ex-trusion of thin bar from which tin-bronze wireis produced by rolling and drawing.
The production of tin-bronze wire today ismainly by this continuous casting process[Bau 76].
5.16.7 Extrusion of Copper-AluminumAlloys (Aluminum Bronzes)
5.16.7.1 The Different Alloys, TheirProperties, and Applications
The composition of copper-aluminum alloys,usually referred to as aluminum-bronzes, is cov-ered by DIN 17665 (see Table 5.11 for the Eurostandard). The DKI information sheet i.6 [DKIe]describes aluminum-bronzes in detail (see Cop-per Development Association).
Binary aluminum-bronzes up to approxi-mately 9% aluminum have a homogeneousstructure with the fcc �-phase, as can be seen
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 259
Fig. 5.54 Hot tensile strength curves of copper-aluminumalloys and SF-Cu [Wie 86]Fig. 5.53 Copper-aluminum phase diagram [Ray 49]
from the phase diagram in Fig. 5.53. In contrast,complex aluminum-bronzes with aluminumcontents of approximately 10% and additions ofiron, nickel, manganese, and silicon individuallyor together usually have a heterogeneous struc-ture.
Aluminum-bronzes are corrosion- and oxi-dation-resistant alloys because of the formationof Al2O3-containing surface layers. They arealso very resistant against erosion-corrosion andcavitation and are particularly suitable for sea-water applications. Aluminum-bronzes are alsofound in the chemical industry and in highlystressed machine components.
5.16.7.2 Binary Copper-Aluminum Alloys
Materials and Extrudability. The two stan-dardized alloys CuAl5 and CuAl8 have a fcc �-phase structure. They can be readily coldworked. Their hot workability depends on thealuminum content. Figure 5.54 shows the tem-perature dependence of the hot tensile strengthof the aluminum-bronzes.
The poor extrudability is revealed by the ex-trusion data in Table 5.9.
The phase diagram in Fig. 5.53 shows that theboundary line �/�-b is displaced to higher alu-minum contents with decreasing temperature.The b-content thus decreases during coolingfrom the extrusion temperature. Even with nor-mal cooling to room temperature from the ex-trusion temperature, residues of the bcc b-phaseare retained with the 8% aluminum-bronze sothat only limited cold working is possible. Ifsmaller final cross sections are to be produced
from larger extruded cross sections by coldworking, then only the 5% aluminum bronzescan be considered.
Extrusion and Extrusion Defects of Cop-per-Aluminum Alloys. The oxidized billet sur-face, mainly with Al2O3, does not act as a lu-bricant in the container in contrast to copperoxide. The friction resistance is so high that it ispossible to extrude with a stable shell. Becausethe cast billets frequently have surface defects,machining is recommended. Testing the crosssection for cracks and cavities is also advisable.
If the billet heated to more than 750 �C coolsduring extrusion—it can only be extruded rela-tively slowly—variations in the �-b-ratio occurover the length of the extrusion along with vari-ous forms of the �-phase, resulting in differ-ences in the mechanical properties.
As with all difficult-to-extrude materials, ahigher extrusion ratio for the same press powercan be attained with the aluminum-bronzes byextruding with a lubricated container without ashell through a conical die. This process is usedonly in exceptional cases.
If the aluminum content is 8% or more, thenthe structure at the high extrusion temperatureabove 900 �C consists of an �-b mixed structure,the flow stress of which is less than the pure �-structure. However, in the direct extrusion ofthese alloys, there is the risk of flow type C oc-curring and the formation of the piping defect atthe end of extrusion in the same way as withbrass. The countermeasures are the same asthose mentioned previously; the piping defectcan be controlled by a suitable short billet and
260 / Extrusion, Second Edition
Fig. 5.55 Copper-nickel phase diagram [Han 58]
long discard. Fracture tests of the ends of theextrusion are recommended in any case.
Materials, Structure, and Properties. Theproperties of alloys CuAl8Si, CuAl9Mn,CuAl10Fe, CuAl10Ni, and CuAl10Fe3Mn2 aregiven in the standards.
The mechanical, physical, and chemical prop-erties of the complex aluminum-bronzes dependon the composition. The aluminum-bronzes withiron, manganese, and silicon additions are usu-ally extruded in the b-phase where they have alower flow stress than CuAl8 (see Fig. 5.54). Onthe other hand, CuAl10Ni has an extremely highflow stress (about 70% higher than CuAl8 at 700�C). This alloy is extremely difficult to extrude.
Extrusion of Complex Aluminum-Bronzes.Complex aluminum-bronzes have a strong ten-dency to stick to the container wall. In contrastto brass, the shear stress needed to shear awaythe shell is very high at the extrusion tempera-ture of 600 to 700 �C. The related high stem loadneeded to shear the shell and the high flow stressresult in the complex aluminum-bronzes beingvery difficult to extrude in spite of the high b-phase component. Often, if small cross sectionshave to be extruded there is no alternative toworking with a lubricated container without ashell and with a conical die entry.
A special case in the complex aluminum-bronzes is the copper-aluminum-nickel-shapememory alloy (about 13% aluminum, 4%nickel) in which the composition, billet pretreat-ment, and the extrusion conditions have to beexactly maintained to obtain the desired shapememory properties [Don 92].
The sticking of complex aluminum-bronzes tothe extrusion tooling can sometimes cause prob-lems in tube extrusion if the mandrel lubricationor the mandrel taper is insufficient. The internalsurface of the tube can then exhibit cracks orflaking.
Another almost typical defect with the com-plex copper-aluminum alloys is the “wood grainfracture” attributable to severe gas porosity ofthe billet or the inclusion of aluminum oxidefilms. The billets therefore have to be carefullychecked (e.g., by penetrant check) for sound-ness.
The complex aluminum-bronzes in direct un-lubricated extrusion usually flow according to
type C similar to CuAl8, so that allowance hasto be made for the piping defect.
Further Processing. The cold workability ofthe complex aluminum-bronzes is very re-stricted compared with the binary alloys becauseof the high b-phase component. They are, there-fore, normally supplied as extruded.
Similar to special brasses, the complex alu-minum-bronzes can be heat treated [Ben 93].Higher mechanical properties can be obtainedwith CuAl10Fe and CuAl10Ni by solution heattreatment in the �-b-region followed by quench-ing and age hardening at 500 to 650 �C. Onquenching a b�-martensite forms that partlybreaks down on aging. The quenching can takeplace at the press, but better values are obtainedby a separate heat treatment after extrusion.
5.16.8 Extrusion of Copper-Nickel Alloys
5.16.8.1 Materials, Structure,Properties, and Applications
The copper-nickel alloys contain between 4and 50% nickel, usually approximately 5 to30%. Nickel forms a continuous solid solutionwith copper (see Fig. 5.55) so that all copper-nickel alloys are single phase and have a fcc �-structure.
The most important copper-nickel alloys arecovered by DIN 17664 (ASTM E 75). They areCuNi10Fe, CuNi20Fe, CuNi30Fe, and CuNi30Mn1Fe.
The most important alloy technically isCuNi30Fe, which is used for tubes in ship heatexchangers, seawater desalination plants, and air
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 261
Fig. 5.56 Hot tensile strength curves of copper-nickel alloysand SF-Cu [Wie 86]
and oil coolers. However, CuNi10Fe alloys alsofind applications here. Whereas pure binary al-loys are used primarily for the production ofcoins, the extruded alloys used for other appli-cations, e.g., condenser tubes, have additions ofiron and manganese to improve the properties.These elements increase the corrosion and ero-sion resistance and raise the strength as well asthe recrystallization temperature. The DKI in-formation sheet i.14 [DKIf] describes the cop-per-nickel alloys in detail. (See UNS C/70100to C/72950 and ASTM B 151.)
5.16.8.2 Extrusion of Copper-NickelAlloys
The melting point and recrystallization tem-perature increase with increasing nickel content,and the cold and hot workability decrease. Ascan be seen in Table 5.9, the extrusion tempera-ture increases with increasing nickel content.The range is between 850 and 1000 �C and isthe highest of all the copper extruded alloys.Figure 5.56 shows the hot strength of the copper-nickel alloys as a function of the temperature. Itdoes fall within reasonable limits at the high ex-trusion temperatures mentioned, but when de-termining the extrudability, the service life of thetooling plays the decisive role.
Copper-nickel alloys should and can be ex-truded as quickly as possible because of the highextrusion temperature and the associated ther-momechanical stressing of the tooling. Usually,relatively thick-walled tubes are extruded andthen further processed on cold pilger machinesand by drawing with intermediate annealing be-cause the copper-nickel alloys have good cold-working properties as a result of their fcc lattice.
Extrusion is usually carried out with a shelland without container lubrication through flatdies. This is not possible if small cross sections,i.e., a high extrusion ratio, have to be extrudedin these difficult-to-extrude alloys. Container lu-brication is then recommended with extrusionthrough conical dies and without a shell. Theresultant significantly lower friction between thebillet and the container enables higher extrusionratios to be produced or lower extrusion tem-peratures to be used. However, attention has tobe paid to having a high-quality container boreand a good billet surface to avoid defects on thesurface of the extrusion.
The increase in hydrogen solubility with in-creasing nickel content and thus the risk of gasporosity with inadequate melt treatment has to
be taken into account when casting the billetsinto water-cooled molds or by continuous cast-ing.
A homogenization heat treatment of the bil-lets is not considered to be necessary in spite ofthe nickel concentration variations resulting tosome degree from the casting process. The bil-lets often have to be machined to remove thecast skin.
5.16.9 Extrusion of Copper-Nickel-ZincAlloys (Nickel-Silver)
5.16.9.1 Material, Structure,Properties, and Applications
Alloys of copper, nickel, and zinc are referredto as nickel-silver because of their silver color.The copper content of the technically most com-mon alloys can range from 45 to 62% and thenickel content 7 to 26%, with zinc forming theremainder. Alloys suitable for turning and drill-ing with an �-b-phase contain up to 2.5% leadas a chip breaker similar to brass. Small addi-tions of manganese are usual to reduce the riskof cracking at high temperatures (“annealingcracking”).
The copper-nickel-zinc alloys are covered byDIN 17663 (see ASTM B 151).
Figure 5.57 shows the copper corner of thecopper-nickel-zinc system. The solid lines in thephase diagram show the room temperature state;the dashed lines apply at 850 �C.
In the majority of the commercial nickel-sil-ver alloys, the fractions of nickel and zinc are
262 / Extrusion, Second Edition
Fig. 5.58 Hot tensile strength curves of copper-nickel-zincalloys and SF-Cu [Wie 86]
completely dissolved in the copper so that only�-solid solution exists. As with brass, only thehigh-zinc-containing alloys are heterogeneousand consist of �-b mixed crystals.
Tableware and spring elements are producedfrom lead-free nickel silver. It is also used forframes of glasses and zippers.
The �-b-nickel silvers, also with lead addi-tions, are used for all kinds of turned compo-nents and for fine mechanical fittings. The DKIinformation sheet Nr i.13 [DKIg] describes thenickel-silver alloys in detail (Also refer toASTM B 151).
5.16.9.2 Extrusion of Nickel-Silver Alloys
The extrusion data for the nickel-silver alloysis also given in Table 5.9. As with the brassesthe b-component determines the hot and coldworkability of the different alloys. A lead addi-tion hardly reduces the hot workability of the b-containing nickel-silvers but reduces it signifi-cantly in the �-alloys. The hot tensile strengthcan be obtained from Fig. 5.58 for some Cu-Nialloys. These alloys can be readily extruded inthe temperature range between 600 and 700 �C.In principle, the lowest possible extrusion tem-perature should be used to avoid oxidation, hot
cracking, and a structure with an excessive grainsize.
The billet surface of the nickel-silver alloys isusually turned off and extrusion is carried outwith a thin shell. Because a defect-free sectionsurface requires the lowest possible extrusiontemperature, it is frequently possible to extrudeonly with a large cross section, i.e., a low-extru-sion ratio. Further processing is then carried outby rolling and drawing.
Fig. 5.57 Copper corner of the copper-nickel-zinc system [Sch 35]
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 263
5.16.9.3 Competition from Wire Casting
Lead-free nickel-silver is—similar to tin-bronze—occasionally cast in wire format andthen cold worked without hot working. This pro-cess is economically superior to the manufactureof wire by extrusion but frequently produces de-fects in the strand, which reduce the quality[Bau 76].
Titanium with a melting point (Ts) of 1668 �Cis one of the high-melting-point metals. Al-though it is the fourth most abundant metal inthe earth’s crust after aluminum, iron, and mag-nesium, the high cost, in particular, of the re-duction to the pure metal but also of casting andfurther processing, makes titanium products ex-pensive [Sib 92]. Its low density of 4.5 g/cm3
compared with iron and, simultaneously, thevery high strength of some of its alloys makesit particularly useful where a favorable ratio ofstrength to density is required, i.e., in the aero-space industry. The very good corrosion resis-tance against numerous media is attributable tothe strongly adhering oxide film that forms evenat room temperature. It therefore finds numerousapplications in the chemical and petrochemicalindustries. Titanium alloys can be classified asone of the exotic extruded metals because of thehigh cost. Because its industrial application onlystarted in the 1950s, all production and appli-cation possibilities have certainly not been ex-hausted.
Special mention should be made of supercon-ductors of titanium-niobium alloys, shape-mem-ory alloys with titanium and nickel as the mainconstituents, as well as intermetallic phases oftitanium and aluminum for high-temperature ap-plications. The book Titanium and Titanium Al-loys by U. Zwicker gives an overview [Zwi 74].
*Extrusion of Semifinished Products in Titanium Alloys,Martin Bauser
With extrusion temperatures of 850 to 1150�C, the extrusion process largely corresponds tothat of iron and nickel alloys. Titanium alloysare, therefore, usually extruded on steel extru-sion presses. An analogous technology is used.
Similar to steels and nickel alloys forging, hotand cold rolling are preferred when possible tothe more expensive extrusion process for theproduction of bar material of titanium. Thin-walltube in unalloyed titanium is also usually madefrom strip with longitudinal welding. Extrusionis, however, the most suitable method of pro-duction for thick-wall titanium tubes, titanium-alloy tubes, and for sections.
5.17 Materials, Their Properties,and Applications
5.17.1 The Structure and ItsInfluence on the Properties
The example of the phase diagram for tita-nium with aluminum (Fig. 5.59) shows that puretitanium has a close-packed hexagonal lattice upto 882 �C, the so-called �-phase. Above thisthere is the bcc b-phase. In alloys there is a moreor less wide field within which the �- and b-phases can coexist.
Titanium alloys are classified into threegroups according to the structure at room tem-perature after the most common deformationand heat treatment processes: �-alloys, �-b-alloys, and b-alloys.
b-alloys in general have a lower strength andelongation as well as an inferior fatigue perfor-mance compared with the �-b-alloys. However,the b-alloys are superior to the �-b-alloys interms of creep strength and fracture toughness[IMIa 88].
The �-alloys are relatively difficult to work atroom temperature [Mec 80], whereas b-alloyscan still be readily deformed.
The alloying constituents of the technicallymost important materials are aluminum, whichincreases the strength and stabilizes the �-phase,as well as chromium, manganese, molybdenum,vanadium, copper, tin, and zirconium. Most ofthese elements stabilize the b-phase and reducethe �/b transformation temperature and are con-tained in the precipitates after age hardening. Tinas an alloying element, which slightly stabilizesthe �-phase, and zirconium have a high solubil-ity in the �-phase and harden it [Mec 80]. Zir-conium increases the hot and creep strengths.
The undercooling capability of the b-phaseand the reducing solubility with the decreasingtemperature of numerous alloying elements re-sult in interesting hardening and tempering pos-sibilities. Solution heat treatment is usually car-ried out in the b-region or just below followedby rapid cooling so that the �-phase or otherprecipitates can form only in a finely distributedmanner after subsequent age hardening. Agehardening cannot be carried out on �-materials[Lam 90].
The often complicated transformation kineticsof the �-b alloys are represented in time-tem-perature-transformation diagrams similar tosteel [Zwi 74].
The lowering of the transformation tempera-ture by b-stabilizing additions is important forhot working because the body-centered cubic b-phase has better forging and extrusion propertiesthan the hexagonal �-phase.
The niobium-titanium alloy with 52% Nb and48% Ti used for superconductors has a relativelylow transformation temperature and can there-fore be extruded at approximately 900 �C.
5.17.2 The Most ImportantTitanium Alloys and Applications
Table 5.13 shows the most important titaniumalloys with their properties and applicationareas.
The strength is not a decisive criterion forthin-wall tube for liquid transport in the chemi-cal or petrochemical industry and, for example,in water desalination plants and power stationcooling towers. The requirement is far more forthe outstanding corrosion resistance of titaniumagainst a range of media and its oxidation resis-tance at normal temperatures. In these applica-tions different grades of pure titanium are usedthat differ from each other only in their oxygencontent (Table 5.13).
As with iron alloys the move is away from theproduction of soft tubes by extrusion, cold pilgermills, and tube drawing. This area has had to beconceded to the more economic longitudinalwelding of cold-rolled strip.
Bar and section in commercially pure tita-nium can be extruded and is produced by thisroute but the demand is small.
The aerospace industry is the main user ofsemifinished products in titanium alloys apartfrom special products, including racing wheelsand sports cars. In aircraft, they are used for en-gine components, fuselage, and wing elements[Pet 92] (Table 5.13). Wherever the strength andmechanical properties of aluminum alloys areinsufficient, titanium alloys are used for light-weight fabrications.
Extruded sections, tubes, and bars are pro-duced from the higher-strength materials as well
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 265
Tabl
e5.
13So
me
tita
nium
allo
yssu
itab
lefo
rex
trus
ion,
prop
erti
es,a
ndap
plic
atio
ns(I
MIa
88,I
MIb
88,a
ndpe
rson
alco
mm
unic
atio
n)
Low
-tem
pera
ture
allo
ys
Mat
eria
lC
ompo
siti
onSt
ruct
ure
atro
omte
mpe
ratu
reSt
reng
thR
m,N
/mm
2E
long
atio
n,A
5,%
Pro
pert
ies
App
licat
ion
Ti1
–T
i3(a
)U
nallo
yed
titan
ium
with
min
99–9
9.5%
Ti
�-p
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266 / Extrusion, Second Edition
Table 5.14 Data for the extrusion of some titanium alloys (Lau 76, Zwi 74, IMI personalcommunication)
Alloy b-phase boundary Extrusion temperature Structure at extrusion temperature Deformation at extrusion temperature
as sheet and strip. The alloy TiAl6V4 is the mostcommon not only as a construction material be-cause of its high strength, but also in surgery forimplants because of its good biocompatibility.
5.17.3 Titanium Alloys Produced byPowder Metallurgy
Titanium alloys produced by powder metal-lurgy (P/M) are being offered to an increasingextent. This occurs on one hand when high-alloymaterials, e.g., with intermetallic titanium alu-minide, cannot be produced by melting metal-lurgy or cannot be or only to a limited extent behot worked and, on the other hand, if it is dis-persion-hardened material in which the disper-soids can only be added to the metal using P/M.These materials are described in Chapter 4.
5.18 Billet Production, Extrusion
5.18.1 Casting, Billet Preparation,Billet Heating
The billet pre-material is usually obtainedfrom vacuum arc furnaces with melting elec-trodes of titanium foam and recycled material[Kra 82]. If the ingot diameter is too large, it hasto be forged to the final billet diameter. This hasthe advantage that an originally coarse grainstructure with low hot workability can be re-crystallized with a fine-grain structure by carefulselection of the forging temperature intervals.
Mechanical surface processing is the finalstage: a smooth billet surface is the requirementfor a good product surface in the same way aswith steel extrusion because in direct extrusionwithout a shell, with lubrication and a conicaldie entry, the billet surface becomes the productsurface.
The billets have to be rapidly heated in aninduction furnace or a protective atmosphere(argon) because titanium easily oxidizes above
700 �C and can become brittle in hydrogen-con-taining atmospheres. Very fast heating is op-posed by the poor thermal conductivity of tita-nium. Heating in a salt bath (e.g., with bariumchloride as a deoxidant) is less environmentallyfriendly and is therefore rarely used.
5.18.2 Extrusion, Lubrication
Table 5.14 shows the important data for theextrusion of titanium alloys produced by meltingmetallurgy.
Titanium alloys are preferentially extruded inthe b-phase region where the flow stress is sig-nificantly lower than in the �-b mixed region (orin the pure �-phase). The uniform structure,which is obtained by the recrystallization thatoccurs during extrusion in the b-region, is alsoadvantageous. If the temperature in the b-regionis too high, there is the risk of coarse grain for-mation, which has to be avoided at all costs.With some alloys it is possible to obtain a mar-tensitic structure if the section is not cooledslowly enough from the b-phase. Subsequentannealing is then required.
Because titanium becomes brittle not only inhydrogen but also nitrogen and oxygen atmo-spheres, the extrusion temperature should be aslow as possible without crossing the �/b tem-perature limit.
If, on the other hand, deformation is carriedout just below the �/b phase boundary, the var-iations in heating during extrusion over the crosssection and along the length of the section resultin a changing mixed structure that produce var-iations in materials properties over the cross sec-tion and along the length.
Titanium alloys are extruded in the tempera-ture range 850 to 1150 �C. Above 1000 �C,which is in the b-region, it is necessary to useglass lubrication similar to steel and nickelalloys.
The glass film hinders the contact between thetitanium material and the extrusion tooling and
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 267
Fig. 5.60 Flow stress kf as a function of the temperature[Zwi 74]
thus the destruction of the bearing surfaces bycorrosion, to which titanium tends in contactwith steel. The molten glass also acts as a ther-mal insulation barrier and hinders the heat flowfrom the hot material into the colder tooling. Theglasses 4 and 5 for the lower temperature rangeare preferred from the recommended glassmixtures for stainless steel tubes (see Table 5.18and [Mar 74]).
The extrusion technology with titanium alloysdiffers slightly from that for iron-base alloys: ti-tanium alloys are, if possible, coated with a glasspowder suspension at 80 to 150 �C. During thesubsequent heating to the extrusion temperature,the glass coating can act as a barrier against thereaction of the billet surface with air constituents[Mar 74].
However, the method described for stainlesssteels, rolling the billet heated to the extrusiontemperature over a table in glass powder is com-mon, particularly with tube extrusion and wheninduction furnaces are available for heating.
Extrusion is usually carried out quickly, di-rectly through a die with a conical entry with aglass disc in front to minimize the cooling of thematerial during the extrusion process and to en-sure that the tooling is subjected to the high tem-peratures for only a short time. At a high speedthe temperature of the emerging section caneven increase from the front to the back.
If the speed is too high, in particular with sec-tions, there is the risk that the glass film will tear.The instantaneous failure of the tool workingsurfaces is the result [Mar 74]. The extrusionspeed is in the range 0.5 to 5 m/s.
There are also alloys that have to be extrudedin the �-b-mixed region with temperatures be-tween 700 and 950 �C, especially if subsequentcold working is not carried out and a fine-grainstructure is needed in the as-extruded condition[Zwi 74]. The turned billet is then wrapped in athin copper sheet and extruded with a lubricatedcontainer, in this case, a graphite grease mixture,through conical dies. At the upper temperaturerange of 850 to 950 �C, an iron foil is recom-mended as an intermediate layer to avoid a re-action between the copper and the titanium.
With adequate lubrication the friction be-tween the billet and the container is, similar toglass lubrication, so low that a quasi-stationarydeformation with laminar material flow occurs,and the surface copper layer at the start and endof the section has an approximately constantthickness. This would be even better with indi-rect extrusion, but usually only direct extrusionpresses are available.
The pickling away of the copper film in a mix-ture of nitric and hydrofluoric acid is unpleasantand is the reason why cladding in copper isavoided as much as possible. It has been re-ported that an extrusion in the �-b-mixed regionis possible without a copper cladding in spite ofthe severe adhesion tendency between titaniumand the tooling surface if a special graphitegrease lubrication is used [Boy 89].
5.18.3 Flow Stress, Extrusion Defects
Compared with many stainless steels, the ti-tanium alloys have a high flow stress at the ex-trusion temperatures mentioned, and they aretherefore difficult to extrude (see Fig. 5.60). Thismeans that on a press usually used for extrudedsemifinished products in steel, titanium can beextruded only with a low deformation. Extrusionratios of V � 10 to V � 100 are used.
The flow stress is—particularly in the �-b-mixed region—very temperature dependent: Itincreases rapidly with falling temperature asshown in Fig. 5.60. Care must therefore be takento ensure that the billet is not given time in thecontainer to cool, producing a large radial tem-perature gradient. Otherwise, the material from
268 / Extrusion, Second Edition
the billet interior will flow first producing a pip-ing defect at the end of the section. If the extru-sion is too slow there is also the risk of the bil-let’s “sticking.”
The dependence of the flow stress on the rateof deformation is higher for titanium than forsteel. Nevertheless, extrusion has to be carriedout quickly for the reasons just mentioned.
5.19 Tooling, Further Processing
The tooling is discussed in Chapter 7 in thesection on tooling for the extrusion of titaniumalloys.
The tooling loading is similar to the extrusionof stainless steels. When extruding in the b-region with glass lubrication—particularly insection extrusion—only one or two billets canbe extruded before the die has to be reworked.The die service life is significantly longer withcopper sheathed billets and lower extrusion tem-peratures. Chapter 7 describes coated sectiondies that are much more expensive but have asignificantly longer service life.
Normally, the lowest limit for the wall thick-ness of sections is 2 mm. Sections used in air-craft construction often have to have thinnerwalls (down to 1 mm wall thickness). Becausecold working of the common alloys is possibleonly to a limited extent, drawing and stretchingof the extruded sections at a higher temperatureis utilized [Mar 74]. However, 350 �C should notbe exceeded to avoid undesired structuralchanges.
Pure zirconium and, in particular, zirconiumalloys with tin (ZrSn1.5 � zircaloy) and nio-
*Extrusion of Semifinished Products in Ziroconium Alloys,Martin Bauser
bium (ZrNb2.5) are used as construction mate-rials in nuclear reactors, especially for the casingmaterial for the fuel rods because of the low ab-sorption of thermal neutrons, good heat transfer,good corrosion resistance, and high hot strength.The chemical industry also uses zirconium-alloysemifinished products for specific critical cor-rosion conditions.
The production of the very pure metal nec-essary for processing is costly, and zirconiumalloys are therefore correspondingly expensive.
Zirconium melts at 1852 �C and has a densityof 6.5 g/cm3. Pure zirconium has a hexagonal �-lattice up to 862 �C. Above this transformationtemperature, there is a bcc b-phase. With al-loys—also with small quantities of element ad-ditions—there occurs a more or less wide tem-perature interval with the simultaneous presenceof the �- and the b-phases [Web 90]. The struc-ture is similar to that described for titanium al-loys, and the properties during hot and coldworking are comparable.
5.21 Billet Production, Extrusion
5.21.1 Casting, Billet Preparation
Similar to titanium production, the startingmaterial is melted from zirconium sponge invacuum or protected atmosphere furnaces withself-consuming electrodes. After forging or roll-ing the large-diameter cast ingots, the billets forthe extrusion of tubes have to be turned on theoutside to remove surface defects and also beprebored. Because zirconium has the same prop-erty as titanium of readily welding to the toolingduring deformation and also reacts with oxygen,hydrogen, and nitrogen at high temperature, thebillets are normally externally and internallyclad. Copper sheet material is usually usedsometimes with an intermediate steel layer whenthe temperature is high to prevent diffusion ofthe copper into the zirconium base material. Thecopper cladding can, however, be applied byplasma spraying. To avoid an expensive zirco-nium discard, a copper disc can be attached tothe back of the billet to form the discard, at theend of extrusion. The cladding of the billet canbe avoided by using special lubricants.
5.21.2 Extrusion, Influenceof the Structure
If zirconium, for which there is no great de-mand, is processed on presses installed for steel
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 269
or copper alloys, then the technology used fol-lows that for the corresponding main material.
The flow stress of most zirconium alloys isnot very high at the extrusion temperature, andthe dependence on the temperature is tolerableso that the extrusion temperature and speed canbe varied within relatively wide boundaries withno risk.
Because the b-phase has the best hot work-ability, it makes sense from the deformationpoint of view to extrude in the temperature range800 to 1100 �C, where the flow stress is rela-tively low. This, however, rarely occurs becausethe billets would then have to be clad in steel.In this case, lubrication is carried out with aglass powder mixture similar to steel and tita-nium.
In practice zirconium alloys are extrudedpreferentially in the temperature range 675 to800 �C to reduce the risk of gas absorption,which severely impairs the toughness of the ex-truded semifinished product. Just below the �/btransformation temperature and (in the case ofalloys) in the �-b mixed phase region the work-ability is very good [Sch 93].
The best method, but also the most expensive,is cladding the billets in copper. Clad billets canbe heated in an induction furnace and lubricationwith oil-graphite suffices similar to standardcopper alloys. Dies with conical entries are nec-essary to obtain a predominantly laminar flowand thus to obtain a uniform cladding thicknessover the length of the extrusion.
If extrusion is carried out without cladding, itis important that the billets are brought rapidlyto temperature and protected as far as possiblefrom atmospheric influences during heating. Saltbath heating is, therefore, preferred over othermethods (75% BaCl, 25% NaCl is referred to in[Lus 55]).
Special glass mixtures are used as lubricantswhen extruding without cladding at approxi-mately 800 �C. They have a low viscosity evenat this low temperature and substantially protectthe billets from gas absorption.
5.22 Tooling, Further Processing
When selecting the tooling materials and thetooling design, the experience gained from theextrusion of copper alloys and—when extrudingat high temperatures—of steels can be utilized.
As a general rule, conical dies (2� � 140 to90 �C) are used to produce tubes and round bars.
If extrusion is carried out with copper clad-ding, the cladding material has to be removedfrom the extruded product either mechanicallyor with nitric acid. Zirconium is not attacked bynitric acid. Coarse grain billet material leaves anorange peel effect on the pickled surface as animage of the grain structure. A fine-grain start-ing material is therefore preferred.
The subsequent processing of zirconium alloytubes is carried out on precision cold pilger ma-chines [Jun 93]. Drawing requires careful prep-aration by bonderizing and lubrication becausethe material has a tendency to weld to the tool-ing. More recently, drawing has been carried outusing an ultrasonic vibrating mandrel, which im-proves the internal surface. Zircalloy claddingfor fuel rods can be produced from extrudedtubes without drawing by multiple cold pilgeroperations with intermediate annealing (70% re-duction per cycle can be achieved).
Extrusion of Iron-AlloySemifinished Products
Martin Bauser*
5.23 General
5.23.1 Process Basics
The high melting point of iron alloys (pureiron: 1535 �C, density 7.9 g/cm3) corresponds toa high recrystallization and hot-working tem-perature. Hot working usually is carried out inthe fcc austenite region, depending on the alloy,between 1000 and 1300 �C. This is associatedwith high tool wear because of the high thermaland mechanical stresses and, depending on thecomposition, a more or less severe oxide for-mation. As a result, it was relatively late beforethese alloys could be successfully extruded.
The oil graphite lubricants known from cop-per alloys, and which were used for steel extru-sion around 1930, were not really suitable. Theywere initially replaced with mixtures of oil,graphite, and cooking salt, which could be ef-
*Extrusion of Iron-Alloy Semifinished Products, MartinBauser
270 / Extrusion, Second Edition
fective only with very short contact times be-tween the hot material and the shape-formingtooling. This method of lubrication is used onlyrarely today for the production of mild steeltubes on vertical mechanical presses (see section5.24).
A significant extension to the application ofthe extrusion process followed the developmentof the Ugine-Sejournet process in 1950 in whichglass of a specific composition was used for lu-brication [Sej 56]. The molten glass not onlyprotects the heated billet from oxidation and actsas a lubricant between the material and the tool-ing, but also acts as thermal insulation so thatthe die and container heat more slowly than withthe lubricants previously used. It was now pos-sible to produce alloy steel tubes and steel sec-tions on horizontal extrusion presses with a lu-bricated container.
Steels are extruded using the direct extrusionprocess with lubricated containers without ashell. With this process it is possible to achieveshort contact times with fast extrusion speeds—important for the necessary high extrusion tem-peratures. The use of conical dies result in a ma-terial flow in which the billet surface forms thesurface of the extruded product (see Chapter 3,the section on material flow in direct extrusion,Fig. 3.31).
5.23.2 Importance ofSteel Extrusion Today
The growth in the extrusion of steel in the1950s and 1960s was followed by a continuousdecline. The production of seamless tubes inmild steels and low-alloy structural steels onvertical presses has largely been replaced bymore economic continuous rolling processes.Seamless tubes in these steel grades are now re-placed whenever possible by the less-expensivelongitudinally welded tubes.
Today, the extrusion of stainless steel tubesand steel sections is used only when the material,the section shape, or the low volume requiredcannot be produced by other processes or onlywith significant expense.
The reasons for extruding steel tubes are:
● Crack-free production of long products evenin materials that are difficult to hot work andthat tend to crack during rolling
● Production of small volumes. If unusual di-mensions or materials are involved, then fre-quently the setting and operation of rollingprocesses designed for mass production is
uneconomic. The tooling costs can also bevery high. In contrast, extrusion can be via-ble for quantities as low as three billets.
● Experimental or pilot production of tubesand sections that will later be produced inlarge quantities more economically byrolling
Approximately only 30% of all steel tubes areproduced as seamless and of these, less than10% are produced by extrusion [Bil 79].
Three product groups are described in detailsubsequently:
The numerous vertical mechanical pressespreviously used in the technically highly devel-oped countries are no longer in operation. Car-bon steel tubes are usually produced on contin-uous production lines that have a significanthigher productivity. However, these mechanicalpresses are still used in other countries.
Carbon steels, free cutting steels, low alloyhot and cold working steels and high speedsteels can be processed on mechanical presses.
5.24.2 Application of Mechanical Presses
Mechanical extrusion presses are similar tothe machines used in the drop-forging industry.
The billets are pierced and extruded in oneoperation on vertical mechanical presses. Thebillets are first heated to 1100 to 1300 �C andloaded into the vertical lubricated container.Then, at the upper top dead center the crankshaftis connected to a continuously operating electri-cally driven flywheel with a horizontal axis. Inone revolution the stem and piercing mandrelfall and the billet is extruded down into a curvedchannel and the stem and mandrel retracted. Thediscard is sheared with a shearing tool and re-moved from above.
The high deformation temperature and the ex-trusion time, which lasts for only a few secondson the mechanical press, enables extruded tubesin mild and free-cutting steels to be hot reduced
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 271
Table 5.15 Production ranges for the differentmethods for producing seamless steel tubes(Man 86)
Process Tube diameter, mm
Extrusion 35–250Tube continuous rolling 20–180Diagonal rolling and Pilger process 160–660Plug rolling procedure 180–400
directly after extrusion on a stretch reducing millwithout reheating.
The glass lubrication used on horizontalpresses to protect the tooling (see section5.25.4.2) is not really applicable for verticalpresses because the glass powder does not ad-here securely to the billet surface in the verticalposition.
The older method of lubrication with a vis-cous oil-graphite salt mixture has to be used.This partly evaporates and can even burn duringextrusion. Today’s environmental requirementsare difficult to fulfill even with careful extrac-tion.
5.24.3 Dimensional Range andThroughput
Mechanical tube presses are restricted in theirpress loads because there is a limit to the loadthat can be economically transferred mechani-cally. They are, therefore, used only for extru-sion loads up to a maximum of 17 MN. Becausethe upper limit of the billet weight naturally de-pends on the extrusion load that can be devel-oped, the maximum billet weight that can be de-formed is 120 kg with a diameter of 200 mm.
The entire extrusion process lasts no longerthan 3 s so that it is possible to have up to 200working cycles in one hour (average 120 to130). The mandrel length is restricted to 5 to 7times the mandrel diameter because longer man-drels can deflect sideways within the press dur-ing piercing.
The dimensional range of the extruded tubesextends from 40 to 120 mm external diameterand 2.5 to 5 mm wall thickness [Sar 75].
5.25 Seamless Alloy Steel Tubes
5.25.1 Extrusion in Competition withOther Hot-Working Processes
There has also been a large reduction in theextrusion of seamless alloy steel tubes over thelast few decades and numerous presses have hadto be closed down. The extrusion of seamlessalloy steel tubes has to compete with a range ofmore economic rolling processes, the productioncapacity of which can be seen in Table 5.15.
Economic analysis has shown that extrusionis inferior in output to the continuously operat-ing rolling processes because of the low billetweight and the long dead cycle times. The dies
for extrusion can be produced relatively inex-pensively but wear much more rapidly than thetooling in the rolling process.
The most important method of producingseamless alloy steel tubes in the same dimen-sional range as the extrusion press is the contin-uous tube-rolling process in which a forgedround ingot formed to a hollow billet in a pierc-ing mill is hot rolled over a mandrel throughnumerous profiled roll pairs and then brought tothe finished size on a stretch reducing mill afterreheating. This process has at least 4 times thethroughput of an extrusion press [Bil 79].
The extrusion process today is restricted to:● High-alloy stainless ferritic and austenitic
The dimensional range of these tubes is be-tween 35 and 250 mm external diameter and awall thickness of 5 to 50 mm with a minimuminternal diameter of 30 to 40 mm [Ric 93]. Theextruded tubes are usually processed further byrolling and/or drawing.
To produce the infrequently required dimen-sions over 200 mm external diameter, an alloysteel tube extrusion press would have to be somassive that the high cost would generally pro-hibit the use of the extrusion process [Bil 79]. A55 MN press designed for this purpose was sup-plied to Russia several years ago.
Horizontal water-driven tube presses and onlythe direct process without a shell are used. Fig-ure 5.61 shows the extrusion principle.
Alloy steel tubes produced by extrusion aresold as hot finished or further cold worked. Ap-plications are in the chemical industry, plantmanufacture, nuclear technology, and the pet-rochemical industry.
5.25.2 Alloys and Extrusion Properties
5.25.2.1 Alloy Groups, Structure,Properties, and Applications
Table 5.16 shows a selection of the alloysteels currently extruded.
272 / Extrusion, Second Edition
Fig. 5.61 Extrusion of alloy steel tubes on a horizontal press [Sar 75]
Table 5.16 Examples of stainless heat-resistant and high hot strength steels that are processed byextrusion (Ric 93)
Chemical composition, %
Material No. C Cr Ni Mo Other Similar UNS No./Common Name
1.4845 �0.15 25.0 20.5 . . . . . . S31000/3101.4841 �0.20 25.0 20.5 . . . Si 2.0 S31400/3141.4876 �0.12 21.0 32.0 . . . � Al � Ti
High-temperature steels (austenite)
1.4910 �0.04 17.0 13.0 2.5 N 0.14 S31653/316LN1.4961 �0.10 16.0 13.0 . . . Nb 10 � %C � 1.20 S34700/3471.4959 0.07 20.0 32.0 . . . Al � Ti, V 0.07 . . .
The heat-resistant materials are covered byDIN EN 10095, the other materials by DIN EN10216-5 and DIN 17456 (the European standard[EN] has replaced the previous DIN standards).
The following material groups are classifiedby the hot-working temperature range [Ric 93]:
● Ferritic steels with bcc �-iron structure. Allferritic chromium steels over 12% Cr contentas well as the steels alloyed with molybde-num and/or titanium belong to this group andhave a bcc �-iron structure. They are char-acterized by a flow stress that decreases rap-idly with increasing temperature so that theycan be readily hot worked.
However, this alloy group tends to brittle-ness due to precipitated phases and to graincoarsening during the hot working of thestarting material by rolling or forging beforeextrusion. The embrittlement reduces the
notch impact values. Suitable billets for ex-trusion are therefore difficult to produce freefrom cracks in the high-chromium-contain-ing materials.
Hot-brittle lead and sulfur-containingfree-machining steels can only be hot rolledwith difficulty. Only piercing and rollingover a mandrel are suitable for processingthese alloys apart from extrusion, which isstill the most common process used for thismaterial group.
● Austenitic steels have chromium contentsover 16% and nickel contents exceeding 8%.At the extrusion temperature the structure ofthese alloys is the fcc c-iron lattice. The aus-tenite is, therefore, usually characterized byvery good hot workability as well as a lowtendency to embrittlement. The undesirablecoarse grain formation found with the ferriticsteels does not occur with the austenitic ma-
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 273
Fig. 5.62 Phase diagram according to Schaeffler-DeLong [Ric 93]
terials so that they can also be readilywelded.
The good workability and the good cor-rosion resistance against many media pro-vide the austenitic materials with a widerange of applications in chemical plant man-ufacture and in energy production.
The higher the nickel content, the higheris the hot strength of austenitic materials andthe more difficult they are to extrude. High-nickel-containing materials can conse-quently be extruded only at low extrusionratios.
● Austenitic–ferritic alloys with chromiumcontents of 18–25% and nickel contents of4–7% are used for a range of applications.They combine to some extent the advantagesof both the alloy groups described previ-ously. They have a lower susceptibility toembrittlement than do ferritic materials andare more easily worked than the austeniticmaterials and also offer advantages in spe-cific types of corrosion attack. The mostwell-known representative of this group isthe material 1.4462 with 22% Cr, 5.5% Ni,and 3% Mo, which is an alloy steel withmany applications in the petrochemical in-dustry.
An overview of the structure of the three ma-terial groups is given by the phase diagram ac-cording to Schaeffler-DeLong (Fig. 5.62), whichshows the structure as a function of the chro-
mium and nickel content. The effect of other al-loying elements is determined from their chro-mium or nickel equivalent.
In austenitic steels, d-ferrite is particularlydangerous because its occurrence limits theextrusion temperature; otherwise, transversecracking occurs in the extruded section.
5.25.2.2 Extrusion Properties, Defects
The extrudability and extrusion temperaturerange of the different materials are given in Ta-ble 5.17.
The fewer the precipitates, the better the ex-trudability of ferrite and austenite. Single-phasematerials—particularly in the c-range with a fccstructure—are particularly easy to extrude [Bur70, Ben 73].
At the normal extrusion temperatures of 1100to 1350 �C, the flow stress is in the range 150MPa (materials with high extrudability) to 400MPa (materials with low extrudability) for aus-tenite and ferrite. The extrusion ratio (V) is usu-ally 8 to 40.
Examples of the relationship of the flow stressand the deformation capability on the composi-tion and the temperature are shown by the curvesfor different steels in Fig. 5.63.
Whereas the workability of homogeneousstructures always increases with increasing tem-perature, a marked decrease can occur with Cr-Ni steels as a result of the formation of d-ferriteabove approximately 1200 �C. This results in a
274 / Extrusion, Second Edition
Fig. 5.63 Flow stress and workability of steels measured from the number of turns to failure in torsion tests as a function oftemperature. (a) Flow stress. (b) d. (c) Deformation capacity [Ben 73]
Table 5.17 Extrudability of different material groups (examples) (Ric 93)
Group Characterization Nominal flow stress kf MPa Example, alloy (DIN No.) Billet preheat temperature, �C
risk of transverse cracking during extrusion. Thelimit is considered to be 2 to 3% d-ferrite atroom temperature (corresponding to approxi-mately 8% at the extrusion temperature).
The calculation of the extrusion load is dis-cussed in detail in Chapter 3.
The high extrusion temperature produces alarge temperature difference between the mate-rial and the container and a rapid loss of heat. Itis necessary to extrude quickly to prevent thebillet freezing during the extrusion process. Thisalso aids the die life.
In the deformation zone immediately in frontof the die, an adiabatic temperature increase ofup to 150 �C occurs during extrusion. The higherthe flow stress of the material and the faster theextrusion speed, then the higher the temperatureincrease will be, i.e., the closer the process ap-proaches adiabatic conditions. It is thereforenecessary to carefully match the temperature andspeed for materials with low-melting-point con-stituents to avoid melting and thus cracks in theextruded section.
Because the extrusion temperature is abovethe solution heat treatment temperature of con-
stituents that can form precipitates, the extrudedsection usually has a fully recrystallized struc-ture.
At low extrusion ratios, i.e., large tube cross-sectional areas, the core region tends to accel-erate. The resultant internal longitudinal stressescan result in lamination such as internal crackingwithin the extruded section that cannot be de-tected externally. These defects can only be dis-covered using ultrasonic testing.
In contrast, in thin-wall tubes, these stressesresult in cracks originating at the surface.
5.25.3 Billet Production
5.25.3.1 Melting, Casting, Hot Working
Alloy steels are melted today by the followingprocesses [Ric 93]:
● Electric arc melting process with down-stream argon-oxygen-decarburization (AOD)or vacuum-oxygen-decarburization [VOD]
● Induction melting in air or in a vacuum● Remelting using the electroslag refining pro-
cess (ESR)
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 275
The latter guarantees a high cast purity and anextensively homogeneous distribution of the ele-ments that tend to segregation such as niobiumand molybdenum.
Continuous casting is carried out if the chargesize permits; otherwise, mold casting is used.Only pure and low-alloy carbon steels can bedirectly extruded without previous hot working.Higher-alloyed materials have to first be forgedor hot rolled to destroy the columnar cast struc-ture and produce a homogeneous structure [Man91, Bil 79, Bur 70].
5.25.3.2 Homogenizing, BilletPreparation
Homogenizing of the rolled or forged billetsis rarely carried out and only when the homo-geneity or grain distribution is inadequate fordefect-free extruded bars [Ric 93]. The need forhomogenization increases with increasing con-tent of the elements molybdenum, niobium, andtitanium, which result in the formation of seg-regations.
Because the billet surface in direct extrusionwith lubrication without a shell and using coni-cal dies becomes the surface of the extrusion,the 8- to 12-m-long forged or hot-rolled roundbars have to be cross sectioned and carefully ma-chined by turning or peeling. Coarse turningmarks result in fish-bone patterns on the ex-truded tube surface. A chamfer on the front faceof the billet simplifies the uniform flow of theglass lubricant during extrusion (see below).
Billet diameters from 200 to 300 mm and bil-let lengths up to 700 mm are standard.
Whereas in the extrusion of copper alloy tubeslarge billets are pierced in the press, this processis not possible with alloy steels, which are alsoprocessed on hydraulic horizontal presses. Themandrels cannot withstand the severe thermo-mechanical stresses and would rapidly wear be-cause of the absence of the lubricating film. Thebillets are therefore bored, although for diame-ters up to 60 mm, a deep hole without expansionis sufficient. The billet length to bore ratioshould not exceed the value of 7:1; otherwise,the bore will deviate. With larger bore diameters,the predrilled billet is expanded in a separatepiercing press [Bil 79]. The hole diameter is al-ways larger than the mandrel diameter so thatthe lubricant spread in the bore is not strippedoff by the mandrel.
5.25.4 Billet Heating, Lubrication
5.25.4.1 Billet Heating
Apart from the uniform through heating of thebillet, it is important during heating to avoid ox-idation and surface decarburization of the heattreatable steels.
Billet heating to the required temperature of1100 to 1300 �C is carried out in rotary hearthfurnaces with protective atmospheres, in induc-tion furnaces, or in a combination of both witha gas furnace for preheating to approximately700 �C and final heating in an induction furnace.
The slower heating in the rotary hearth fur-nace with its more uniform through heating canhave a homogenizing effect on the structure ofsome steels.
The cost advantage of gas heating also playsa role, particularly with infrequent alloychanges. The possibility of a rapid temperaturechange with small batch sizes of different steelsis an argument for induction heating.
Induction furnaces are more expensive to op-erate but heat more quickly, and the temperaturecan be more accurately controlled, which is thereason why induction furnaces are used in par-ticular for high-alloy steels with exactly speci-fied preheat temperatures. Complex alloy steelsoften have only a narrow extrusion temperaturerange. The upper limit is determined by the sol-idus temperature and the lower limit by the presspower.
The temperature of the billet core lags behindduring the rapid induction heating, which canresult in core cracking in crack-susceptiblesteels, e.g., heat-resistant ferrites with high chro-mium contents. In this case the preheating in ro-tary hearth furnaces mentioned previously isrecommended.
Although induction furnaces with the shorterheating times of 5 to 15 minutes largely heat freeof oxidation, a protective atmosphere is oftenpassed through the induction coil with the oxide-susceptible steels. This can be necessary, in par-ticular, with furnaces with a vertical axis inwhich the billets oxidize more severely becauseof the larger air throughput from the chimneyeffect.
Salt bath ovens for billet heating are rarelyused. Their advantage is the capability of exacttemperature control and the avoidance of oxi-dation. They are, however, unwieldy and expen-sive and the salt carryover presents a large en-vironmental risk [Sar 75, Lau 76, Kur 62]. The
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Table 5.18 Glasses used successfully as lubricants for extrusion (Deg 74)
Composition Extrusion temperature
No. Na2O K2O MgO CaO BaO Al2O3 B2O3 SiO2 Maximum HK(a), �C Minimum DET(b), �C
(a) Hemispherical temperature (HK) is the temperature at which a specific quantity of the glass powder melted in a heated microscope forms a hemisphere. Viscosityis approximately 2 � 104 poise, i.e., highly fluidic. (b) Compressive softening temperature (DET) is the temperature at which the glass has a viscosity of approximately1010 poise and is extremely tough but no longer brittle.
diffusion of nitrogen damages the surface zonesof austenite and, as a result, the extruded tubessometimes have to be ground over.
If the turned billet has to be pierced beforeextrusion, this process is carried out in verticalpiercing presses linked to a vertical single-billetfurnace in order to balance out the heat lossesand to exactly control the extrusion temperature.In the piercing press, for example, the existingbore diameter is expanded from 35 mm to arange of 53 to 74 mm, increasing the billetlength by up to 20%.
Carbon steels and low-alloy steels can oxidizeso severely that removing the thick oxide layerwith a strong jet of high pressure water afterbillet heating is recommended. This process isso quick that hardly any heat is lost.
The high billet temperature of high-alloysteels necessitates a fast transport from the ovento the press. Otherwise, the material would cooltoo quickly, particularly with small billet diam-eters where there is a large surface to volumeratio.
The use of radiation protective sheaths hasbeen reported in the United States. These haveto be removed just before extrusion when thebillet is loaded into the container [Lau 76]. How-ever, a thick glass layer, which surrounds the hotbillet as a lubricant, also acts as insulation.
5.25.4.2 Lubrication
Similar to other difficult-to-extrude materials,e.g., the nickel alloys, the development of lubri-cation technology was an important requirementfor the economic extrusion of alloy steel tubes.The graphite and salt containing oil suspensionsinitially used as lubricants would carburize thesurface of low-carbon steels, among other dis-advantages. Glass lubrication technology wasrapidly adopted and had to meet numerous re-quirements [Ben 73]:
● The lubricant with its good slip propertieshas to form a closed lubricant film that does
not break up even when being extrudedthrough the die aperture. It should preventcontact with the tool working surfaces aswell as reduce the load.
● It should form an insulating layer betweenthe billet and the extrusion tooling and thusincrease the ability of the tooling to with-stand the thermomechanical stressing. A lowthermal conductivity is desirable.
● The lubricant applied to the billet surface im-mediately after heating should protect thisand the emerging extrusion from oxidationwith a thick film. Thin oxide coatings thathave already formed should as far as possiblebe removed or absorbed by the lubricant.
● Glassy lubricants have a larger volume con-traction on solidification and cooling thansteel, and the lubricant can readily break off,particularly with rapidly cooled extrusions.
Glasses that are optimally suited for all steelswith their different extrusion temperatures andflow resistances do not exist. Therefore, glasseswith different compositions and thus a differentviscosity temperature dependence are recom-mended for the various material groups (Table5.18).
The criterion for the upper limit of the tem-perature application of glasses in extrusion is theso-called “hemispherical” temperature (see foot-note in Table 5.18). The viscosity is then ap-proximately 2 � 104 poise. The lower limit ischaracterized by the compressive softening tem-perature (CST) where the glass is extremelytough (approximately 1010 poise) but no longerbreaks in a brittle manner. The extrusion tem-perature should be at least 200 to 300� above theCST.
Standardized glasses are available from vari-ous manufacturers. Today only a few types areused in extrusion with SiO2 as the main constit-uent. They contain oxides of sodium, potassium,calcium, magnesium, aluminum, and boron.Barium oxide is also sometimes added. These
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 277
additions lower the melting point and stabilizethe glasses.
The billet from the preheating furnace isrolled over a sloping table covered in the glasspowder and also has powder spread in the boreusing a spoon. The powder immediately meltsand protects the billet from further oxidation andcan even dissolve any oxide that has formed. Ifthe billet is not pierced before it is loaded intothe press, the procedure for applying the glasspowder has to be repeated (after the reheating).Different glasses are sometimes used for internaland external application. The grain size of theglass also plays a role because fine grain-sizepowder melts more quickly than coarse grain.
In front of the die there is a pressed disc ofthe same glass powder or fiber glass (with waterglass as the binding agent) that slowly melts dur-ing the extrusion process and encloses the frontof the extrusion. The thickness of the glass layeris of the order of 10 lm.
Careful matching of the type of glass, the ex-trusion temperature, and the extrusion speed, aswell as the quantity of glass is important to pro-duce a perfect glass, coating on the extrusion. Iftoo little glass flows through the die, groovesform; if the quantity is too high, the surface ofthe section exhibits bulges or a so-called “orangepeel surface” corresponding to the individualgrains [Bur 70].
It should be mentioned that the removal of theglass film from extruded tubes is expensive anddid not have to be carried out with the previousgraphite-containing lubricants.
5.25.5 Extrusion Process
A high extrusion speed is possible for the ma-jority of iron alloys. Depending on the materialand the extrusion ratio, ram speeds in the rangeof 20 to 300 mm/s are attained so that the billetsare extruded within a few seconds. High extru-sion speeds are necessary to keep the stresses onthe tooling to a minimum. The tooling tempera-ture should not exceed 500 �C. The meltingproperties of the glass used for lubrication pre-vents the use of the maximum extrusion speedsso that in practice, the minimum ram speed is 50mm/s and the upper limit is 200 mm/s.
High extrusion speeds can be achieved onlywith hydraulic accumulator drives. In the steelindustry, direct oil operating systems are notused.
The construction of the horizontal hydraulictube extrusion presses are basically similar tothose used for copper tube extrusion.
The stem loading is, as usual, restricted to amaximum of 1200 N/mm2. In order to achievethis, there are presses with several hydraulic cyl-inders that can be individually switched off toavoid overloading with small billet diameters.The press capacity of these presses is 35 to amaximum of 50 MN. Extrusion ratios (V) of 10to 40 are standard.
Because extrusion is carried out without ashell the dummy blocks used have to be matchedas closely as possible to the container bore; acleaning pad is not required.
A laminar quasi-stationary material flow isachieved by using dies with a conical entry inletangle of 120 to 150� or dies with a curved inlet.A fast die-change system is needed to avoidoverheating of the dies and to enable die re-working after every extrusion. If a die has to bereused, it is cooled to approximately 250 �C ina water bath.
The container is not heated during extrusionbecause the heat transferred from the hot billetis sufficient to keep it warm. The container onlyhas to be maintained at approximately 500 �Cbefore starting extrusion and during productioninterruptions. There are extrusion presses withrotating container devices in which two contain-ers are used in turn: while one container is beingused for extrusion, the other can be cooled andcleaned.
The extrusion mandrel moves with the stemduring extrusion so that no mandrel cross sectionhas to have prolonged contact with the hot de-formation zone in the die. The mandrel is cooledinternally as with copper materials or sprayedexternally with water between extrusions.
In order to remove the glass lubricant fromthe tooling, the following operations have to becarried out between extrusions:
● The mandrel is sprayed externally with wa-ter.
● The container is cleaned with a steel brush.● The die is quenched in a water bath.
The discard and tube are separated with a hotsaw or a shear between the opened container andthe die. The extruded length is restricted to amaximum of 20 m for steel tube extrusion toensure that the contact time between the materialand the die is not too long. Runout tables up to50 m, which are used for aluminum and copperalloys, are not possible with steel. The runouttrough for steel can also be fitted with poweredrollers.
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In the majority of cases, quenching with wateris carried out directly behind the die becausemost of the alloys produced have to be cooledas quickly as possible to avoid precipitates ofcarbides and intermetallic phases such as the r-and v-phases. Precipitated phases result in em-brittlement and in numerous cases, unfavorablecorrosion behavior. Water quenching also hasthe advantage that the glass film breaks up sothat it can be easily removed. Quenching shouldbe avoided only with materials where there is atendency for cracking with rapid cooling andwith ferrites where martensite can form [Ric 93].
5.25.6 Tooling
Tooling design and other details are coveredin Chapter 7.
The shape-forming tools—die and mandrel—are usually made in steel 1.2343 (H11, UNST20811). The die with the conical entry is usu-ally located in a die holder with a truncatedshaped external face, which mates with the con-ical container seat. Container liners are also fre-quently manufactured in the steel 1.2343.
Dies that are removed and cooled can last be-tween 20 and 40 extrusions and can then beopened up to another dimension. The mandrellife is around 400 extrusions and that of the con-tainer about 4000 extrusions, with the possibilityof reworking.
The use of high-alloy materials for dies andliners in the extrusion of alloy steels has beentried but for economic reasons is not used[Ben 73].
5.25.7 Further Processing, Testing
5.25.7.1 Further Processing
Steel, ball shot blasting (VacuBlast) is used,followed by pickling in nitric acid with hydro-fluoric acid additions to remove residual glasslubricant from the internal and external surfacesof the tube. Hot molten sodium and calcium saltsare used to some extent for surface cleaning, butthey are expensive and environmentally un-friendly because salt carryover is difficult tooavoid.
Heat treatment after extrusion is necessary ifit is not possible to arrange the cooling of thehot extrusion so that a precipitation-free struc-ture is obtained. The heat treatment after extru-sion then has the nature of a homogenization ora solution heat treatment.
Extruded and metallic clean semifinishedproducts can be directly used as hot-finishedsemifinished products after finishing (straight-ening and cutting to the finished length). If nec-essary, the surface has to be ground.
In other cases, the extruded tubes are furtherprocessed by cold pilgering and/or cold draw-ing.
5.25.7.2 Testing
In order to prevent d-ferrite occurring in aus-tenitic steels, the billets are tested randomly bya magnetic balance. The permitted upper limit is3%. Crack testing of the billet surface (penetranttesting) is required only occasionally.
As well as the standard visual inspection ofthe tube surface, ultrasonic testing is required,particularly with high-alloy materials and forsome critical applications. This is carried outwith either fixed test heads and rotating tubes orwith static tubes and rotating test heads. Wateris used as the coupling agent.
The danger of internal cracks occurs only withthe highest-alloyed materials (molybdenum-containing). Internal inspection with an endo-scope is specified only for these.
5.26 Steel Sections
5.26.1 General
5.26.1.1 Extrusion Process, Materials
As with alloy steel tubes, only horizontal hy-draulic extrusion presses are used for steel sec-tions. The laminar extrusion takes place withglass lubrication and without a shell. In this casethe competing processes, which are economi-cally more competitive for suitable quantitiesand specific sections, again have a higher marketshare than the extrusion process.
In principle, all steel grades that can be hotworked can be extruded to sections. Neverthe-less, the tool wear increases drastically with in-creasing extrusion temperature. The higher theflow stress at the extrusion temperature, thelower the possible extrusion ratio will be andalso, as a rule, the length of the extrusion.
5.26.1.2 Competition with OtherDeformation Processes
The processes used to produce sections are:
● Hot rolling: The dimensional range fallswithin a circumscribing circle of 250 mm di-
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 279
Fig. 5.64 Extruded steel profiles [Hoe 90]
ameter. The possible weight per meter is 1to 7 kg with minimum wall thickness of 3mm. The minimum wall thickness toleranceis �0.3 mm. In the hot-rolling process,cross-sectional undercuts are impossible andhollow sections cannot be produced. A rela-tively large tonnage is required to justify thecost of the manufacture of the profiled rollpairs needed for the sequential tools.
● Machining from solid material: The highmaterial loss and the expensive process en-sure that this process usually follows a non-machining deformation and only when othermethods of producing the final shape haveto be excluded.
● Cold profile forming from steel strip: Thisprocess requires material that can be coldbent and the section must have a uniformwall thickness. The final shape is producedfrom the flat sheet using several roll sets inthe bending machine. The form-shaping toolpairs are expensive, so the process is eco-nomic only for large quantities.
● Joining of part sections by longitudinalwelding, riveting, or bolting: Extruded sec-tions are frequently used to produce morecomplex sections or larger cross-sectionalareas.
● Extrusion: The possible dimensional rangefalls within a circumscribing circle of ap-proximately 250 mm diameter with a weightper meter of 1.5 to 100 kg/m and a wallthickness of at least 3.5 mm. The thicknesstolerance that can be achieved is �0.5 mm.Complicated sections and also hollow sec-tions can be produced (Fig. 5.64).
Obviously, given the severe thermal stressingof the form-producing tooling, the degree ofcomplexity and the range of sections that can beproduced cannot be compared with aluminumsections. Sharp edges are impossible because ofthe risk of tooling failure and the thermome-chanical localized stresses. External edgesshould, therefore, have a minimum radius of atleast 1.5 mm and internal edges on hollow sec-tions a minimum of 4 mm (Lin 82). Tolerancesthat are too wide for the application can oftenbe reduced by subsequent cold drawing.
Extrusion is used even for sections that canbe produced by hot rolling when it is not eco-nomical to produce the roll sets needed for themultistand mills because of the small volumerequired. Extrusion can then be used for the pro-duction of prototypes and first series.
Extrusion is also preferred for sections thatcan be more economically produced by weldingbut which cannot have any weld for safetyreasons.
The average lot size is 10 billets per order andis therefore generally small. If larger quantitiesare required, an alternative method of produc-tion is usually sought for cost reasons, e.g., hotrolling, even if the shapes have to be slightlymodified and simplified.
Of the steel sections produced in a sectionmill, only 8 to 10% are extruded. Approximately65% of sections are hot rolled and the others areproduced using other processes.
5.26.2 Materials, StartingMaterial, Process
Extrusion produces the same mechanicalproperties as hot rolling. In both processes thehot-working temperature is higher than the re-crystallization temperature, and a fine-grain re-crystallized structure is produced.
Structural steels are processed along with heattreatable steels and alloy steels (stainless, heatresistant as well as tool grades), as well asnickel-base alloys and, more rarely, cobalt-basealloys and titanium alloys.
The starting material for carbon steels is con-tinuously cast and supplied in approximately 10m lengths from the foundry cleaned by picklingor shot blasting.
The starting material for alloy steels has to behot rolled or forged in the same way as for tube
280 / Extrusion, Second Edition
production for homogenizing and to achieve afine-grain, crack-free structure.
The extrusion process with a lubricated con-tainer and without a shell resembles that de-scribed for tube production. The large billetsused for heavy sections and, in the case of hol-low sections, prebored billets also have to bechamfered on the end surfaces to achieve theuniform flow of the glass lubricant.
5.26.3 Billet Production,Extrusion, Further Processing
5.26.3.1 Billet Preparation,Heating, Lubrication
As soon as an order is passed to production,the bars delivered to billet production are cross-cut, peeled, turned or ground and chamfered.The billets for hollow sections have also to bebored. The hole is larger than the circumscribingcircle of the mandrel cross section. The billetsare heated to 1000 to 1300 �C in a rotary hearthfurnace with a reducing protective gas burner orin an induction furnace similar to steel tubes.Salt bath heating is also used.
After the billets have been removed from thefurnace, they are rolled down a table with glasspowder. The glass film formed from the moltenpowder prevents high thermal losses and oxi-dation of the billet surface and can even dissolvethe oxide that has formed. A 4 mm-thick glasspowder disc bonded with water glass is placedin front of the conical die. The type of glass usedis the same as for the extrusion of steel tubes(e.g., type 3 in Table 5.18 for carbon steels).
Extrusion. The tube and solid extrusionpresses used have a capacity of 15 to 25 MN.The diameter of the billets varies from 150 to250 mm, depending on the section cross-sec-tional area, and the length can extend to 900mm. Extrusion ratios up to 100 are possible butrarely used.
Hollow sections are extruded over round orprofiled mandrels that are not internally cooledand that move with the stem during the extrusionprocess. The thickness should be at least 20 mmto be able to withstand the large thermal stresses.It is not possible to use bridge dies as with alu-minum and copper because of the high extrusiontemperature, the relatively high flow stress, andthe resultant thermomechanical stresses on thetooling.
The billet loaded into the container is ex-truded to a discard length of 10 to 20 mm. After
the container has been opened, the discard is cutfrom the section with a hot saw. The section ispulled back through the die and then removedon a powered roller conveyor. After pushing outthe discard together with the dummy block, thedie is changed for a new or reworked die usinga rotating arm or a slide. The die has to bechecked for the dimensional tolerances aftereach extrusion and, if necessary, reworked be-cause of the high thermomechanical stresses.
The length of the section that is extruded asquickly as possible has a maximum length of 20m so that the die that is deformed by the tem-perature effect during the extrusion does not ex-ceed the extrusion tolerances toward the end ofthe extrusion.
The required high ram speeds of up to 300mm/s can only be achieved by a water hydraulicsystem.
The sections cool in free air but bend andtwist significantly in the longitudinal directionand deform in the transverse direction becauseof the different flow behaviors of different cross-sectional areas in the die and the faster coolingof thin legs after extrusion. To avoid accidentsfrom moving sections, the runout table is occa-sionally covered to form a tunnel.
In multihole extrusion, the extruded sectionsusually have varying lengths because of the dif-ferent amounts of lubricant in the die openings.
5.26.3.2 Further Processing
The extruded sections have to be straightenedand detwisted by stretching 2 to 3% on stretcherswith rotating stretcher heads and capacities ofup to 3000 kN. Hot stretching is also used forthe higher-strength alloys (alloy steels, Ni, andTi alloys). This stretching process is sufficientwith carbon steels to break off the 10 to 20 lm-thick glass film from the lubrication. Alloysteels, however, have to be pickled and some-times first shot blasted.
After stretching, it may be necessary to carryout further straightening on a roller correctionmachine or even on a straightening press withprofiled tools. This straightening process addssignificantly to the costs of producing steel ex-truded sections [Lin 82].
If a coarse grain structure or, in the case ofalloy steels, excessive mechanical properties aredetected, a subsequent annealing treatment is re-quired.
Tight tolerances are obtained by bright draw-ing. If necessary, sections can also be ground.
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 281
5.26.4 Tooling
Information on the tooling for steel sectionscan be found in Chapter 7 in the section on tool-ing for the glass-lubricated extrusion of tita-nium, nickel, and iron alloys.
Extrusion of SemifinishedProducts in Nickel Alloys(Including Superalloys)
Martin Bauser*
5.27 General
Nickel alloys are used for many applicationsin machinery, chemical engineering, industrialfurnaces, electrical engineering, electronics, andin power stations. Extrusion is used to producetubes and wire as well as bars for feedstock forthe manufacture of turned, forged, and impact-forged components.
High-alloy nickel materials (in particular withiron, cobalt, and molybdenum) are referred to assuperalloys and are suitable for applications attemperatures of more than 1000 �C. They areused in gas turbines and jet engines. These mul-tiphase alloys can often hardly be referred to asnickel alloys, but they do not belong to any spe-cific alloy group. Extrusion is important for theproduction of bars and rods in these high-strength alloys because they are almost impos-sible to forge [Vol 70].
The extrusion process is largely identical tothat described for alloy steel tubes.
Pure nickel melts at 1453 �C and has a densitycomparable to iron and copper of 8.9 g/cm3.Nickel and many technically important nickelalloys have a fcc lattice up to the melting pointand therefore have good hot and cold workabil-ity. The flow stress varies considerably depend-ing on the alloy.
The high-alloy superalloys are very difficultto extrude because of both the high extrusiontemperature (up to 1300 �C) as well as the highextrusion loads needed.
*Extrusion of Semifinished Products in Nickel Alloys (In-cluding Superalloys), Martin Bauser
Copper forms a continuous solid solution withnickel. Molybdenum increases the strength bythe formation of a solid solution. Intermetallicphases (mainly Ni3Al) increase the strength asdo carbides and carbon-nitrides in conjunctionwith titanium, niobium, molybdenum, and chro-mium.
5.28 Materials, Properties,and Applications
Table 5.19 refers to the nickel alloy DIN stan-dards. Table 5.20 shows some important andtypical nickel alloys that are processed by extru-sion.
Pure nickel and low-alloy nickel materialshave properties that are particularly suited tochemical processes and electronic applications.Nickel alloys are corrosion resistant to many re-ducing chemicals and cannot be bettered for re-sistance to strong alkalis. The food industry isan important application. Nickel also has a highelectrical conductivity, a high Curie tempera-ture, and good magnetostrictive properties. Bat-tery components and spark electrodes are appli-cation examples.
Low-alloy nickel materials are often used inheat exchangers because of the good thermalconductivity. Good workability and good weld-ability mean they can be readily worked.
Monel, i.e., nickel-copper alloys, are the mostwidely used high-nickel-containing alloys andhave been used for over a hundred years. Theirstrength is higher than pure nickel, but in spiteof this, they can be easily worked and possessgood corrosion resistance against many environ-mental factors. Their good resistance to acids isutilized in the chemical industry and the goodthermal conductivity relative to other nickel al-loys in heat exchangers. The seawater resistanceof Monel is useful in ship construction.
Certain nickel-iron alloys have a special co-efficient of expansion property, which makesthem suitable for use with glasses. The soft mag-netic behavior of nickel-iron alloys is also util-ized.
Nickel-chromium alloys and nickel-iron-chromium alloys are characterized by their goodcorrosion resistance, good mechanical proper-ties, and excellent oxidation resistance at hightemperatures. Combined with their good work-ability, these alloys have a wide range of appli-cations in heat treatment furnaces, incineration
282 / Extrusion, Second Edition
Table 5.19 German Standardization Institute (DIN) standards for nickel alloys
Standard DesignationComments
to DIN
SimilarASTM
standard Title
DIN 17740 Nickel in semifinishedproducts
Composition B39 Standard Specification for Nickel
DIN 17741 Low-alloyed nickelwrought alloys
Composition . . . . . .
DIN 17742 Nickel wrought alloyswith chromium
Composition B167 Standard Specification for Nickel Chromium-IronAlloys (UNS N06600, N06690, N06025)Seamless Pipe and Tube
DIN 17743 Nickel wrought alloyswith copper
Composition B164 Standard Specification for Nickel-Copper AlloyRod, Bar, and Wire
DIN 17744 Nickel wrought alloyswith molybdenum andchromium
Composition B335 Standard Specification for Nickel Molybdenum-Alloy Rod
DIN 17745 Wrought alloys of nickeland iron
Composition B407
B408
Standard Specification for Nickel-Iron-ChromiumAlloy Seamless Pipe and Tube
Standard Specification for Nickel-Iron-ChromiumAlloy Rod and Bar
DIN 17751 Tubes in nickel andnickel wrought alloys
Dimensions,mechanicalproperties
B161 Standard Specification for Nickel Seamless Pipeand Tube
DIN 17752 Bar in nickel and nickelwrought alloys
Dimensions,mechanicalproperties
B160 Standard Specification for Nickel Rod and Bar
DIN 17753 Wire in nickel and nickelwrought alloys
Dimensions,mechanicalproperties
B473
B475
Standard Specification for UNS N08020, UNSN08024, and UNS N08026 Nickel Alloy Barand Wire
Standard Specification for UNS N08020, UNSN08024, and UNS N08026 Nickel RoundWeaving Wire
plants, steam generators, and resistance heatingelements.
The superalloys contain cobalt, molybdenum,iron, titanium, and aluminum, in addition tochromium and nickel. A wide range of appli-cations are possible at high and very high tem-peratures. These include gas turbines, jet en-gines, and nuclear power stations. Hot-workingtooling is also produced from these alloys.
There are high-strength dispersion-hardenednickel alloys that have to be produced by powdermetallurgy. These alloys are discussed in thesection “Extrusion of Powder Metals.”
5.29 Billet Production
Depending on the composition and the ma-terial requirements, various routes are used toproduce the starting material and these havebeen described in section 5.41.5 [Ric 93].
High-alloy materials are melted in an electricarc furnace and, if a specific purity is required,subjected to the Vacuum-oxygen-decarburiza-tion (VOD) process in an evacuated ladle. Thisremoves the carbon and nitrogen and reduces thesulfur content drastically.
Nickel alloys can be continuously cast. If thecharge size is insufficient for continuous casting,chill mold casting with all its disadvantages(coarse, radial columnar, and frequently crack-susceptible structure) is preferred. For this rea-son, as well as reducing the large chill mold cast-ing to the billet dimensions, they are usuallybroken down by forging or rolling. This alignsthe grains axially and frequently produces a fine-grained recrystallized structure. It is then nec-essary with the high-alloy materials to followthis with a homogenization heat treatment tominimize the segregation.
The feedstock material supplied to the pressas long bars is sawn to length, turned, and cham-fered. If tubes are being produced, the billetsusually have to be bored. A good billet surfaceis necessary as is the case with alloy steels be-cause it will form the surface of the extrusion asa result of the glass lubrication (see below) andthe resultant laminar material flow.
5.30 Billet Heating
The billets have to be heated in a low-sulfurfurnace atmosphere because of the sensitivity of
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 283
nickel and its alloys to intercrystalline attack bysulfur. It should be weakly reducing in gas-firedfurnaces because nickel and nickel-copper al-loys in particular tend to intercrystalline corro-sion and thus embrittlement in an oxidizing at-mosphere even without sulfur [Vol 70].
Because the oxide layer of severely oxidizingnickel-iron and nickel-copper alloys also re-duces the effect of the lubricant in extrusion andproduces a poor surface quality, these materialsshould be heated as quickly as possible—at best,in an induction furnace.
With high-alloy, crack-sensitive alloys on theother hand, and with coarse grain cast billets, aslow heating rate is important because if theheating is too rapid the thermal stresses can re-sult in grain-boundary cracking. Some nickel-chromium-cobalt alloys cannot be directlycharged into the hot-gas-fired furnace if they ex-hibit coarse grain or severe segregation but haveto be slowly heated from a low temperature tothe extrusion temperature so that the temperaturegradient remains low in all phases of heating[Pol 75]. Reducing continuous, chamber or ro-tary furnaces are used for this.
High-alloy materials that are not crack sensi-tive and those that have been well homogenizedand have a fine-grain structure can, however, beheated in an induction furnace that guaranteesthe exact final temperature. With complex al-loys, the exact setting of the final temperature isvery important if a low-melting-point eutectichas formed (e.g., with niobium).
To save energy and to ensure the slow heatingof crack-sensitive materials mentioned above, agas furnace can be used for the base heating (upto 1000 �C) and an induction furnace for the finalstage [Ric 93]. Salt bath heating cannot be usedbecause of the risk of the diffusion of embrittlingelements.
5.31 Extrusion
5.31.1 Billet Preparation, Lubrication
As with alloy steels, prebored or pierced bil-lets are used for the production of tubes to avoidhigh mandrel wear and to obtain a low eccen-tricity [Eng 74]. With large internal diametersthe billets are again expanded on a separatepiercing press and then reheated in an inductionfurnace.
Whereas nickel, low-alloy nickel materials,NiCu, and NiFe can be lubricated with graphite
oil for low extrusion ratios and thus low extru-sion temperatures, higher-alloyed material bil-lets are only lubricated with glass using the Se-journet process similar to alloy steels [Pol 75].Suitable glasses for the corresponding tempera-ture are selected from Table 5.18. The glass withthe highest melting point is used for the high-alloy materials with extrusion temperatures ofalmost 1300 �C. The glass powder is appliedeven with two-stage heating (gas furnace-induc-tion furnace) only after the billet has left the in-duction furnace.
5.31.2 Deformation Behaviorand Extrusion Data
Figure 5.65 shows the tensile strengths ofvarious nickel alloys as a function of tempera-ture.
The flow stress of pure nickel is low enoughfor extrusion even below 1000 �C. Similarly, thesoft alloys of nickel with copper or with up to20% Cr have a relatively wide temperature rangefor hot working. The complex alloys on theother hand have narrow deformation tempera-ture ranges, in particular, the molybdenum-con-taining and the high-strength nickel-chromiumalloys, and these must be accurately maintainedto avoid cracking.
The addition of strength-increasing elementsto improve the mechanical properties at high
Fig. 5.65 Tensile strength of nickel alloys as a function ofthe temperature [Inc 88]
284 / Extrusion, Second Edition
Tabl
e5.
20So
me
impo
rtan
tty
pica
lnic
kela
lloys
that
are
extr
uded
(a)
Nic
kela
ndlo
w-a
lloye
dni
ckel
mat
eria
ls
Mat
eria
lSt
anda
rdB
riti
sh/U
.S.
Ni
Fe
Mn
Al
Oth
erSe
mifi
nish
edpr
oduc
tSt
reng
thP
rope
rtie
sA
pplic
atio
n
Ni
99.2
DIN
1774
02.
4066
NA
11A
lloy
200
�99
.2�
0.4
�0.
35..
...
.R
,S,D
370–
590
Hig
hel
ectr
ical
and
ther
mal
cond
uctiv
ity,
corr
osio
nre
sist
ant
Cor
rosi
on-r
esis
tant
com
pone
nts,
light
bulb
s,el
ectr
ode
tube
s
NiM
n2D
IN17
741
2.41
10..
.�
97.0
...
1.5–
2.5
...
...
S,D
...
...
Lig
htbu
lbs,
elec
trot
ubes
,spa
rkpl
ugs
(b)
Nic
kel-
copp
eral
loys
(Mon
el)
Stan
dard
Bri
tish
/U.S
.N
iC
uF
eA
lO
ther
Sem
ifini
shed
prod
uct
Stre
ngth
Pro
pert
ies
App
licat
ion
NiC
u30F
eD
IN17
743
2.43
60N
A13
Allo
y40
0�
6328
–34
1.5–
2.5
...
�2.
0M
nR
,S,D
450–
700
Goo
dst
reng
th,b
est
high
-co
rros
ion
resi
stan
ceC
orro
sion
resi
stan
tcon
stru
ctio
nun
its,s
hip
cons
truc
tion,
chem
ical
indu
stry
NiC
u30A
lD
IN17
743
2.43
75N
a18
Allo
yK
500
�63
27–3
40.
5–2.
02.
2–3.
50.
3–1.
0T
iR
,S,D
600–
800
Age
hard
enin
gC
hem
ical
indu
stry
,pet
roch
emic
alin
dust
ry
(c)
Nic
keli
ron
allo
ys
Mat
eria
lSt
anda
rdB
riti
sh/U
.S.
Ni
Fe
Oth
erSe
mifi
nish
edpr
oduc
tSt
reng
thP
rope
rtie
sA
pplic
atio
n
Ni
42D
IN17
745
1.39
17A
lloy
4240
.0–4
3.0
Rem
aind
er..
.R
,D..
.L
owte
mpe
ratu
reex
pans
ion
Gla
ss/m
etal
cera
mic
bond
s,e.
g.,f
orin
tegr
ated
circ
uits
Ni
48D
IN17
745
1.39
22/2
6/27
Allo
y48
46.0
–49.
0R
emai
nder
...
R,S
,D..
.B
est
coef
ficie
ntof
expa
nsio
n,sl
ight
lym
agne
ticR
elay
com
pone
nts,
tran
sfor
mer
s
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 285
(d)
Nic
kel-
chro
miu
m(i
ron)
allo
ys
Mat
eria
lSt
anda
rdB
riti
sh/U
.S.
Ni
Cr
Fe
Ti
Oth
erSe
mifi
nish
edpr
oduc
tSt
reng
thP
rope
rtie
sA
pplic
atio
n
NiC
r20T
iD
IN17
742
2.49
51A
lloy
75�
7218
–21
�5
0.2–
0.5
...
S,P,
R,D
650
Hig
hox
ide
and
corr
osio
nre
sist
ance
Furn
ace
com
pone
nts,
heat
ing
cond
ucto
r,ga
stu
rbin
esN
iCr1
5Fe
DIN
1774
22.
4816
NA
14A
lloy
600
�72
14–1
76–
10..
...
.S,
R,D
550
Hea
tre
sist
ant,
corr
osio
nre
sist
ant
Furn
aces
,che
mic
alin
dust
ry,s
park
plug
s
NiC
r20T
iAl
DIN
1774
22.
4952
Allo
y80
A�
6518
–21
...
2.3
1.0–
1.5A
l1.
9Ti
S,P,
R,D
1100
Age
hard
enin
g,hi
ghho
tst
reng
thC
ompo
nent
ssu
bjec
ted
tocr
eep
load
ing
(e)
Nic
kela
lloys
wit
hch
rom
ium
,mol
ybde
num
,and
/or
coba
ltsu
per
allo
ys
Mat
eria
lSt
anda
rdB
riti
sh/U
.S.
Ni
Cr
Fe
Co
Mo
Oth
erSe
mifi
nish
edpr
oduc
tSt
reng
thP
rope
rtie
sA
pplic
atio
n
NiM
o16C
r16T
iD
in17
744
2.46
10A
lloy
C4
Rem
aind
er14
–18
�3
�2
14–1
7..
.S,
R,D
690–
850
Exc
elle
ntco
rros
ion
prop
ertie
sC
hem
ical
indu
stry
,pap
erin
dust
ryN
iMo1
6Cr1
5WD
IN17
744
2.48
19A
lloy
C27
6R
emai
nder
14.5
–16.
54.
0–7.
0�
215
–17
3.0–
4.5W
S,R
,D69
0–85
0E
xcel
lent
corr
osio
npr
oper
ties
Che
mic
alin
dust
ry,p
aper
indu
stry
NiC
r22M
o9N
bD
IN17
744
2.48
56A
lloy
625
Rem
aind
er20
–23
...
...
8–10
3.15
–4.1
5Nb
S,R
,D69
0–83
0H
igh
stre
ngth
with
out
age
hard
enin
g,co
rros
ion
resi
stan
t
Che
mic
alin
dust
ry,
airc
raft
NiC
r23C
o12M
oD
IN17
744
2.46
63A
lloy
617
Rem
aind
er20
–23
...
10–1
58–
10A
l,T
iS,
R,D
�70
0St
able
upto
high
tem
pera
ture
,res
ista
ntto
oxid
atio
n
Gas
turb
ine,
petr
oche
mic
alin
dust
ry,c
hem
ical
indu
stry
NiC
r20C
o18T
i2.
4632
NA
19A
lloy
9058
20�
1.5
18..
.2.
5T
i1.5
Al
S,P,
R�
1100
Cre
epre
sist
antu
pto
900
�CG
astu
rbin
e,ho
t-w
orki
ngto
ols
NiC
r15C
o14M
oTiA
l2.
4636
Allo
y11
551
15..
.14
44T
i5A
lS,
P�
1200
Cre
epre
sist
antu
pto
1010
�CJe
ten
gine
286 / Extrusion, Second Edition
Table 5.21 Data for the extrusion of some nickel alloys (see Table 5.20 for examples indicated by(b), (c), (d), and (e) (Lau 7, Vol 70, Ric 93)
Group Characterization Approximate Kf value, N/mm2 Billet preheat temperature Examples
1 Easily extruded 150 900–1100 �C All nickel and the low-alloyed nickel materials2 Good extrudability 200 1050–1150 �C Monel(b)
Nickel iron(c)3 Difficult to extrude 250 1100–1250 �C Ni-Cr materials(d)4 Very difficult to extrude 320 1150–1280 �C Ni-Cr-Mo materials
Super alloys(e)
temperatures results, on one hand, in a reductionin the liquidus temperature and, on the otherhand, an increase in the flow stress.
These difficult-to-extrude alloys require ahigh degree of homogeneity in the starting ma-terial (see section 5.29), uniform billet heating(see section 5.30), correct lubrication condi-tions, and exact maintenance of the extrusionconditions. Economic production is, therefore,not possible without an extrusion plant with acorrespondingly high extrusion power [Inc 86].
Table 5.21 gives data for the extrusion ofsome nickel alloys.
In the high-strength nickel alloys, there aresome with a low-melting-point eutectic (e.g.,with niobium) that can only be extruded with amaximum of 1150 �C and thus with a low ex-trusion ratio (e.g., Inconel 625).
Some superalloys, for example, the high-molybdenum-containing alloys, have to be ex-truded above 1400 �C to obtain low flowstresses. Preferably, extrusion is carried out be-low 1300 �C because of the high tool wear andthe low viscosity of the glasses used, and theresultant low extrusion ratio is accepted. Thisnaturally means more expensive further pro-cessing.
5.31.3 Defects and Their Prevention
Narrow limits are placed on the extrusionspeed, particularly for the complex molybde-num-containing nickel alloys. If the extrusionspeed is too slow, the lubricant film breaks up,resulting in a rough surface on the extrudedproduct and/or the extrusion process “freezes.”If the extrusion speed is too high, the exit tem-perature can increase to such an extent thattransverse hot cracking occurs or—particularlywith thick bars—lap defects occur. Conse-quently, especially for the slowly extruded com-plex high-alloy steels with extrusion speeds be-low 10 mm/s, the extrusion speed has to becontrolled to ensure that the exit temperature re-
mains between the two limits. In other words, ifthe plant is capable, “isothermal extrusion” isrequired. Extrusion presses built especially fornickel have a suitable speed control system. Oilhydraulic presses are preferred to water hydrau-lic ones [Lau 76]. A high press power (up to 60MN) simplifies the extrusion of the high-alloynickel materials with high flow stresses [Eng74]. Because the billet rapidly cools at this slowspeed with the risk of freezing, only short billetscan be used.
Ultrasonic testing of extruded products is rec-ommended for the crack-sensitive materials.
5.32 Tooling, Further Processing
Information on the tooling used and the tool-ing design is given in Chapter 7.
The dies are, as described for alloy steel tubes,conical to obtain a laminar flow with the glasslubrication.
The further processing is also similar to thatdescribed for alloy steel. The glass lubricant filmis removed by shot blasting with steel grit fol-lowed by pickling. If the glass skin is broken upby quenching at the press (to suppress precipi-tation), shot blasting and pickling may not berequired. Cold pilgering or drawing is then car-ried out, possibly with intermediate annealing.
Extrusion of SemifinishedProducts in Exotic Alloys
Martin Bauser*
All the materials described in this section arerarely used in long lengths and are therefore not
*Extrusion of Semifinished Products in Exotic Alloys, Martin Bauser
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 287
Fig. 5.66 Elongation to failure and tensile strength of com-pacted beryllium as a function of the temperature
[Sto 90, Kau 56, Lau 76]
extruded in Germany. However, they are in-cluded for completeness.
5.33 Beryllium
5.33.1 Properties and Applications
There are special areas of application for be-ryllium in optical components, precision instru-ments, and space travel because of its unusualcombination of physical and mechanical prop-erties. Selection criteria are the low weight (den-sity 1.85 g/cm3), a high E-modulus, and low ra-diation absorption.
Beryllium melts at 1283 �C and has a hex-agonal lattice. It is characterized by a high ther-mal capacity and heat resistance combined withgood corrosion resistance and high strength andis therefore used in reactor technology. The verypoisonous metal can only be melted and pro-cessed under the strictest conditions [Sto 90].
5.33.2 Billet Production
Cast beryllium has a coarse grain, is brittle,and tends to porosity. It can consequently be fur-ther processed only with difficulty. For this rea-son, plus the importance of a fine grain for theproperties, beryllium is usually prepared bypowder metallurgy.
This also applies to extrusion (see the section“Extrusion of Powder Metals”). Usually, beryl-lium powder with the minimum possible oxygencontent is consolidated to billets by hot-isostaticcompaction in vacuum [Sto 90].
If extrusion has to be carried out at a hightemperature (approximately 1000 �C), a billetclad in a steel jacket has to be used. The com-paction of the beryllium powder directly into thejacket with a ram, the subsequent welding of thejacket and the evacuation (to prevent oxidation)has been described [Kur 70].
5.33.3 Deformation Behavior, Extrusion
According to Fig. 5.66, beryllium exhibitstwo maxima in the elongation to fracture, one at400 �C and another one at approximately 800�C. From experience, the elongation to fracturecan be taken to be a measure of the workability.Consequently, it is possible to differentiate be-tween “warm” extrusion with billet temperaturesfrom 400 to 500 �C and “hot” extrusion withbillet temperatures from 900 to 1065 �C. In thefirst case, no recrystallization takes place during
extrusion and a texture favorable for the me-chanical properties is formed. In “warm” extru-sion, the compacted and possibly sintered billetcan be lubricated with graphite or molybdenumsulfide. Warm extruded bars and tubes can bedrawn to the finished sizes.
The “hot” extrusion process is selected forlarge extrusion ratios because of the lower ex-trusion loads. The billets sheathed in steel areextruded like alloy steel with glass lubrication.The sections leave the press fully recrystallizedand largely texture free.
5.34 Uranium
5.34.1 Properties, Applications
The best-known application is the extensiveuse of uranium in the form of oxide powder asnuclear fuel in nuclear power stations. Uraniumis also sometimes used in other areas as a puremetal and dilute alloys because of the very highdensity (19.1 g/cm3, which is 68% higher thanlead) and the good radiation absorption. Typicalnonnuclear applications are radiation protectiveshields and counterweights [Eck 90].
Natural uranium contains up to 99.3% of theweakly radioactive isotope U238 and only up to0.7% of the nuclear fuel U235. Whereas this lowradioactivity has little effect on the workability,the poisonous nature and the ease of oxidationof uranium necessitates special measures.
Uranium melts at 1689 �C. It has a rhombiclattice up to 665 �C (�-phase) and a tetragonallattice (b-phase) above this temperature. At 771�C this lattice transforms into the body-centeredcubic (bcc) c-phase. Because the tetragonal b-phase is difficult to deform, extrusion cannot be
288 / Extrusion, Second Edition
Fig. 5.67 Hot tensile strength and elongation of uranium asa function of the temperature [Eck 90]
carried out between 650 and 790 �C because ofthe risk of cracking.
5.34.2 Deformation Behavior, Extrusion
The billets are melted in induction furnacesunder a vacuum and cast in molds.
The deformation behavior of uranium isshown in Fig. 5.67. The extrusion in the lower�-region (between 550 and 650 �C) avoids thedifficulties of high-temperature extrusion de-scribed subsequently but does require a highpress power because of the higher flow stress.The extruded sections are fine-grain recrystalli-zed. It is possible to work in this temperaturerange without cladding with the usual graphite-grease lubrication. Preheating the billets in a saltbath prevents oxidation. They should, however,be subjected to the atmosphere for only a shorttime at the extrusion temperature.
In the c-region (between 800 and 1000 �C),where significantly higher extrusion ratios canbe used, uranium reacts very rapidly with iron,nickel, and other metals and is very susceptibleto oxidation. The billets for this hot extrusionare usually clad in copper, evacuated, and thenextruded using graphite grease as a lubricant.
Only a few low-alloy-content uranium alloyswith higher mechanical properties and bettercorrosion resistance are known (with titanium,zirconium, molybdenum, and niobium) that can
be melted in vacuum induction melting furnacesor in vacuum arc furnaces. Because the structureis similar, the extrusion is the same as for pureuranium [Eck 90]. Because the b/c phase bound-ary is displaced to lower temperatures by thesealloying additions and because in the c-regionsecondary phases are held in solution, the extru-sion of alloys is easier.
5.35 Molybdenum, Tungsten
5.35.1 Molybdenum
Molybdenum is usually used as an alloyingelement in steels and high-alloyed materials.However, it is also important as a pure metal anda low-alloy material. Tools in TZM (Mo-0.5 Ti-0.17Zr), for example, can withstand tempera-tures well over 1000 �C. Further application ar-eas are cathodes, electric resistance elements (upto 2200 �C), and high-temperature componentsin space travel and for rockets. The highest ap-plication temperature is 1650 �C. Molybde-num’s good resistance to hydrochloric acid is ofinterest to the chemical industry. The density ofmolybdenum is 10.2 g/cm3 [Joh 90].
The high melting point of 2622 �C permitsonly powder metallurgical processing. The pow-der is obtained by hydrogen reduction of mo-lybdenum oxide and then cold compacted andsintered. These sintered billets can then be eitherdirectly extruded or melted in a vacuum arc fur-nace.
The bcc lattice of molybdenum is the reasonbehind the excellent deformation characteristics.This can be deduced from the hot tensilestrength curves in Fig. 5.68.
Pure molybdenum is extruded in the range1065 to 1090 �C and the most common alloyTZM (Mo-0.5Ti-0.1Zr) at 1120 to 1150 �C. Thebillets are heated in conventional gas or oil-firedfurnaces or by induction. Above 650 �C, molyb-denum oxidizes as a gas with a weight loss of 1to 5% without an oxide layer forming. Rapidheating obviously reduces the loss from oxida-tion. Similar to steel, glass has to be used forextrusion. Bar, tube, and simple shapes can beproduced by extrusion [Joh 90]. The glass ap-plied as powder after the billet has left the fur-nace melts and protects it from oxidation.
5.35.2 Tungsten
Tungsten melts at the extremely high tem-perature of 3380 �C. The structure and properties
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 289
Fig. 5.68 Hot tensile strength of some high-melting-pointmetals and alloys as a function of temperature
[Kie 71]
are similar to molybdenum. With its high densityof 19.3 g/cm3, it dominates where high weightis a main requirement. The high melting pointcombined with its high electrical resistancemakes it the most common material for heatingconductors and filaments in lamps [Joh 90a].
In contrast to molybdenum, a significantlyhigher hot-working temperature of 1500 to 1800�C is required, as shown in Fig. 5.68, whichmakes tungsten unsuitable for extrusion. Hotforging of sintered powder billets is preferred[Par 91].
5.36 Niobium and Tantalum
Niobium melts at 2468 �C and at room tem-perature has a bcc structure with a density of 8.4g/cm3. It can be readily worked at room tem-perature. Niobium alloys have a wide range ofapplications in space travel because of their rela-tively low weight and high hot strength. Nio-bium and its alloys are also in demand in thechemical industry because of the resistance tospecific corrosive media [Ger 90].
Similar to other high-melting-point metals,the extrusion billets are made by a powder met-allurgical route.
Figure 5.68 shows for niobium a relativelylow hot-working temperature. Extrusion tem-peratures between 1050 and 1200 �C with extru-sion ratios of V � 4 to V � 10 have been de-scribed for niobium alloys with zirconium,hafnium, and tungsten [Ger 90]. However, nio-
bium oxidizes severely above 425 �C so that thebillets have to be protected from oxidation bycladding and evacuating.
Niobium forms a continuous solid solutionwith titanium. Niobium with 46.5% titanium isthe alloy most commonly used as a super con-ductor [Kre 90]. Fine wires of this niobium alloyare embedded in a copper matrix (see the section“Extrusion of Semifinished Products from Me-tallic Composite Materials”). The niobium-tita-nium starting material is extruded to bars usingglass as a lubricant in the same way as steel andtitanium. These are then clad with copper andfurther processed in several stages.
5.36.1 Tantalum
Tantalum has similar structure and propertiesto niobium. It also has a bcc structure but firstmelts at 3030 �C and is twice as heavy with 16.6g/cm3.
The widest application of tantalum—apartfrom an alloying element in steel like niobium—is as an anode material in electrolytic cells. Inthe chemical industry it is known for its resis-tance to nitric acid, hydrochloric acid, and sul-furic acid. The high melting point ensures ap-plications in the high temperature region[Pok 90].
Processing usually follows a powder metal-lurgical route. Electron beam cast tantalum isusually too coarse grained for further processingand has to be forged before it can be extruded.
The extrusion of compacted and sinteredpowders has been mentioned but does not haveany great importance because the recrystalliza-tion temperature, and thus the extrusion tem-perature, is very high [Pok 90].
Extrusion ofPowder Metals
Martin Bauser*
5.37 General
Oxide-containing aluminum powder was ex-truded in the 1940s under the name sintered alu-
*Extrusion of Powder Metals, Martin Bauser
290 / Extrusion, Second Edition
minum powder (SAP). In the 1950s the advan-tages of the extrusion of powder was known forreactor materials and beryllium. This processhas only recently found a wide application, par-ticularly with aluminum alloys but also high-alloyed and dispersion-hardened materials.
A good overview of powder metallurgy (P/M) can be obtained from the textbook on powdermetallurgy by W. Schatt [Sch 86]. The work ofRoberts and Ferguson gives detailed informationon the extrusion of powder metals [Rob 91].
5.37.1 Main Application Areasof Powder Metallurgy
Processing by extrusion plays an importantrole compared with other processes in P/M. Themost important is the processing of metal pow-ders to near final shape and thus the economicproduction of molded components with a pieceweight below 2 kg, and mainly in iron alloys.The powder is compacted on vertical compac-tion presses in dies by a stem and then sinteredat a high temperature, during which the particlesform a solid bond. For large pieces with simplegeometries (e.g., forging billets), the compactionis carried out by fluid pressure applied on allsides (cold isostatic pressing, or CIP) beforethey are sintered. Compaction and sintering car-ried out in a single operation by hot isostaticpressing (HIP) is also used occasionally.
5.37.2 Advantages of MetalPowder Extrusion
Where long semifinished products can be con-ventionally produced by casting and extrusion,powder extrusion is usually less favorable be-cause the production of suitably mixed andsieved powders is usually too expensive.
The production of extruded semifinishedproducts by the powder route is worthwhilewhen:
● The material cannot be processed conven-tionally by melting and casting.
● An extremely fine grain size and finely dis-tributed precipitates have to be achieved.Rapidly quenched powder can have a signifi-cantly extended solid-solution range[Tus 82].
● A uniform distribution of very small inclu-sions to achieve specific properties is re-quired (dispersion hardening).
This last-mentioned processing of metal pow-ders with mixed or reaction produced nonme-
tallic particles as dispersions by extrusion is alsodiscussed in this chapter (metal matrix compos-ites, or MMCs). Chapter 4 discusses the physicalproperties, in particular those at high tempera-tures, and the processes in the deformation ofdispersion-hardened materials. Frequently, me-tallic or nonmetallic fibers are mixed with themetal powder to achieve certain properties andthen extruded [Boe 89]. The resultant so-calledfiber composite materials are described in thesection “Extrusion of Semifinished Productsfrom Metallic Composite Materials.”
5.37.3 Powder Production
Atomization of the melt using water or gasstreams is the most common of all the differentcasting, mechanical, and chemical processesused to produce metal powders. Water gives thefastest cooling rate, but the powder particles arecoarser (approximately 500 lm) and in a spat-tered format. If, however, atomization is carriedout in an air or protective gas jet, more roundedshapes and smaller particles are produced (downto a few lm, depending on the process param-eters) (Fig. 5.69). The output per hour of atom-ization plants is relatively low. Because a re-stricted particle size is usually required and thepowder usually falls in a wide particle spectrum,the particles of different sizes have to be sepa-rated by sieves. The powder costs are corre-spondingly high. A desired material composi-tion can be obtained by mixing powders if itdoes not exist in the atomized melt.
“Reaction milling” in ball mills is a new pro-cess that enables the mechanical alloying ofmetal components. Nonmetallic particles canalso be finely dispersed by this method. Thepowder particles are repeatedly broken downand welded between steel balls. A fine structureis obtained after a certain milling time [Inc 86].
5.37.4 Spray Compaction
In the new process of spray compaction de-veloped by the company Osprey [Cra 88], thegas-atomized material from the melt is sprayedonto a rotating block with a vertical axis ([Man88] (Fig. 5.70). The slowly sinking block growslayer by layer. This process saves precompac-tion, encapsulation, and evacuation.
Hard particles can be blown into the spray byan injector and form a dispersion in the finishedbillet.
The disadvantage of spray compaction is thata considerable part of the spray droplets miss the
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 291
Fig. 5.69 Example of a gas atomization plant for metal powder [Wei 86]
Fig. 5.70 Schematic of spray compaction [Arn 92]
block and fall to the bottom as powder “overspray,” which can be remelted or used as apowder.
The powder particles cool on the solidifiedblock so slowly in spray compaction that a struc-ture similar to casting forms, but with signifi-cantly better homogeneity and with finely dis-tributed particles and dispersed particles.
Because the billet diameter varies slightly, ithas to be turned to the extrusion dimension. Thespray-compacted billets can be extruded likecast billets—also to tubes with piercing man-drels. Plants are in operation producing alumi-num, copper, and steel.
5.38 Powder Extrusion Processes
5.38.1 General
The individual powder particles are plasti-cally deformed in the extrusion direction duringthe extrusion of metal powders, usually at thesame temperature as cast billets, and the surfacearea increases. Oxides and other films on theparticle surface break up and release new reac-
292 / Extrusion, Second Edition
Fig. 5.71 The classic processes for powder extrusion. (a) Ad-dition of loose powder. (b) Precompaction outside
the press. (c) Encapsulation before extrusion [Rob 91]
Fig. 5.72 (a) Encapsulation of powder. Cladding sealed atthe back and with evacuation tube. (b) Evacuation
[Rob 91]
tive metallic surfaces. The powder is compactedduring extrusion and the newly formed surfacesbonded by pressure welding. Even with low-density powders, a complete compaction andporous-free material is obtained by extrusion ifthe particles can be sheared sufficiently. This as-sumes that no gas porosity can form, e.g., frommoisture.
Several types of powder extrusion are known,including (a) the rarely used shaking of theloosed powder into the vertical container of anextrusion press, (b) the precompaction outsidethe press, and (c) the encapsulating of the pow-der before extrusion (Fig. 5.71). The new pro-cess of spray compaction is described above.
5.38.2 Shaking of Loose Powder into aVertical, Heated Container andDirect Extrusion (Version a)
The loosely filled powder has a low bulk den-sity (maximum 50% of the theoretical density),because the large number of interstitial space.Therefore, a relatively long container is neces-sary. The stem first compacts the powder beforethe actual extrusion process commences. Thisapparently economic process has a low through-put if the powder first has to be heated in thecontainer. It can be used only rarely. An exampleis the extrusion of magnesium-alloy pellets withgrain sizes from 70 to 450 lm to rods [Rob 91].
5.38.3 Precompaction of the Powderoutside the Press
“Green” compacts can be produced by coldisostatic pressing (CIP in autoclaves), particu-larly from angular particles or flakes. These areso stable that they do not break up during han-dling before extrusion. The powder filled into aplastic container is subjected by a pressurizingliquid to a uniformly applied high pressure(2000 to 5000 bar max), which increases theoriginal powder density of 35 to 50% of the den-sity of a cast billet to 70 to 85%. The risk of thefracture of the precompacted billet during han-dling can be reduced further if necessary (e.g.,with round particles) by sintering before extru-sion by heating. An alternative to this two-stageprocess is to carry out the compaction itself atan elevated temperature (HIP). In this case, thepowder has to be filled into a thin-walled metalcan that does not melt at the pressing tempera-ture (special case of version c).
5.38.4 Encapsulation of the Powderbefore Extrusion (Version c)
Usually the metal powder is compacted in themetal sleeve before sealing. After this it is oftenevacuated (possibly at an elevated temperature)and then vacuum sealed (Fig. 5.72).
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 293
The reasons for encapsulation include:
● Excluding a reactive powder material fromair and extrusion lubricants
● Protection from poisonous materials duringhandling (e.g., Be and U)
● Risk of breaking up of green compacts ofround powders or other particles that are dif-ficult to compact in a billet shape
● Improved lubrication and friction behaviorand better flow through the die by the correctselection of the container material
● Keeping the base material away from the dieand the zone of severe shear deformation.This is important only for materials with lowductility [Rob 91].
With high-purity materials, processing has tobe carried out in a clean room with careful clean-ing of the can and evacuation at an elevated tem-perature.
A large disadvantage of this process is thatthe can material remains on the surface of theextruded product and can be difficult to removeby machining or by pickling. There has beenreference to a degassed and hot-compacted alu-minum powder in an aluminum capsule, whichis removed from the billet before extrusion bymachining [Sha 87].
5.39 Mechanism andFlow Behavior in theExtrusion of Metal Powders
If loose powder is extruded (version a), thecontainer first has to be sealed on the die side toensure good compaction by the stem. The die isthen opened.
With precompacted green blanks (version b),a “nose” or a disc of the cast material corre-sponding to the powder is often placed in frontof the die. This compensates for the risk of thesurface of the extrusion tearing due to insuffi-cient initial pressure. Indirect extrusion is pre-ferred because of the more uniform materialflow. Extrusion with a lubricated container isavoided because of the risk of lubricant penetra-tion of the green blank.
The thermal conductivity in the green blankis relatively low. Too-rapid billet heating there-fore results in localized melting with the risk ofcracking. Slow heating in a chamber furnace isthe most suitable. If heating has to be carried outin an induction furnace, allowance has to be
made for an equalization time before loading thebillet into the furnace (possibly in an equaliza-tion furnace).
Encapsulated powders (version c) are usuallyextruded with lubricated containers and conicaldies (90–120�) or indirectly. This gives a laminarmaterial flow and a uniform cladding materialthickness on the extrusion. The selection of thelubricant depends on the cladding material (e.g.,aluminum: unlubricated; copper: graphite-oil;iron: glass). Nevertheless, a uniform laminarmaterial flow can be achieved only with roundbar or tubes and simple section shapes. Sectionextrusion is successful only when a suitable dieinlet has been developed.
A billet produced from powder usually has alower density than a cast billet. A longer billetis therefore needed for the same section weight.In extrusion the stem initially pushes togetherthe still not highly compacted powder in the con-tainer before the particles are subjected to ashear deformation and friction between eachother in the entry (shear) zone to the die and inthe die itself. The bonding of the grains on thenewly formed surface occurs by friction (orpressure) welding under high pressure and takesplace more quickly than the more conventionalsintering process used in powder metallurgy.
The extrusion process can produce a betterbond between individual particles than sintering.This applies in particular to aluminum alloys,the powder particles of which always exhibitsurface films of aluminum oxide because of thehigh reactivity of the element aluminum. Thesefilms are broken up to such an extent in extru-sion that the higher the extrusion ratio thegreater the number of newly formed surfacesavailable for particle bonding. An extrusion ratioof at least 20 is often stated to be certain ofachieving a perfect bond between the powderparticles [Sha 87a]. The resultant structure of theextruded powder corresponds to the structureformed during the extrusion of solid billets.
Encapsulated powder has to be well com-pacted and degassed. If the strength of the pow-der material and the encapsulating material dif-fer too severely, or if the can is too thin and thepowder inadequately compacted, there is the riskof folds forming during extrusion (Fig. 5.73).Diffusion between the can material and the pow-der material should be prevented as far as pos-sible, and it should not be possible for a eutecticto form between them. When selecting the canmaterial, consideration should always be givento the fact that it later has to be removed.
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Fig. 5.73 (a) Fold formation during extrusion of insufficientlyprecompacted metal powder. (b) Avoiding fold
formation by advance of the capsule back wall [Rob 91]
Fig. 5.74 Load variation in the extrusion of powder andspray compacted material compared with cast
An interesting variation of the extrusion ofmetal powders is the hydrostatic extrusion of sil-ver alloy powders. Presintered billets can be ex-truded without a can because the pores closeduring the pressure buildup, preventing the pen-etration of the pressure medium and the produc-tion of defects.
As mentioned previously, spray-compactedbillets behave similarly to cast billets—the pow-der compaction measures then no longer apply.
5.40 Load Variation
The variation in the load in the extrusion ofpowders differs considerably from that with castbillets because there is a less severe increase inthe load during compaction in the first part ofthe stem movement. The gradient of the load-displacement curve varies with the degree ofcompaction.
The high load peak often seen with aluminumpowders during upsetting can cause problems,and there is still no definite explanation for it(Fig. 5.74).
Additional work also has to be carried outduring extrusion to overcome the friction be-tween the grains and for their bonding. It istherefore often assumed that the extrusion loadin extruding powders is somewhat higher thanin the extrusion of cast billets. In a few cases alower load has been mentioned [Nae 69]. How-
ever, it is rare for exactly the same material tobe extruded as a cast billet or as a powder, whichprevents exact comparisons.
The use of indirect extrusion has proved ben-eficial in the extrusion of powders in order toreduce the extrusion load.
5.41 Examples of Powder Extrusion
In almost every powder metallurgically pro-duced material group there are materials withsolid insoluble inclusions (dispersoids). Thesedispersion-hardened materials can be producedonly by powder metallurgical processes.
5.41.1 Aluminum Alloys
It is practically impossible to produce alumi-num powder without an oxide skin because ofthe high affinity of aluminum for oxygen. Nor-mal sintering is therefore hindered or preventedwith aluminum alloys. Extrusion, therefore, re-mains as the only practical possibility of pro-ducing defect-free dense materials by P/M ofaluminum alloys because the oxide skins on thepowder particles are torn away to leave oxide-free, easily weldable surfaces. Pure aluminumpowder can be bonded to a metallic dense ma-terial with at least 80% cold deformation[Gro 73].
The starting material is usually quenchedpowder from a gas stream atomization plant thatis first compacted in a cold isostatic press or, for
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 295
Fig. 5.75 Temperature dependence of the tensile strength ofdifferent aluminum alloys. 1, aluminum 99% soft;
2, AlZnMgCu1.5; 3, sintered aluminum powder (SAP) with 10%Al2O3; 4, Al with 4% C (as Al4C3) [Sch 86]
small dimensions, in a cylindrical compressiondie to a green blank with 75 to 80% of the theo-retical density. In the extrusion press the greenblank is almost 100% compressed with an ini-tially closed die and then extruded using the di-rect or the indirect process at 450 to 500 �C [Sha87]. Placing a “nose” or a disc of aluminum infront of the powder billet can promote sufficientcompaction during upsetting and also preventthe start of the extrusion from breaking up. Theabrasive effect of hard particles in a powder mix-ture on the shape-forming tools is also reduced.Encapsulation with degassing at 500 �C and hotcompaction is required only if absolute freedomfrom hydrogen is specified. The cladding can beremoved by machining prior to extrusion.
In practice, the extrusion of aluminum powdermaterials is worthwhile only if it produces a ma-terial that cannot be produced by casting tech-nology. Most attention is paid to the high-alloymaterials with high room temperature strengthas well as dispersion-hardened materials thathave a much higher strength and better mechan-ical properties at higher temperatures than nat-urally hard and precipitation-hardened alumin-ium alloys (Fig. 5.75).
In the 1940s, a process was developed for pro-ducing and working an oxide-containing powderfrom aluminum powder by a milling process.This product was referred to as sintered alumi-num powder (SAP) [Irm 52, Jan 75]. Originally,the extruded aluminum material contained 12 to15% Al2O3; today less than 5% Al2O3 is used
because the dispersed particles are finer and bet-ter distributed.
Milling aluminum powder with electrogra-phite followed by heat treatment for completetransformation of the carbon into Al4C3 pro-duces a dispersion-hardened aluminum materialwith embedded Al4C3 and Al2O3 particles mar-keted under the trade name Dispal [Arn 85]. Invariations of the material solid-solution hard-ening AlMg5 or AlSi12 is used as the base ma-terial instead of pure aluminum to achieve bettermechanical properties even at lower tempera-tures. Whereas the solid-solution hardening andprecipitation hardening are lost at higher tem-peratures, the dispersion hardening is retained.
Dispal materials with high silicon content arepreferably processed by spray compaction to ex-trusion billets. As well as saving the cost of coldisostatic pressing of powders, the main advan-tage is the fine distribution of the embedded par-ticles. After extrusion, rapidly solidified alumi-num powder with iron and nickel additionscontain finely distributed particles of interme-tallic phases of the types Al3Fe and Al3Ni (me-tallic dispersoids) [Sha 87]. Alloys based onAlFeCe, AlFeMo, AlCrMnZr, and AlNiFeMnwith additions of copper, magnesium, and tita-nium, which also form intermetallic dispersoids,are used to achieve high hot strengths. This re-sults in alloys that have an approximately 100%higher hot strength in the temperature range 250to 350 �C, compared with conventional materi-als. The fatigue strength of such materials canalso be significantly improved. The E-modulusis also increased by 20 to 30%. These improve-ments in the mechanical properties are, however,obtained with an increase in density to 2.8 to 3.0g/cm3 because of the increased content of heavyelements [Sha 87a]. In powder metallurgicallyproduced aluminum alloys with silicon contentsof 10 to 30%, the eutectic and the primaryphases are very finely formed in the structure(�25 lm). This structure has an advantageouseffect on the hot and fatigue strengths [Sha 87a].The alloys can be readily mechanically workedand are characterized by a low coefficient ofthermal expansion. For bearing elements, the ex-trusion of powder- and spray-compacted mate-rials produces an uneven finer distribution ofhard particles than that achieved by casting. Thewear properties are significantly improved. Ma-terials that contain other elements in addition tosilicon are being tested in engine construction[Arn 92a], where, however, the high manufac-turing cost is a major barrier in spite of the sig-
296 / Extrusion, Second Edition
nificantly better properties compared with con-ventionally produced components. Abreakthrough is high-silicon-containing alumi-num tubes for cylinder sleeves that have recentlybeen extruded from spray-compacted billets[Hum 97].
Other aluminum alloys produced by mechan-ical alloying with oxides and carbides as disper-soids and that contain 1 to 4% copper and mag-nesium are commercially available. They are hotcompacted and then extruded. However, onlysimple cross sections are available. After extru-sion they can be further processed by forging,hammering, rolling, or drawing [Rob 91].
5.41.2 Copper and NobleMetal-Base Alloys
If powders of electrolytic copper and alumi-num oxide are cold isostatically compacted, sin-tered, and extruded to rods, the very fine Al2O3particles (approximately 0.3 lm), which shouldbe finely distributed, prevent recrystallization sothat a high strength is retained up to 1000 �C[Zwi 57].
Copper alloys with 1.1% aluminum oxide aresupplied as wire and rods and processed to spotwelding tips. These tips have a very good elec-trical conductivity and a high flow stress at thewelding temperature. At room temperature thematerial can be cold worked like copper withoutintermediate annealing.
The strength-increasing effect up to very hightemperatures of the aluminum oxide particles asdispersoids requires extremely fine particles (3–12 nm) and a uniform distance between them ofless than 0.1 lm. This is achieved by a so-calledinternal oxidation of copper-aluminum powders,i.e., an oxidizing annealing in which the oxygentransforms the aluminum to oxide, whereas themore noble copper is retained as a metal.
Dispersion-hardened copper with 0.1 to 0.5lm large TiB2 particles (3 vol %) is producedby the following original process.
Two different copper melts are reacted to-gether in a reaction vessel whereby the dispersedphase is produced by a precipitation reaction insitu in the melt. This melt is then atomized topowder, which is filled into a copper can andevacuated [Sut 90]. This material can also beextruded and processed to spot welding elec-trodes.
CuSn and CuSnNi alloys with high tin con-tents are known as bearing materials but cannotbe extruded because of the coarse d-phase pre-
cipitates. However, with spray compaction, theparticles are so finely distributed that the billetscan be readily extruded without difficulty to rodand tube. Graphite particles, which improve theslip properties, can be included by an injectorduring the spray compaction (see Fig. 5.70). Afurther application of spray-compacted-pro-duced tubes of high-alloy bronze is as super con-ductors [Mur 96].
Dispersion-hardened silver materials (e.g.,with AgNi2 or with cadmium oxide) have bettererosion properties, a lower contact resistance,and less tendency to welding than homogeneousmaterials. Silver and nickel are almost com-pletely insoluble in each other at room tempera-ture. AgNi alloys, therefore, cannot be producedby melting metallurgy but only by powder met-allurgy. A uniform distribution of the embeddednickel is obtained by currentless nickel platingof silver powder followed by extrusion[Mue 87].
As described previously for copper, finely dis-tributed particles of cadmium oxide can beformed as a dispersoid in silver by internal ox-idation of alloy powders of the noble metal sil-ver, which practically cannot be oxidized, andthe base metal additive cadmium. The fractionof cadmium oxide is very high, up to 25%.
Platinum and its alloys, which are used forspecial heating wires and other high-temperaturecomponents, are dispersion hardened wherebythey are extruded as powder mixtures with tho-rium oxide, yttrium oxide, or zircon oxide asdispersoids [Rob 91].
5.41.3 Titanium Alloys
A dispersion-hardened titanium alloy is, forexample, a modification of Ti-6242 with 6% Al,2% Sn, 4% Zr, 2% Mo, 0.1% Si, and 2% Er,which is produced by internal oxidation duringthe annealing of the powder Er2O3, with ex-tremely small particles. Of interest is that thestructure obtained by very rapid solidification isnot changed by extrusion [Rob 91].
5.41.4 Iron Alloys
The production of chromium-nickel-steelsand chromium-aluminum-steels by extrusion ofsteel powder produced by water atomization wasreported in 1969. The process did not go intoproduction [Nae 69] because, normally, evenwith iron alloys the production, or steels usingpowder, and compaction is more expensive thanby melting and casting.
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 297
Table 5.22 Composition of some iron and nickel superalloys produced by powder metallurgy(addition in wt%)
Designation Fe Ni Cr Co Mo W Ti Al Nb Ta Zr B C Y2O3
(a) Inco Alloys International. (b) High temperature metal GmbH. (c) Nuclear Metals Inc. Source: Sch 86, Rue 92
However, because fewer operating steps areneeded than between casting and the productionof the bar and a better homogeneity is obtained,powder extrusion is again being promoted forsome iron alloys. A Swedish manufacturer of-fers powder metallurgically produced tubes instainless chromium-nickel-steels as well asnickel and cobalt alloys [Asl 81, Rob 91]. Themelt is atomized under a protective gas and thepowder then cold isostatically compacted in aniron can before being extruded over a mandrel.The hollow billet also has to be internally en-capsulated to produce tubes. The lubricant isglass, the same as with conventional extrusion,and extrusion is carried out at approximately1200 �C. The process is supposed to have eco-nomic advantages over the conventional one[Tus 82].
Some high-speed and tool steels that are se-verely segregated when produced by the normalprocess are extruded as bar from powder to elim-inate segregations and to retain elements in so-lution above the equilibrium value. This is notpossible with molten metallurgical production.Because atomization is usually carried out undera protective atmosphere, the round particles thatdo not readily bond have to be encapsulated forcompaction. Water-atomized material can becold compacted without a can and then sinteredbefore extrusion [Rob 91].
The dispersion hardened iron based alloysMA956 and PM2000 with chromium and alu-minium as the main alloying elements and yt-trium oxide as the dispersoid are described inthe next section.
5.41.5 Nickel and Cobalt-BaseHigh-Temperature Alloys
Alloys with high contents of nickel, chro-mium, and cobalt are necessary for high-tem-perature applications, particularly in gas turbinesand in the aerospace industries. The high
strength is usually obtained by precipitationhardening.
The molten metallurgical production route isthe most economic solution when possible. It is,however, accompanied with coarse precipitatesand reduced hot workability and is not possibleat all with wide melting intervals.
Oxide dispersion strengthened (ODS) alloysare materials in which fine particles, in contrastto precipitates, are also resistant to high tem-peratures, act as strength increasing dispersions.There is no melting metallurgical alternative topowder metallurgical production. Oxides of yt-trium are mainly used [Rob 91].
Table 5.22 summarizes the superalloys pro-duced by P/M.
In the ODS process, the rapidly quenched(more than 103 K/s) powder produced by at-omization in an inert gas stream after sievingand mixing is encapsulated in a steel can, whichhas to be evacuated at a high temperature toeliminate micropores. If a particularly uniformprecipitation-free structure is required, coolinghas to be carried out extremely quickly. Specialrapid solidification rate (RSR) processes havebeen developed (up to 106 K/s). Extrusion hasproved to be the most suitable compactionmethod with these materials because the largedeformation breaks up oxide films as describedfor aluminum, disperses impurities present, andprovides very good compaction. The encapsu-lated powder is precompacted by HIP or forgingto approximately 85% before it is extruded. Thebars extruded at an extrusion ratio of 5–7 withglass lubrication at approximately 1200 �C arefurther processed by forging and rolling after re-moval of the can. It is possible with extrusion tosignificantly reduce the residual porosity andthus the notch impact sensitivity. The extrusiontemperature with these alloys has to be verycarefully controlled to prevent the risk of coarseprecipitates forming at high temperature (e.g.,carbide in Rene 95) [Rob 91].
298 / Extrusion, Second Edition
The alloy Rene 95 and Nimonic AP1 are pre-cipitation-hardening alloys where solid-solutionhardening is also effective at moderate tempera-tures because of the addition of molybdenum.They are used for gas turbine discs.
In the ODS alloys Ma 965 and PM2000 (Fe-Cr-basis), as well as Ma754, Ma 6000, and PM3000 (Ni-Cr-basis), the Y2O3 is worked into themetal powder by mechanical alloying in high-energy ball mills and finely dispersed. The fibertexture obtained by extrusion is utilized by suit-able heat treatment (coarse-fiber crystals fromzone recrystallization) to achieve a very goodfatigue and creep strength.
MA956 and PM2000 with high oxidation andhot corrosion strength up to 1100 �C are used incombustion chambers, in gas turbines, and othercases demanding maximum resistance at hightemperatures.
MA6000 and PM3000 are used in the first andsecond stages of jet engines and can be hot andcold rolled after extrusion.
5.41.6 Exotic Materials
Beryllium tubes are the ideal construction ma-terial in communication satellites because oftheir low density (1.85 g/cm3), the high E-mod-ulus, and their considerable elongation. In orderto achieve a fine structure, P/M processing ispreferred to melting and casting. The materialbeing processed is encased in carbon steel be-cause of the poisonous nature of beryllium andthe susceptibility to oxidation. This can be re-moved by pickling with hydrochloric acid afterextrusion with glass lubrication. The workabilityin the extrusion direction is increased by the tex-ture formed in extrusion of this hexagonal crys-tallizing material [Rob 91].
Ceramic uranium oxide powder has to beheated to 1750 to 2000 �C for plastic flow. Be-cause no deformation tooling can withstand thishigh temperature, experiments have been re-ported in which the hot powder is filled into rela-tively cold steel cans at approximately 700 �Cand then extruded.
This two-temperature process has also beeninvestigated for the production of chromiumtube and bar whereby hot chromium powder isfilled into a colder steel can and immediatelyextruded. The aim is to ensure that the very dif-ferent deformation behaviors of the two mate-rials at the same temperature are matched to eachother.
Intermetallic phases such as Ni3Al4 or Ti3Al4can only be deformed at approximately 1100 �C.
Because the production of alloy powder wouldbe very expensive, element powders are mixedand cold compacted. The green blanks are en-cased in aluminum and extruded at approxi-mately 500 �C. This prevents the extreme exo-thermic reaction from the formation of theintermetallic phases occurring during the extru-sion. It takes place—possibly after further de-formation steps—in high-vacuum furnaces or inan HIP unit. Another method is “reaction extru-sion” at such a high temperature that the phaseformation actually occurs during extrusion.
Extrusion of SemifinishedProducts from MetallicComposite Materials
Klaus Muller*
Metallic composite materials are microscopi-cally heterogeneous, macroscopically homoge-neous-appearing materials that consist of two ormore components intimately connected witheach other and in which at least the componentwith the highest volume is a metal or an alloy[Rau 78]. Its structure can be matched to thestresses of the component.
Pure metals and alloys have a defined prop-erty spectrum so that the determination of aproperty for a given material also determines allother properties. In composite materials, in con-trast, the properties of different components arecombined, resulting in a new extended propertyspectrum. The spatial arrangement of the com-ponents in the composite gives rise to typicalcomposite properties, the so-called structuralproperties. Composite materials can also exhibitproperties resulting from interactions betweenthe components, the so-called product proper-ties. Always present are the cumulative proper-ties, i.e., the resultant properties from the addi-tion of the component properties. This is shownin Fig. 5.76 [Sto 86].
The application of metallic composite mate-rials in place of conventional alloys produces inmost cases economic as well as technical advan-
*Extrusion of Semifinished Products from Metallic Com-posite Materials, Klaus Muller
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 299
Fig. 5.76 Properties of metallic materials
tages. Composite materials are produced today,the metallic components of which form solid so-lutions or intermetallic phases corresponding tothe phase diagram or are insoluble in each other.The application of the composite materials is de-termined by the adhesion at the boundary facesof the individual components. The general con-ditions for a satisfactory boundary surface ad-hesion with common deformation of heteroge-neous materials include:
● Adequate face pressure (compressive stressin the deformation zone)
● Adequate increase in the surface area duringthe deformation
● A limited inhomogeneity of the flow in thedeformation zone that is still sufficient forthe structures to conform to each other but,on the other hand, is not so high that thecomposite material is subjected to unaccept-able internal stresses. The composite com-ponents have to be capable of flowing to-gether under the process-specific stressconditions.
For longitudinally orientated semifinishedproducts (section, bar, wire, and tubes), the cri-teria described previously for stresses, surfaceincrease, and flow field formation are fulfilledby the deformation processes rolling, drawing,extrusion, and (to a limited extent) by forging.
However, extrusion provides the most favor-able conditions with reference to the three basicrequirements:
● Compressive deformation● Large strains in one operation
● The capability of influencing the deforma-tion zone by die design
Depending on the materials combination, thematerials structure, the application, and the ex-trusion temperature, metallic composite materi-als can be produced by direct, indirect, or hy-drostatic extrusion.
5.42 Terminology and Examples
The metalic composites can be classified ac-cording to the spatial arrangement of the com-ponents:
Composite materials in which fibers of theother components aligned or randomly orientedare embedded, aligned, or randomly orientatedin the matrix of the predominating componentby volume are referred to as fiber composite ma-terials. Examples are:
● Directionally solidified eutectics● Copper-sheathed aluminum conductors for
electrotechnology● Superconducting materials, including NbTi
multifilaments in a copper matrix
300 / Extrusion, Second Edition
Fig. 5.77 Spatial arrangement of the components in composite materials [Lan 93]
● Electrical contact material, including Ag/C,Ag/Cu/C, and Cu/Pd
● SiC or B fiber-reinforced light metal mate-rials
Composite materials in which the other com-ponents are embedded without a marked pre-ferred orientation in the matrix of the predomi-nating component by volume are referred to asparticle composite materials. They are discussedin the section “Extrusion of Powder Metals.”
Composite materials in which the variouscomponents form an intermingled structure arereferred to as penetration composite materials.
Composite materials that have a laminatedstructure of the composite materials are referredto as laminated composite materials.
5.43 Flow Behavior in the Extrusionof Fiber Composite Materials
The following cases can be differentiatedfrom the point of view of the deformation:
● Only one or not all of the components aredeformed (aluminum-steel bus bars, metalpowder combinations with ceramic addi-tives).
● All the components involved are deformedtogether whereby different strains can occurwithin the components.
Because the composite materials produced byextrusion are predominantly metallic fiber com-posite materials and the production of dispersioncomposite materials are described in the section“Extrusion of Powder Metals,” the basic prin-ciples are described using this type of material.The range of possible material combinations,which are not fully utilized today, are illustratedby way of an example in Fig. 5.78 [Mue 91].
Single or multicore wires can be produced aswell as wires with solid or powder cores. Insome cases, viscous wire fillers (glasses duringhot working) can be used. If thermal materialinfluences and diffusion are taken into accountas well as the structure along with the associatedmetallurgical possibilities, the wide range of ma-terial technical possibilities is clear [Mue 82].
Metallic fiber composite materials, corre-sponding to the structure described (outer tubeand one or more wire cores), can only be pro-duced, depending on the application, by indirector hydrostatic extrusion because the outer tubecannot have any distortion relative to the corematerial resulting from adhesion or friction withthe container liner.
Both in indirect extrusion and hydrostatic ex-trusion there is no friction between the billet and
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 301
Fig. 5.78 Schematic structure of metallic fiber composite materials
Fig. 5.79 Influence of the extrusion ratio and core fractionon the type of deformation. Material: sheath Al,
core Cu, flow stress ratio 2.7, die opening angle 2 alpha (m �45�). Area for achieving homogeneous deformation [Osa 73]
the container. Under comparable stress condi-tions in the actual deformation zone both pro-cesses can be differentiated by the billet upset-ting. This is important because the billet used toproduce a core filled sheathed tube contains anempty volume of between 15 and 23%. In hy-drostatic extrusion, this empty volume is re-moved by the hydrostatic pressure conditions; inindirect extrusion this occurs by the reduction ofthe billet length and the increase in the billetdiameter. This can result in twisting and buck-ling processes that can produce defects in thecomposite material.
The risk of twisting and buckling increaseswith an increasing length/diameter ratio of thecore wire in the tube sheath. The structure canbe so severely distorted that further extrusionresults in elongated folds and doublings that caninitiate cracks and thus render the composite ma-terials unusable [Mue 80].
5.43.1 Criterion forHomogeneous Deformation
In the combined extrusion of metallic mate-rials with different flow stresses kf, the materialflow can take place in different ways. The aimof composite material production by extrusionmust be, in addition to pure cladding, to trans-form the individual components into a compactundamaged material. This requires that the in-dividual components have the same flow stresskf in the deformation zone. This condition canbe achieved by the selection of the followingparameters:● Extrusion ratio● Volume distribution or volume fraction
● Die opening angle● Flow stress ratio kf1/kf2/........../kfm● Extrusion speed● Lubricant (influences the friction conditions
in the die in particular)● Temperature control of the extrusion process● Bonding quality in the initial billet
These relationships are shown in Fig. 5.79for the two-component-composite aluminum-copper.
Figure 5.80 represents a homogeneous defor-mation in which the individual components aresubjected to the same reduction for the three-partcomposite CU/Ni/Cu with the relevant HV 0.1values in the billet and the extruded section.
302 / Extrusion, Second Edition
Fig. 5.80 Homogeneous deformation of the composite Cu/Ni/Cu
Fig. 5.81 Example of core fracture. The core material usedis periodically broken.
A nonhomogeneous deformation in which theindividual components are extruded with differ-ent reductions can result in the failure of thecomposite with the pure cladding material notbeing deformed with the core material.
5.43.2 Deformation with Failureof the Composite Material
The most common failures are core andsheath fracture, if defects from undesired reac-tions during the deformation are excluded. Ex-amples of core failure are shown in Fig. 5.81 forboth a two- and three-component composite.However, there is no external visible defect onthe extrusion. This type of failure occurs withthe combination of hard core material and softsheath material. Sheath failure can be seen ex-ternally at least for the case of the two-part com-posite with the material composition hard sheathmaterial and soft core material. In a three-com-ponent composite, the damage need not be visi-ble at the surface. The innermost core can havecore oscillations, i.e., periodic irregular defor-mation. Figure 5.82 shows examples. The worksof [Ahm 78, Rup 80, Hol 78] give various so-lutions to avoid these failures for the compositeCu/Al.
5.44 Production of MetallicComposite Materials
Indirect and hydrostatic extrusion offer thefollowing favorable conditions for production:● It is possible to extrude composite billets
with thin casing tubes without the risk of the
casing cracking because of the absence offriction between the billet and the container.
● The bonding of the composite componentsto one another is strongly promoted by thesurrounding compressive stress state duringthe deformation.
● Because the process can be carried out overa wide temperature range, boundary surfacereactions can be suppressed by suitable tem-perature control where they reduce the prop-erties and promoted where they improve thecomposite material.
Depending on the geometry of the billet used,composite materials with a fibrous, particle andlaminated structure can be produced. Figure5.83 illustrates this for the composite copper-palladium [Sto 87].
The production of a metallic fiber compositematerial corresponding to the schematic struc-ture shown in Fig. 5.78 and consisting of a cas-
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 303
Fig. 5.82 Examples of sheath failure
ing tube and one or more core wires is possibleby extruding a number of thin core wires (thenumber of wires corresponding to the desirednumber of fibers in the finished product, simpleextrusion) or by extruding a less number of thickcore wires in a casing to a relatively thick sec-tion, a small number of sections of which arethen bundled into a common casing and reex-truded (repetitive extrusion).
Handling of thin wires can be difficult. It cantherefore be assumed that a core wire thicknessof approximately 1 mm is the lower limit for
acceptable handling. Thus, in simple extrusion,the number of fibers in the end product is deter-mined by the maximum possible internal diam-eter of the casing tube.
The total production is largely determined bythe cost of the production of the casing tube.These include casings in the form of closed cups(produced by drawing and redrawing); casingsmachined with a solid or hollow tip, the angleof which is matched to the angle of the die; andcasings that consist of flanged tube sections. Thesimplest case from the production point of viewis shown in Fig. 5.84.
A tube is used for the casing material, theshort inlet of which is conically upset outsidethe press. The upper and lower closure of thecore wire packet is made from embeddedstamped blanks (indirect extrusion) or from astamped blank and an embedded radial plug thatsimultaneously can act as the guide for the billetin the container (hydrostatic extrusion). Withsuitable materials selection, this component canbe reused.
In the application of composite billets thatconsist of only two components and thus havealmost no volume gaps (e.g., copper-clad alu-minum bus bar conductor), there are no addi-tional influences resulting from the geometry ofthe initial material apart from those described.
This is not the case when multicore compositebillets are used (simplest billet preparation, noprecompaction of the billets). In the case of hy-drostatic extrusion, the billet is radially com-pacted at the start of extrusion because upsettingcannot occur. Twisting and bending of the wirebundle does not occur from experience; how-ever, with thick wires their contour can be seenin the form of corrugations in the encasing tube.With thin wires, the compaction results in thegeometry of the billet deviating from a circularshape.
In both cases there is nonuniform flow in thedie and explosive lubricant breakdowns in thedie aperture, resulting in the extrusion shatter-ing. In the case of hydrostatic extrusion of non-compacted multicore billets, with simple billetpreparation, these problems have to be expectedif very small core wires (1 mm diam.) are notused [Mue 81].
Under the stress conditions of indirect extru-sion, the composite billet is not radially com-pacted as in hydrostatic extrusion but upsetswith a reduction in length and increase in crosssection to the diameter of the container. The re-
304 / Extrusion, Second Edition
Fig. 5.83 Structure of extruded copper/palladium-composite material. Micrograph image width is approximately 3.6 mm
duction in length of the core wire bundle cantake place in three ways:
● Twisting of the core wire bundle
● Buckling of the individual wires or the entire
bundle● Compression of the core wires
Fig. 5.84 Multicore billet preparation for (a) hydrostatic extrusion and (b) indirect extrusion
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 305
The first two mechanisms do not initially con-tain any cross-section increase of the individualwires. There is the risk of the casing buckling orbending giving rise to extrusion defects. Thetwisting represents energetically the lowest stateand can therefore always be expected. Whethercompression or buckling occurs after twistingdepends on the length-diameter (L/d) ratio of theindividual rods as well as their mutual support
in the rod bundle, the support of the casing, andthe mechanical properties of the material.
[Mue 81] shows that twisting and bucklingprocesses can be identified at an L/d ratio of 20for a fiber composite material of AlMgSi0.5wires; however, no material defect occurs. At anL/d ratio of 100, the casing material exhibitsfolding and a defect-free product cannot be ob-tained.
Fig. 5.85 Schematic of indirect repetitive extrusion
306 / Extrusion, Second Edition
Therefore, the so-called indirect repetitive ex-trusion is a particularly suitable variation of theindirect extrusion for the production of compos-ite materials with complex structures. A billetmade from a specific number of core wires(mantle wires) is extruded to a rod, which is thensectioned and the cross sections reextruded. Thisprocess can be repeated as often as required toproduce the desired structure. The process se-quence is shown schematically in Fig. 5.85.
The process for the production of Ag/C and
Ag/Cu/C contact materials is described in moredetail in this chapter [Mue 85]. Silver or copperbillets were drilled and filled with graphite pow-der for the production of silver/graphite and cop-per/graphite composite materials. The diameterof the bores was determined by the graphite con-tent that had to be achieved. The billets weresealed, preheated to extrusion temperature, andindirect extruded to bars. After cutting the sec-tions to length, a specific number of the crosssections was arranged in a thin-walled silver or
Fig. 5.86 Different structures in Ag/C (billet diam, 30 mm; bar diam, 12 mm). ZF, number of fibers in sheath tube
Fig. 5.87 Cross section after progressive extrusion stages (bar diam, 12 mm). ZF, number of fibers in the sheath tube
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 307
copper casing. The number of bar sections de-pends on the extrusion ratio.
Depending on the extrusion ratio and thenumber of extrusion stages, different structurescan be obtained Fig. 5.86.
The casings were also sealed at both ends andextruded like the powder filled billets. The ex-truded multicore bars were again cut to lengthand reprocessed as described previously. The se-quential structures can be seen in Fig. 5.87.
The relationship between the casing used, thewire diameter, the extrusion ratio, and the num-
ber of fibers in the composite is shown inFig. 5.88.
5.45 Application Examples
The applications described below subse-quently for metallic composite materials pro-duced by extrusion can illustrate only a few ar-eas of the comprehensive range. There has beenno attempt to provide a full coverage of all pos-sible and, to some extent, still-experimental ma-terials.
Fig. 5.88 Geometric arrangements in indirect repetitive extrusion
Fig. 5.89 Process principle for the production of Al-alloy steel composite bus bar
308 / Extrusion, Second Edition
5.45.1 Simple Structures and Coatings
Lead sheathing of electrical cables can beconsidered as the oldest and also the most well-known example of the production of a metalliccomposite material. In the mid-1960s, the com-posite extrusion of bar, tube, and section becametechnically important [Ric 69], particularly forthe processing of reactive metals including be-ryllium, titanium, zirconium, hafnium, vana-dium, niobium, and tantalum. The problem ofattack of the extrusion tooling is a major prob-lem in the processing of these metals and theiralloys. Technically, this means that the standardlubricants used in extrusion are not able to guar-antee reliable separation between the metals be-ing extruded and the extrusion tooling so thatlocalized welding occurs. Billets of reactivemetals have to be clad with a metal that providesthe lubrication as a “lost shell.” Ti/Cu ignitionelectrodes are produced this way.
The development of suitable lubricants, andof low-melting-point glasses in particular, en-ables conventional extrusion without a claddingmaterial to be carried out today in many cases.Cladding with, for example, 18/8 chromium-nickel steel, is used today for the extrusion ofcast intermetallic materials including NiAl,Ni3Al, and TiAl to counteract embrittlement byoxygen absorption at the high extrusion tem-perature of approximately 1250 �C.
Since 1963, laminated composite materials ofcopper-clad aluminum for conductors have beendeveloped as a substitute for copper [Hor 70,Nyl 78, Fri 67]. The aluminum core consists ofEC aluminum, the copper cladding of electro-lytic copper. For technical and economic rea-sons, the copper cladding is 15% of the cross-sectional area. This material is produced only toa limited extent today because of the changes inmetal prices.
In contrast, the process developed by the com-pany Alusingen for the production of compositeelectrical bus bars conductors in aluminum-steelfor high-speed and underground trains is com-mercially important [Mie 87]. The basic processis that two alloy steel strips are fed into an ex-trusion die. The steel strips are fed from decoil-ers. They pass through stations for chemical andmechanical pretreatment to apply corrosion pro-tection. They are then fed into the extrusion dieand turned through 90�. Both metals bond to-gether in the welding chamber under the influ-ence of the high extrusion pressure and the in-creased temperature as well as the relative
movement between the steel and the aluminum.In this way, two-mirror image, composite busbars conductor rails are extruded with a metallicbond between the steel and the aluminum. Thereason for simultaneously extruding two com-posite sections is to avoid friction between thesteel strip and the die. In this process the steelband does not contact the die surface butemerges with no wear. This is achieved as thesteel strips are completely surrounded by thealuminum and never come into contact with thedie bearings. Because the steel strips do not stickto each other, the two sections can be easilyseparated from each other. Figure 5.89 shows theprocess principles.
Simple composite structures are also found inthe area of steel- and iron-alloy composite tubesand sections [Deg 89, Gut 91, Hug 82, Lat 89,Vil 92], as well as aluminum-clad steel wire [Hir84] that has excellent corrosion resistance evenin marine atmospheres and is used as wire clothand wire netting.
Outstanding examples in this area are the in-dustrial metallic superconductors NbTi andNb3Sn [Web 82, Bre 79, Hil 84]. Very differentcomplex structures of usually three or more in-dividual components are produced depending onthe desired current-carrying capacity. The ex-amples in Fig. 5.90 shows examples of niobiumwires embedded in a copper-tin matrix.
Copper forms the core of the composite ma-terial and is separated from the CuSn of the su-perconductor by a tantalum barrier. Structureswith up to 10,000 individual filaments and a fil-ament diameter between 3 and 5 lm in wire di-ameter of 0.4 to 2.0 mm are obtained by multiplecombination and followed by multiple extru-sion, comparable to the repeated indirect extru-sion already described but with a packing den-sity of 95 to 98%, followed by drawing.
After the final operation to produce the re-quired dimensions, the material is heat treatedto produce the superconducting compoundNb3Sn by diffusion of the tin from the copper-tin matrix and reaction with the niobium wires.The copper core remains unaltered (protected bythe tantalum barrier). Because the supercon-ducting intermetallic phase Nb3Sn cannot be de-formed, this production route has to be used.
The possibility of influencing the materialand properties by changing the structure and by
Fig. 5.90 Cross section of a filament super conductor [Web 82]
Fig. 5.91 Property comparison between Cu/Pd composite materials (VW) and corresponding Pd/Cu alloys (electrical conductivityand hardness after cold working)
310 / Extrusion, Second Edition
specific heat treatments can be applied to othermaterials. The work by [Sto 87] and [Mue 86]describes the contact material copper palla-dium. Palladium-copper alloys with 15 or 40wt% Cu are used extensively for contact ma-terials for switching direct current because oftheir high resistance to material movement. Pal-ladium and copper form a continuous solid-so-lution series with superstructure phases. Thisgoverns the electrical conductivity of the alloys.There are significant technical and economicadvantages in using Cu/Pd composite materialsinstead of conventional PdCu alloys. Careshould be taken to avoid the formation of solid-solution zones at the boundary surface betweencopper and palladium in the manufacture of thecomposite materials. It is possible, by control-ling the structure and the temperature in the de-formation process, to obtain the microscopicheterogeneous composite structure. The indirectextrusion of coils of copper and palladium stripis particularly suited for this. This producescontact materials that exhibit a completely dif-ferent property spectrum compared with con-ventional alloys of the same composition. Theelectrical and thermal conductivities of Cu/Pdcomposite materials are up to 10 times higherthan the corresponding Cu-Pd alloys. The ab-sence of solid-solution hardening gives the Cu/
Pd composite alloys a high ductility (Fig. 5.91).This provides excellent further processing pos-sibilities, providing savings in the noble metalsand economic production processes for com-plex components.
5.45.3 Metallic Fiber CompositeMaterials with P/M Structures
This application area includes all those fibercomposite materials that can be produced frompowder starting materials. These are mainly alu-minum-base fiber-reinforced materials [Sch 87,Ros 92, Bei 90, Moo 85], metal matrix compos-ites (MMC) [Sta 88], and materials for electricalcontacts [The 90]. The production of fibers [Boe88] also provides a means of materials strength-ening.
Alloys based on the intermetallic phase TiAlhave a large potential for use as high-tempera-ture, light structural components. The deforma-tion potential of this phase, which is usually brit-tle under a conventional stress state, is verylimited. Semifinished and structural componentscan be produced economically by reaction pow-der metallurgy [Mue 90, Wan 92, Mue 93] (seealso the section “Extrusion of Powder Metals”).The production of titanium aluminides by reac-
Fig. 5.92 Schematic process for reaction powder metallurgy. (a) Longitudinal and transverse structure of Ti48.5Al, unreacted.(b) Longitudinal and transverse sections in the extruded state
Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 311
tion powder metallurgy (Fig. 5.92a) includes thefollowing steps:
1. Mixing the initial powders (element and/oralloy powders)
2. Precompaction of the powder mixture (e.g.,by CIP)
3. Production of semifinished product by extru-sion
4. Finishing of component5. Reaction heat treatment
Figure 5.92(b) shows longitudinal and trans-verse sections in the extruded state. Suitablecontrol of the reaction heat treatment producesthe phase TiAl expected from the phase diagram.
In the future, it should be possible for startingmaterials that have been produced by mechani-cally alloyed powder particles or by a spraycompaction process to be used for the applica-tion of new composite materials with excellentproperties.
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