Titanium for Automotive Applications: Challenges andOpportunities in Materials and Processing

ANIL K. SACHDEV,1,6 KAUSTUBH KULKARNI,2 ZHIGANG ZAK FANG,3

RUI YANG,4 and VLADIMIR GIRSHOV5

1.—GM Global R&D, Warren, MI, USA. 2.—GM Global R&D, Bengaluru, India. 3.—Universityof Utah, Salt Lake City, UT, USA. 4.—Institute of Metal Research, Shenyang, China.5.—St. Petersburg Polytechnic, St. Petersburg, Russia. 6.—e-mail: anil.k.sachdev@gm.com

Titanium offers performance as well as mass saving benefits in automotivecomponents subjected to reciprocating and suspension loads and in compo-nents subjected to extreme temperatures and gradients. However, theextensive use of titanium is hampered by the high cost of the raw material andthe special handling that is needed. This article outlines the technological andeconomic challenges faced and highlights some example materials and processdevelopments that attempt to address these hurdles.

INTRODUCTION

Weight reduction of automotive subsystems hasreceived much attention in recent years because ofits strong influence on improving fuel economy. It isgenerally accepted that a 10% reduction in weighttranslates to a fuel economy improvement ofapproximately 6% depending on the vehicle size andthe level of discipline for driving mass decom-pounding into the vehicle design process as early aspossible.1 A detailed mass decompounding analysishas shown that a reduction of 1 kg in a subsystemcan take out as much as an additional 0.95 kg if allsubsystems are available for redesign.2 Among thevarious components that can be downsized relatedto the additional mass savings is the opportunity todownsize the engine.

With achievable tensile strengths in the range of1,000 MPa, rivaling those of Gen 3 steels but atapproximately 60% of its density (4.5 g/cm3 versus7.8 g/cm3), the much higher specific strength oftitanium can drive a weight reduction of at least40%. Furthermore, the strength can be retained tohigh temperatures. Another key attribute of tita-nium is its excellent corrosion resistance partly aresult of passivation by its intrinsic oxide that formson exposure to air in even fresh cracks, thus obvi-ating the need for expensive surface treatment forcorrosion protection.

These attributes have driven several automotiveapplications during the past several years.3–7 Thevarious applications fall into one of three broad

categories driven by two key product features:components contributing to unsprung mass forreduced momentum transfer into the passengercompartment and components subjected to hightemperatures and highly cyclic loading for inertialmass benefits.

1. Reciprocating components: The application of Tivalves and connecting rods in the Corvette Z06V8 engine, for example, enabled the maximumengine rpm to increase from 6,600 rpm to7,100 rpm.8 The large 56-mm-diameter Ti intakevalves were 21 g lighter than the stainless steelvalves they replaced while providing 22% greatersurface area coverage, whereas the forged Ti-6Al-4V connecting rods weighed only 464 g each foran approximate 30% weight reduction comparedwith forged powder metal ferrous connectingrods. The lighter connecting rods also resultedin decreased loading on the rod end and mainbearings, thereby allowing the bearings to bedesigned for minimal friction.

2. Extreme temperatures and thermal gradients:Turbochargers especially benefit from the lowmass of titanium. Turbochargers use the hotexhaust gas to power the turbine wheel, whichdrives the compressor to pressurize the inlet airleading to more power and cleaner fuel burn,which translates to improved fuel economy. Forthe turbocharger to deliver the power boost, theturbine wheel (and shaft) must operate at extraor-dinarily high rotational speeds (depending on size

JOM, Vol. 64, No. 5, 2012

DOI: 10.1007/s11837-012-0310-8� 2012 TMS

(Published online April 27, 2012) 553

they can spin at speeds greater than250,000 rpm) and at temperatures reaching�950�C. To rapidly reach these high speeds,the turbine wheel needs to be lightweight toreduce rotational inertia and to reduce thepower boost time known as turbo lag. Turbo-charger wheels are predominantly made from anickel-based superalloy (Inconel 713), which hasa high density of �8 g/cm3. A lighter alloy,based on gamma titanium aluminide with adensity of �3.8 g/cm3, led to a perceptible 0.2 sshaving off of the turbo lag.9

3. Suspension components: Springs, for example,exploit the combination of high strength and lowshear modulus that requires approximately halfas many turns for titanium compared with steel.This along with the lower density of titanium canprovide a weight savings of 60–70%. In addition,the free height of the spring can be reduced to 50–80% that of a comparable steel spring,10 whichtranslates to styling opportunities. Extendingtitanium to other suspension components thatare part of the mass below the springs anddampers (unsprung mass as opposed to sprungmass that includes the body and powertrain)reduces momentum transfer into the passengercompartment for a smoother ride; aka Maglevtrains. Reducing unsprung mass and inertia alsoimproves performance by increasing the ability ofthe tire to closely follow road inputs and contours,contributing to a smooth ride as well.

In all cases, a specific benefit of the particular sub-system has justified the application: obtainingeither more power from the engine or better rideand handling rather than just mass reduction of thetotal vehicle. However, the high cost of titanium(approximately 20 times more expensive than car-bon steel and approximately 5 times more expensivethan stainless steel11) has hindered its use inautomotive applications.12 The following reasonsdrive this high cost:

� The extraction process is currently not highlyautomated and is only performed using lowvolume batch production.13

� A strong affinity for oxygen requires a well-controlled atmosphere during melting and high-temperature processing to minimize oxidationand oxygen pickup that decreases ductility.

� Need for specialized mold materials because of itsreactivity with conventional mold materials.

� Excessive tool wear during machining caused bylow thermal conductivity and high work-harden-ing rate that result in large forces and hightemperatures at the tool work interface, leadingto increased manufacturing cost.14,15 The excessivewear is generally a consequence of thermochemicalwear mechanisms present in the tool-chip contactand shear plane regions.15 These issues are com-pounded by the occurrence of segmentation in chip

formation because of flow localization and shearbanding16,17 as well as the greater part deflectionthan steel because of its lower modulus.

� Galvanic corrosion when joining to other materials.� Special care required to avoid impurity contam-

ination during recycling and the need to segregatethe various alloys.

Unless the cost decreases greatly, titanium willremain a material that will be limited primarily tolow volume and/or special niche vehicles, or it will beused in unique and high-end applications. A notableeffort to reduce alloy cost by 50% was the develop-ment of low-cost beta (TIMETAL LCB, TitaniumMetals Corporation, World Headquarters, Dallas,TX) alloy for springs where the expensive V and Zradditions routinely used in titanium alloys werereplaced with Fe and Mo.5,18 Despite this develop-ment, there is only limited use of titanium in springs.

Numerous conferences and reports have beendevoted to low-cost titanium to foster its greaterautomotive use.19–25 Much research and developmentis reported in the areas of primary metal productionand powder metallurgy (PM) processing. In addition,considerable interest is observed in advanced tita-nium aluminide alloys26–28 and in ‘‘meltless’’ processesthat can directly produce high-quality titanium pow-der without the need to atomize the melt, which is anadditional and costly processing step.29

Coupling low-cost powder with cost-effective andefficient PM processing for net shape manufactur-ing could provide the winning combination forbringing titanium into the realm of volume usage inautomotive applications. The following sectionsdescribe some recent advances in titanium alumi-nides, the processing of low-cost powders and recy-cled machining chips, and hydride powderprocessing that show promise to reduce cost.

TITANIUM ALUMINIDES

The high-temperature capability and low density(�3.8 g/cm3) of gamma TiAl make it an attractivematerial for rotating and reciprocating parts inengines. Furthermore, the presence of more than30 wt.% aluminum contributes to lower raw mate-rial cost compared with conventional titaniumalloys. In addition, TiAl-based alloys exhibit goodoxidation resistance and retention of strength tohigh temperatures.

Much research has occurred in the past 10 yearsthat has led to significant improvements both in thesupply of high-quality casting stock and in thecontrol of metallurgical quality of cast and thermo-mechanically processed billets.26 A key developmentis in the understanding of the relationship of prop-erties to microstructural parameters including col-ony size, lamellar thickness, and type of lamellarboundaries, as well as how these can be modifiedwith a combination of microalloying and heattreatment. Such an understanding is crucial for

Sachdev, Kulkarni, Fang, Yang, and Girshov554

achieving a balance of mechanical properties,including room-temperature tensile elongation andcreep and fatigue properties at high temperatures.The typical properties of a TiAl-based alloy with afully lamellar microstructure are shown in Table Iand are compared with well-known titanium alloysTi-6Al-4V and Ti-6242.30 What is impressive is thatmaterials with a fully lamellar microstructure mayhave fatigue strength (FS) (at 107 cycles in rotatingbending tests) to yield strength (YS) ratios exceed-ing 1.0, and up to a maximum of 1.28,31 comparedwith a typical value of 0.55 for conventional tita-nium alloys. For example, Ti-46.5Al-5Nb microal-loyed with B and Si, provided a room temperatureYS and ultimate tensile strength of 470 MPa and520 MPa, respectively, in the cast + hot isostaticpressed state, yet its FS in rotating bending isapproximately 525 MPa (higher than its YS).32 ThisFS, especially at high temperatures, makes thismaterial most suitable for inertia-related parts ininternal combustion engines.

Titanium aluminides have observed increasinginterest in the past 15 years in the several auto-motive applications, as discussed next.

Turbochargers

Reducing turbo lag has been a key driver for usinggamma titanium aluminide (c-TiAl) for this appli-cation. The major benefits of c-TiAl compared withtitanium alloys are the higher elastic modulus andlower density, as well as the tensile strength thatcan be retained up to a high temperature.33–35

Turbochargers impose additional challenges, forexample, oxidation, creep, and fatigue, during tem-perature transients.

Early developments of gamma titanium alumi-nide (c-TiAl) exhibited strengths marginally belowthat of Inconel 713, which is the typical alloy usedfor these applications. However, later developmentsincluded Nb additions, which enhanced high tem-perature strength and also improved oxidationresistance and creep.9 In both TiAl alloys (with andwithout niobium), a fully lamellar microstructurebased on the constitutive phases c and a2 wasshown to provide the best combinations of high YSand FS. This process led to a niobium-containingTiAl alloy developed in the late 1990s that not onlyexhibited superior specific strength up to �950�Ccompared with that of Inconel 713 but also showedsignificant increases in fracture strength and ero-sion, creep resistance, and oxidation resistance. Itwas found that niobium increased the YS and FSmainly from solid-solution strengthening and short-range order of point defects and, because of nio-bium’s sluggish diffusion kinetics, helped retainthis lamellar microstructure also to improve creepresistance.36,37 The viability of c-TiAl(Nb) for massproduction has been demonstrated by the steadyincrease in automobiles equipped with TiAl turbo-chargers.9,38 T

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Titanium for Automotive Applications: Challenges and Opportunities in Materials and Processing 555

Exhaust Valves

Exhaust valves produced with Ti-33.5Al-0.5Si-1Nb-0.5Cr (wt.%) have shown excellent propertiesand potential for improved engine performance(�40 kW increase in power output).39 Newer alloyswith lower Al and higher Nb contents and smallboron additions have been developed recently thathave better properties.40 The B addition was shownto refine the as-cast grain size, whereas the addi-tional Nb contributed to the high temperatureproperties.

One approach for cost reduction has been the useof CaO crucibles that have led to a reduction in thetotal energy required for melting and that has alsoavoided the need for water cooling required whenmelting in copper crucibles.41 The CaO cruciblesalso enabled greater superheating of the melt, morethan 100�C compared with only 50�C that is possiblewith the cold-hearth furnace normally used formelting and casting TiAl alloys. This improvedmetallurgical quality and increased yield of goodquality cast valves by almost 30% (Fig. 1).40 A dis-cussion of some of the technical issues includingperformance, tolerance, and cost of such cast valvesis provided by Gebauer.42

Connecting Rods

While titanium alloys are 40% lighter than steeland can enable increased rpm and thus higherspecific output, their low elastic modulus comparedwith steel is a concern as connecting rods aredesigned for stiffness, in particular near the big end.(Ti-6Al-4V, for example, has an elastic modulus of

only 110 GPa compared with steel of 210 GPa.)Although titanium alloy connecting rods have beenused in limited applications in high-performancevehicles, effort still continues to reduce their costand improve their performance. Gamma TiAl-basedalloys provide an alternative approach because theyare lighter than Ti-6Al-4V and also have a higherelastic modulus.43,44 Ti-46.5Al-5Nb, for example,has an elastic modulus of 170 GPa, approachingthat of steel, and it could be considered for thisapplication because of its superior fatigue propertiesas well.

Exhaust Systems

Alloys based on Ti2AlNb, another intermetalliccompound in the Ti-Al system, have a sustainedtemperature capability of 700�C, with short excur-sions up to 800�C, making it a candidate materialfor replacing nickel-based superalloys for exhaustsystems. Producing high-quality sheets of Ti2AlNbmaterial with a thickness of approximately 1 mm isa great challenge. The Institute of Metal Research,Shenyang, has developed a process involving acombination of hot rolling and packed cold-rolling tomanufacture 1 m 9 2 m sheets with a thickness ofapproximately 0.5 mm. Tungsten inert gas weldingof these sheets as well as welding to tubes have beendemonstrated. The PM route is being explored,which allows essentially 100% material usage byremoving all of the processing steps and cost asso-ciated with ingot breakdown, as well as rolling andannealing. The cost of Ti2AlNb is, however, higherthan TiAl, because of the higher amount of Ti andNb that is present. Its density of 5.3 g/cm3 is alsohigher than the 3.8 g/cm3 for gamma TiAl.

POWDER METALLURGY

Titanium alloys and aluminides can be processedby routine casting, wrought approaches, and PMapproaches, with the choice driven by a tradeoffbetween cost and performance. Wrought processesprovide the best properties, but at a high cost,because of the excessive amount of material that isremoved during machining and the sluggish natureof the machining process itself. Castings offer alower cost approach because the parts can be madeto near net shape, although the cost is still not lowenough for volume production. The extreme reac-tivity of titanium drives the need for a cold crucible,which consumes much energy during melting andcasting. One approach for cost reduction has beenthe use of CaO crucibles described earlier.

PM, unlike conventional wrought processes,promises lower cost because of the ability to producenear net shape parts, although high powder costremains a barrier for its widespread use. Althoughmuch has been written about PM Ti since the1970s,45,46 the only high-volume automotive PMtitanium part in production is the Ti-TiB compositeintake and exhaust engine valves,47,48 which are

Fig. 1. (a) As-cast Ti-44Al-8Nb-1B (at.%) exhaust valve after roughmachining. (b) X-ray image showing no porosity. A CaO crucible wasused for melting and a 160 K superheat was used.40

Sachdev, Kulkarni, Fang, Yang, and Girshov556

currently produced at a volume of �10,000 pieces/month. Nevertheless, the interest in powder pro-cessing remains high because of its key metallur-gical advantage of providing feedstock that hasminimal long range compositional segregation asoccurs in castings.

Low-Cost Ti Powders

PM processing of titanium has gained renewedattention recently because of the potential avail-ability of low-cost titanium powder. These powdersare produced by ‘‘meltless’’ processes from, forexample, titanium chloride, without the need tomelt and atomize. These processes can also producealloy powders by introducing the appropriate ratiosof chlorides of the desired elements into the processstream. McCracken et al.29 provided a good reviewof these processes, whereas a detailed technical costmodel (TCM) for a few promising powder productionapproaches are provided in a study reported byKirchain and Roth.24 The low cost of powders pro-duced by meltless routes stem from the fact thatthey require low capital investment, which allowsmall economically efficient plants to be con-structed, promising high-grade titanium powder atabout $8.00 per kg.24 The TCM model showed thatin producing a titanium tube by flow forming aninitially hot isostatically pressed preform, that thelow-cost powder accounted for more than 50% of thetotal cost. The next two important contributors tocost were the labor intensive container fabricationand hot isostatically pressing steps. Reducing pow-der price further was shown to have a large effect onthe cost of the product.

Powder Processing

PM Ti is generally classified into two groups:blended elemental (BE) and prealloy (PA).49,50 TheBE powders are softer than the PA powders, whichallows them to be cold pressed to a green density of�85–90% after cold pressing at 415 MPa.51 PApowders are generally not cold pressed because of

the excessively high pressure required to achieveacceptable green density and so are normally hotisostatically pressed after filling, evacuating, andsealing the powders in a flexible metal die or cap-sule.52 Although PM parts can achieve equal orhigher tensile strength than wrought materials,their ductility and fatigue properties are usuallylower because of the presence of residual porosity, acoarse microstructure, and/or contaminants likeoxygen and chlorine in the sintered product. Pow-ders in general have higher oxygen and interstitialslike carbon and nitrogen, whereas residual chlorinecontents of 0.12–0.15 w/o can be present in the BEpowders.53–55 Although the ASTM standard foraerospace Ti-6Al-4V alloy requires oxygen contentto be less than 0.2%, most PM parts have higheroxygen levels because of the large surface area ofthe powders and the strong affinity of Ti for oxygen,which is not a favorable situation for fine pow-ders.52,56–58 The surface oxides decompose at hightemperatures with oxygen dissolving into the Timatrix leading to an increase in strength but adecrease in ductility (Fig. 2).59

Although the BE approach affords lower cost thanusing PA powders, the mechanical properties of theformer are, in general, lower even if hot isostaticpressing is used.60 The microstructure (and thus,mechanical properties) of PM Ti can be tailored toapproach those of IM Ti if postsintering hot isostaticpressing can be introduced to produce full densifi-cation, but this adds to the cost. The overall thermalexposure has to address two outcomes: (I) It mustprovide for bonding and homogenization duringsintering especially if the BE approach is taken and(II) it must produce the specific size, aspect ratio,and distribution of the phases for optimum proper-ties. Multiple heat treatments and thermomechan-ical processing may thus be needed, e.g., sinteringmay be conducted in the beta phase field becausethe diffusivities of many alloying elements, includ-ing the self-diffusivity of titanium, are higher in thebeta phase than in the alpha phase.52,61 The post-sintering heat treatment will need to be carried out

Fig. 2. Effect of oxygen content on strength and elongation of sintered PM Ti-6Al-4V.

Titanium for Automotive Applications: Challenges and Opportunities in Materials and Processing 557

in the alpha, beta, or alpha + beta phase fieldsdepending on the final microstructure desired.Achieving both low cost and high performance will,therefore, remain the key challenge in PM process-ing of titanium and its alloys.

Porosity always remains when BE powders areused, especially if the powder has a relatively highresidual chlorine content, which volatilizes andcreates insoluble gas bubbles within the materialduring sintering.53–55 These pores cannot be easilyeliminated even by secondary pressing after sin-tering.49,52,54 Low chlorine powders are thus neces-sary for achieving high-density PM Ti products withBE powders.62

Sintering times are also generally longer with theBE approach because of the need for completeinterdiffusion of all elements present, which will addcost. The diffusivities of Ti and Al at different tem-peratures and in various phases and intermetallicsof binary Ti-Al are presented in Table II. The datashow that the diffusivity of Al in all phases except inAl3Ti is much lower than Ti; the diffusivity of Al inAl3Ti is approximately an order of magnitude higherthan that of Ti. Such differences in diffusivities be-tween Ti and Al in various phases also lead to Kir-kendall porosity in the BE sintered products,63–65

which needs to be eliminated to enable higherstresses in the component for mass efficiency. Notethat the values listed in Table II are for self andimpurity diffusion coefficients, and data on multi-component diffusion in the Ti-alloys are lacking. Theavailability of such multicomponent diffusion datawill be essential to establish alloy guidelines and todesign sintering and homogenization approachesbased on mutlicomponent diffusion kinetics.

Although much research has been reported on theeffect of alloying in accelerating densification,68–70

the controlling mechanisms are not well under-stood. More innovation is needed to accelerateinterdiffusion kinetics during sintering; for exam-ple, Fe is known to be a fast diffuser in Ti, and it hasbeen shown that the addition of Fe to the powdercompact does improve sintering kinetics.71,72

Another aspect of cost reduction is the feasibilityof achieving nearly fully dense Ti alloys by a singlecompressing-sintering step.72 One approach is theuse of an external field during sintering; e.g., sparkplasma sintering (SPS), which has been shown toenhance diffusion kinetics and reduce sinteringtimes. SPS also allows the sintering operation tooccur at lower temperatures than encountered incommon sintering processes.73–76 Work is alsoreported on using transient liquid phase to enhancediffusion during sintering.72,77–80 However, themain challenge is to sustain this liquid phase for alonger time to maximize the faster diffusion throughthe liquid. This is because liquid aluminuminstantly reacts with Ti to form Al3Ti intermetallicat the interface between Al and Ti, which preventsfurther intermixing in the liquid phase.63,81 Hence,any alloy design or process modification that canretain the liquid phase for longer times to allowliquid phase diffusion to occur has a great potentialfor improving sintering kinetics.

The reduction of diffusion distances and enhancedsurface diffusion by reducing initial powder size isalso an approach for enhancing sintering kinetics.However, reducing powder size also means anincreased risk of oxidation. Whereas doping pow-ders with rare-earth elements have been shown toreduce oxygen by scavenging, which also enhancesparticle bonding during sintering resulting inincreased ductility, this process again adds to thecost.72,82

Hydride Powder Processing

One approach for controlling oxygen content invacuum sintered products is to use titanium hydride(TiH2) powders instead of metallic powders.56 TiH2

is formed at relatively low temperatures above350�C in the presence of hydrogen and is relativelybrittle, which allows it to be effectively crushed tofine powder in normal PM processing. The hydridedpowders are also readily compacted because of eas-ier cold welding between TiH2 particles and because

Table II. Diffusivities of Ti and Al in various Ti-Al phases

Matrix Element

Diffusivity (m2/s)

Comment800�C 1,000�C

a-Ti Ti 2.5 9 10�18 5.2 9 10�16 Estimated from values of frequency factorand activation energies summarized

in the review by Mishin and Herzig66

for self and impurity diffusion coefficients

Al 6.6 9 10�19 2.2 9 10�16

b-Ti Ti 2.7 9 10�14 1.8 9 10�13

Al 1.1 9 10�14 8.6 9 10�14

Ti3Al Ti 2.1 9 10�19 3.4 9 10�17

Al 1.7 9 10�20 1.7 9 10�17

TiAl Ti 1.0 9 10�18 8.2 9 10�17

Al 8.3 9 10�20 4.5 9 10�17

Al3Ti Ti 2.8 9 10�15 4.8 9 10�14 Intrinsic diffusivities from Tardy and Tu67

Al 4.5 9 10�14 9.7 9 10�13

Sachdev, Kulkarni, Fang, Yang, and Girshov558

of fragmentation of the powder during compactionleading to higher green densities. The TiH2 dehy-drogenates under high vacuum during ramp-upheating for sintering, leaving active Ti surfacesexposed that can rapidly sinter at the selected sin-tering temperature.59,83 Ivasishin84 showed that thefinal oxygen content of 0.21 w/o after direct sinter-ing of TiH2 was significantly lower than in conven-tional sintered titanium (0.39 w/o). The loweroxygen levels in the final product have been ex-plained as follows: (I) The interstitial sites in thetitanium lattice are occupied by hydrogen atoms,which reduces the chance of dissolution of oxygen,59

and (II) hydrogen released during sintering isassumed to have a cleansing effect by removingoxygen from the particle surfaces.

Sintered TiH2, including hydrides with alloyingadditions, have shown higher final densities of

98.5–99.5% compared with only 90–95% obtainedwith metallic powders.85 The higher densitiesobtained with the hydrogenated powders have beenpostulated to be caused by defects formed duringdehydrogenation that accelerate mass trans-port.56,86,87 The final sintered TiH2 powder showedtensile and fatigue properties similar to that pro-duced by ingot metallurgy.59

Eliminating post-sintering hot isostatic pressingor any hot working and heat treatment would bevery desirable for reducing the cost of PM compo-nents. One process for obtaining the desired micro-structure in the as-sintered state is called hydrogensintering and phase transformation (HSPT).88 Thisprocess is described in Figs. 3 and 4, and it relies onsintering in the b-Ti(H) regime in a controlledhydrogen/argon atmosphere as opposed to in thepure b regime in vacuum where the hydrogen isdriven off before the system reaches the sinteringtemperature. Next, the HSPT process induces aeutectoid phase transformation that cannot occur ifall of the hydrogen is driven out of the powder.Table III compares phase transformations thatoccur with the HSPT process with that which occurwith standard vacuum sintering.

The HSPT sintering process has demonstrated adensity greater than 99% in Ti powders in theas-sintered state with a grain size that is approxi-mately 1 lm.88 Reasons for this improvement indensification are explained by the fact that a solidsolution of H atoms in titanium can reduce theactivation energy of Ti self-diffusion, promotingdensification rate.89,90 In addition, it has beenreported that Al and V atoms have higher diffusioncoefficients in b-Ti(H) than in b-Ti without hydro-gen.91,92

The HSPT method can be extended to BE powdersas well. Figure 5 compares as-sintered microstruc-tures of blended TiH2 and 60Al-40V powders in aproportion to produce Ti-6Al-4V in both HSPT andvacuum sintered conditions. Both processes resultedin �99% relative density after sintering, as alsoFig. 3. Comparing processing paths in the Ti-H phase diagram.

Fig. 4. Comparison of sintering profiles between (a) simple direct sintering and (b) the proposed hydrogen sintering of TiH2.

Titanium for Automotive Applications: Challenges and Opportunities in Materials and Processing 559

confirmed by the absence of pores in the scanningelectron microscope (SEM) images. However, theresultant microstructures are significantly different.Specifically, vacuum sintering, which does not takeadvantage of sintering in well-controlled phasefields, shows a coarse (a + b) lamellar microstruc-ture (a in dark and b in bright contrast, and inter-granular b phase at the a boundaries), which istypical for this process. In contrast, the microstruc-ture following the controlled-hydrogen HSPT pro-cess shows a clearly different microstructure thatconsists of ultrafine broken b phases (bright con-trast) in a matrix of refined a (dark contrast). TheHSPT process clearly refines the microstructure asconfirmed from the SEM and transmission electronmicroscope images and shows the mean grain size ofb phases to be approximately 0.5 lm compared witha mean grain size of the a phase to be approximately1 lm. These sizes are clearly much finer than the

approximately 20 lm for the a phase obtained withvacuum sintering alone. This refined microstructureresults in better tensile properties (Table IV),88 andit is expected that fatigue properties will also behigher than vacuum sintered Ti as has been dem-onstrated in wrought alloys with a refined micro-structure compared with a coarse lamellarmicrostructure.93 Although the oxygen levels weresimilar in the two cases after sintering, they met theASTM standards, as did the carbon and nitrogencontents. Full densification is always desirable.Although hot isostatic pressing is expensive, oneoption for automotive applications that could besubject to high-volume manufacturing is couplinglow-cost BE powders and HSPT, with postsinteringone-step operations like die-forging, coining, orpneumatic isostatic forging, which could become alower cost option for producing fully dense partsthan hot isostatic pressing.94,95

Fig. 5. Microstructures produced by (a) vacuum sintering of TiH2 (SEM image); (b and c) HSPT of TiH2 (SEM image and STEM image);(d) typical wrought processes (SEM image), and (e) vacuum sintering of Ti metal powder (SEM image).

Table III. Phase transformations of simple direct sintering and hydrogen sintering of TiH2

Vacuum sintering of TiH2 Controlled hydrogen sintering of TiH2 (HSPT)

Heating d-TiH2 fi b-Ti(H) fi a-Ti fi b-Ti d-TiH2 fi b-Ti(H)Sintering b-Ti b-Ti(H)Cooling b-Ti fi a-Ti b-Ti(H) fi a-Ti(H) + d-TiHx fi a-Ti

Sachdev, Kulkarni, Fang, Yang, and Girshov560

PROCESSING OF RECYCLEDTITANIUM CHIPS

A process to convert titanium chips obtained fromsecondary manufacturing operations into fabricatedparts is described by Girshov96 (Fig. 6). The processcontains the following steps: grinding, cleaning,briquetting, encapsulation, and hot extrusion. Thechips are separated from foreign subjects (especiallymetals) before grinding in a specially designed cone-inertial crusher.97 The ground chips are washed andmagnetic particles are separated, after which theyare thermovacuum degassed (TVD),96,98 whichpurifies the chips from volatile contaminants as wellas decreases their hardness to reduce the pressurenecessary for briquetting. Although the TVD pro-cess reduces the hydrogen and carbon content sig-nificantly, the oxygen and nitrogen content does not

change much from their as-received condition(Table V).

The briquettes are placed in thick steel capsulesand hot extruded to produce 25- and 40-mm diam-eter rods. Computer models were developed tooptimize the extrusion process.99–103 In contrast tomaterials pressed in a closed mold, extruding asealed capsule does not allow free flow of thematerial deforming inside the capsule. Mathemati-cal models of deformation of these sealed porousmaterials were performed on the basis of ther-momechanics relations of contact interactionaccounting for physical and geometrical nonlinear-ity.104 Figure 7 compares simulations with actualexperiments. The model shows that the pressure ispredominantly perceived by the steel capsule at thebeginning of the extrusion, and the steel capsule canbreak thereafter. Using the model for understanding

Fig. 6. Processing titanium chips: 1—grinding, 2—washing, 3—magnetic separation, 4—vacuum annealing, 5—briquetting, 6—encapsulation,and 7—hot extrusion.

Table V. Interstitial impurities after the TVD process of 30 min at 600�C followed by air cooling

Chips condition

Impurity content

Oxygen Carbon Nitrogen Hydrogen

Before TVD 0.138 0.109 0.023 0.0053After TVD 0.133 0.034 0.024 0.0016

Table IV. Impurity concentrations and tensile properties of vacuum-sintered and hydrogen-sinteredTi-6Al-4V

Tensilestrength (MPa)

0.2% YS(MPa)

El(%)

RA(%) O (wt.%) H (wt.%) C (wt.%) N (wt.%)

ASTM B348 895 828 10 25 0.20 0.015 0.08 0.05Vacuum sintered 982 859 12 18 0.302 ± 0.044 0.004 ± 0.002 0.080 ± 0.012 0.025 ± 0.007HSPT 1,036 943 15 27 0.308 ± 0.07 <0.003 – –

Titanium for Automotive Applications: Challenges and Opportunities in Materials and Processing 561

this breakage is important for producing a rod witha clean surface and for determining the parametersthat provide ‘‘scalping’’ of the thin steel capsule bythe titanium briquette leading to the clean sur-face.98 Mechanical properties of the hot extrudedsamples compared with that produced from stan-dard titanium ingot are presented in Table VI. Thedata show that whereas the strength of the tubesextruded from canned titanium chips exceeded that

produced from ingot, the former had a ductility thatvaried between 2% and 10%.

A microstructural analysis (Fig. 8) showed thatthe presence of sharp interparticle boundaries withpores and microcracks in the extruded and subse-quently forged samples were the cause of the lowplasticity. To increase the plasticity to greater than10%, the briquettes were first vacuum arc remelted(VAR) and then cast and hot deformed (extruded).

Table VI. Mechanical properties of samples in various processed conditions96,105

No. Sample rB (MPa) r0.2 (MPa) d %

1 Standard sample (technical titanium ingot after deformation) 329–539 ‡343 ‡202 As-extruded 695 685 0.43 Extruded and annealed at 800�C for 1 h 780 760 1.04 Extruded and annealed at 1,200�C for 1 h 662 640 1.45 Extruded, annealed at 1,200�C for 2 h, rolled and annealed at 800�C for 1 h 750 734 4.86 Extruded and forged 800–810 780–790 2–107 VAR briquettes and cast 650–670 580–590 15–188 VAR briquettes that were cast and then hot deformed (rod) 710–715 610–615 18–209 VAR briquettes forged annealed at 800�C for 1 h 910–949 18–20

Fig. 7. (a) Experiments, (b) computer modeling results at the beginning of the extrusion, and (c) computer simulation of the breakage of thecapsule formation of the extruded rod.100

Fig. 8. (a) Cross-section and (b) fracture surface of sample extruded and annealed at 800�C for 1 h (25 mm diameter rod, sample 3,Table VI).96,98

Sachdev, Kulkarni, Fang, Yang, and Girshov562

The microstructure and fracture of the samplesafter VAR are shown in Fig. 9, whereas the prop-erties obtained are listed in Table VI. Better prop-erties could be obtained if the briquettes could beextruded before VAR compared with only pressedbriquettes because that would allow for homogeni-zation of the alloy composition. Corrosion studies inmineral and organic acids and sodium chlorideshowed the corrosion resistance of these rods to besimilar to industrial titanium. This process can alsoproduce 2-mm-thick to 6-mm-thick sheets, shapedcastings, and ingots, as well as TiH2 powders andsintered filters.

SUMMARY

Although much development has occurred overthe past decade, the improved manufacturingmethods and lower cost of titanium have still notbeen sufficient to displace conventional materials.In response to the cost pressures, much innovationis occurring in the following areas, which bearpromise for increased penetration into automotiveapplications, including advanced titanium alumi-nides, low-cost powders, metal injection molding,106

melt processing, use of hydride powders as in HSPT,metal mold castings (that can produce about 1,500shots per die),107 recycling of machining chips andother offal, etc., all of which touch on the fringes ofproduction in automotive volumes. Near-net-shapemanufacturing will be key for reducing the amountof expensive value added material that wouldotherwise be introduced into the recycling stream.Coupling development efforts with fundamentalresearch is also needed, especially in multicompo-nent thermodynamics and kinetics to design betteralloys computationally to respond to developmentsin conventional materials that will constantlychallenge emerging materials like titanium. Muchwork is required in the area of multicomponentkinetic and thermodynamic models with a focus oninterdiffusion kinetics and phase equilibria to opti-mize processes based on an understanding of thephysical phenomena controlling the process. One

class of materials that is intriguing for automotiveapplications is nanostructured and ultrafine-grained materials as they show potential forexceptional properties for some of the demandingapplications being considered.108,109 Finally, mate-rials and processing developments should be cou-pled with integrated multifunctional design110 forsustained use rather than a simple material sub-stitution that skirts design space. TCM coupled withvarious designs and corresponding product benefitswill become important for product decisions.

ACKNOWLEDGEMENTS

One author (A.K.S.) would like especially to thankseveral people who provided material to make thisarticle possible: Mike Holly and Peter Sarosi ofGM; Randy Kirchain of MIT; Laine Mears ofClemson University; Osman Ertorer of UC Davis;S. Chandrasekar of Purdue; and Pei Sun ofUniversity of Utah. Finally, the management ofGM Global R&D is thanked for their support.

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