Metal ForMing PDF

Nikhil R. Dhar, Ph. DProfessor, IPE Department

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IPE-3209: METAL FORMING AND

SHEET METALWORKING

Department of Industrial & Production Engineering

Course Outlines

Fundamental of Metal Forming: Overview of metal forming,material behavior in metal forming, temperature in metal forming,friction and lubrication in metal forming.

Bulk Deformation Processes in Metal Working:

Rolling and Other deformation processes related to rolling

Forging and Other deformation processes related to forging

Extrusion and Other deformation processes related to forging

Sheet Metal Working: Cutting operations, bending operations,drawing, Other sheet metal forming operations, precision formingprocesses; various features of different types of metal forming dies;principles of powder forming.

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Suggested Reading

Manufacturing Processes for Engineering Materials - S. Kalpakjian& S. R. Schmid

Materials and Processes in Manufacturing - E.P. Degarmo, J.T.Black & R.A. Kohser

Fundamentals of Modern Manufacturing - M.P. Groover

Processes and Design for Manufacturing - S.D.EI Wakil

Metal Cutting Principles - M. C. Shaw

Metal Cutting - E. Trent

Manufacturing Technology B. Kumar

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Marks Distribution

Total Marks: 100

Class Test (20%)

Class Attendance[10%]

Final Examination[70%]

1 2 3 4

10 10 10 10 10% 70%

Quiz-01 : Fundamental of Metal Forming

Quiz-02 : Bulk Deformation Processes - Rolling

Quiz-03 : Bulk Deformation Processes - Forging

Quiz-04 : Sheet Metal Forming Processes

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LECTURE-01: FUNDAMENTAL OF

METAL FORMING


Introduction

Large group of manufacturing processes in which plastic deformation isused to change the shape of metal workpieces

The tool, usually called a die, applies stresses that exceed the yieldstrength of the metal

The metal takes a shape determined by the geometry of the die

Forming processes tend to be complex systems consisting

Independent Variables,

Dependent Variables, and

Independent-dependent Interrelations.

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Independent Variables: Independent variables are those aspects of theprocess over which the engineer has direct control, and they are generallyselected or specified when setting up the process. Consider some of theindependent variables in a typical forming process:

Starting material : The engineer is often free to specify the chemistryand condition. These may also be chosen for ease in fabrication or theymay be restricted by the final properties desired for the product.

Starting geometry of the workpiece: This may be dictated by previousprocessing or it may be selected by the engineer from a variety ofavailable shapes. Economics often influence this decision.

Tool or die geometry : This are has many aspects such as the diameterof a rolling mill roll, the die angle in wire drawing and the cavitydetails when forging. Since tooling will produce and control the metalflow, success or failure of a process often depends on tool geometry.

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Lubrication: Since lubricants also acts as coolants, thermal barriers,corrosion inhibitors, and parting compounds, their selection is anaspect of great importance. Specification includes type of lubricantamount to be applied and the method of application.

Starting temperature: Many material properties vary greatly withtemperature, so its selection and control may well dictate the successor failure of an operation.

Speed of operation: Since speed can directly influence the lubricanteffectiveness, the forces required for deformation and the timeavailable for heat transfer. It is obvious that its selection would besignificant in a forming operation.

Amount of deformation: While some processes control this variablethrough die design, others, such as rolling permits its selection at thediscretion of the engineer.

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Dependent Variables: After the engineer specifies the independentvariables, the process then determine the nature and values for a second setof variables. Known as dependent variables, these, in essence, are theconsequences of the dependent variable selection. Consider some of thedependent variables in a typical forming process:

Force or power requirements: Engineers cannot directly specify theforce or power; they can only specify the independent variables andthen experience the consequences of the selection. The ability topredict the forces or powers however is extremely important for onlyby having this knowledge will the engineer be able to specify or selectthe equipment for the process.

Material properties of the product: The customer is not interested inthe starting properties but is concerned with our ability to produce thedesired final shape with the desired final properties

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Exit (or Final) temperature: Engineering properties can be altered byboth the mechanical and thermal history of the material thus it isimportant to know and control the temperature of the materialthroughout the process

Surface finish and precision: Both are characteristics of the resultantproduct that are dependent on the specific details of the process.

Nature of the material flow: Since properties depend on deformationhistory, control here is vital the customer is satisfied only if the desiredgeometric shape is produced with the right set of companion propertiesand without surface or internal defects.

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Independent-Dependent Interrelations: The following Figure illustrate a majorproblem facing the metal-forming engineer. On one side are the independentvariables, those aspect of the process for which control is direct and immediate. Onthe other are the dependent variables, those aspects for which control is totallyindirect. It is the dependent variables that we want to control, but the dependentvariables are determioned by the process, as consequences of the independentvariable selection. If we want to change a dependent variable, we must determinewhich independent variable is to be changed, in what manner, and by how much. Thusit is important for us to develop a knowledge of the independent variable-dependentvariable interrelations.

details flow Material

precision lDimensiona

finish Surface

ratureExit tempe

propertiesProduct

trequiremenPower Forceor

Modeling

Experiment

Experience

ndeformatio ofAmount

ndeformatio of Speed

re temperatuStarting

nLubricatio

geometry Tool

geometry Starting

material Starting

variablesDependent variablest Independen

Schematic of the metal-forming systemshowing independent variables,dependent variables and the variousmeans of relating the two

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The ability to predict and control dependent variables can be obtained inthree distinct ways:

Experience: This requires long time exposure to the process and isgenerally limited to the specific materials, equipment and productsencountered in the realm of past contact.

Experiment: While possibly the least likely in error direct experimentis both time consuming and costly.

Process modeling: Here one approaches the problem with a high speedcomputer and one or more mathematical models of the processnumerical values are provided for the various independent variablesand the models are used to compute predictions for the dependentvariables . Most techniques rely on the applied theory of plasticity withvarious simplifying assumptions.

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General Parameters

While much metal-forming knowledge is specific to a given process, there arecertain features that are common to all processes, and these will be presentedhere.

Friction and Lubrication: An important consideration in metaldeformation processes is the friction developed between the tool and theworkpiece. For some processes, more than 50% of the input energy isspent in overcoming friction. The surface finish and dimensional precisionof the product are often directly related to friction. Changes in lubricationcan alter the mode of material flow during forming and in so doing, createor eliminate defects, or modify the properties of the final product.Production rate, tool design, tool wear and process optimization alldepend on the ability to determine and control process friction.

Temperature Concerns: In general, an increase in temperature brings outa decrease in strength, an increase in ductility, and a decrease in the rateof strain hardening - all effects that would tend to promote ease ofdeformation.

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Forming processes tend to be classified as hot working, cold working or warmworking based on both the temperature and the material being formed.

Hot Working:

Elevated temperatures bring about a decrease in the yield strength of ametal and an increase in ductility. At the temperatures of hot working,recrystallization eliminates the effects of strain hardening, so there isno significant increase in yield strength or hardness, or correspondingdecrease in ductility.

The plastic deformation of metals above their recrystallizationtemperature; it is important to note, however, that the recrystallizationtemperature varies greatly with different materials

In addition, the elevated temperatures promote diffusion that canremove or reduce chemical inhomogeneities; pores can be welded shutor reduced in size during the deformation; and the metallurgicalstructure can often be altered through recrystallization to improve thefinal properties.

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Structure and Property Modification by Hot Working: When metalssolidify, particularly in the large sections that are typical cast strands,coarse structures tend to form with a certain amount of chemicalsegregation. The size of the grains is usually not uniform, andundesirable grain shapes can be quite common, such as the columnargrains. Small gas cavities or shrinkage porosity can also form duringsolidification.

Temperature Variations: The success or failure of a hot deformationprocess often depends on the ability to control the temperatures withthe workpiece. To minimize problems, it is desirable to keep theworkpiece temperatures as uniform as possible.

Cold Working:

Plastic deformation of metals below the recrystallization temperatureis known as cold working. The process is usually performed at roomtemperature, but mildly elevated temperatures may be used to provideincreased ductility and reduced strength.

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Advantages of cold working: No heating is required Strength, fatigue and wear properties are improved through strain hardening Superior dimensional control is achieved, so little, if any, secondary machining

is required Better surface finish is obtained Products possess better reproducibility and interchangeability Directional properties can be imparted Contamination problems are minimized

Disadvantages of cold working: Higher forces are required to initiate and complete the deformation Less ductility is available Intermediate anneals may be required to compensate for the loss of ductility

that accompanies strain hardening Heavier and more powerful equipment is required Metal surfaces must be clean and scale-free Imparted directional properties may be detrimental Undesirable residual stresses may be produced

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Warm Working:

Deformation produced at temperatures intermediate to hot and coldworking.

Compared to cold working, it offers the advantages of reduced loads on thetooling and equipment, increased material ductility, and a possiblereduction in the number of anneals due to a reduction in the amount ofstrain hardening.

Compared to hot forming, the lower temperatures of warm workingproduce less scaling and decarburization, and enable production ofproducts with better dimensional precision and smoother surfaces.

The warm regime generally requires less energy than hot working due tothe decreased energy in heating the workpiece, energy saved throughhigher precision and the possible elimination of post forming heattreatments.

Tools last longer, for while they must exert 25 to 60% higher forces, thereis less thermal shock and thermal fatigue.

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Stresses in Metal FormingStresses to plastically deform the metal are usually compressive Examples: rolling, forging, extrusion

However, some forming processes Stretch the metal (tensile stresses) Others bend the metal (tensile and compressive) Still others apply shear stresses

Material Properties in Metal FormingDesirable material properties: Low yield strength High ductility

These properties are affected by temperature: Ductility increases and yield strength decreases when work

temperature is raisedOther factors: Strain rate and friction

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Basic Types of Deformation Processes

Bulk deformation Processes

Rolling

Forging

Extrusion

Wire and bar drawing

Sheet metalworking

Cutting or Shearing

Bending

Deep drawing

Miscellaneous processes

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ForgingRolling

Extrusion Drawing

Bending DrawingShearing

Bulk deformation Processes

Sheet Metalworking

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LECTURE-02: BULK DEFORMATION

PROCESSES - ROLLING


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Rolling

Rolling is the most widely used deformation process. It consists of passingmetal between two rollers, which exert compressive stresses, reducing themetal thickness. Where simple shapes are to be made in large quantity,rolling is the most economical process. Rolled products include sheets,structural shapes and rails as well as intermediate shapes for wire drawing orforging. Circular shapes, I beams and railway tracks are manufactured usinggrooved rolls.

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Practically all metals, which are not used in cast form are reduced to somestandard shapes for subsequent processing.

Manufacturing companies producing metals in form of ingots which areobtained by casting liquid metal into a square cross section.

Slab (500-1800 mm wide and 50-300 mm thick)

Billets (40 to 150 sq mm)

Blooms (150 to 400 sq mm)

Sometimes continuous casting methods are also used to cast the liquidmetal into slabs, billets or blooms.

These shapes are further processed through hot rolling, forging orextrusion, to produce materials in standard form such as plates, sheets,rods, tubes and structural sections.

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Sequence of operations

Schematic layout ofvarious flat and shaperolling processes

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Basic Principles of Rolling

When a piece of metal is rolled in between two rolls, the thickness is reducedas a result of the compressive stresses exerted by the rolls and it can be treatedas a two-dimensional deformation in the thickness and length directionsneglecting the width direction. This is due to the fact that the length of contactbetween the rolls and workpiece is generally much smaller than the width ofthe sheet passing through and the undeformed material on both sides of theroll gap is restraining the lateral expansion along the width direction.

The metal piece experiences both vertical and horizontal stresses caused by thecompressive load from the rolls and the restrains by the portions of the metalpiece before and after the material in contact with the roll respectively.

As the rolls exert a vertical stress on the metal piece, the metal piece exerts thesame amount of stress back onto the rolls itself. As such the rolls are subjectedto elastic deformation due to this stress induced by the workpiece. As shown inthe figure below, the rolls in a 4-high rolling mill are subjected to four kinds ofdeformation:

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Deflection of the back-up rolls, Deflection of the work rolls, Flattening of the work rolls caused by contact with the back-up rolls

and workpiece Flattening of the back-up rolls caused by contact with the work rolls.

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Rolling is the process of reducing the thickess or changing the cross-section of a long workpiece by compressive forces applied through a setof rolls. The rolling processes can be done by

Flat Rolling

Shape Rolling

Production of Seamless Tubing & Pipe

Flat Rolling:

Metal strip enters the roll gap

The strip is reduced in size by the metal rolls

The velocity of the strip is increased the metal strip is reduced in size

Factors affecting Rolling Process

Frictional Forces

Roll Force and Power Requirement

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Department of Industrial & Production Engineering 107/28


Flat-Rolling Practice

Hot rolling The initial break down of an ingot Continuously cast slab Structure may be brittle Converts the cast structure to a wrought structure

Finer grainsEnhanced ductility

Reduction in defectsContinuous Casting Is replacing traditional methods Faster & better

Product of the first hot-rolling operation - Bloom or slab Square cross section of 150mm (6in) on one side Processed father by shape rolling

I-beamsRailroad rails

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Billets smaller than blooms and rolled into bars and rods

Cold rolling

carried out at room temperature

Produces sheet and strip metal

Better surface finish less scale

Pack rolling when two or more layers of metal are rolled together

Changes in grain structure during hot-rolling

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Rolling Defects in Sheets and Plates

The elastic deflection of the work rolls results in an uneven widthwisedistribution of the workpiece thickness in such a way where the thicknessis greater at the center of the width and smaller at the edges. In order tosolve the bending of the work rolls, several methods can be adopted.

Smaller work rolls are more prone to greater bending under high roll-separating forces from the vertical stresses induced by the workpiece.As such, back-up rolls are often used to counter this phenomenon.

Another method to reduce or eliminate elastic roll deflection is to usematerials of high elastic modulus, such as sintered carbide, for thework rolls.

A more common method to counter the effects of roll bending is theusage of cambered rolls. The degree of cambering depends on thewidth of the metal piece, flow stress of the material and the reductionper pass. However certain problems arise with improper work rollscambering.

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Lack of camber or insufficient cambering of the work rolls results inproducing a workpiece that has a thicker center than the edge. The thickercenter implies that the edges are plastically elongated more than the center.This induces a residual stress pattern of compression at the edges and tensionalong the centerline of the workpiece (Figure a). The consequences of thisuneven distribution of stress within the workpiece can be centerline cracking(Figure b), warping (Figure c) or edge wrinkling (Figure d) of the final metalsheet.

Figure a

Figure d Figure c

Figure b

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In the case where the work rolls are over-cambered, the edges of theworkpiece will be thicker than the center and the residual stress pattern isexactly the opposite of that of insufficient cambering, i.e. tension at the edgesand compression along the centerline (Figure e). Possible undesirable resultsof the workpiece being produced in such a manner are edge cracking (Figuref), splitting (Figure g) or centerline wrinkling (Figure h).

Figure e Figure f

Figure g Figure h

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Schematic Illustration of Various Roll arrangements

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Schematic Illustration of various roll arrangements: (a) Two-high; (b) Three-high;

(c) Four-high; (d) Cluster mill

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Shape-Rolling OperationsVarious shapes can be produced by shape rolling Bars Channels I-beams Railroad rails

Roll-pass design requires considerable experience in order to avoidexternal and internal defects

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Stages in Shape Rolling of an H-section part. Various other structuralsections such as channels and I-beams, are rolled by this kind of process.

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Ring Rolling

A thick ring is expanded into a large diameter ring The ring is placed between the two rolls One of which is driven The thickness is reduced by bringing the rolls together

The ring shaped blank my be produced by: Cutting from plate Piercing Cutting from a thick walled pipe

Various shapes can be produced by shaped rollsTypical applications of ring rolling: Large rings for rockets Gearwheel rims Ball-bearing and roller-bearing races

Can be carried out at room temperatureHas short production timeClose dimensional tolerances

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Thread Rolling

Cold-forming processStraight or tapered threads are formed on round rods by passing the pipethough diesTypical products include Screws and Bolts

Threads are rolled in the soft conditionThreads may then be heat treated, and subjected to final machining orgrindingUncommon or special-purpose threads are machined

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Production of Seamless Pipe & Tubing

Rotary tube piercing (Mannesmann process)

Hot-working process

Produces long thick-walled seamless pipe

Carried out by using an arrangement of rotating rolls

Tensile stresses develop at the center of the bar when it is subjected tocompressive forces

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Continuous Casting & Integrated Mills and MinimillsContinuous casting Advantages

Highly automatedReduces product costCompanies are converting over to this type of casting

Integrated Mills utilize everything from the production of hot metal to thecasting and rolling of the finished productMinimills Scrap metal is melted Cast continuously Rolled directly into specific lines of products Each minimill produces one kind of rolled product

RodBarStructural steel

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Continuous Casting

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Salient Points about Rolling

Rolling is the most extensively used metal forming process and its share isroughly 90%

The material to be rolled is drawn by means of friction into the tworevolving roll gap

The compressive forces applied by the rolls reduce the thickness of thematerial or changes its cross sectional area

The geometry of the product depend on the contour of the roll gap

Roll materials are cast iron, cast steel and forged steel because of highstrength and wear resistance requirements

Hot rolls are generally rough so that they can bite the work, and cold rollsare ground and polished for good work finish

In rolling the crystals get elongated in the rolling direction. In cold rollingcrystal more or less retain the elongated shape but in hot rolling they startreforming after coming out from the deformation zone

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The peripheral velocity of rolls at entry exceeds that of the strip, which isdragged in if the interface friction is high strip enough.

In the deformation zone the thickness of the strip gets reduced and itelongates. This increases the linear speed of the strip at the exit.

Thus there exist a neutral point where roll speed and strip speeds areequal. At this point the direction of the friction reverses.

When the angle of contact exceeds the friction angle the rolls cannot drawfresh strip

Roll torque, power etc. increase with increase in roll work contact lengthor roll radius

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LECTURE-03 :BULK DEFORMATION

PROCESSES - FORGING


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Forging

Forging is a deformation process in which the work is compressed betweentwo dies, using either impact or gradual pressure to form the part. Today,forging is an important industrial process used to make a variety of high-strength components for automotive, aerospace, and other applications. Thesecomponents include engine crankshafts and connecting rods, gears, aircraftstructural components, and jet engine turbine parts. In addition, steel andother basic metals industries use forging to establish the basic forms of largecomponents that are subsequently machined to final shape and dimensions.

Either impact or gradual pressure is used in forging. The distinction derivesmore form the type of equipment used than differences in process technology.A forging machine that applies an impact load is called a forging hammer,while one that applies gradual pressure is called a forging press. Anotherdifference among forging operations is the degree to which the flow of thework metal is constrained by the dies. By this classification there are threetypes of forging operations like

Open-die forging

Impression or Close die forging

Flashless Forging.

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Most forging processes begin with open die forging. Opendie forging is hot mechanical forming between flat orshaped dies in which the metal flow is not completelyrestricted. The stock is laid on a flat anvil while the flatface of the forging hammer is struck against the stock. Theequipment may range from the anvil and hammer to gianthydraulic presses.Open-die hot forging is an important industrial process.Shapes generated by open-die operations are simple;examples include shafts, disks, and rings. In someapplications, the work must often be manipulated (forexample, rotating in steps) to effect the desired shapechange. Open-die forging process is shown in the followingFigure. The skill of the human operator is a factor in thesuccess of these operations. Operations classified as open-die forging or related operations include:

FulleringEdging, andCogging

Open-Die Forging

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Fullering is a forging operation performed toreduce the cross section and redistribute the metalin a workpart in preparation for subsequent shapeforging. It is accomplished by dies with convexsurfaces. Fullering die cavities are often useddesigned into multicavity impression dies so thatthe starting bar can be rough formed before finalshaping.

Edging is similar to fullering, except that the dieshave concave surfaces.

Cogging operation consists of a sequence of forgingcompressions along the length of a workpiece toreduce cross section and increase length. It is usedin the steel industry to produce blooms and slabsfrom cast ingots. It is accomplished using open dieswith flat or slightly contoured surfaces. The termincremental forging is sometimes used for thisprocess.

Fullering

Edging

Cogging

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Advantages and Limitations

Advantages Simplest type of forging Dies are inexpensive Wide range of part sizes, ranging from 30-1000lbs Good strength qualities Generally good for small quantities

Limitations Simple shapes only difficult to hold close tolerances machining necessary low production rate poor utilization of material high skill required

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In impression-die forging,so0metimes called closed die forging,the die surfaces contain a shape orimpression that is imparted to thework during compression, thusconstraining metal flow to asignificant degree as shown infollowing Figure. In this type ofoperation, a portion of the work metalflows beyond the die impression toform flash and must be trimmed offlater. The process is shown in thefollowing Figure as a three stepsequence. The raw workpiece isshown as a cylindrical part similar tothat used in the previous open-dieoperation.

Impression or Close Die Forging

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Advantages Good utilization of material Better properties than Open Die Forgings Dies can be made of several pieces and inserts to create more advanced

parts Presses can go up to 50,000 ton capacities Good dimensional accuracy High production rates Good reproducibility

Limitations High die cost Machining is often necessary Economical for large quantities, but not for small quantities

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Flashless Forging

Flashless forging is sometimes called closed-die forging inindustry terminology. However, there is a technical distinctionbetween impression-die forging and true closed-die forging. Thedistinction is that in closed-die forging the raw workpiece iscompletely contained within the die cavity during compression,and no flash is formed. This process is shown in the followingfigure. Flashless forging imposes requirements on process controlthat are more demanding than impression-die forging. Mostimportant is that the work volume must equal the space in the diecavity within a very close tolerance. If the starting blank is toolarge, excessive pressures may cause damage to the die or even thepress. If the blank is too small, the cavity will not be filled.Because of the special demands made on flashless forging, theprocess lends itself best to part geometries that are usually simpleand symmetrical and to work materials such as aluminum andmagnesium and their alloys. Flashless forging is often classified asa precision forging process.

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Advantages

Close dimensional tolerances Very thin webs and flanges are possible Very little or no machining is required Little or no scrap after part is produced Cheaper to produce from less finishing operations and faster

production Typical applications are gears, connecting rods, and turbine blades Common materials used in precision forging are aluminum,

magnesium alloys, steel, and titanium

Limitations

High forging forces Thus higher capacity equipment is required Intricate dies leading to increased die cost Precise control over the Blanks volume and shape Accurate positioning of the Blank in the die cavity

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Other Forging Operations

Coining: Coining is a forgingprocess by which very fine andintricate details can be createdon the surface of a metal workpiece. Coining may be used tocontrol surface quality anddetail on parts. One commonuse of coining, as the namesuggests, is in the production ofcoins.

This is a flashless, precision forging operation, that due to the requiredaccuracy of the process, is performed cold. Lubrication is not used, since anysubstance between the die and work would hinder the reproduction of themost accurate details that are to be formed on the work's surface. In thecoining process, a large amount of force is exerted on the forging, over a shortdistance. Mechanical presses are often used for these operations.

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Upsetting: Upsetting is a deformation operation in which a cylindricalworkpart is increased in diameter and reduced in length. However, as anindustrial operation, it can also be performed as closed-die forging, as shownin the following Figure. Upsetting is widely used in the fastener industry toform the heads of nails, bolts, and similar hardware products.

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Heading: The following Figure illustrates a variety of heading applications,indicating various possible die configurations. Owing to these types ofapplications, more parts are produced by upsetting than any other forgingoperation. It is performed as a mass production operation - cold, warm, orhot - on special upset forging machines, called headers or formers.

Care must be taken so that work piece does not buckle

Can be highly automated

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Swaging and Radial Forging: Swaging and radial forging are forgingprocesses used to educe the diameter of a tube or solid rod. Swaging is oftenperformed on the end of a workpiece to create a tapered section. The swagingprocess shown is accomplished by means of rotating dies that hammer aworkpiece radially inward to taper it as the workpiece is fed into the dies.

Radial forging is similar to swaging in its action against the work and isused to create similar shapes. The difference is that in radial forging the diesdo not rotate around the workpiece; instead , the work is rotated at it feedsinto the hammering dies.

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Roll Forging: Roll forging is a deformation process used to reduce the cross sectionof a cylindrical (or rectangular) workpiece by passing it through a set of opposing rollsthat have grooves matching the desired shape of the part. The typical operation isshown in the following Figure. Roll forging is generally classified as a forging process,even though it utilizes rolls. The rolls do not turn continuously in roll forging, butrotate through only a portion of one revolution corresponding to the desireddeformation to be accomplished on the part. Roll-forged parts are generally strongerand possess favorable grain structure compared to competing processes, such asmachining, that might be used to produce the same part geometry.

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Forging Machines

Equipment used in forging consists of forging machines, classified asforging hammers and presses, and forging dies, which are thespecial tooling used in these machines. In addition, auxiliary equipment isneeded, such as furnaces to heat the work, mechanical devices to load andunload the work, and trimming stations to cut away the flash inimpression-die forging.

Forging Hammers: Forging hammers operate by applying an impactload against the work. The term drop hammer is often used for thesemachines, owing to the means of delivering impact energy. Drop hammersare most frequently used or impression-die forging. The upper portion ofthe forging die is attached to the ram, and the lower portion to the anvil.In the operation, the work is placed on the lower die, and the ram is liftedand then dropped. When the upper die strikes the work, the impact energycauses the part to assume the form of the die cavity.

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Drop hammers can be classified asgravity drop hammers and powerdrop hammers.

Gravity drop hammers achievetheir energy by the falling weight of aheavy ram. The force of the blow isdetermined by the height of the dropand the weight of the ram.

Power drop hammers acceleratethe ram by pressurized air or steam.One disadvantage of the drophammers is that a large amount of theimpact energy is transmitted throughthe anvil and into the floor of thebuilding. This results in a great dealof vibration for the surrounding area.

Power drop hammers

Gravity drop hammers

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Forging Presses: Presses apply gradual pressure, rather than suddenimpact, to accomplish the forging operation. Forging presses include

Mechanical Presses

Hydraulic Presses, and

Screw Presses

Mechanical presses typically operate by means of eccentrics, cranks, orknuckle joints, which convert the rotating motion of a drive motor into thetranslational motion of the ram. These mechanisms are very similar tothose used in stamping presses. Mechanical presses typically achieve veryhigh forces at the bottom of the forging stroke.

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Knuckle Joint Press Crank Press Eccentric Press

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Hydraulic presses: The basic working principlesof the hydraulic press are simple, and rely ondifferences in fluid pressure. Fluid is pumped intothe cylinder below the piston, this causes the fluidpressure under the piston to increase.Simultaneously fluid is pumped out of the topchannel, causing the fluid pressure above the pistonto decrease. A higher pressure of the fluid below thepiston than the fluid above it causes the piston torise. In the next step, fluid is pumped out frombelow the piston, causing the pressure under thepiston to decrease. Simultaneously fluid is pumpedinto the cylinder from the top, this increases thefluid pressure above the piston. A higher pressureof the fluid above the piston, than the fluid below it,moves the piston downward.

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Screw Presses: Forging screw pressesuse the rotational energy of a motor toturn a large screw. Typically a friction diskis used to translate the force from thedrive shaft to the screw's head. The screwpushes a ram with great mechanicaladvantage. Screw presses are similar tohydraulic presses in that they arerelatively slow and require a longercontact with the work. Screw presses arealso similar to hydraulic presses in thatthey can produce a constant amount offorce over a long stroke. Some screw pressmachines in modern industry can produce31,000 tons, (62,000,000 lbs), of force.

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LECTURE-04: BULK DEFORMATION

PROCESSES - EXTRUSION


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Extrusion Process

Extrusion is a process that forces metal or plastic to flow through a shapedopening die. The material is plastically deformed under the compression in thedie cavity. The process can be carried out hot or cold depending on the ductilityof the material.The tooling cost and setup is expensive for the extrusion process, but theactual manufactured part cost is inexpensive when produced in significantquantities.Materials that can be extrudes are aluminum, copper, steel, magnesium, andplastics. Aluminum, copper and plastics are most suitable for extrusion.

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Classification of Extrusion Processes

Depending on the ductility of the material used extrusions can be caries outvarious ways:

Hot Extrusion: Extrusion carried out at elevated temperatures

Forward or direct extrusion and

Backward or indirect extrusion

Cold Extrusion: Extrusion carried out a ambient temperature. Oftencombined with forging operations

Hydrostatic Extrusion: Pressure is applied by a piston throughincompressible fluid medium surrounding the billet

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Hot Extrusion

Extrusion is carried out at elevated temperatures-for metals and alloys that do not havesufficient ductility at room temperature, or in order to reduce the forces required. In thisextrusion, die wear can be excessive and cooling of the hot billet in the chamber can be aproblem, which results in highly non-uniform deformation. To reduce cooling of the billetand to prolong die life, extrusion dies may be preheated, as is done in hot forgingoperations. Hot billet causes the following problems:

Because the billet is hot, it develops an oxide film unless heated in an inert-atmosphere furnace. This film can be abrasive and it can affect the flow pattern ofthe material.

It also results in an extruded product that may be unacceptable in cases in whichgood surface finish is important.

In order to avoid the formation of oxide films on the hot extruded product, the dummyblock placed ahead of the ram is made a little smaller in diameter than the container. As aresult, a thin cylindrical shell, consisting mainly of the oxidized layer, is left in thecontainer. The extruded product is thus free of oxides; the skull is later removed from thechamber. Hot extrusion can be done by

Forward or direct extrusion process

Backward or indirect extrusion process

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Direct Extrusion: In this extrusion process, the heated billet is placed in the container. Aram towards the die pushes it. The metal is subjected to plastic deformation, slides alongthe walls of the container and is forced to flow through the die opening. At the end of theextruding operation, a small piece of metal, called butt-end scrap, remains in the containerand cannot be extruded.

Indirect Extrusion: For the production of solid part, the die is mounted on the end of ahollow ram and enters the container as shown in the following Figure, the outer end ofcontainer being closed by a closure plate. As the ram travels, the die applies pressure on thebillet and the deformed metal flows through the die opening in the direction opposite to theram motions and the product is extruded through the hollow ram. In indirect extrusion,there is practically no slip of billet with respect to the container walls.

Direct ExtrusionExtrusion

Indirect Extrusion107/71


Cold Extrusion

This process is similar to hot extrusion except that the metals worked possess theplasticity necessary for successful forming without heating them. Usually, these metalshave a high degree of ductility. Cold extrusion is also done to improve the physicalproperties of a metal and to produce a finished part. Cold extrusion is done mostly onvertical mechanical presses because they are fast and simple. The method is fast,wastes no or little materials and gives higher accuracy and tolerance. The widelyemployed cold extrusion method is Impact extrusion. Impact extrusion isperformed at higher speeds and shorter strokes than conventional extrusion. It is formaking discrete parts. For making thin wall-thickness items by permitting largedeformation at high speed.

Backward impact extrusion Forward impact extrusion Combined impact extrusion

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Hydrostatic Extrusion

With the hydrostatic extrusion the billet in the container issurrounded with fluid media, is called also hydrostaticsmedium. The container space is sealed on the stem side andon the die side, so that the penetrating stem can compress thehydrostatics medium on pressing power, without the stemtouches the billet. Also during extrusion the stem does nottouch the billet. The rate, with which the billet moves whenpressing in the direction of the die, is thus not equal to theram speed, but is proportional to the displaced hydrostaticsmedium volume. For this process it is substantial that thebillet seals the container space on applying the pressingpower in the hydrostatics medium against the die, sinceotherwise the pressing power cannot be developed.

It is thus a conical die and a careful sharpening billet a prerequisite of the process.Since the billet does not touch the container's wall, but between billet and containerhydrostatics medium exists, prevails negligibly small friction of a liquid at the billetsurface. Only the friction between billet and die is of importance for the deformingprocess. Likewise pressing of the billet is unnecessary at the press begin.

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Tube-Drawing

Tube-drawing operations, with and without an internal mandrel. Note that a variety ofdiameters and wall thicknesses can be produced from the same initial tube stock.

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Extrusion Defects

Surface Cracking: Cracking on billet materials occurs due totemperature, friction, punch speed. High Temperatures

Crack from along the grain boundaries. Typically occur inaluminum, magnesium, zinc alloys

Cold TemperaturesCaused by sticking of billet material at the die landKnown has the Bamboo Defect because of its similar appearanceto bamboo

Pipe: The metal-flow pattern tends to draw oxides and impurities towardthe center of the billetInternal Cracking: Center of extruded product develops cracks. Attributed to a state of hydrostatic tinsel stress Cracks increase with increasing die angle, impurities, and decreasing

extrusion ratio and friction

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Advantages of Extrusion Processes

The range of extruded items is very wide. Cross-sectional shapes not possible byrolling can be extruded, such as those with re-entrant sections.

No time is lost when changing shapes since the dies may by readily removed andreplaced.

Dimensional accuracy of extruded parts is generally superior to that of rolled ones.

In extrusion, the ductility of the metals is higher as the metal in the container is incomposite compression, this advantage being of particular importance in workingpoorly plastic metals and alloys.

Very large reductions are possible as compared to rolling, for which the reductionper pass is generally 2.

Automation in extrusion is simpler as items are produced in a single passing.

Small parts in large quantities can be made. For example, to produce a simplepump gear, a long gear is extruded and then sliced into a number of individualgears.

It does not need draft or flash to trim and needless machining as it is more accuratethan forging.

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Disadvantages of Extrusion Processes

Process waste in extrusion is higher than in rolling, where it is only 1 to 3%

In-homogeneity in structure and properties of an extruded product isgreater due to different flows of the axial and the outer layers of blanks.

Service life of extrusion tooling is shorter because of high contact stressesand slip rates.

Relatively high tooling costs, being made from costly alloy steel.

In productivity, extrusion is much inferior to rolling, particularly to itscontinuous varieties.

Cost of extrusion are generally greater as compared to other techniques

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Applications of Extrusion Processes

Extrusion is more widely used in the manufacture of solid and hollowsections from poorly plastic non-ferrous metals and their alloys(aluminum, copper, brass and bronze etc.)

Steel and other ferrous alloys can also be successfully processed with thedevelopment of molten-glass lubricants.

Manufacture of sections and pipes of complex configuration.

Medium and small batch production

Manufacture of parts of high dimensional accuracy

The range of extruded items is very wide: rods from 3 to 250 mm indiameter, pipes of 20 to 400 mm in diameter and wall thickness of 1 mmand above and more complicated shapes which can not be obtained byother mechanical methods.

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LECTURE-05: SHEET METAL FORMING

PROCESSES


BUET


Introduction

Sheet metal forming is a grouping of manycomplementary processes that are used toform sheet metal parts. One or more of theseprocesses is used to take a flat sheet of ductilemetal, and mechanically apply deformationforces that alter the shape of the material.Before deciding on the processes, one shoulddetermine whether a particular sheet metalcan be formed into the desired shape withoutfailure. The sheet metal operations done on apress may be grouped into two categories,cutting (shearing) operations andforming operations.

Sheet Metal Forming

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Cutting (Shearing) Operations

In this operation, the workpiece is stressed beyond its ultimate strength. Thestresses caused in the metal by the applied forces will be shearing stresses.The cutting operations include:

Punching (Piercing)

Blanking

Notching

Perforating

Slitting

Lancing

Parting

Shaving

Trimming

Fine blanking

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Punching (Piercing): It is a cutting operation by which various shaped holes aremade in sheet metal. Punching is similar to blanking except that in punching, thehole is the desired product, the material punched out to form the hole being waste.

Blanking: Blanking is the operation of cutting a flat shape sheet metal. The articlepunched out is called the blank and is the required product of the operation. Thehole and metal left behind is discarded as waste.

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Notching: This is cutting operation by which metal pieces are cut from the edge of a sheet,strip or blank.

Perforating: This is a process by which multiple holes which are very small and closetogether are cut in flat work material.

Slitting: It refers to the operation of making incomplete holes in a workpiece.

Lancing: This is a cutting operation in which a hole is partially cut and then one side is bentdown to form a sort of tab. Since no metal is actually removed, there will be no scrap.

Parting: Parting involves cutting a sheet metal strip by a punch with two cutting edges thatmatch the opposite sides of the blank.

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Shaving: The edge of blanked parts is generally rough, uneven and unsquare.Accurate dimensions of the part are obtained by removing a thin strip of metalalong the edges.

Trimming: This operation consists of cutting unwanted excess material from theperiphery of previously formed components.

Fine blanking: Fine blanking is a operation used to blank sheet metal parts withclose tolerances and smooth, straight edges in one step.

(a) Shaving a sheared edge. (b) Shearing and

shaving, combined in one stroke.Fine blanking

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Shearing Dies

Because the formability of a sheared part can be influenced by the quality of its shearededges, clearance control is important. In practice, clearances usually range between 2% and8% of the sheets thickness; generally, the thicker the sheet, the larger is the clearance (asmuch as 10%). However, the smaller the clearance, the better is the quality of the edge.Some common shearing dies are describe below:

Punch and Die Shapes: As the surfaces of the punch and die are flat; thus, the punchforce builds up rapidly during shearing, because the entire thickness of the sheet issheared at the same time. However, the area being sheared at any moment can becontrolled be beveling the punch and die surfaces, as shown in the following Figure.This geometry is particularly suitable for shearing thick blanks, because it reduces thetotal shearing force.

Examples of the use of shear angles on punches and dies.

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Compound Dies: Several operations on the same strip may be performed in onestroke with a compound die in one station. These operations are usually limited torelatively simple shearing because they are somewhat slow and the dies are moreexpensive than those for individual shearing operations.

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Progressive Dies: Parts requiring multiple operations, such as punching,blanking and notching are made at high production rates in progressive dies. Thesheet metal is fed through a coil strip and a different operation is performed at thesame station with each stroke of a series of punches.

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Transfer Dies: In a transfer die setup, the sheet metal undergoes differentoperations at different stations, which are arranged along a straight line or acircular path. After each operation, the part is transfer to the next operation foradditional operations.

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Forming Operations

In this operation, the stresses are below the ultimate strength of the metal. In thisoperation, there is no cutting of the metal but only the contour of the workpiece ischanged to get the desired product. The forming operations include:

Bending: In this operation, the material in the form of flat sheet orstrip, is uniformly strained around a linear axis which lies in theneutral plane and perpendicular to the lengthwise direction of thesheet or metal. The bending operations include:

V-bendingEdge bendingRoll bendingAir bendingFlangingDimpling

Press break formingBeadingRoll formingTube formingBulgingStretch forming

Drawing: This is a process of a forming a flat workpiece into a hollow shape by meansof a punch, which causes the blank to flow into die cavity.

Squeezing: Under this operation, the metal is caused to flow to all portions of a diecavity under the action of compressive forces.

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V-bending Edge bending Roll bending

Bending in 4-slide machine Air bending

Bending of Flat Sheet and Plate

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Flanging : Flanging is a process of bending the edges of sheet metals to 90o

Shrink flanging subjected to compressive hoop stress.

Stretch flanging subjected to tensile stresses

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Dimpling:

First hole is punched and expanded into a flange Flanges can be produced by piercing with shaped punch When bend angle < 90 degrees as in fitting conical ends its called

flanging

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Press Break Forming: Sheet metal or plate can be bent easily with simplefixtures using a press. Long and relatively narrow pieces are usually bent in a pressbreak. This machine utilizes long dies in a mechanical or hydraulic press and issuitable for small production runs. The tooling is simple and adaptable to a widevariety of shapes.

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(a) Bead forming with a single die

Beading: In beading the edge of the sheet metal is bent into the cavity of a die.The bead gives stiffness to the part by increasing the moment on inertia of theedges. Also, it improves the appearance of the part and eliminates exposed sharpedges

(b) Bead forming with two dies, in a press brake.

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Roll-forming process

Roll Forming: For bending continuous lengths of sheet metal and for largeproduction runs, roll forming is used. The metal strip is bent in stages by passing itthrough a series of rolls.

Stages in roll forming of a sheet-metal door frame. In Stage 6, the rolls may be shaped as in A or B.

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Bulging: The basic forming process of bulging involves placing tabular, conical orcurvilinear part into a split-female die and expanding it with, say, a polyurethane plug.The punch is then retracted, the plug returns to its original shape and the part isremoved by opening the dies.

(a) Bulging of a tubular part with a flexible plug. Water pitchers can be made by this method

(b) Production of fittings for plumbing byexpanding tubular blanks with internal pressure.

(c) Manufacturing of Bellows.

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Examples of the bending and the embossing of sheet metal with a metal punch and with a flexible pad serving as the female die.

Rubber Forming: In rubber forming , one of the dies in a set is made of flexiblematerial, such as a rubber or polyurethane membrane. Polyurethanes are usedwidely because of their resistance to abrasion, long fatigue life and resistance todamage by burrs or sharp edges of the sheet blank. In bending and embossingsheet metal by the rubber forming method, as shown in the following Figure, thefemale die is replaced with a rubber pad. Parts can also be formed with laminatedsheets of various nonmetallic material or coatings.

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Hydroform Process: In hydroforming or fluid forming process, the pressureover the rubber membrane is controlled throughout the forming cycle, withmaximum pressure reaching 100 Mpa. This procedure allows close control of thepart during forming to prevent wrinkling or tearing. Hydroforming processes havethe following advantages: Low tooling cost Flexibility and ease of operation Low die wear No damage to the surface of the sheet and Capability to form complex shapes.

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Explosive Forming Process: Explosive forming, is distinguished fromconventional forming in that the punch or diaphragm is replaced by an explosivecharge. The explosives used are generally high explosive chemicals, gaseousmixtures, or propellants. There are two techniques of high explosive forming suchas Contact technique and Stand -off technique.

Contact Technique: The explosive charge in theform of cartridge is held in direct contact with thework piece while the detonation is initiated. Thedetonation builds up extremely high pressures (upto30,000MPa) on the surface of the work pieceresulting in metal deformation, and possiblefracture. The process is used often for bulging tubes.

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Standoff Technique: The sheet metal work piece blank is clamped over a dieand the assembly is lowered into a tank filled with water. The air in the die ispumped out. The explosive charge is placed at some predetermined distance fromthe work piece. On detonation of the explosive, a pressure pulse of very highintensity is produced. A gas bubble is also produced which expands spherically andthen collapses. When the pressure pulse impinges against the work piece, themetal is deformed into the die with as high velocity as 120 m/s.

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Deep Drawing: Drawing operation is the process of forming a flat piece ofmaterial (blank) into a hollow shape by means of a punch, which causes the blankto flow into the die-cavity. Round sheet metal block is placed over a circular dieopening and held in a place with blank holder & punch forces down into the diecavity. Wrinkling occurs at the edges.

Deep-drawing process on a circular sheet-metal blank

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Ironing Process: If the thickness of the sheet as it enters the die cavity is morethan the clearance between the punch and the die, the thickness will have to bereduced; this effect is known as ironing. Ironing produces a cup with constant wallthickness thus, the smaller the clearance, the greater is the amount of ironing.

Schematic illustration of the ironing process. Note that the cup wallis thinner than its bottom. All beverage cans without seams areironed, generally in three steps, after being deep drawn into a cup.

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Redrawing Operations: Containers or shells that are too difficult to draw in one operationare generally redrawn. In reverse redrawing, shown in following Figure, the metal issubjected to bending in the direction opposite to its original bending configuration. Thisreversal in bending results in strain softening. This operation requires lower forces thandirect redrawing and the material behaves in a more ductile manner.

Conventional redrawing Reverse redrawing.

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Beverage Can

Steps in Manufacturing an

Aluminum Can

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Aluminum Two-Piece Beverage Cans

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THANK YOU FOR YOUR ATTENTION

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