Cold ExtrusionRevised by Murali Bhupatiraju, Metaldyne and Robert Greczanik, American Axle and Manufacturing

COLD EXTRUSION is a push-through com-pressive forming process with the startingmaterial (billet/slug) at room temperature. Dur-ing the process, however, the deforming materialundergoes deformation heating (conversion ofdeformation work to heat) to several hundreddegrees. Typically, a punch is used to applypressure to the billet enclosed, partially or com-pletely, in a stationary die. Aluminum and alu-minum alloys, copper and copper alloys, carbonsteels, alloy steels, and stainless steels can becold extruded.

Based on the punch and die design and theresulting material flow, cold extrusion can beclassified into three primary processes: forwardextrusion, backward extrusion, and lateralextrusion. In forward extrusion, the materialflows in the same direction as the punch dis-placement. In forward extrusion, the diameterof a rod or tube is reduced by forcing it throughan orifice in a die (Fig. 1a). In backward extru-sion, the material flows in the opposite directionof the punch displacement. The billet is enclosedin a die and is forced to flow backward throughthe annular region between the punch and thedie (Fig. 1b). Backward extrusion is differ-entiated from impact extrusion where typically anonferrous material is extruded backward by arapidly moving punch and a shallow die withminimal material contact. Forward and back-ward extrusion can also be simultaneouslyachieved through die design (Fig. 1c). In lateralextrusion, the material flows perpendicular to thedirection of punch displacement. The material isenclosed by the die and the punch and is forcedthrough radially placed orifices (Fig. 1d). Hookerextrusion is a variation of the forward extrusionprocess where a tubular billet is forced through aforward extrusion die with a punch that acts as apusher and a mandrel to reduce the outer dia-meter and elongate the tubular portion (Fig. 1e).Although not strictly a compressive formingoperation, draw wiping or ironing is also com-monly used as a cold extrusion process. In iron-ing, the wall thickness of a tubular billet isreduced by forcing it under tension (and orcompression) through a die similar to a forwardextrusion die (Fig. 1f). Figure 2 shows variousparts made by these cold forging operations.

Cold extrusion, being a forging operation, hasthe typical advantages of material savings, work

hardening (strengthening), and grain flow ordirectional strengthening. Compared to otherforging operations cold extrusion is particularlyattractive for the following reasons: dimensionalprecision, superior surface finish, net-shapedfeatures, lower energy consumption, higherproduction rates, and cleaner work environment.Drawbacks of cold extrusion are higher loads,lubrication cost, limited deformation, and lim-ited shape complexity.

Extrusion Pressure

The punch pressure in extrusion depends onthe flow stress of the material being extruded, thedegree of deformation (strain), billet geometry,billet/die interface friction, and die design.Punch speed available in conventional presseshas little effect on extrusion pressure. Punchspeed, however, affects the extrusion process inother ways: tool heating, lubricant deterioration,and dynamic loading. Hardness of the material isrepresentative of its flow stress, and conse-quently softer materials require lower extrusionpressure. Thermal softening processes ofannealing and normalizing also reduce extrusionpressure. Apart from hardness, the degree ofdeformation has the greatest impact on extrusionpressure and is expressed as either extrusion ratioR or reduction ratio r :

R=A0

Af

r=A07Af

A0

·100

where A0 is the initial cross-sectional area and Af

is the final cross-sectional area.Billet geometry factors such as length-

to-diameter ratio are significant in forwardextrusion. Die design aspects such as die entryangle (forward extrusion) and punch geometry(backward extrusion) also affect extrusion pres-sure. Bethlehem Steel developed the followingequations for extrusion pressure:

Forward extrusion:

Pf ¼ 0:5½sys þ Kðln RÞn�ðaf þ bf ln RÞðe4mZÞ

Backward extrusion:

Pb=0:4[sys+K( lnR)n] (ab+bb lnR)R

R71

� �

where

af=1:15a

57:3 sin2 a7 cota

� �+4my

and

bf ¼ 1:1 þ mð1 þ 0:5 ln RÞ cota

where ab=0.28; bb=2.36; s is the 0.2% yieldstrength, psi; K is the true flow strength at unitstrain, psi; R is the extrusion ratio; n is the strain-hardening exponent; m is the coefficient of fric-tion; Z is the ratio of preform length to die borediameter at entry; a is the die half angle, degrees;and y is the ratio of length to diameter for theextrusion die land.

Nomograms and empirical equations havetraditionally been used to calculate extrusionpressure; more recently, however, finite-elementanalysis has provided another method for esti-mation of extrusion pressure especially forcomplex shapes. Figure 3 shows that extrusionpressure increases with extrusion ratio. It alsoshows that extrusion ratio has a larger effect onram pressure in the forward extrusion of carbonsteel than either carbon content or type ofannealing treatment. Figure 4 illustrates theeffect of tensile strength on extrudability in termsof ram pressure for both the backward and for-ward extrusion of low-carbon and medium-carbon steels of the 1000, 1100, and 1500 seriesat different extrusion ratios.

Steel for Cold Extrusion

Carbon steels up to 0.3% C can be easily coldextruded. Higher-carbon steels up to 0.5% C canalso be extruded, but the extrusion ratios arelimited and spheroidize annealing may berequired. Backward extrusion generally requiresspheroidize annealing for both low- and high-carbon steels. Alloy steels are harder thantheir carbon steel counterparts and hencerequire higher pressures for extrusion. They alsowork harden more rapidly, thus limiting their

ASM Handbook, Volume 14A: Metalworking: Bulk Forming S.L. Semiatin, editor, p405-418 DOI: 10.1361/asmhba0004005

Copyright © 2005 ASM International® All rights reserved. www.asminternational.org

extrudability and intermediate annealing is oftenrequired to restore extrudability. Alloying ele-ments differ in their effects on strength andhardenability. If possible, it is desirable tochoose alloying elements so as to minimizestrengthening while achieving the requiredhardenability: for example, boron increaseshardenability with minimal strengthening. Steelsin the AISI 4000, 4100, 5000, 5100, 8600, and8700 series can be cold extruded without diffi-culty up to 0.35% C. The AISI 4300, 4600, and4800 steels are more difficult to extrude and less

desirable for cold extrusion. Free-cutting resul-furized steels also have lower forgeability thantheir carbon steel counterparts, as they are moresusceptible to rupture during cold forging due totheir higher occurrence of sulfide inclusions.Sulfur is typically limited to 0.02%. Low-carbonresulfurized steels can be extruded if care istaken to keep metal in compression throughoutthe process. Internal purity of steel is critical incold extrusion especially at high extrusion ratios.Central segregations increase the tendency forinternal fracture along the axis of the extrusion

(chevrons). Killed steels are specified for coldforging to ensure homogeneous structure. Alu-minum-killed steel is preferred over silicon-kil-led steel for difficult extrusions due to thereduction in strain hardening and reduction instrain aging achieved. Silicon is kept in the lowrange of 0.2%. Silicon-killed steels, however,have better surface quality, which might be cri-tical in any postextrusion operations. Seams,laps, and scratches on steel surface can be tol-erated up to 1% of the bar diameter if coldextrusions are machined at the surface. However,if net-shaped features are being cold extruded,then the seams and laps have to be removed priorto forging by peeling or turning the steel bars.Although cold extrusion is a compressive pro-cess, it is typically preceded or followed byprocesses of cold heading and heat treatments,which require defect-free surfaces. The steelmanufacturer often certifies steels meeting thecold extrusion requirements as “cold extrusionquality” or “cold working quality.” To ensuresurface quality of steel hot-scarfing duringsemifinshed state (blooms) and eddy-currenttesting in the finished state (bars) is sometimesrequired.

Steel bars are available as normal hot rolled,precision hot rolled and cold finished. Normalhot-rolled bar is made to standard AISI toler-ances and is the least costly form of steel formaking slugs. It is also likely to have deepersurface seams and greater depth of decarburizedlayers. In addition, the variation in the outsidediameter of hot-rolled bars will cause consider-able variation in weight or volume of theslug, despite close control in cutting to length.Whether or not the surface seams and decarbur-ization can be tolerated depends largely on theseverity of extrusion and the quality require-ments of the extruded part. In many applications,acceptable extrusions can be produced with slugscut from hot-rolled bars. Precision hot-rolledbars have 50% better tolerances on size thannormal hot-rolled bars and smaller decarburiza-tion layer. These bars are made by performing aspecial precision-sizing operation during hotrolling.

Cold-finished bars are made by taking the hot-rolled bar through a costly series of cold-drawingsteps to give them tighter dimensional tolerances(25% of normal hot-rolled bar tolerances).Therefore, the size variation in cold-finished barsis considerably less than that in hot-finished bars.However, some seams and decarburization willalso be present in cold-finished bar stock unlessremoved by grinding, turning, or other means.Some users gain the advantage of cold-drawnbars by passing hot-rolled bars or rods through acold-drawing attachment directly ahead of theslug-cutting operation. Turning, peeling, orgrinding of cold-finished bars will eliminate thedifficulties caused by decarburization and seams.For some extrusions, especially those subjectedto surface treatments that cannot tolerate a dec-arburized layer, previously machined bars ormachined slugs must be used. Another practice isto turn and burnish normal hot-rolled bars to

(a) (b) (c)

(d) (e) (f)

Fig. 1 Displacement of metal in cold extrusion. (a) Forward extrusion. (b) Backward extrusion. (c) Combined backwardand forward extrusion. (d) Lateral extrusion. (e) Hooker extrusion. (f ) Ironing

406 / Cold Heading and Cold Extrusion

remove surface defects. These practices aremandatory for precision net spline/gear formingor products requiring induction hardening, whichcannot withstand a decarburized surface.

Equipment

Mechanical presses and hydraulic presses arespecifically designed for cold extrusion withhigh rigidity, accurate alignment, and longworking strokes. Mechanical presses are pre-ferred because of lower maintenance and higherproduction rates. Mechanical presses alsorequire higher investment and are preferred forlarge production volumes and large batch sizes.Horizontal mechanical presses with bar or coilfeeds with multiple stations and integrated billetshearing are used for small forgings. Thesepresses are capable of applying loads up to 1000metric tons (1100 tons) and producing up to 150parts/min. Vertical mechanical presses can be

single station or multistation and are typicallyused to make larger forgings with loads around1000 to 2000 metric tons (1100 to 2200 tons) at25 parts/min. For annual production volumes ofmore than 500,000, these vertical presses aretypically automated with loading and transfersystems. The drive mechanisms on mechanicalpresses also vary: crank, knuckle, link, andeccentric. Knuckle-joint presses offer lower andmore constant velocities during work stroke thancrank-drive presses, reducing dynamic loads.However, the working stroke and loads availableabove bottom dead center with knuckle drivesare lower than with a crank press. Link-drivepresses have similar forming velocities toknuckle-joint presses and have longer strokes.Eccentric presses fill the gap between knuckleand crank presses. It is important to analyze theforce-displacement curve of the press and of theprocess to ensure that sufficient deformationenergy is available during the cycle. In multi-station transfer presses, careful study of time-displacement curves of the press, transfer, part,and ejector is essential to ensure proper transfer.

Hydraulic presses are typically vertical, lesscomplex, more versatile, and have longer workstrokes than mechanical presses and are usuallyselected for long or large extrusions. They alsoprovide full-rated tonnage throughout the stroke.Hydraulic presses operate at lower speeds andare less suitable for automation than mechanicalpresses; consequently, they are typically used forlower production volumes.

Tooling

Tool design is critical in cold extrusion notonly for the success of the process, but also forthe safety of the operator. Although partdimension is the predominant factor in tooldesign the following also have to be considered:alignment, excess volume, friction (load, heat-ing, and wear), lubricant availability, balancedmetal flow, uniform metal flow velocity, ease ofassembly, stress concentration, load distribution,and elastic deflection. Cold extrusion tooling canbe separated into perishable and nonperishable.

The perishable toolings are typically in directcontact with metal flow and are highly stressed.These include punches, forming dies, guidesleeves, and mandrels. The nonperishable tool-ings are used to support the perishable toolingand are not directly exposed to the forgingpressure. These include shrink rings, backupplates, spacers, and retainer rings. Nonperishabletools are designed to be flexible and are usedacross different tooling setups, whereas perish-able tooling tends to be part specific. Figure 5shows a typical tool layout for backward extru-sion. The punch, the die, and the shrink rings arethe most critical components of cold extrusiontooling.Punch Design. The backward extrusion

punch (Fig. 6) is subjected to high pressuresapproaching 3000 MPa (450 ksi). Punches madetypically from AISI tool steels M2 and M4material heat treated to a hardness of 62 to 66HRC provide the required strength. Tungstencarbide is also used when high loads and stiffnessare required. At these high loads, it is alsoimportant to limit the effective length-to-diameter ratio of the extruded hole to around 3 to1 for rigidity. The punch nose contour controlsthe metering of the lubricant during the process.A hemispherical nosed punch, although desir-able from a load point of view, results in rapiddepletion of the lubricant. A tapered punch nosewith a 170� included angle is found to be optimalfor controlling the lubricant escape to avoidlubricant depletion before the end of the processand prevent punch-splitting failures. The edgeradius of the punch controls the material over-shoot as it negotiates the corner. This overshootaffects the extruded diameter and the tendency toform folds. Typically, a waterfall radius of 5% ofbearing land diameter is used. The bearing landof the punch should be minimal to reduce fric-tion, yet long enough to impart dimensionalcontrol to the extruded diameter. The commonpractice is to use 1.5 to 4.5 mm (1/16 to 3/16 in.)long land. The punch stem and shoulder shouldbe designed with gradual angles and radii todecrease stress concentration. Surface finishon the punch is also critical. Grinding marksshould be removed, and working surfaces shouldbe lapped in the direction of metal flow.

D0

h0

h0 /D0 = 0.500(slug)

0.19 SpheroidizedMill annealedHot rolled

0.26 SpheroidizedMill annealedHot rolled

0.34 SpheroidizedMill annealedHot rolledSpheroidizedMill annealedHot rolled

Extrusion ratio, R :

0.38

Carbon, % Condition

R = (D0/De)2(extruded part)

126°27°

De

R R = 2= 1.5

0 700

Ram pressure, MPa

1400 2100 2800

R = 4

Fig. 3 Effect of carbon content, annealing treatment,and extrusion ratio on maximum ram pressure in

the forward extrusion of the carbon steel part from thepreformed slug

Fig. 2 Parts made by (a) forward extrusion, (b) backward extrusion, and (c) lateral extrusion. Courtesy of Metaldyne Corporation

Cold Extrusion / 407

A 0.125 mm (5 min.) finish has been found to besatisfactory. In forward extrusion, the punchdesign is simpler because the stress seen is muchlower and the metal flow against the punch isminimal. Issues of stress concentration andstrength have to be considered, and the punchnose is not as critical as in backward extrusion. Itis typically flat with a bearing land of 3.25 mm(1/8 in.) and diameter that is very close to thebore of the die (0.025 to 0.125 mm, or 0.001 to

0.005 in., smaller) to prevent metal squirtbetween the punch and die. The closing-in of thedie bore when the die is assembled in shrink ringsshould be considered when designing this dia-meter.

Die Design. The high forming pressure incold extrusion leads to high hoop stress in thecylindrical dies. Rather than increase the amountof material in the die to resist these loads, shrink-fit or press-fit rings are used to induce favorablecompressive stresses in the die. Shrink-fitassemblies are made by heating the outer ringand allowing it to cool around the die. Morecommonly, the die insert is force fittedmechanically into the shrink ring, using taperedinterference surfaces and molybdenum disulfide

as a lubricant. The most commonly used taperangle is 1/2 to 1�. In general, no further advantageis gained by making the outside diameter ofshrink-ring more than four to five times the diediameter. The holding power of a shrink fit istypically greater than that of press fit becausegreater interference can be achieved. The exter-nal pressure from the shrink ring on the die bal-ances the internal pressure from extrusion on thedie. The design of the interferences and the dia-meters for the rings is based on Lame’s theory ofthick compound cylinders. The tool designermust calculate the hoop stresses on the inner diewall and provide adequate reinforcement withshrink rings. Ordinarily, pressures of less thanabout half the yield strength of the die do notrequire reinforcement, while those in excess ofthis value do require reinforcement. The reduc-tion of stress in the die due to shrink rings alsoprolongs the dies fatigue life. Often an inter-mediate shrink ring is added to the assembly for amore efficient design. Commercially availablemultiple-ring shrink rings utilizing strip windingare also used in severe forming applications.Outer shrink rings are made from 4340 or H13heat treated to 46 to 48 HRC. Care must be takento ensure that the yield strength of the outer ringis not exceeded during die assembly. Premiumgrades of H13 are typically used to provide thegreatest fatigue life and safe assembly. Figure 5shows a typical backward extrusion die in ashrink ring assembly. The die is designed so thatthe anvil absorbs the axial load, and the loadon the die is minimized. This is required to pre-vent cracking at the die corners. The length ofanvil is designed to allow for elastic deflectionunder extrusion load. The forward extrusion diedesign involves more factors (Fig. 7). The

0 20 40 60 80 100 120600

500

400

300

200

100 700

1400

2100

2800

3500

27°

42000 20 40 60 80 100 120

600

500

400

300

200

100

000 150

Tensile strength of steel being extruded, MPa

Tensile strength of steel being extruded, ksi

300 450 600 750 9000

900750600450300

Tensile strength of steel being extruded, MPa

Tensile strength of steel being extruded, ksi

1500

(a) (b)

0

700

1400

170°

R = D20 /(D2

0 − D 2p)

2100

2800

Ram

pre

ssur

e, M

Pa

Ram

pre

ssur

e, k

si

Ram

pre

ssur

e, M

Pa

Ram

pre

ssur

e, k

si

3500

h0

D0

R = 4R = 2

R = 1.5Dp

h0/D0 = 0.400

R = (D0/De)2

h0/D0 = 0.500

D0

h0

De

R = 4

R = 2

R = 1.5

4200

Fig. 4 Effect of tensile strength on ram pressure required for backward (a) and forward (b) extrusion of low- and medium-carbon steels at different extrusion ratios. Data are for AISI1000, 1100, and 1500 series steels containing 0.13 to 0.44% C.

Retainer

Punch

Punch guide

Shrink ring

Die

Anvil

Locator ring

Ejector

Fig. 5 Tools constituting a typical setup for the back-ward extrusion of steel parts

D

BC

A

Fig. 6 Backward extrusion punch. A, nose angle;B, punch land; C, corner radius; D, relief angle

408 / Cold Heading and Cold Extrusion

internal diameters are prescribed by the productrequirements. The design of the extrusion dieangle (Fig. 8) is dependent on the extrusion ratio.Typically, the extrusion die angle (half angle)varies from 5 to 30�. Standards published byBethlehem (Ref 1) and Chrysler (Ref 2) arecommonly used for designing the extrusion dieangle. When using higher angles, care has tobe taken to prevent chevrons during multipleextrusions. Chevrons or central bursts are inter-nal arrow-shaped defects occurring along theaxis of a forward extrusion (Fig 9). Typically,lower angles are preferred for higher reductions.The safe zone decreases with work hardening;therefore, extra care is required in designingmultiple forward extrusions. For high reductions(435%) a radius is preferred in place of theangle because it requires lower extrusion loads.The die land is typically 0.75 to 3.25 mm (1/32 to1/8 in.) long. For high reductions (435%), thebillet has to be fully contained within the dieand consequently a long lead die is used forthis purpose. An angle of 30 to 60� is preferredfor extruding hollow parts, the angle varying in-versely with wall thickness. Extrusion dies areusually made from M2 (60–62 HRC). Tungstencarbide is also used for high volume or criticalextrusions. Ejection pressure on the workincreases with decreasing die angle, because

greater friction must be overcome due to thegreater surface area. The ejection pressure alsoincreases with an increase in the length of thepart. Extrusion pressure also causes elasticexpansion of the die, which shrinks when thepressure is discontinued. Accordingly, very highwall pressures are developed, and these requirecorrespondingly high ejection pressures. Thishas to be considered when designing ejector pins.Ejector pins are typically made from S7 heattreated to 54 to 56 HRC, and M2 heat treated to60 to 62 HRC can be used in higher-pressureapplications.

Process Limits. Free (open-die) forwardextrusion of solid bars is limited to 30 to 35%reduction in area. However, if the billet is fullyenclosed in a die (closed die), the reduction inarea can be increased to 70 to 75%. Figure 1shows the difference between open- and closed-die forward extrusions. The ratio of length offorward extruded parts to diameter of the billet isusually limited to 8 to 1. In backward extrusion,the reduction ratio should be kept between aminimum of 20 to 25% and a maximum of 70 to75%. The maximum depth of extruded hole islimited to three times the hole diameter. In mostcases, the bottom thickness of the cup has to beequal to or greater than the wall thickness of thecup. In lateral extrusion, up to 60% reduction inarea is achievable. The width or the diameterof the extrudate has to be at least half thestarting blank diameter. Draw ironing is usuallylimited to 30%; however, if the tube is beingpushed instead of being drawn this can beincreased to 50%.

Preparation of Slugs

The preparation of billets/slugs often repre-sents a substantial fraction of the cost of produ-cing cold-extruded parts.

Billet Cutoff. Sawing and shearing are thetwo common methods for creating the billet. Theadvantages of sawing are dimensional accuracy,freedom from distortion, and minimal workhardening. The disadvantages are material lossas saw kerf and slower production rates. The useof circular saws instead of band saws and doublecuts per cycle has considerably improved theproduction rates. The cycle time and materiallosses increase with billet diameter. The qualityand roughness of the cut has an important effecton quality of the extrusion. Shearing is a chiplessprocess and is a more economical means ofproducing billets due to much higher productionrates. Variation in the sizes and shape of the billetis a major disadvantage of shearing. Extrusionprocess design has to allow for this variation. Ifprecise shape is required, then the billets have tobe coined to desired dimensions in a press. Inshearing, the ends of the billet are work hardenedand consequently their ductility is reduced.Another billet-cutting method is the adiabaticcutoff process involving high-speed/high-energyimpact processing. The cycle time with thisprocess is about a millisecond, so the productionrates can be very high. Production rates of sev-eral hundred parts per minute are possible, basedon the capabilities of the material-handling sys-tem. This process produces precision cutoffblank that is burr-free, with minimum pulldownand end distortion and is also capable of cuttingsteels in hardened condition.Surface Preparation of Steel Slugs. Pho-

sphate coating for cold extrusion is the commonpractice. The primary purposes of this coatingare, first, to form a nonmetallic separating layerbetween the tools and workpiece, and, second,by reaction with or absorption of the lubricant,to prevent its migration from bearing surfacesunder high unit pressures. During extrusion, thecoating flows with the metal as a tightly adherentlayer.

Punch

Shrink ring

Die

Ejector

Half die angle

R

RDie land

Relief

Fig. 7 Tooling for forward extrusion

2 α = 85° 2 α = 50°

Fig. 8 Measurement of die angle in dies for forwardextrusion

00

10

20

30

40

50

60

70

80

90

20 40 60 80 100

Included die angle, degrees

Red

uctio

n in

are

a, %

120 140 160 180

Safe zone

Maximum friction

Good lubrication(As per normalcommercial practice)

No friction

Centralbursting zone

Fig. 9 Chevrons in forward extrusion. Source: Ref 2

Cold Extrusion / 409

The recommended preparation of steel slugsfor extrusion consists of alkaline cleaning, waterrinsing, acid pickling, cold and hot water rinsing,phosphate coating, and rinsing. These methodsare discussed in this section.Alkaline cleaning is done to remove oil,

grease, and soil from previous operations so thatsubsequent pickling will be effective. Alkalinecleaning can be accomplished by spraying theslugs with a heated (65–70 �C, or 150–160 �F)solution for 1 to 2 min or by immersing them insolution at 90 to 100 �C (190 to 212 �F) for 5 to10 min.Water rinsing is done to remove residual

alkali and to prevent neutralization of the acidpickling solution. Slugs are usually rinsed byimmersion in overflowing hot water, but theymay also be sprayed with hot water.Acid Pickling. Most commercial installations

use a sulfuric acid solution (10% by volume) at60 to 90 �C (140 to 190 �F). Pickling can beaccomplished by spraying for 2 to 15 min or byimmersion for 5 to 30 min, depending on surfaceconditions (generally, the amount of scale).Three times are usually sufficient to remove allscale and to permit a good phosphate coating.Bright annealing or mechanical scale removal,such as shot blasting, as a substitute for picklinghas proved unsatisfactory for severe extrusion ifsignificant scale is present. However, the use ofa mechanical scale removing method prior topickling can reduce pickling time, and for pro-ducing extrusions of mild severity the mechan-ical (or bright annealing) methods have oftenbeen used without subsequent pickling or com-bined with cold pickling process.Cold and hot water rinsing can be carried out

by immersion or spraying for 1/2 to 1 min foreach rinse. Two rinses are used to ensure com-plete removal of residual pickling acid and ironsalts. Cold water rinsing is usually of shortduration, with heavy overflow of water toremove most of the residual acid. Hot water atabout 70 �C (160 �F) increases the temperatureof the workpiece and ensures complete rinsing.Phosphate coating is performed by immer-

sion in zinc phosphate at 70 to 80 �C (160 to180 �F) for 3 to 5 min. Additional information isavailable in the article “Phosphate Coating” inVolume 5 of ASM Handbook.Rinsing with cold water, applied by spraying

for 1/2 min or by immersion for 1 min, removesthe major portion of residual acids and acid saltsleft over from the phosphating solution. Thisrinse is followed by a neutralizing rinse appliedby spraying or immersion for 1/2 to 1 min using awell-buffered solution (such as sodium carbon-ate), which must be compatible with the lubri-cant. In the second rinse, the remaining residualacid and acid salts in the porous phosphatecoating are neutralized so that absorption of, orreaction with, the lubricant is complete.

Stainless steels are not amenable to conven-tional phosphate coating, oxalate coatings havebeen developed with reactive soaps; copperplating of stainless steel slugs is preferred. Limecoating is sometimes substituted successfully for

copper plating. In extreme cases, the stainlesssteel can be zinc plated and then coated with zincphosphate and a suitable soap lubricant. Methodsof surface preparation for nonferrous metals arediscussed in the sections “Cold Extrusion ofCopper and Copper Alloy Parts” and “ColdExtrusion of Aluminum Alloy Parts” in thisarticle.

Lubricants for Steel

A soap lubricant has traditionally providedthe best results for the extrusion of steel. Slugsare immersed in a dilute (45 to 125 mL/L, or6 to 16 oz/gal) soap solution at 65 to 90 �C(145 to 190 �F) for 3 to 5 min. Some soapsare formulated to react chemically with thezinc phosphate coating, resulting in a layer ofwater-insoluble metal soap (zinc stearate) onthe surfaces of the slugs. This coating has ahigh degree of lubricity and maintains a filmbetween the work metal and tools at the highpressures and temperatures developed duringextrusion.

Other soap lubricants, with or without filleradditives, can be used effectively for the mildextrusion of steel. This type of lubricant does notreact with the phosphate coating, but is absorbedby it.

Although the lubricant obtained by the reac-tion between soap and zinc phosphate is optimalfor extruding steel, its use demands precautions.If soap accumulates in the dies, the workpieceswill not completely fill out. Best practice is tovent all dies so that the soap can escape and keepa timed air blast into the dies to remove the soap.Polymer lubricants are gaining wider use for allbut the most severe applications where coatingbuildup in the dies is a concern.

When steel extrusions are produced directlyfrom coiled wire (similar to cold heading), theusual practice is to coat the coils with zincphosphate, using the procedure outlined in thesection “Preparation of Slugs” in this article.This practice, however, has one deficiency;because only the outside diameter of the workmetal is coated, the sheared ends are uncoated atthe time of extrusion. This deficiency is partlycompensated for by constantly flooding the workwith sulfochlorinated oil. Because the major axisof a heading machine is usually horizontal, thereis less danger of entrapping lubricant than whenextruding in a vertical press.

Nonphosphate Coatings. New lubricantsare replacing soap-phosphate treatments for thecold forging process. Soap-phosphate treat-ments, although very effective for extrusionprocess, are not conducive to continuous pro-cessing because of the long cycle time (30 min).Another disadvantage is the waste liquid treat-ment and disposal required for the solutions usedin the process. Oil- and water-based lubricantsare available that can be applied through a simpleprocess of tank-dip, air-blowing, and hot airdrying, which can be used in a continuous pro-duction line. The waste liquid treatment is

significantly reduced. The application of thesenew lubricants are gaining acceptance slowly.

Selection of Procedure

The shape of the part is usually the primaryfactor that determines the procedure used forextrusion. Typically, short cuplike parts areproduced by backward extrusion, while solidshaftlike parts and thin-walled hollow shafts areproduced by forward extrusion. Semihollowshapes and thick-walled hollow shafts are madewith both forward and backward extrusion.Other factors that influence procedure are thecomposition and condition of the steel, the pro-cess limits, the required dimensional accuracy,quantity, and cost. For difficult extrusions, it maybe necessary to incorporate several steps and oneor more intermediate annealing operations intothe process. Some shapes may not be completelyextrudable from difficult-to-extrude steels, andone or more machining operations may berequired.

Cold extrusion is ordinarily not consideredunless a large quantity of identical parts mustbe produced. The process is seldom used forfewer than 100 parts, and more often it is usedfor hundreds of thousands of parts or continuoushigh production. Quantity requirements deter-mine the degree of automation that can bejustified and often determine whether the partwill be completed by cold extrusion (assumingit can be if tooling is sufficiently elaborate)or whether, for low quantities, a combinationof extruding and machining will be more eco-nomical.

Cost per part extruded usually determines:

� The degree of automation that can be justified� Whether a combination of extruding and

machining should be used for low-quantityproduction

� Whether it is more economical to extrudeparts for which better-than-normal dimen-sional accuracy is specified or to attain therequired accuracy with secondary operations

It is sometimes possible to extrude a given shapeby two or more different procedures. Under theseconditions, cost is usually the deciding factor.Several procedures for extruding specific steelparts, categorized mainly by part shape, arediscussed in the following sections.

Cuplike Parts

The basic shape of a simple cup is oftenproduced by backward extrusion, although oneor more operations such as piercing or coiningare frequently included in the operationssequence. For cuplike parts that are more com-plex in shape, a combination of backward andforward extrusion is more often used. Example 1describes combined backward extrusion andcoining for the fabrication of 5120 steel valvetappets.

410 / Cold Heading and Cold Extrusion

Example 1: Backward Extrusion and Coin-ing for Producing Valve Tappets. The valvetappet shown in Fig. 10 was made from fine-grain, cold-heading quality 5120 steel. Slugswere prepared by sawing to a length of 25.9 to26.0 mm (1.020 to 1.025 in.) from bar stock 22.0to 22.1 mm (0.867 to 0.871 in.) in diameter.Slugs were tumbled to round the edges, thenphosphated and lubricated with soap.

The slugs were fed automatically into the twoloading stations of the eight-station dial, thenextruded, coined, and ejected. One part wasproduced in each set of four stations (two partsper stroke). This technique helped to keep theram balanced, thus avoiding tilting of the pressram, prolonging punch life, and reducingeccentricity between the outside and inside dia-meters of the extruded part. An eccentricity ofless than 0.25 mm (0.010 in.) total indicatorreading (TIR) was required. The cup could not beextruded to the finished shape in one hit, becausea punch of conelike shape would pierce ratherthan meter-out the phosphate coating. Therefore,two hits were used—the first to extrude and thesecond to coin. Punches are shown in Fig. 10(b)and (c). Axial pressure on the punch was about2205 MPa (320 ksi).

Tubular Parts

Backward and forward extrusion, drawing,piercing, and sometimes upsetting are oftencombined in a sequence of operations to producevarious tubular parts. Example 2 describes aprocedure for extruding a part having a longtubular section.Example 2: Producing Axle-Housing Spin-

dles in Five Operations. An axle-housingspindle was produced from a slug by backwardextruding, piercing, and three forward extrudingoperations, as shown in Fig. 11. The 10 kg(22.5 lb) slug was prepared by sawing and thenannealing in a protective atmosphere at 675 to730 �C (1250 to 1350 �F) for 2 h, followed byair cooling. The slug was then cleaned, phos-phate treated, and coated with soap. Afterbackward extruding and piercing, and again after

the first forward extruding operation, the work-piece was reannealed and recoated.

A 49 MN (5500 tonf) crank press operated at14 strokes/min was used. The punches weremade of D2 tool steel, and the die inserts of A2tool steel.

Stepped Shafts

Three methods are commonly used to coldform stepped shafts. If the head of the shaft isrelatively short (length little or no greater thanthe headed diameter), it can be produced byupsetting (heading). For a head more than about2.5 diameters long, however, upsetting in a sin-gle operation is not advisable; buckling willresult because of the excessive length-to-dia-meter ratio of the unsupported portion of theslug. Under these conditions, forward extrusionor multiple-operation upsetting should be con-sidered.

Forward extrusion can be done in a closed dieor an open die (Fig. 12). In a closed die, the slugis completely supported, and the cross-sectionalarea can be reduced by as much as 70%. Closed-die extrusion gives better dimensional accuracyand surface finish than the open-die technique.However, if the length-to-diameter ratio of theslug is more than about 4 to 1, friction alongthe walls of the die is so high that the closed-die method is not feasible, and an open die mustbe used. In an open die, reduction must be limitedto about 30 to 35%, or the unsupported portionof the slug will buckle. Stepped shafts can,however, be extruded in open dies usingseveral consecutive operations, as described inExample 3.

Example 3: Transmission Output ShaftForward Extruded in Four Passes in anOpen Die. A transmission output shaft wasforward extruded from a sheared slug in fourpasses through a four-station open die, asshown in Fig. 13. Extrusion took place in twodirections simultaneously. Transfer from stationto station was accomplished by a walking-beammechanism.

Air-actuated V-blocks (not shown in Fig. 13)were used to clamp the large diameter of the shaftto prevent buckling. A hydraulic cushion(Fig. 13) contacted the slug at the start of thestroke and remained in contact with the work-piece throughout the cycle. Therefore, extrusioninto the stationary tool holder took place first,ensuring that variation in finished length, causedby variation in stock diameter, was always in themovable tool holder. Each station of the die wasoccupied by a workpiece at all times; a finishedpiece was obtained with each stroke of thepress. The amount of area reduction was aboutthe same for each pass and totaled 65% for thefour passes.

The cold working caused a marked change inthe mechanical properties of the workpiece.Tensile strength increased from 585 to 945 MPa(85 to 137 ksi), yield strength increased from 365to 860 MPa (53 to 125 ksi), elongation decreasedfrom 26 to 7%, and reduction of area decreasedfrom 57 to 25%.

Extrusion Combined with Cold Heading

The combination of cold extrusion and coldheading is often the most economical means ofproducing hardware items and machinery partsthat require two or more diameters that arewidely different (see also the article “ColdHeading” in this Volume). Such parts are com-monly made in two or more passes in some typeof heading machine, although presses aresometimes used for relatively small parts.Presses are required for the heading and extrud-ing of larger parts.

Parts that have a large difference in cross-sectional area and weight distribution cannot be

0.8880.885diam0.69

0.69350.6930diam7

diam

0.28 0.3750.370diam

0.3200.316

spherical diam

(a) Completed workpiece

(b) Extruding punch

(c) Coining punch

30° 2.112.04 0.03 R

15°

R

R0.06

0.09 R

0.10

132

Fig. 10 5120 steel valve tappet (maximum hardness:143 HB) produced by extrusion and coining

with punches shown. Dimensions given in inches

5 516

3 12

5 38

2 716

4

Operation 2Pierced

Operation 1Backward extruded

12

diam

diam

diam

5 38

3 34

diam

diam

2

814

101116 119

16

3

diam

18diam

Operation 3Forwardextruded

Operation 4Forwardextruded

Operation 5Forwardextruded

5 38diam

4 34

diam

Slug

Fig. 11 1030 steel (hardness: 75–80 HRB) axle-hous-ing spindle produced by extruding and pier-

cing in five operations. Dimensions given in inches

Punch

Punch

WorkpieceDie Open die Ejector

Ejector

WorkpieceClosed die

Die

Fig. 12 End of stroke in the forward extrusion of astepped shaft in a closed die and an open die

Cold Extrusion / 411

formed economically from material equivalentin size to the smallest or largest diameter of thecompleted part. The most economical procedureconsists of selecting material of an intermediatesize, achieving a practical amount of reduction ofarea during forward extrusion, and forming thelarge sections of the part by heading. This prac-tice is demonstrated in Example 4.Example 4: Adjusting Screw Blank Pro-

duced by Forward Extrusion and SevereHeading in Three Operations. The blank for aknurled-head adjusting screw, shown in Fig. 14,was made from annealed and cold-drawn rod thatwas coated with lime and a soap lubricant at themill. In this condition, the rod was fed to aheading machine, in which it was first cut to sluglengths. The slugs were then lubricated withan oil or a water-soluble lubricant containingextreme-pressure additives. As shown in Fig. 14,the slug was extruded in one die, and the work-piece was then transferred to a second die, inwhich it was cold headed in two operations: thefirst for stock gathering and the second forcompleting the head (which represents severecold heading). Except for the extrusion die,which was made from carbide, all dies andpunches were made from M2 and D2 steelshardened to 60 to 62 HRC. Tool life for thecarbide components was 1 million pieces; for thetool steel components, 250,000 pieces. Produc-tion rate was 6000 pieces/h.

Extrusion of Hot Upset Preforms

Although the use of symmetrical slugs as thestarting material for extrusion is common prac-tice, other shapes are often used as the startingslugs or blanks. One or more machining opera-tions sometimes precede extrusion in order toproduce a shape that can be more easily extru-

ded. The use of hot upset forgings as the startingmaterial is also common practice. Hot upsettingfollowed by cold extrusion is often more eco-nomical than alternative procedures for produ-cing a specific shape. Axle shafts for cars andtrucks are regularly produced by this practice;the advantages include improved grain flow aswell as low cost. A typical application isdescribed in Example 5.

Example 5: Hot Forging and Cold Extrusionof Rear-Axle Drive Shafts. The fabrication ofrear-axle drive shafts (Fig. 15) for passengercars and trucks by three-operation cold extrusionimproved surfaces (and consequently fatigueresistance), maintained more uniform diametersand closer dimensional tolerances, increasedstrength and hardness, and simplified produc-tion. The drive shafts were hot upset forged toform the flange and to preform the shaft, andthey were cold extruded to lengthen the shaft.The flange could have been upset as a finaloperation after the shaft had been cold extrudedto length, but this would have required morepasses in the extrusion press than space allowed.Hot upsetting and cold extrusion replaced ahammer forging and machining sequence afterwhich the flange, a separate piece, had beenattached.

Steel was extrusion-quality 1039 in 42.9 mm(111/16 in.) diam bars. The bars were sheared tolengths of 757 to 929 mm (2913/16 to 369/16 in.),then hot forged and shot blasted. A continuousconveyor took the hot upset preforms through ahot alkaline spray cleaner, a hot spray rinse, azinc phosphating bath (75 �C, or 165 �F, for5 min), a cold spray rinse, a hot spray rinse, andfinally a soap tank (90 �C, or 190 �F, for 5 min).As shown in Fig. 15, cold extrusion was a three-operation process that increased the length of theshaft and reduced the smallest diameter to33.2 mm (1.308 in.).

Extrusion of Large Parts

Although most cold extrusion of steel is con-fined to relatively small parts (starting slugsseldom weigh more than 11.3 kg, or 25 lb),much larger parts have been successfully coldextruded. For press operations, the practicalextremes of part size are governed by the avail-ability of machinery and tool materials, theplasticity of the work material, and economicalproduction quantities. Bodies for large-caliberordnance shells have been successfully producedby both hot and cold extrusion processes. Theprocedure used in the production of theselarge parts by cold extrusion is described inExample 6.

18.51.94 diam

21.25

22

23.75

1.5041.4341.228diam 1.722 1.982 1.501

Third pass

Second pass

First pass

Sheared slug

Hydraulic cushion Extruding orifice

Ejector

21.25Station 1

Movable toolholder

Fourth pass1

1 3 3

78

334

4

16(a) (b)

1

Station 4

Stationary toolholder

2525

1.229

Fig. 13 4028 steel transmission shaft produced by four-pass forward extrusion in a four-station open die. (a) Shapesproduced in extrusion. (b) Two of the die stations. Dimensions given in inches

0.5302.75

1.34diam

0.3252.08

0.342

Ejector0.340 diam

Punch

DieOperation 1 Extruding

Ejector

Operation 2Heading (gathering)

Operation 3Heading

Completed workpiece

Slug

Fig. 14 1018 steel adjusting-screw blank formed byforward extruding and severe cold heading.

Dimensions given in inches

1.522

1.522

1.4121.308

1.412

1.522

1.522

1.687

Hot upsetpreform

Operation1

Operation2

Operation3

Fig. 15 1039 steel rear-axle drive shaft produced bycold extruding an upset forging in three

operations. Billet weight: 9.6 kg (21.2 lb). Dimensionsgiven in inches

412 / Cold Heading and Cold Extrusion

Example 6: Use of Extrusion in Multiple-Method Production of Shell Bodies. Figure 16shows the progression of shapes resulting fromextrusion, coining, and drawing in a multiple-method procedure for producing bodies for

155 mm shells from descaled 1012 steel billets190 mm (71/2 in.) in diameter that weighed36 kg (79.5 lb) each. The sequence of operationsis listed with Fig. 16. Production of these shellbodies was designed for semicontinuous opera-tion that included annealing, cleaning, andapplication of lubricant between press opera-tions.

Dimensional Accuracy

In cold extrusion, the shape and size of theworkpiece are determined by rigid tools thatchange dimensionally only from wear. Becausetool wear is generally low, successive parts madeby cold extrusion are nearly identical. Theaccuracy that can be achieved in cold extrusiondepends largely on the size and shape of thegiven section. Accuracy is also affected by toolmaterial, die compression, die set design, toolguidance, and press drive rigidity.

Tolerances for cold extrusion are commonlydenoted as close, medium, loose, and open.Definitions of these tolerances, as well asapplicability to specific types of extrusions, arediscussed in this section.

Close tolerance is generally considered to be+0.025 mm (+0.001 in.) or less. Close toler-ances are usually restricted to small (525 mm,or 1 in.) extruded diameters.

Medium tolerance denotes +0.13 mm(+0.005 in.). Extruded diameters of larger parts(up to 102 mm, or 4 in.), headed diameters ofsmall parts, and concentricity of outside andinside diameters in backward extruded parts aretypical of dimensions on which it is practical tomaintain medium tolerance.

Loose tolerance denotes +0.38 mm(+0.015 in.). This tolerance generally applies toshort lengths of extruded parts less than about89 mm (31/2 in.) long.

Open tolerance is generally considered tobe greater than +0.38 mm (+0.015 in.). This

tolerance applies to length dimensions of large,slender parts (up to 508 mm, or 20 in., andsometimes longer).Variation. With reasonable maintenance of

tools and equipment, the amount of variation of agiven dimension is usually small for a productionrun. Some drift can be expected as the tools wearand work metal properties vary from lot to lot.

Causes of Problems

The problems most commonly encountered incold extrusion are:

� Tool breakage� Galling or scoring of tools� Workpieces sticking to dies� Workpieces splitting on outside diameter or

cupping in inside diameter� Excessive buildup of lubricant in dies

Table 1 lists the most likely causes of theseproblems.

Cold Extrusion of AluminumAlloy Parts

Aluminum alloys are well adapted to cold(impact) extrusion. The lower-strength, moreductile alloys, such as 1100 and 3003, are theeasiest to extrude. When higher mechanicalproperties are required in the final product, heattreatable grades are used.

The cold extrusion process should be con-sidered for aluminum parts for the followingreasons. High production rates—up to 4000pieces/h—can be achieved. However, even whenparts are large or of complex shape, lower pro-duction rates may still be economical. Theimpact-extruded part itself has a desirablestructure. It is fully wrought, achieving max-imum strength and toughness. It is a near-net

5 diam

7

6

6

4

5 diam5 diam

15

7 5 6 diamdiamdiam

Forward extruded Coined

Sequence of operations

1. Cold saw the billet. 2. Chamfer sawed edges. 3. Apply lubricant as follows: Degrease in boiling caustic; rinse. Pickle in sulfuric acid; rinse. Apply zinc phosphate. Apply zinc stearate. 4. Cold size indent (see illustration above). 5. Induction normalize (925 to 980 °C, or 1700 to 1800 °F). 6. Apply lubricant as in step 3. 7. Backward extrude (see illustration). 8. Induction normalize (see step 5). 9. Apply lubricant as in step 3.10. Forward extrude in two stages to shape in illustration.11. Anneal lip by localized induction heating (815 to 830 °C, or 1500 to 1525 °F).12. Apply lubricant as in step 3.13. Coin base and form boat tail to finish dimension and coin bottom (see illustration).14. Final draw (see illustration).15. Turn and recess lip.16. Induction anneal nose (790 to 815 °C, or 1450 to 1500 °F).17. Apply lubricant as in step 3.18. Expand bourrelet in No. 6 press.19. Form nose.20. Anneal for relief of residual stress.

Drawn

1817

22 20

diamdiam

Billet IndentedBackward extruded

7

118

31

134

2

8

8

2

2

8 837

22 8

5111

1

34

diam12

7 diam12

Fig. 16 1012 steel 155 mm (6 in.) shell body pro-duced by a multiple-step procedure that

included cold extrusion. Billet weight: 36 kg (79.5 lb).Dimensions given in inches

Table 1 Problems in cold extrusion and some potential causes

Problem Potential cause

Tool breakage Slug not properly located in dieSlug material not completely annealedSlug not symmetrical or not properly shapedImproper selection or improper heat treatment of tool materialMisalignment and/or excessive deflection of tools and equipmentIncorrect preloading of diesDamage caused by double slugging or overweight slugs

Galling or scoring of tools Improper lubrication of slugsImproper surface finish of toolsImproper selection or improper heat treatment of tool materialImproper edge or bend radii on punch or extrusion die

Workpieces sticking to die No back relief on punch or dieIncorrect nose angle on punch and incorrect extrusion angle of dieGalled or scored tools

Workpieces splitting on outside diameter orforming chevron on inside diameter

Slug material not completely annealedReduction of area either too great or too smallExcessive surface seams or internal defects in work materialIncorrect die angles

Excessive buildup of lubricant on dies Inadequate vent holes in dieExcessive amount of lubricant usedLack of a means of removal of lubricant, or failure to prevent lubricant

buildup by spraying the die with an air-oil mist

Cold Extrusion / 413

shape. There is no parting line, and all that maybe required is a trim to tubular sections. Surfacefinish is good. Impacts have zero draft angles,and tolerances are tight. Once impacted, sectionscan be treated in the same manner as any otherpiece of wrought aluminum.

From a design standpoint, aluminum impactsshould be considered:

� For hollow parts with one end partially ortotally closed

� When multiple-part assemblies can bereplaced with a one-piece design

� When a pressure-tight container is required� When bottoms must be thicker than the walls

or the bottom design includes bosses, tubularextensions, projections, or recesses

� When a bottom flange is required� When bottoms, sidewalls, or heads have

changes in section thickness

Aluminum provides the characteristics of goodstrength-to-weight ratio, machinability, corro-sion resistance, attractive appearance, and highthermal and electrical conductivity. It is alsononmagnetic, nonsparking, and nontoxic.

Although nearly all aluminum alloys can becold extruded, the five alloys listed in Table 2 aremost commonly used. The alloys in Table 2are listed in the order of decreasing extrudabilitybased on pressure requirements. The easiestalloy to extrude (1100) has been assigned anarbitrary value of 1.0 in this comparison.Temper of Work Metal. The softer an alloy

is, the more easily it extrudes. Many extrusionsare produced directly from slugs purchased in theO (annealed, recrystallized) temper. In otherapplications, especially when slugs are machinedfrom bars, the slugs are annealed after machiningand before surface preparation. The raw materialis often purchased in the F (as-fabricated) temperto improve machinability, and the cut or punchedslugs are then annealed before extrusion.

When extruding alloys that will be heat trea-ted, such as 6061, common practice is to extrudethe slug in the O temper, solution treat the pre-form to the T4 temper, and then size or finishextrude. This procedure has two advantages.First, after solution treatment, the metal is rea-sonably soft and will permit sizing or additionalworking, and, second, the distortion caused bysolution treatment can be corrected in final siz-ing. After sizing, the part can be aged to the T6temper, if required.Size of Extrusions. Equipment is readily

available that can produce backward and forwardextrusions up to 406 mm (16 in.) in diameter.Backward extrusions can be up to 1.5 m (60 in.)long. The length of forward extrusions is limitedonly by the cross section of the part and thecapacity of the press. Irrigation tubing with a152 mm (6 in.) outside diameter and a 1.47 mm(0.058 in.) wall thickness has been produced inlengths up to 12.2 m (40 ft). Small-outside-diameter tubing (525 mm, or 1 in.) has beenproduced by cold extrusion in 4.3 m (14 ft)lengths.

Hydraulic extrusion and forging presses, sui-tably modified, are used for making very largeextrusions. Parts up to 840 mm (33 in.) in dia-meter have been produced by backward extru-sion from high-strength aluminum alloys in a125 MN (14,00 tonf) extrusion press. Similarextrusions up to 1 m (40 in.) in diameter havebeen produced in large forging presses.

Presses. Both mechanical and hydraulicpresses are used in the extrusion of aluminum.Presses for extruding aluminum alloys are notnecessarily different from those used for steel.There are, however, two considerations thatenter into the selection of a press for aluminum.First, because aluminum extrudes easily, theprocess is often applied to the forming of deepcuplike or tubular parts, and for this application,the press should have a long stroke. Again,because aluminum extrudes easily, the process isoften used for mass production, which requiresthat the press be capable of high speeds.

The press must have a stroke that is longenough to permit removal of the longest part tobe produced. Long shells are sometimes coldextruded in short-stroke knuckle-type presses, inwhich the punch is tilted forward or backward forremoval of the workpiece.

Because of their high speeds, mechanicalcrank presses are generally preferred for produ-cing parts requiring up to about 11 MN (1200tonf) of force. Production of as many as 70extrusions/min (4200/h) is not unusual, andhigher production rates are often obtained.Therefore, auxiliary press equipment is usuallydesigned for a high degree of automation whenaluminum is to be extruded.

Cold-heading machines are also used for thecold extrusion of aluminum parts. Hollow alu-minum rivets are formed and extruded in coldheaders in mass-production quantities. In gen-eral, the extruded parts are small and usuallyrequire an upsetting operation that can be doneeconomically in a cold header.

Tooling. Tools designed especially forextruding aluminum may be different from thoseused for steel, because aluminum extrudes moreeasily. For example, a punch used for the back-ward extrusion of steel should not have a length-to-diameter ratio greater than about 3 to 1;however, this ratio, under favorable conditions,can be as high as 17 to 1 for aluminum (althougha 10-to-1 ratio is usually the practical max-imum).Dies. Three basic types of dies for extruding

aluminum are shown in Fig. 17. Solid dies are

usually the most economical to make. Generally,a cavity is provided in each end so that the die canbe reversed when one end becomes cracked orworn.

Holder-and-sleeve dies are used when extru-sion pressures are extremely high. This type ofdie consists of a shrink ring or rings (the holder),a sleeve, and an insert (button). The die sleeve isprestressed in compression in the shrink ring tomatch the tension stress expected during extru-sion.

Horizontal split dies are composed of as manyas four parts: a shrink ring, a sleeve (insert), and aone-piece or two-piece base. Figure 17 identifiesthe one-piece base as a die bottom, and thecomponents of the two-piece base as a holder anda backer.

Compared to the die cavities used in thebackward extrusion of steel, the die cavities foraluminum are notably shallow, reflecting a majordifference in the extrusion characteristics of thetwo metals. Steel is more difficult to extrude,requiring higher pressures and continuous diesupport of the workpiece throughout the extru-sion cycle. In contrast, aluminum extrudesreadily, and when the punch strikes the slug inbackward extrusion, the metal squirts up thesides of the punch, following the punch contourswithout the external restraint or support affordedby a surrounding die cavity.Punches. Typical punches for forward and

backward extrusion are shown in Fig. 18. In the

Table 2 Relative pressure requirements forthe cold extrusion of annealed slugs of fivealuminum alloys (alloy 1100=1.0)

Alloy Relative extrusion pressure

1100 1.03003 1.26061 1.62014 1.87075 2.3

Sleeve

Die Backward extrusion Forward extrusionHolder-and-sleeve diesSolid die

Ring

Die bottom

Sleeve Sleeve Button

Holder Button Extrusion lip

HolderHorizontal split dies

Backer

Fig. 17 Three types of dies used in the cold extrusionof aluminum alloy parts

Extrusiontip

0.002 to 0.015clearance

Mandrel

Forward extrusionBackward extrusion

Fig. 18 Typical punches for backward and forwardextrusion of aluminum alloy parts. Clearance

given in inches

414 / Cold Heading and Cold Extrusion

backward extrusion of deep cuplike parts, spe-cially designed punches must be used to facilitatestripping.Tool Materials. Typical tool materials and

their working hardnesses for the extrusion ofaluminum are given in Table 3.Stock for Slugs. Slugs for extrusions are

obtained by blanking from plate; by sawing,shearing, or machining from bars; or by casting.In general, the methods for preparing aluminumslugs are similar to those for preparing slugsfrom other metals and are therefore subject to thesame advantages and limitations (see the section“Preparation of Slugs” in this article).

Rolled aluminum alloy plate is widely used asa source of cold extrusion stock. The high speedat which slugs can be prepared is the majoradvantage of blanking from rolled plate. Whenslug thickness is greater than about 50 mm(2 in.) or when the thickness-to-diameter ratio isgreater than about 1 to 1, blanking from plateis uneconomical, if not impossible. Blankingis also excessively wasteful of metal, whichnegates a principal advantage of the cold extru-sion process.

Sawing from bars is widely used as a methodof obtaining slugs. More accurate slugs are pro-duced by sawing than by blanking; however, asin blanking, a considerable amount of metal islost.

When “doughnut” slugs are required, they canbe sawed from tubing, or they can be punched,drilled, or extruded. Machined slugs (such asthose produced in an automatic bar machine) aregenerally more accurate but cost more than thoseproduced by other methods.

Cast slugs can also be used; the selection of acast slug is made on the basis of adequate qualityat lower fabricating cost. Compositions that arenot readily available in plate or bar stock cansometimes be successfully cast and extruded.There is often a savings in metal when a preformcan be cast to shape.Tolerance on slug volume may vary from

+2% to +10%, depending on design and eco-nomic considerations. When extrusions aretrimmed, as most are, slug tolerance in the upperpart of the above range can be tolerated. Whenextrusions are not trimmed and dimensions arecritical, the volume tolerance of the slugs mustbe held close to the bottom of the range. In thehigh-quantity production of parts such as thin-wall containers, the degree to which slug volumemust be controlled is often dictated by metalcost.Surface Preparation. Slugs of the more

extrudable aluminum alloys, such as 1100 and3003, are often given no surface preparationbefore a lubricant is applied prior to extrusion.For slugs of the less extrudable aluminum alloysor for maximum extrusion severity or both, sur-face preparation may be necessary for retentionof lubricant. One method is to etch the slugs in aheated caustic solution, followed by water rin-sing, nitric acid desmutting, and a final rinse inwater. For the most severe extrusion, slug sur-faces are given a phosphate coating before the

lubricant is applied. Additional information onthe alkaline etching, acid desmutting, and phos-phate coating of aluminum alloys is available inthe article “Cleaning and Finishing of Aluminumand Aluminum Alloys” in Volume 5 of ASMHandbook.Lubricants. Aluminum and aluminum alloys

can be successfully extruded with such lubri-cants as high-viscosity oil, grease, wax, tallow,and sodium-tallow soap. Zinc stearate, appliedby dry tumbling, is an excellent lubricant forextruding aluminum. In applications in which itis desirable to remove the lubricant, water-solu-ble lubricants are used to reduce the wash cycle.

The lubricant should be applied to metal sur-faces that are free from foreign oil, grease, anddirt. Preliminary etching of the surfaces (seeabove) increases the effectiveness of the lubri-cant. For the most difficult aluminum extrusions(less extrudable alloys or greater severity orboth), the slugs should be given a phosphatetreatment, followed by application of a soap thatreacts with the surface to form a lubricating layersimilar to that formed when extruding steel.

Impact parts range from simple cuplike partssuch as compressed air filter bowls, switchhousings, and brake pistons to such complexparts as aerosol cylinders and ribbed cans, elec-trical fittings, motor housings, and home appli-ance parts. Numerous examples and designcriteria are given in Ref 3.Shallow cuplike parts can be easily extruded

from most of the wrought aluminum alloys. If thewall thickness is uniform and the bottom isnearly flat, shallow cups can be produced in onehit (blow) at high production rates; if the shape ismore complex, two or more hits may be needed.In the following example two hits were used toproduce a part with an internal boss.

Example 7: Use of a Preform for Producinga Complex Bottom. The aluminum alloy 1100-O housing shown in Fig. 19 required two extru-

sion operations on a hydraulic 3 MN (350 tonf)press because of the internal boss, which wasformed by backward extrusion in a secondoperation, as shown in Fig. 19. The blendedangle in the preform functioned as a support forthe finishing punch during extrusion of theinternal boss. This counteracted the side pressurethat was created as the metal flowed into thecavity of the finishing punch.

The slug was sawed from bar and annealed;zinc stearate lubricant was used. The productionrate was 350 pieces/h for the preforming opera-tion and 250 pieces/h for finish forming. Mini-mum tool life was 100,000 pieces.Deep Cuplike Parts. Although cups having a

length as great as 17 times the diameter havebeen produced, this extreme condition is seldomfound in practice, because a punch this slender islikely to deflect and cause nonuniform wallthickness in the backward-extruded product. Thelength of the cup and the number of operations(use of preform) are not necessarily related.Whether or not a preform is required dependsmainly on the finished shape, particularly of theclosed end. When forming deep cups from heattreatable alloys such as 6061, if the amount ofreduction is 25% or more in the preform, theworkpiece should be reannealed and relubricatedbetween preforming and finish extruding.Parts with Complex Shapes. Producing extru-

sions from aluminum and aluminum alloys in asingle hit is not necessarily confined to simpleshapes. The extrusion described in Example 8was produced in a single hit despite its relativelycomplex shape. For extrusions with longitudinalflutes, stems, or grooves, the use of one of themost extrudable alloys, such as 1100, is helpfulin minimizing difficulties. Sometimes, however,a less extrudable alloy can be used to form acomplex shape in one hit.

Table 3 Typical tool steels used in extrudingaluminum

Tool AISI steel Hardness, HRC

Die, solid W1 65–67Die sleeve(a) D2 60–62

L6 56–62H13 48–52

Die button(b) H11 48–50H13 48–50L6 50–52H21 47–50T1 58–60

Ejector D2 55–57S1 52–54

Punch S1 54–56D2 58–60H13 50–52

Stripper L6 56–58Mandrel, forward S1 52–54

H13 50–52Holder H11 42–48

H13 42–484130 36–444140 36–44

(a) Cemented carbide is sometimes used for die sleeves. (b) Maragingsteel is sometimes used for die buttons.

2.9952.990 5.750

3.000diam

0.070

2.600diam

Completed workpiece

Preform

Workpiece

Punch

Die

Ejector

Preforming Finish forming

Tolerance onfinish dimensions, ±0.010 in.

5.250

Slug 1.310

Fig. 19 Aluminum alloy 1100-O housing that wasextruded in two operations because of an

internal boss. Dimensions given in inches

Cold Extrusion / 415

The successful extrusion of complex shapes,especially in a single hit, depends greatly on tooldesign and slug design. Some developmentalwork is usually required for each new job beforeit can be put into production.Example 8: Maximum Extrudability for a

Complex Shape. The hydraulic cylinder bodyshown in Fig. 20 was extruded from a solid slugin one hit. Aluminum alloy 1100, which hasmaximum extrudability, was required for thispart because of the abrupt changes in section ofthe cylinder body. Surface cracks and lapsresulted when more difficult-to-extrude alloyswere used. The different wall thicknesses andsteps in this design represent near-maximumseverity for extruding in one hit, even with themost extrudable alloy. During the developmentof this part, it was necessary to change the faceangles, shorten the steps, and blend the outsideribs more gradually to ensure complete fillout.The part was produced on a 7 MN (800 tonf)mechanical press set at 4.4 MN (500 tonf). Theslug was sawed from bar, annealed, and lubri-cated with zinc stearate. Production rate in asingle-station die was 300 pieces/h, and mini-mum tool life was 70,000 pieces.Dimensional Accuracy. In general, alumi-

num extrusions are manufactured to close toler-ances. The closeness depends on size, shape,alloy, wall thickness, type and quality of tooling,and press equipment. Lubrication and slug fit inthe die are also important.

Wall thickness tolerances range from +0.025to +0.13 mm (+0.001 to +0.005 in.) forrelatively thin-wall cylindrical shapes of mod-erate size extruded from low-strength alloys, butmay be as great as +0.25 to +0.38 mm(+0.010 to +0.015 in.) for large parts of high-strength alloys. Wall-thickness tolerances forrectangular shells range from +0.13 to+0.38 mm (+0.005 to +0.015 in.), dependingon size, alloy, and nominal wall thickness.Diameter tolerances typically range from+0.025 mm (+0.001 in.) for small parts to+0.25 to +0.38 mm (+0.010 to +0.015 in.)for large high-strength alloy parts. Closer controlof diameter can be achieved on small heavy-wallparts by centerless grinding of the extrusions(provided the alloy is one that can be groundsatisfactorily). Dimensional tolerances in the

forged portion of the impact are influenced bythe same variables as those listed above, buta range of +0.13 to +0.38 mm (+0.005 to+0.015 in.) is typical. Variations in extrudedlength usually necessitate a separate trimmingoperation.

Surface finish typically ranges from 0.5 to1.8 mm (20 to 70 min.). Smoother surfaces cansometimes be obtained by using extreme care insurface preparation and lubrication of the workmetal and by paying close attention to the surfacecondition of the tools.

Cold Extrusion of Copper andCopper Alloy Parts

Oxygen-free copper (Copper DevelopmentAssociation alloy C10200) is the most extrud-able of the coppers and copper-base alloys. Othergrades of copper and most of the copper-basealloys can be cold extruded, although there arewide differences in extrudability among the dif-ferent compositions. For example, the hardercopper alloys, such as aluminum-silicon bronzeand nickel silver, are far more difficult to extrudethan the softer, more ductile alloys, such as car-tridge brass (alloy C26000), which can satisfac-torily withstand cold reduction of up to 90%between anneals.

Alloys containing as much as 1.25% Pb can besuccessfully extruded if the amount of upset ismild and the workpiece is in compression at alltimes during metal flow. Copper alloys contain-ing more than 1.25% Pb are likely to fracturewhen cold extruded.

The pressure required for extruding a givenarea for one of the more extrudable coppers orcopper alloys (such as C10200 or C26000) is lessthan that required for extruding low-carbon steel.However, the pressure required for extrudingcopper alloys is generally two to three times thatrequired for extruding aluminum alloys(depending on the copper or aluminum alloybeing compared).

The length of a backward-extruded section islimited by the length-to-diameter ratio of thepunch and varies with unit pressure. This ratioshould be a maximum of 5 to 1 for copper. A ratioof 10 to 1 is common for the extrusion of alu-minum, and ratios as high as 17 to 1 have beenused. The total reduction of area for copper orcopper alloys, under the best conditions, shouldnot exceed 93%.

Equipment and Tooling. Copper and copperalloys can be extruded in hydraulic or mechan-ical presses or in cold-heading machines. Tool-ing procedures and tool materials for theextrusion of copper alloys are essentially thesame as those for extruding steel.

Preparation of Slugs. Sawing, shearing, andmachining are the methods used to preparecopper and copper alloy slugs. Each method hasadvantages and limitations. Sawing or shearingis generally used to produce solid slugs.Machining (as in a lathe) or cold forming in

auxiliary equipment is seldom used unless a holein the slug, or some other modification, isrequired.Surface Preparation. In applications invol-

ving minimum-to-moderate severity, copperslugs are often extruded with no special surfacepreparation before the lubricant is applied.However, for the extrusion of harder alloys(aluminum bronze, for example) or for max-imum severity or both, best practice includes thefollowing surface preparation before the lubri-cant is applied:

1. Cleaning in an alkaline cleaner to remove oil,grease, and soil

2. Rinsing in water3. Pickling in 10 vol% sulfuric acid at 20 to

65 �C (70 to 150 �F) to remove metal oxides4. Rinsing in cold water5. Rinsing in a well-buffered solution, such as

carbonate or borate, to neutralize residualacid or acid salts

Lubrication. Zinc stearate is an excellentlubricant for extruding copper alloys. Commonpractice is to etch the slugs as described aboveand then to coat them by dry tumbling in zincstearate.Examples of Practice. The following exam-

ples describe typical production practice forextruding parts from copper and brass. The partdescribed in Example 9 could have been made byforging, casting, or machining; however, coldextrusion produced more accurate dimensionsthan forging or casting, consumed less materialthan machining, and was the lowest-cost method.Example 9: Shearing, Heading, Piercing,

Extruding, and Upsetting in a Header. Theplumbing fitting shown in Fig. 21 was made ofelectrolytic tough pitch copper (alloy C11000)rod cold drawn (about 15% reduction of area) toa diameter of 26.9 mm (1.06 in.). The pipe-taperdiameter and the 22.2 mm (0.875 in.) diameterof the tube socket were critical, being specifiedwithin 0.064 mm (0.0025 in.).

Manufacture of the fitting consisted of feedingthe rod stock into the cold-heading machine,which cut the stock into slugs 20.3 mm (0.80 in.)long and transferred the slugs progressively todies for heading, backward extruding, piercing,forward extruding, and upsetting (Fig. 21). Onlytrimming on each end and tapping were requiredfor completion. The extrusion equipment con-sisted of a five-die cold-heading machine.

The final cross-sectional area of the thin endafter extrusion was 16.4% of the 30.7 mm(1.21 in.) diam headed preform from which thefitting was made. A reduction of this magnitudecould have been made in one operation if acylindrical rod were being extruded from thepreform. The shape, however, was not suitablefor production in one operation. Therefore, thefitting was made by backward and forwardextrusion and mild upsetting. Production rate at100% efficiency was 3600 pieces/h, and mini-mum life of the D2 tool steel dies was 200,000pieces.

3.000 diam 2.75 diam10.00

6.384.380diam

3.380Slug

Total tolerances:OD, ±0.005 in.; ID, ±0.005 in. Completed workpiece

3.500diam

0.87

Fig. 20 Aluminum alloy 1100-O hydraulic cylinderbody extruded in one hit. The complexity of

this part is close to the maximum producible for one-hitextrusion of alloy 1100-O. Dimensions given in inches

416 / Cold Heading and Cold Extrusion

Difficult Extrusions. The part described inthe following example represents a difficultextrusion for two reasons. First, the metal (tell-urium copper, alloy C14500) is one of the moredifficult-to-extrude copper alloys, and second,the configuration (12 internal flutes and 12external ribs) is difficult to extrude regardless ofthe metal used.Example 10: Extrusion Versus Brazed

Assembly for Lower Cost. The rotor shown inFig. 22 was originally produced by brazing amachined section into a drawn ribbed and flutedtubular section. By an improved method, thisrotor was extruded from a sawed, annealed slugin one hit in a 1.7 MN (190 tonf) mechanicalpress. A lanolin-zinc stearate-trichloroethylenelubricant was used to produce 1800 pieces/h. The

extruded rotor was produced at less cost and hadbetter dimensional accuracy than the brazedassembly, and there were fewer rejects. Mini-mum tool life was 50,000 pieces.

Impact Extrusion ofMagnesium Alloys

Impact extrusion is used to produce symme-trical tubular magnesium alloy workpieces,especially those with thin walls or irregularprofiles for which other methods are not prac-tical. As applied to magnesium alloys, theextrusion process cannot be referred to as coldbecause both blanks and tooling must be pre-heated to not less than 175 �C (350 �F); work-piece temperatures of 260 �C (500 �F) arecommon.

Length-to-diameter ratios for magnesiumextrusions may be as high as 15 to 1. There is nolower limit, but parts with ratios of less thanabout 2 to 1 can usually be press drawn at lowercost. A typical ratio is 8 to 1, and parts withhigher length-to-diameter ratios are moreamenable to forward extrusion than to backwardextrusion. At all ratios, the mechanical propertiesof magnesium extrusions normally exceed thoseof the blanks from which they are made, becauseof the beneficial effects of mechanical working.

Equipment and Tooling. Mechanicalpresses are faster than hydraulic presses and aretherefore used more often for impact extrusion,except when long strokes are needed. Presseswith a capacity of 900 kN (100 tonf) and a strokeof 152 mm (6 in.) are adequate for most extru-sion applications. Up to 100 extrusions/min havebeen produced. Extrusion rate is limited only bypress speed.

Dies for the impact extrusion of magnesiumalloys differ from those used for other metals,because magnesium alloys are extruded at ele-vated temperature (usually 260 �C, or 500 �F).Common practice is to heat the die with tubularelectric heaters. The die is insulated from thepress, and an insulating shroud is built around thedie. The top of the die is also covered, except forpunch entry and the feeding and ejection devices.The punch is not heated, but it becomes hotduring continuous operation; therefore, thepunch should be insulated from the ram.

Punches and dies are usually made of a hot-work tool steel, such as H12 or H13, heat treated

to 48 to 52 HRC. In one application, tools madeof heat treated H13 produced 200,000 extrusions.Carbide dies can be used and can extrude up to10 million pieces.

The sidewalls of the die cavity shouldhave a draft of approximately 0.002 mm/mm(0.002 in./in.) of depth, which prevents theextrusion from sticking in the cavity. In normaloperation, the part stays on the punch and isstripped from it on the upward stroke.The procedure for the preparation for

extrusion and extrusion practice is outlined in thefollowing paragraphs.Preparation of Slugs. Magnesium alloy slugs

are prepared by the same methods as othermetals—sawing from bar stock or blanking fromplate, if rough edges can be tolerated. Slugs canalso be made by casting. Slugs must be uniformin size and shape for centering in the die in orderto ensure uniform wall thickness on the extru-sion, which in turn depends on the clearancebetween die and punch. Slugs are lubricated bytumbling in a graphite suspension for 10 minuntil a dry coat develops.

For automatic impact extrusion of magnesiumparts, the lubricated slugs are loaded into ahopper feed. The slugs are heated by an electricheater as they pass along the track between thehopper and the die.Extrusion Practice. The heated slug is loaded

onto the heated die, and the press is activated toproduce the extrusion. Operating temperaturesfor the extrusion of magnesium alloys rangefrom 175 to 370 �C (350 to 700 �F), dependingon composition and operating speed. The oper-ating temperature should be held constant inorder to maintain tolerances.

In practice, slugs and dies are usually heated to260 �C (500 �F) for feeding by tongs, becausethe rate of operation is slow. In automatic feed-ing, the slug and die temperature can be as low as175 �C (350 �F), because speed is greater; diesabsorb heat during operation and can increase intemperature by as much as 65 �C (150 �F).When a decrease in properties is not important,operating temperatures can be higher.Extrusion pressures for the impact extru-

sion of magnesium alloys are about half thoserequired for aluminum and depend mainlyon alloy composition, amount of reduction,and operating temperature. Table 4 showsthe pressures required to extrude several mag-nesium alloys to a reduction of area of 85% at

1.21

1.06diam

0.80

Sheared Headed Backward extruded

1.251.00

0.8750

1.50

UpsetForward extrudedPierced

Taperedfor

tapping

0.98

Fig. 21 Copper alloy C11000 plumbing fitting pro-duced by the operations shown, including

cold forward extrusion. Dimensions given in inches

1.2551.245

0.43

Slug

0.093

4.25 2.81

A

A

Completedworkpiece

Total tolerances:major diameter, 0.010 in.,minor diameter, 0.004 in.

1.250

View A-A

Fig. 22 Copper alloy C14500 rotor produced bycombined backward and forward extrusion.

Dimensions given in inches

Table 4 Pressures required for the impact extrusion of four magnesium alloys at varioustemperaturesTestpieces were extruded to a reduction in area of 85%.

Extrusion pressure at temperature

230 �C (450 �F) 260 �C (500 �F) 290 �C (550 �F) 315 �C (600 �F) 345 �C (650 �F) 370 �C (700 �F) 400 �C (750 �F)

Alloy MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi

AZ31B 455 66 455 66 414 60 372 54 359 52 345 50 317 46AZ61A 483 70 469 68 455 66 441 64 428 62 414 60 400 58AZ80A 496 72 483 70 441 68 455 66 441 64 428 62 414 60ZK60A 469 68 455 66 441 64 428 62 400 58 372 54 359 52

Cold Extrusion / 417

temperatures ranging from 230 to 400 �C (450 to750 �F).Thermal Expansion. Magnesium has a rela-

tively high coefficient of thermal expansioncompared to steel. Therefore, in order to ensurethat the magnesium extrusion, when cooled toroom temperature, will be within dimensionaltolerance, it is necessary to multiply the room-temperature dimensions of steel tools by acompensatory factor for the temperature atwhich the magnesium alloy is to be extruded.The tolerances for magnesium alloy extru-

sions are influenced by the size and shape of thepart, the length-to-diameter ratio, and the pressalignment. Table 5 gives typical tolerances for amagnesium part with a length-to-diameter ratioof 6 to 1.

Cold Extrusion of Nickel Alloys

Cold heading and cold extrusion are mostoften used in the production of fasteners andsimilar cold upset parts. For nickel alloys, coldextrusion is rarely done except in conjunctionwith cold heading (see the article “Cold Head-ing” in this Volume).

The high-strength and galling characteristicsof nickel alloys require slow operating speedsand high-alloy die materials. Because of the highstrength and work-hardening rates of the nickelalloys, the power required for cold forming maybe 30 to 50% higher than that required for mild

steels. Tools should be made of oil-hardening orair-hardening die steel. The air-hardening types,such as AISI D2, D4, or high-speed steel (M2 orT1), tempered to 60 to 63 HRC, are preferred.

To prevent galling, high-grade lubricants mustbe used in cold heading of nickel alloys.

Lime and soap are usually used as a basecoating on alloy 400. Better finish and die lifecan be obtained by using copper plating 7.5 to18 mm (0.3 to 0.7 mils) thick as a lubricant car-rier.

Copper plating may also be used on thechromium-containing alloys 600 and 800, butoxalate coatings serve as an adequate substitute.

Regardless of the type of carrier, a baselubricant is best applied by drawing it on in alight sizing pass to obtain a dry film of thelubricant. Any of the dry soap powders of thesodium, calcium, or aluminum stearate types canbe applied this way.

If the wire rod is to be given a sizing or tem-pering pass before the cold-heading operation,the heading lubricant should be applied duringdrawing.

ACKNOWLEDGMENT

This article has been adapted from P.S.Raghupathi, W.C. Setzer, and M. Baxi, ColdExtrusion, Forming and Forging, Volume 14 ofASM Handbook, 1988, pages 299 to 312.

REFERENCES

1. “Forging, Forming and Extrusion ProcessRequirements,” Engineering Standard,Chrysler Corporation, 1972

2. “Preventing of Central Bursting in ColdExtrusion,” Technical Report, BethlehemSteel, 1970

3. “Aluminum Impacts Design Manual andApplication Guide,” Aluminum Associa-tion, 1979

SELECTED REFERENCES

General

� T. Altan, S. Oh, and H. Gegel, Metal FormingFundamentals and Application, AmericanSociety for Metals, 1983

� B. Avitzur, Conventional Extrusion: Directand Indirect and Impact Extrusion, Handbookof Metal Forming Processes, Wiley-Inter-science, 1983

� J.L. Everhart, Impact and Cold Extrusion ofMetals, Chemical Publishing, 1964

� H.D. Feldmann, Cold Forging of Steel,Hutchinson Scientific and Technical, 1961

� “Impact Machining,” Verson Allsteel PressCo., 1969

� K. Lange, Ed., Fundamentals of Extrusion andDrawing and Cold and Warm Extrusion,Handbook of Metal Forming, McGraw-Hill,1985

� K. Lange, Handbook of Metal Forming,McGraw-Hill, 1985

� K. Laue and H. Stenger, Extrusion: Processes,Machinery, Tooling, American Society forMetals, 1981

� Metal Forming Handbook, Schuler PressenGmbH and Springer-Verlag, 1998

� Z. Zimmerman, and B. Avitzur, Analysis ofEffect of Strain Hardening on Central Burst-ing Defects in Drawing and Extrusion, Trans.ASME B, Vol 92 (No. 1), 1970

Aluminum Alloys

� F.L. Church, Impacts: Light, Tough, Precise;Reduce Machining; Replace Assemblies,Mod. Met., Vol 37 (No. 2), March 1981, p 18–20, 22, 24

� P.J.M. Dwell, Impact Extrusion of Aluminumand Its Alloys, Alum. Ind., Vol 2 (No. 4), Sept1983, p 4, 6–7

� Encyclopedia of Materials Science and Engi-neering, Vol 1, Pergamon Press, 1986, p 704–707

� P.K. Saha, Aluminum Extrusion Technology,ASM International, 2000

Table 5 Typical tolerances for a magnesiumalloy extrusion with a length-to-diameterratio of 6 to 1

Dimension Tolerance, mm (in.)

Diameter +0.05 (+0.002)(a)Bottom thickness +0.13 (+0.005)(b)Wall thickness, mm (in.)0.5–0.75 (0.020–0.029) +0.05 (+0.002)0.76–1.13 (0.030–0.044) +0.076 (+0.003)1.14–1.50 (0.045–0.059) +0.10 (+0.004)1.51–2.54 (0.060–0.100) +0.13 (+0.005)

(a) Per 25 mm (1 in.) of diameter. (b) All thicknesses

418 / Cold Heading and Cold Extrusion