Forging of Aluminum Alloys - NIST

Forging of Aluminum AlloysG.W. Kuhlman, Metalworking Consultant Group LLC

ALUMINUM ALLOYS are forged into avariety of shapes and types of forgings with abroad range of final part forging design criteriabased on the intended application. Aluminumalloy forgings, particularly closed-die forgings,are usually produced to more highly refined finalforging configurations than hot-forged carbonand/or alloy steels, reflecting differences in thehigh-temperature oxidation behavior of alumi-num alloys during forging, the forging engi-neering approaches used for aluminum, and thehigher material costs associated with aluminumalloys in comparison with carbon steels. For agiven aluminum alloy forging shape, the pres-sure requirements in forging vary widely,depending primarily on the chemical composi-tion of the alloy being forged, the forging processbeing employed, the forging strain rate, the typeof forging being manufactured, the lubricationconditions, and the forging workpiece and dietemperatures.Figure 1 compares the flow stresses of some

commonly forged aluminum alloys at 350 to370 �C (660 to 700 �F) and at a strain rate of 4 to10 s�1 to 1025 carbon steel forged at an iden-tical strain rate but at a forging temperaturetypically employed for this steel. Flow stress ofthe alloy being forged represents the lower limitof forging pressure requirements; however,actual forging unit pressures are usually higherbecause of the other forging process factorsoutlined previously. For some low- to inter-mediate-strength aluminum alloys, such as 1100and 6061, flow stresses are lower than those ofcarbon steel. For high-strength alloys—particu-larly 7xxx series alloys such as 7x75, 7010, 7040,7x49, 7050, 7085, and others—flow stresses, andtherefore forging pressures, are considerablyhigher than those of carbon steels. Finally, otheraluminum alloys, such as 2219, have flowstresses quite similar to those of carbon steels. Asa class of alloys, however, aluminum alloys aregenerally considered to be more difficult to forgethan carbon steels and many alloy steels. Thechemical compositions, characteristics, andtypical mechanical properties of all wroughtaluminum alloys referred to in this article arereviewed in the articles “Aluminum mill andEngineered Wrought Products” and “Propertiesof Wrought Aluminum and Aluminum Alloys”in Properties and Selection: Nonferous Alloys

and Special-Purpose Materials, Volume 2 ofASM Handbook, 1990.

Forgeability

Compared to the nickel/cobalt-base alloys andtitanium alloys, aluminum alloys are con-siderably more forgeable, particularly whenusing conventional forging process techniqueswhere dies are heated to 540 �C (1000 �F) orless. Figure 2 illustrates the relative forgeabilityof ten aluminum alloys that constitute themajority of aluminum alloy forging production.This arbitrary unit is based principally on thedeformation per unit of energy absorbed in therange of forging workpiece temperatures typi-cally employed for the alloys in question. Alsoconsidered in this index is the difficulty of

achieving specific degrees of severity in defor-mation, as well as the cracking tendency of thealloy under given forging process conditions.There are wrought aluminum alloys, such as1100 and 3003, whose forgeability would berated significantly above that of those alloyspresented; however, these alloys have limitedapplication in forged products because theycannot be strengthened by heat treatment.Effect of Temperature. As shown in Fig. 2,

the forgeability of all aluminum alloys improveswith increasing metal temperature. However,there is considerable variation in the effect oftemperature for the alloys plotted. For example,the high-silicon alloy 4032 shows the greatesttemperature effect, while the high-strength Al-Zn-Mg-Cu 7xxx series alloys display the leasteffect of workpiece temperature. Figure 3 pre-sents the effect of temperature on flow stress, at astrain rate of 10 s�1 for alloy 6061, a highly

150

1257075 at 370 °C; 10 s–1

2219 at 370 °C; 10 s–1

6061 at 370 °C; 10 s–1

Flo

w s

tres

s, k

si

Flo

w s

tres

s, M

Pa

100

75

50

25

00 10 20 30 40 50

Strain, %

60 70 80 90 1000

5

10

20

152014 at 370 °C; 10 s–1

1025 steel at 1205 °C; 10 s–1

1100 at 350 °C; 4 s–1

Fig. 1 Flow stresses of commonly forged aluminum alloys and of 1025 steel at typical forging temperatures and variouslevels of total strain

ASM Handbook, Volume 14A: Metalworking: Bulk Forming S.L. Semiatin, editor, p299-312 DOI: 10.1361/asmhba0003996

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

forgeable and widely used aluminum alloy.There is nearly a 50% decrease in flow stress forthe highest metal temperature plotted, 480 �C(900 �F), the top of the recommended forgingrange for 6061, when compared with a work-piece temperature of 370 �C (700 �F), which isbelow the minimum forging metal temperaturerecommended for 6061. For other, more diffi-cult-to-forge alloys, such as the 2xxx and 7xxxseries, the change in flow stress associated withvariation in workpiece temperature is evengreater, illustrating the principal reason whyforging aluminum alloys requires maintainingrelatively narrow metal temperature ranges.Recommended preheating forging metal

temperature ranges for aluminum alloys that arecommonly forged, along with recently devel-oped alloys, are listed in Table 1. All of thesealloys are generally forged to the same severity,although some alloys may require more forgingpower and/or more forging operations than oth-ers. The preheating forging metal temperaturerange for most alloys is relatively narrow, gen-erally555 �C (5100 �F), and for no alloy is therange greater than 85 �C (155 �F). Achieving

and maintaining proper preheating metal tem-peratures in the forging of aluminum alloys is acritical process variable that is vital to the suc-cess of the forging process. However, die tem-peratures and deformation rates play key roles indetermining the actual workpiece metal tem-perature achieved during the forging deforma-tion sequence.

Effect of Deformation Rate. Aluminumalloy forgings are produced on a wide variety offorging equipment (see the section “ForgingEquipment” in this article). The deformation orstrain rate imparted to the deformingmetal variesconsiderably, ranging from very fast (for exam-ple, i10 s�1 on equipment such as hammers,mechanical presses, screw presses, and high-energy-rate machines) to relatively slow (forexample, j0.1 s�1 on equipment such ashydraulic presses). Therefore, deformation orstrain rate is also a critical process element thatmust be controlled for successful forging of anygiven alloy and forging configuration.

Figure 4 presents the effect of two strainrates—10 and 0.1 s�1—on the flow stresses oftwo aluminum alloys—6061 and 2014—at

370 �C (700 �F). It is clear that higher strainrates increase the flow stresses of aluminumalloys and that the increase in flow stress withincreasing strain rate is greater for more difficult-to-forge alloys, such as the 2xxx and 7xxx series.For 6061, the more highly forgeable alloy, theincrease in flow stress with the rapid strain rate isof the order of 70%; for 2014, the higher strainrate virtually doubles the flow stress. Althoughaluminum alloys are generally not considered tobe as sensitive to strain rate as other materials,such as titanium and nickel/cobalt-base super-alloys, selection of the strain rate in a givenforging process or differences in deformationrates inherent in various types of equipmentaffect the forging pressure requirements, theseverity of deformation possible, and thereforethe sophistication of the forging part that can beproduced.In addition to influencing the flow stress of the

alloy being forged, strain rate during the forgingprocess may also affect the temperature of theworkpiece. Most wrought aluminum alloys aresusceptible to deformation heating in forginghot-working processes. The extent of deforma-tion heating does, however, depend on the spe-cific alloy and the strain rate conditions present,with rapid strain rates, for example, greater than10 s�1, inducing greater changes (increases) inworkpiece temperature. Consequently, whenforging “hard,” more difficult to forge 2xxx and7xxx series alloys in rapid strain rate forgingequipment such as hammers, mechanical andscrew presses, and so forth, preheating metaltemperatures are reduced to the low end of theranges in Table 1. Some high-strength 7xxxalloys are intolerant of the temperature changespossible in rapid strain rate forging, and as aconsequence this type of equipment is notemployed in the fabrication of forgings in thesealloys.Effect of Die Temperature. Unlike some

forging processes for carbon and alloy steels, thedies used in virtually all hot-forging processesfor aluminum alloys are heated in order tofacilitate the forging process. Therefore, dietemperature is another critical process elementaffecting the forgeability and forging processoptimization of this alloy class. Table 2 sum-marizes the die temperature ranges typicallyused for several aluminum forging processes andtypes of forging equipment. The criticality of dietemperature in the optimization of the forgingprocess depends on the forging equipment beingemployed, the alloy being forged, the severity ofthe deformation, and/or the sophistication of theforging design. For slower deformation pro-cesses, such as hydraulic press forging, the alu-minum alloy workpiece rapidly assumes thetemperature of the dies. As a consequence, dietemperature controls the actual workpiece tem-perature during deformation. In fact, aluminumalloys forged in hydraulic presses are iso-thermally forged; that is, the workpiece and thedies are at the same temperature during defor-mation. Therefore, the recommended die tem-peratures employed for hydraulic press forging

Forging temperature, °F

Forging temperature, °C

700 750 800

Alloy 6061

2025

4032

2014

2618

5083

7010, 7075,7049, 7050

350 375 400 425 450 475 500

850 900

Rel

ativ

e fo

rgea

bilit

y

Fig. 2 Forgeability and forging temperatures of variousaluminum alloys

Strain, %

150

125

100

75

50

25

00 10 20 30 40 50 60 70

0

5

10

15

20260 °C

370 °C

480 °CFlo

w s

tres

s, M

Pa

Flo

w s

tres

s, k

si

Fig. 3 Flow stress versus strain rate for alloy 6061 atthree temperatures and a strain rate of 10 s�1

Table 1 Recommended forgingtemperature ranges for aluminum alloys

Aluminumalloy

Forging temperature range

�C �F

1100 315–370 600–7002014 420–460 785–8602025 420–450 785–8402219 425–470 800–8802618 410–455 770–8503003 315–370 600–7004032 415–460 780–8605083 405–460 760–8606061 430–480 810–9006069 440–490 825–9156556 440–490 825–9157010 370–445 700–8307033 380–440 720–8207039 380–440 720–8207040 360–440 680–8207049 360–440 680–8207050 360–440 680–8207068 380–440 720–8207075 380–440 720–8207175 380–440 720–8207085 360–440 680–820

Strain, %

150

125

100

75

50

25

00 10 20 30 40 50 60 70

0

5

10

15

20

2014, 10 s–1

6061, 10 s–1

6061, 0.1 s–1

2014, 0.1 s–1

Flo

w s

tres

s, M

Pa

Flo

w s

tres

s, k

si

Fig. 4 Flow stress versus strain rate for alloys 2014and 6061 at 370 �C (700 �F) and two different

strain rates

300 / Forging of Nonferrous Metals

aluminum alloys are much higher than thosetypical of more rapid deformation processes,such as hammers and mechanical or screwpresses. Die heating techniques are discussed inthe section “Heating of Dies” in this article.

Forging Methods

Aluminum alloys are produced by all of thecurrent forging methods available, includingopen-die (or hand) forging, closed-die forging,upsetting, roll forging, orbital (rotary) forging,spin forging, mandrel forging, ring rolling, andforward and reverse extrusion. Selection of theoptimal forgingmethod for a given forging shapeis based on the desired forged shape, thesophistication of the forged-shape design, andcost. In many cases, two or more forging meth-ods are combined in order to achieve the desiredforging shape and to obtain a thoroughlywroughtstructure. For example, open-die forging fre-quently precedes closed-die forging in order toprework the alloy (especially when cast ingotforging stock is being employed) and in order topreshape (or preform) themetal to conform to thesubsequent closed dies and to conserve inputmetal.Open-die forging is frequently used to pro-

duce small quantities of aluminum alloy forgingswhen the construction of expensive closed dies isnot justified or when such quantities are neededduring the prototype fabrication stages of a for-ging application. The quantity that warrants theuse of closed dies varies considerably, dependingon the size and shape of the forging and on theapplication for the part. However, open-die for-ging is by no means confined to small or proto-type quantities. In some cases, it may be the mostcost-effective method of aluminum forgingmanufacture. For example, as many as 2000pieces of biscuit forgings have been produced inopen dies when it was desired to obtain theproperties of a forging but closed dies did notprovide sufficient economic benefits.Open-die forgings in aluminum alloys can be

produced in a wide variety of shapes, ranging

from simple rounds, squares, or rectangles tovery complex contoured forgings (see the article“Open-Die Forging” in this Volume). In the past,the complexity and tolerances of the open-dieforging of aluminum and other materialsdepended on the skill of the press operator;however, with the advent of programmablecomputer-controlled open-die forging presses, itis possible to produce such shapes to overallthickness/width tolerances bands of 1.27 mm(0.050 in.). Because the open-die forging ofaluminum alloys is also frequently implementedto produce preforms for closed-die forgings,these state-of-the-art forging machines alsoprovide very precise preform shapes, improvingthe dimensional consistency and tolerances ofthe resulting closed-die forging and reducingclosed-die forging cost through further inputmaterial conservation. More information onopen-die forging is available in the article“Open-Die Forging” in this Volume.

Closed-Die Forging. Most aluminum alloyforgings are produced in closed dies. The fourtypes of aluminum forgings shaped in closed diesare blocker-type (finish forging only), conven-tional (block and finish forging or finish forgingonly), high-definition (near-net shape producedby forging in one or more blocker dies followedby finish forging), and precision (no draft, netshapes produced by forging with or withoutblocker dies followed by two or more finishforging steps in the finish dies). These fourclosed-die forging types are illustrated in Fig. 5,which includes a description of key design anddimensional tolerancing parameters for eachforging type.Blocker-type forgings (Fig. 5a) are produced

in relatively inexpensive, single sets of dies. Indimensions and forged details, they are lessrefined and require more machining than con-ventional or high-definition closed-die forgings.A blocker-type forging costs less than a com-parable conventional or high-definition forging,but it requires more machining.Conventional closed-die forgings (Fig. 5b)

are the most common type of aluminum forging.They are produced with either a single set offinish dies or with block-and-finish dies,depending on the design criteria. Conventionalforgings have less machine stock and tightertolerances than blocker-type forgings but requireadditional production costs, both for the addi-tional die set and for additional forging fabrica-tion steps required to produce this type.High-Definition Forgings. With the advent of

state-of-the-art forging press and supportingequipment and enhanced forging process con-trol, as are discussed below, high-definition,near-net-shape, closed-die forgings illustrated inFig. 5(c) can be produced. High-definitionclosed-die forgings offer superior forging designsophistication and tolerances over conventionalor blocker-type forgings and therefore enableeven further reduction in final componentmachining costs. High-definition forgings areproduced with multiple die sets, consisting ofone or more blocker dies and finish dies, and are

frequently used in service with some as-forgedsurfaces remaining unmachined by the pur-chaser.Precision forgings (Fig. 5d) represent the

most sophisticated aluminum forging designproduced. These forgings, for which the forgermay combine forging and machining processesin the fabrication sequence, cost more than otheraluminum forging types. However, by definitionprecision forgings require no subsequentmachining by the purchaser and thereforemay bevery cost effective. Net-shape aluminum for-gings are produced in two-piece, three-piecethrough-die, and/or multiple-segment wrap-diesystems to very restricted design and tolerancesnecessary for assembly. Net-shape aluminumforgings are discussed more thoroughly in thesection “AluminumAlloy Precision Forgings” inthis article and in the article “Precision HotForging” in this Volume. More information onthe closed-die forging process is available in thearticle “Closed-Die Forging in Hammers andPresses” in this Volume.Upset forging can be accomplished in spe-

cialized forging equipment called upsetters (aform of mechanical press) or high-speed, multi-ple-station formers. Upset forging is frequentlyused to produce forging shapes that are char-acterized by surfaces of revolution, such as bolts,valves, gears, bearings, and pistons. Upset for-ging may be the sole process used for the shape,as is the case with pistons, or it can be used as apreliminary operation to reduce the number ofimpressions, to reduce die wear, or to save metalwhen the products are finished in closed-dies.Wheel and gear forgings are typical productsfor which upsetting is advantageously used inconjunction with closed-die forging. As a rule,in the upset forging of aluminum alloys, theunsupported length of forgings must not exceedthree diameters for a round shape or threetimes the diagonal of the cross section for arectangular shape. The article “Hot Upset For-ging” in this Volume contains more informationon upsetting.Roll forging can be used as a preliminary

preform operation to preshape the material andreduce metal input or to reduce the number ofsubsequent closed-die operations. In roll forging,the metal is formed between moving rolls, eitheror both containing a die cavity, and the process ismost often used for parts, such as connectingrods and suspension components, where partproduction volumes are high and relativelyrestricted cross-sectional variations typify thepart. Roll forging is discussed at length in thearticle “Roll Forging” in this Volume.Orbital (rotary) forging is a variant of

closed-die mechanical or hydraulic press forgingin which one or both of the dies is caused torotate, usually at an angle to the other die, leadingto the incremental deformation of the workpiecebetween the moving and stationary die. Orbitalforging is used to produce parts with surfaces ofrevolution (such as impellers and discs) withboth hot and cold forging processes for alumi-num alloys. Orbital forging provides highly

Table 2 Die temperature ranges for theforging of aluminum alloys

Forging process/equipment

Die temperature

�C �F

Open-die forging

Ring rolling 95–205 200–400Mandrel forging 95–205 200–400

Closed-die forging

Hammers 95–150 200–300Upsetters 150–260 300–500Mechanical presses 150–260 300–500Screw presses 150–260 300–500Orbital (rotary) forging 150–260 300–500Spin forging 150–315 200–600Roll forging 95–205 200–400Hydraulic presses 315–430 600–800

Forging of Aluminum Alloys / 301

refined, close-tolerance final shapes. Additionalinformation on orbital forging is available in thearticle “Radial Forging” in this Volume.Spin forging combines closed-die forging

and computer numerically controlled (CNC)spin forgers or spin formers to achieve close-tolerance, axisymmetric hollow shapes includingthose shown in Fig. 6. The forgings in thisfigure were produced using both hot and cold

spin-forging techniques for the aluminum alloyfabricated, illustrating the flexibility of this pro-cess. Because spin forging is generally accom-plished over a mandrel, inside diameter contoursare typically produced to net shape, requiring nosubsequent machining. Outside diameter con-tours can be produced net or with very littlesubsequent machining and to much tighter out-of-round and concentricity tolerances than

competing forging techniques, such as forwardor reverse extrusion (see below), resulting inmaterial savings. Parts with both ends open, oneend closed, or both ends closed can also be pro-duced.Spin forging has been very effectively

employed to fabricate high-volume automobileand light truck wheels. Spin-forging processesfor wheels, primarily in alloy 6061, haveemployed several spin-forging processing tech-niques including hot spin forming of closed-dieforged preforms to the final wheel shape fol-lowed by heat treatment andmachining; multiplecold spin forming steps on preforms to precise,finished dimensions requiring little or no finalmachining; and/or combined hot spin forging ofa preform shape followed by cold spin formingafter solution heat treatment and quench andprior to age for precise shape, out-of-round andtolerance control, and reduction in finalmachining costs.Ring rolling is also used for aluminum alloys

to produce annular shapes. The procedure used toring roll aluminum alloys is essentially the sameas that used for steel (see the article “RingRolling” in this Volume). Both rectangular andcontoured cross section rolled rings, with orwithout subsequent machining by the forger, areproduced in many aluminum alloys. The tem-peratures employed for the ring rolling of alu-minum alloys are quite similar to those for otherforging processes, although caremust be taken tomaintain metal temperature. The deformationachieved in the ring rolling of aluminum typi-cally results in the predominant grain flow in thetangential or circumferential orientation. If pre-dominant grain flow is desired in other direc-tions, such as axial or radial, other ring-makingprocesses, such as hollow-biscuit open-die for-gings, mandrel forging, or reverse/forwardextrusion, can be employed. The economy ofring rolling in aluminum alloys depends on thevolume, size, and contour of the forging. Forsome ring parts, it may be more economical toproduce the shape by mandrel forging or to cutrings from hollow extruded cylinders. Bothtechniques are discussed below.Mandrel forging (Fig. 7) is used in alumi-

num alloys to produce axisymmetric, relativelysimple, hollow ring or cylindrical shapes, inwhich the metal is incrementally forged, usuallyon a hammer or hydraulic press, over a mandrel.In the incremental forging process, the wallthickness of the preform is reduced, and thisdeformation enlarges the diameter of the piece.The mandrel forging of aluminum has beenfound to be economical for relatively low-volume part fabrication and/or in the fabricationof very large ring shapes (up to 3.3 m, or 130 in.,in diameter). With control of the working historyof the input material and the mandrel-forgingprocess, mandrel-forged rings can be producedwith either circumferential or axial predominantgrain orientations.Reverse or forward extrusion, a variant of

closed-die forging for aluminum, can be usedto produce hollow, axisymmetric shapes in

4.3 mm max

4.1 mm max

3.8 mm max

3.8 mm max

3.3 mm max

3.6 mm max

4.3 mm max

3.8 mm max

3.8 mm max

(a) (b)

(d)

(c)

PL

PL

PL

PL

PL

PL

PL

PL

Characteristic Blocker-type Conventional High-definition Precision

Die closure

Mismatch

Flash extensionLength and width

Draft angles

Straightness

+1.5, –0.8+2.3, –1.5(+0.06, –0.03)(+0.09, –0.06)0.5 (0.02)0.5 (0.02)0.8 (0.03)0.8 (0.03)1.5 (0.06)�0.8 (�0.03)

3 (0.12)

5°5°

�0.8 (�0.03)0.8 (0.03)0.5 (0.02)0.25 (0.01)(+0.05, –0.02)+1.25, –0.5

3°

+0.8, –0.25(+0.03, –0.01)0.38 (0.015)0.4 (0.016)0.8 (0.03)+ 0.5, –0.25(+0.02, –0.01)

�0.8 (�0.03)

1°

Tolerance, mm (in.)

Fig. 5 Types of aluminum closed-die forgings and tolerances for each. (a) Blocker-type. (b) Conventional. (c) High-definition. (d) Precision


aluminum alloys with both ends open or with oneend closed. The terminology of reverse or for-ward extrusion refers to the direction of metalmovement in relation to the movement of thepress cross head. In forward extrusion, the metalis extruded (typically downward) in the samedirection as the press head movement. Con-versely, for reverse extrusion, metal movesopposite the motion of the cross head. Selectionof forward versus reverse extrusion is usuallybased on part geometry and the open or shutheight restrictions of the forging press beingused. Some presses are specifically equippedwith openings (circular or rectangular holes) inthe upper cross head and platen to accommodatethe fabrication of very long reverse extrusions,either solid or hollow, that pass through the

moving upper cross head as the deformationprogresses.

Extrusion as a metal deformation processfrequently plays an important role in closed-dieforging of commercially important aluminumalloy parts in addition to the hollow, annularshapes discussed previously, including high-volume automobile and light and heavy truckwheels. In this case, the skirt of the wheel isforward extruded from an appropriately shapedblocker. After hot forward extrusion, the skirt isimmediately hot or warm formed to the requiredfinish forged wheel shape that has appropriatemachining allowance over the final wheeldesign. More information on extrusion is avail-able in the articles “Cold Extrusion” and “Con-ventional Hot Extrusion” in this Volume.

Forging Equipment

Aluminum alloy forgings are produced on thefull spectrum of forging equipment, rangingfrom hammers and presses to specialized forgingmachines. Selection of forging equipment for agiven forging shape and type is based on thecapabilities of the equipment, forging designsophistication, desired forging process, and cost.Additional information on the types of equip-ment used in the manufacture of forgings isavailable in the Section “Forging Equipment andDies” in this Volume.Hammers. Gravity and power-drop ham-

mers are used for both the open-die and closed-die forging of aluminum alloys because of therelatively low fabrication costs associated withsuch equipment, although the power require-ments for forging aluminum alloys frequentlyexceed those for steel. Hammers deform themetal with high deformation speeds; therefore,control of the length of the stroke and of the forceand speed of the blows is particularly useful inforging aluminum alloys, because of their sen-sitivity to strain rate and their exothermic natureunder rapid deformation processes. Power-drophammers are used to manufacture closed-dieforgings if an applied draft of about 5 to 7� can betolerated. Hammers are frequently used as apreliminary operation for subsequent closed-dieforging by other forging processes, and for someproducts, such as forged aluminum propellers,power-assisted hammers are the optimal forgingprocess equipment because of their capacity forconserving input material and their ability to

63.5 mmdiam

13.2 mm

338 mm

(a)

22.1 mm

168.1 mmdiam

465 mm approx

206 mmdiam

(b)

298 mm

129.5 mmdiam

2.8 mm

(c)

330 mmdiam

approx

5.1 mm

318 mm approx

(d)

874 mm approx

400 mmdiam

7.9 mm

(e)

6.1 mm

648 mm approx

366 mmdiam

(f)

536 mm approx

171.5 mmdiam

8.4 mm

(g)

16.8 mm

8.9 mm

339 mm

318 mmdiam

(h)

Fig. 6 Examples of spin-forged aluminum alloy shapes. (a) Ordnance ogive. (b) Ordnance center section. (c) Ordnancefuse. (d) Jet engine spinner. (e) Missile nose cone. (f) Missile center section. (g) Bottle. (h) Missile forward case

(a) (b)

(d)(c)

Fig. 7 Sequence of operations for mandrel/saddle for-ging of a ring. (a) Preform mounted on a saddle/

mandrel. (b) Reduction of preform thickness to increasediameter. (c) Progressive reduction of wall thickness toproduce ring dimensions. (d) Machining to near-net shape.Source: Ref 1


produce a finished blade that has essentially netairfoil contours. State-of-the-art power-assistedhammers with programmable blow sequencingsignificantly improve the repeatability and con-sistency of deformation processes and thereforeenhance the consistency of aluminum forgingsproduced on hammers.Mechanical and Screw Presses. Both

mechanical and screw presses are extensivelyused for the closed-die forging of aluminumalloys. They are best adapted to aluminum for-gings of moderate size, high volume (cost con-sideration), and relatively modest shapes that donot require extensive open-die preforming. Inforging aluminum alloys on mechanical or screwpresses, multiple-die cavities, frequently withinthe same die block, and multiple forging stages,frequently without reheating, are used to enhancethe deformation process, to increase the partdesign sophistication, and to improve tolerancecontrol. The automotive rear knuckle suspensioncomponent in alloy 6061 shown in Fig. 8 illus-trates the complexity of high-volume aluminumalloy forgings that are producible on mechanicaland screw presses. It should be noted that thesuspension component forging in Figure 8requires very limited final machining and thatabout 80% of the surface area of the part is usedin end-product service with as-forged andcleaned surfaces and with no surface treatment.Mechanical and screw presses combine

impact with a squeezing action that is morecompatible with the flow characteristics of alu-minum alloys than hammers. Screw pressesdiffer frommechanical presses in that the formerhave a level of strain rate and blow energy con-trol that can be exploited to enhance the overallcontrol of the deformation in forging aluminumalloys. State-of-the-art mechanical and screwpresses have programmable press operation,press load and operation monitoring and control,and press energy and press operation controlsystems. These systems, combined with auto-mated handling and supporting equipment, suchas reheat furnaces and trim presses, can be usedto achieve full forging process automation andhighly repeatable and precise forging conditions

in order to enhance the uniformity of the result-ing aluminum alloy forgings. Typically, theminimum applied draft for mechanical or screwpress forged aluminum alloys is 3�; however,both press types have been used to manufactureprecision, net-shape aluminum alloy forgingswith draft angles of 1�. Screw presses are parti-cularly well suited to the manufacture of thehighly twisted, close-tolerance aluminum bladesused in turbine engines and other applications.

Hydraulic Presses. Although the fastesthydraulic presses are slower acting thanmechanical or screw presses, hydraulic pressesare frequently best suited to producing eithervery large aluminum closed-die forgings (Fig. 9)or very intricate aluminum alloy forgings. Thedeformation achieved in a hydraulic press ismore controlled than that typical of mechanicaland screw presses or hammers. Therefore,hydraulic presses are particularly well adapted tothe fabrication of conventional, high-definition,and precision no-draft, net-shape aluminumalloy forgings in which slow or controlled strainrates and controlled strain minimize the resis-tance of the aluminum alloy to deformation,reduce unit pressure requirements, and facilitateachieving the desired shape.

State-of-the-art hydraulic forging presses usedto forged aluminum alloys, including very largemachines of up to 715 MN (80,000 tonf),include speed and pressure controls and pro-grammable modes of operation. With organiza-tion of these machines into press cells along withautomated workpiece handling and lubrication,state-of-the-art die preheating and on-press dieheating and supporting equipment, such presscells provide a high degree of forging processautomation and forging process control thatenables forging process and product optimiza-tion and consistency, improved product uni-formity, and significantly enhanced throughput. The minimum applied draft angle for high-definition hydraulic press forged aluminumalloys is 3�; for hydraulic press forged precision,net-shape aluminum forgings, the minimumdraft angle is 0 to 0.5� on outside contours and0.5 to 1� on inside contours.

Die Materials, Design,and Manufacture

For the closed-die forging of aluminum alloys,die materials selection, die design, and diemanufacturing are critical elements in the overallaluminum forging process, because the dies are amajor element of the final cost of such forgings.Further, forging process parameters are affectedby die design, and the dimensional integrity ofthe finished forging is in large part controlled bythe die cavity. Therefore, the forging of alumi-num alloys requires the use of dies specificallydesigned for aluminum because:

� The deformation behavior of aluminum alloysdiffers from that of other materials; therefore,the intermediate and final cavity die designsmust optimize metal flow under given forgingprocess conditions and provide for the fabri-cation of defect-free final parts.

� Allowances for shrinkage in aluminum alloysare typically greater than those for steels andother materials.

� Temperature control of the dies used to forgealuminum alloys is critical; therefore, themethods used for die preheating and main-taining die temperatures during forging mustbe considered in the design.

The die materials used in closed-die forgingof aluminum alloys are identical to thoseemployed in forging steel alloys except that,because of the higher forces applied in aluminumalloy forging and the design sophistication of thealuminum parts produced, the die materials aretypically used at lower hardness levels in order toimprove their fracture toughness. Commerciallyavailable die materials were primarily designedfor the forging of steels and are not necessarilyoptimized for the demands of aluminum alloyforging processes. However, with advancedsteelmaking technology, such as argon oxygendecarburization refining, vacuum degassing,and ladle metallurgy, the transverse ductilityand fracture toughness of available standard andproprietary die steel grades have been improveddramatically. As a result, the performance ofthese grades in the forging of aluminum alloyshas also improved dramatically.Although die wear is less significant with

aluminum alloy forging than with steel and otherhigh-temperature materials, high-volume alu-minum alloy forgings can present die wear pro-blems in cases in which die blocks have reducedhardness in order to provide improved tough-ness. Therefore, higher hardness die inserts and/or cavity surface treatments comparable to thoseused for steel forging dies are often used toimprove wear characteristics in order to maintaindie cavity integrity for aluminum forging dies.The surface treatments employed include car-burizing, nitriding, carbonitriding, and surfacealloying using a variety of state-of-the-art tech-niques.Beyond die wear, the most common cause of

die failure in aluminum forging dies is associatedFig. 8 Complex aluminum alloy automotive suspen-

sion components forged on a mechanical pressFig. 9 Examples of very large blocker-type aluminum

alloy airframe forgings


with die checking or die cracking, which, ifleft unheeded, can lead to catastrophic loss of thedie. Such die checking usually occurs at stressrisers inherent in the die cavity features from thedesign of the forging being produced. Improvedtoughness die steels, improved die-sinkingtechniques (see below), improved die design (seebelow), and lower hardness die blocks serve toreduce the incidence of die checking in dies forforging of aluminum alloys. Numerical model-ing of the dies, using state-of-the-art finite ele-ment methods (FEM) techniques, is widely usedfor analysis of die stress, die strain, and thermalconditions as a function of the die designand forging process conditions. With these ana-lytical models, optimization of the die designand/or forging process conditions can be fullyevaluated prior to actual die sinking and shopfloor use, dramatically increasing die life. Fur-ther, aluminum alloy forging dies with diechecking or cracking are routinely repair weldedusing metal inert gas, tungsten inert gas, or otherwelding techniques. With weld repair andnumerical models of the dies, it is possible,through weld rod composition selection, tomodify the performance capabilities of criticalareas of the die cavity that models have shown tohave high stresses or unavoidably severe stressrisers.For hot upsetting, both gripper dies and

heading tools are usually made of ASM 6G and6F2 grade die steels at a hardness of 42 to 46HRC. This same hardness range applies to 6Gand 6F2 dies when used for mechanical andscrew presses. Grades 6G or 6F2, or proprietaryvariants of these steels, are the most widely useddie materials in all closed-die forging processesfor aluminum alloys. For 6G and 6F2 dies to beused for hammer forging aluminum alloys, ahardness range 36 to 40 HRC is recommended,while for dies used for hydraulic press forging, ahardness range of 38 to 42 HRC is recom-mended. If the quantities to be forged are largeenough to justify the added die cost or if theforging process and the part are particularlydemanding, hot-work tool steels such as H11,H12, H13, or proprietary variants, are employed,usually at 44 to 50 HRC.Die Design. A key element in the cost con-

trol of dies for aluminum forging and in thesuccessful fabrication of aluminum alloy for-gings is die design and die system engineering.Closed-dies for aluminum forgings are manu-factured either as stand-alone die blocks or asinserts into die holder systems to reduce theoverall cost of the dies for any given forging. Dieholder systems may be universal, covering awide range of potential die sizes, forging parts,and customers, or the holder(s) may be con-structed to handle families or parts of similaroverall geometries or for a particular end-pro-duct application. Design of aluminum forgingdies is highly intensive in engineering skills andis based on extensive empirical knowledge andexperience. A complete compendium of alumi-num forging design principles and practices isavailable in Ref 2 to 4.

Because aluminum alloy forging design isengineering intensive, computer-aided design(CAD) hardware and software has had anextensive impact on the aluminum alloy diedesign process. Computer-aided design techni-ques for aluminum forging parts and dies arefully institutionalized within the forging industrysuch that most aluminum alloy die forgings,including blocker-type, conventional, high-definition, and precision forgings, are designedwith this technique. The CAD databases createdare then used, as discussed below, with compu-ter-aided manufacturing (CAM) to produce dies,to direct the forging process, and to assist in finalpart verification and quality control. Both publicdomain and proprietary CAD design softwarepackages are used to design the finished forgingfrom the machined part, including the dies, andto design the critical blocker and preform shapesneeded to successfully produce the finishedshape, including the dies.

Beyond computer-aided design, heuristictechniques, such as artificial intelligence, arebeing used to complement CAD/CAM systemsby capture of extensive aluminum forging designknowledge and experience into expert systems inorder to enhance the speed, accuracy, and effi-ciency of the forging part and die design andmanufacture processes. Complementing CADand expert systems for aluminum alloy forgingdesign is extensive capture of powerful, state-of-the-art finite element process models that includedeformation and thermal analytical modelingtechniques to aid the designer and the forgingengineer in their tasks by enabling evaluation,verification, and optimization of forging part anddie design and forging processing on a computerbefore committing the part, tooling, and processdesign to any costly die sinking or tryout partfabrication. These state-of-the-art computer-aided engineering (CAE) systems for aluminumalloy closed-die forgings have effected sig-nificant collapse of lead and flow times for fab-rication of new forging shapes and improvementin the flow times and consistency of existingforging business.

Die Manufacture. Aluminum alloy forgingdies are produced by a number of machiningtechniques, including hand sinking, copy mill-ing from amodel, electrical discharge machining(EDM), and CNC direct sinking including high-speed and ultrahigh-speed die sinking. Withthe availability of CAD databases, CAM-drivenhigh-speed CNC direct die sinking and EDMdie sinking are at the leading edge of the state-of-the-art in aluminum alloy die manufacturing.These techniques serve to reduce the cost of dies,shorten die manufacturing flow times, and, per-haps more importantly, to increase the accuracyof the dies by as much as 50% compared withthe other techniques. For example, standard die-sinking tolerances are +0.1 mm (+0.005 in.),but with CAM-driven CNC/EDM die sink-ing, tolerances are reduced to +0.07 mm(+0.003 in.) on complex dies.

The surface finish on the cavity in dies used forthe forging of aluminum alloys is more critical

than that for dies used for steel. Therefore, cav-ities are highly polished, frequently with auto-mated equipment, by a variety of techniques inorder to obtain an acceptable finish and toremove the disturbed surface layer resultingfrom such die-sinking techniques as electro-discharge machining. However, state-of-the-arthigh-speed die sinking (e.g., spindle speeds of410,000 rpm) and ultrahigh-speed die sinking(e.g., spindle speeds of 420,000 rpm) havedramatically improved the surface finish of diessunk in the fully hardened state as is the case withdie materials used for aluminum alloy forgings.With high- and ultrahigh-speed die sinking, diecavities are suitable for use in forging aluminumwithout polishing, reducing die manufacturingcost, and flow times.

Processing of AluminumAlloy Forgings

The common elements in the manufacture ofany aluminum alloy forging include preparationof the forging stock, preheating stock, die heat-ing, lubrication, the forging process, trimming,forming and repair, cleaning, heat treatment,and inspection. The critical aspects of each ofthese elements are reviewed in the sections thatfollow.Preparation of Forging Stock. Aluminum

alloy forgings are typically produced from castor wrought stock. The latter includes forged orrolled bar, extruded bar, or plate as primaryexamples. Selection of forging stock type for agiven forging shape is based on the requiredforging processes, forging shape, mechanicalproperty requirements, and cost. Sawing andshearing are the two methods most frequentlyused to cut aluminum alloy forging stock intolengths for forging. Abrasive cutoff can be used,but it is slower than sawing.Sawing with a circular or band saw having

carbide-tipped blades is the fastest and generallythe most satisfactory method. Sawing, however,produces sharp edges or burrs that may initiatedefects when the stock is forged in closed dies.Burrs and sharp edges are typically removed by aradiusing machine. State-of-the-art saws forcutting aluminum alloys are highly automatedand frequently have automatic radiusing cap-ability and control systems that permit veryprecise control of either stock length or stockvolume and therefore stock weight.Shearing is used less for aluminum alloys

than for steel because aluminum alloy billets aresofter and more likely to be mutilated in shearingand because the sheared ends may have unsa-tisfactory surfaces for forging without beingconditioned. Shearing is successfully used forhigh-volume aluminum forgings made fromwrought bar stock generally less than 50 mm(2 in.) in diameter.Preheating for Forging. As noted in the

section “Effect of Temperature” in this article,workpiece temperature is a critical element in thealuminum forging process. Aluminum alloys


form a very tenacious oxide coating upon heat-ing. The formation of this coating is self-limit-ing; therefore, aluminum alloys do not scale tothe same extent as steel does. However, mostaluminum alloys are susceptible to hydrogenpickup during reheating operations such thatreheating equipment and practices are also cri-tical elements of forging process control.Recommended preheating temperatures varywith alloy and are contained in Table 1.Heating Equipment. Aluminum alloys are

heated for forging with a wide variety of heatingequipment, including electric furnaces, fullymuffled or semimuffled gas furnaces, oil fur-naces, induction heating units, fluidized-bedfurnaces, and resistance heating units. Gas-firedsemimuffled furnaces, either batch or con-tinuous, are the most widely used. Heatingequipment design and capabilities necessarilyvary with the requirements of a given forgingprocess. Both oil and natural gas furnaces mustuse low-sulfur fuel. Excessive hydrogen pickupin forged aluminum alloys manifests itself in twoways. The first is high-temperature oxidation,which is usually indicated by blisters on thesurface of the forging. The second is brightflakes, or unhealed porosity, which is usuallyfound during the high-resolution ultrasonicinspection of final forgings. Both types ofhydrogen pickup are influenced by preheatingfurnace practices and/or furnace equipment inwhich water vapor as a product of combustion isthe primary source of hydrogen. Fully muffledgas-fired furnaces or low relative humidityelectric furnaces provide the least hydrogenpickup. Techniques are available for modifyingthe surface chemistry of aluminum alloys toreduce hydrogen pickup in heating equipmentthat has higher levels of relative humidity thandesired. Protective-atmosphere furnaces are sel-dom used to preheat aluminum alloy forgings.Induction heating, resistance heating, and

fluidized-bed heating are frequently used in theforging of aluminum alloys in cases in whichforging processes are highly automated. State-of-the-art gas-fired furnaces can also be linkedwith specially designed handling systems toprovide full automation of the forging process.Temperature Control. As noted in Fig. 1 to 3

and in Table 1, aluminum alloys have a relativelynarrow temperature range for forging. Therefore,careful control of the temperature in preheatingis important. The heating equipment should havepyrometric controls that can maintain +5 �C(+10 �F). Continuous furnaces used to preheataluminum typically have three zones: preheat,high heat, and discharge. Most furnaces areequipped with recording/controlling instrumentsand are frequently surveyed for temperatureuniformity in a manner similar to that used forsolution treatment and aging furnaces.Heated aluminum alloy billets are usually

temperature checked by using either contactmethods or noncontact pyrometry based on dual-wavelength infrared systems. This latter tech-nology, although sensitive to emissivity, hasbeen successfully incorporated into the fully

automated temperature-verification systemsused in automated high-volume aluminum for-ging processes to provide significantly enhancedtemperature control and process repeatability. Inopen-die forging of aluminum alloys, it is gen-erally desirable to have billets near the high sideof the forging temperature range when forgingbegins and to finish the forging as quickly aspossible before the temperature drops exces-sively. Open-die forging and multiple-blow orstroke closed-die forging of aluminum alloys arefrequently conducted without reheating betweenblows or strokes as long as critical metal tem-peratures can be maintained.Heating time for aluminum alloys varies

depending on the section thickness of the stockor forgings and the furnace capabilities. How-ever, in general, because of the increased thermalconductivity of aluminum alloys, the requiredpreheating times are shorter than with otherforged materials. Recording pyrometric instru-ments on furnaces can be used to provide anindication of when the metal has reached thedesired forging temperature. Generally, times attemperature of 10 to 20 min/in. of sectionthickness are sufficient to ensure that aluminumalloy workpieces are thoroughly soaked andhave reached the desired preheat temperature.

Time at temperature is not as critical for alu-minum alloys as for some other forged materials;however, long soaking times offer no particularadvantage, except for high-magnesium alloyssuch as 5083, and may in fact be detrimental interms of hydrogen pickup. Generally, soakingtimes at temperature of 1 to 2 h are sufficient; ifunavoidable delays are encountered such thatsoaking timemay exceed 4 to 6 h, removal of theworkpieces from the furnace is recommended.

Heating of Dies. As noted in the section“Effect of Die Temperature” in this article, dietemperature is the second critical process ele-ment in the aluminum forging process. Dies arealways heated for the forging of aluminumalloys, with die temperatures and die heating forclosed-die forging processes being more critical.As noted in Table 2, the die temperature used forthe closed-die forging of aluminum alloys varieswith the type of forging equipment beingemployed and the type of forging being produced(open- or closed-die, etc.). Both remote and on-press die heating systems are employed in theforging of aluminum alloys. Remote die pre-heating systems are usually gas-fired or infrareddie systems (usually batch-type) capable ofslowly heating and maintaining the die blocks atrecommended temperatures in Table 2. Thesesystems are used to preheat dies to the desiredtemperature prior to assembly into the forgingequipment.

On-press die heating systems range fromrelatively rudimentary to highly engineeredsystems designed to maintain very tight dietemperature tolerances. On-press die heatingsystems include gas-fired equipment, inductionheating equipment, infrared heating equipment,and/or resistance heating equipment. In addition,presses used for the precision forging of alumi-

num alloys frequently have bolsters that haveintegral heating or cooling capabilities. State-of-the-art on-press die heating equipment foraluminum forging can hold die temperaturetolerances to within +15 �C (+25 �F) or better.Specific on-press die heating systems vary withthe forging equipment being used, the size of thedies, the forging process, and the type of forgingproduced. On-press die heating equipment istypically more sophisticated for hydraulic pressforging of aluminum alloys because the forgingprocess occurs over longer period of time underpressure, and thus die temperature establishes thethermal conditions active during the deformationof the workpiece.Lubrication. Die and workpiece lubrication

is the third critical element in the aluminumforging process and is the subject of majorengineering and developmental emphasis, bothin terms of the lubricants themselves and thelubricant application systems.The lubricants used in aluminum alloy forging

are subject to severe service demands. Theymustbe capable of modifying the surface of the dieand workpiece to achieve the desired reductionin friction, enable the desired deformationwithout formation of surface defects, withstandthe high die and metal temperatures and unitpressures employed, and yet leave the forgingsurfaces and forging geometry unaffected.Lubricant formulations are typically highlyproprietary and are developed either by lubricantmanufacturers or by the forgers themselves.Lubricant composition varies with the demandsof the forging process used and the forging type.The major active element in aluminum alloyforging lubricants is graphite; however, otherorganic and inorganic compounds are added tocolloidal graphite suspensions in order toachieve the desired results. Liquid carriers foraluminum alloy forging lubricants vary frommineral spirits to mineral oils to water. The trendin liquid-carrier die lubricants for aluminumforging is away from mineral spirits and mineraloils and is moving to increased use of water-based lubricants that significantly reduce emis-sions, including volatile organic compounds(VOCs). Additionally, powder die lubricantsbased on graphite and other additives are alsoavailable, which not only eliminate VOC emis-sions but also have less impact on die tempera-ture than water-based lubricants are known tohave.Lubricant application is typically achieved

by spraying the lubricant onto the workpiece anddies while the latter are assembled in the press;however, in some cases, lubricants are applied toforging stock prior to reheating or just prior toforging. For liquid carrier lubricants, severalpressurized-air or airless spraying systems areemployed, and with high-volume, highly auto-mated aluminum forging processes, lubricantapplication is also automated by single- or mul-tiple-axis robots. Liquid carrier lubricants maybe applied with or without heating; however,heating can improve the flowability and perfor-mance of the lubricant. For powder lubricants,


electrostatic application techniques are utilizedthat can also be fully automated. State-of-the-artlubricant application systems have the capabilityof applying very precise patterns or amounts oflubricant under fully automated conditions suchthat the forging processes are optimized andrepeatable.Forging Process. The critical elements of the

aluminum forging process—workpiece and dietemperatures, strain rate, deformation mode, andtype of forging process—have been reviewedpreviously, including state-of-the-art forgingprocess capabilities that have served to enhancecontrol of the forging process and therefore theproduct it produces. In addition to the enhancedforging equipment employed in the manufactureof aluminum forging, mention was made ofthe organization of presses and supportingequipment into cells operating as systems; suchsystems are then integrated with advancedmanufacturing and computer-aided manufactur-ing concepts. Aluminum alloy forging has thusentered an era properly termed integrated man-ufacturing, in which all aspects of the aluminumforging process from design to execution on theshop floor are heavily influenced by computertechnology.Trimming, forming, and repair of alumi-

num alloy forgings are intermediate processesthat are necessary to achieve the desired finishshape and to control costs.Trimming. The flash generated in most

closed-die aluminum forging processes isremoved by hot or cold trimming or sawing,punching, or machining, depending on the size,shape, and volume of the part being produced.Hot or cold die trimming technique is ordinarilyused to trim large quantities, especially formoderately sized forgings that are intricate andmay contain several punch-outs. The choice ofhot or cold trimming is largely based on thecomplexity of the part, the potential for distor-tion of the part (greater with hot trimming) andon cost. In cold trimming, two processes prevail:cold trimming after cooling after forging andcold trimming after solution heat treatmentand quench. The former process introduces morerisk of distortion that will have to be corrected instraightening during heat treatment. The latterprocess typically results in less straightening butleaves flash intact through heat treatment, redu-cing throughput in the heat treating processes.The trim presses employed for cold or hot die

trimming are either mechanical or hydraulic.Trimming dies are usually constructed of 6G or6F2 die steel at a hardness of about 444 to477 HB. Tools of these steels are less costlybecause they are often produced from pieces ofworn or broken forging dies. Blades for trimmingand the edges of trimming dies are frequentlyhardfaced to improve their abrasion resistance.In addition to these grades, O1 tool steel and/orhigh-alloy tool steel such as D2 hardened to 58 to60 HRC have also been used to trim aluminumalloy forgings and may offer longer service lives.Hot trimming of aluminum alloys is usuallyaccomplished immediately after forging without

reheating and generally results in shorter flowtimes but increased risk of part distortion thatwill require correction in subsequent processes.Forming. Some aluminum alloy forging

shapes combine hot forging with hot, warm, orcold forming to achieve the shape. As an exam-ple, the 6061 alloy aluminum heavy truck wheelshown in Fig. 10 is closed-die forged, whichincludes forward extrusion of the wheel skirt,and then hot formed to the final shape. Forming isaccomplished on mechanical and hydraulicpresses and on specialized forming equipment,such as spin formers discussed previously, thatare frequently integrated as a part of a forgingcell with the forging press.Repair or conditioning is an intermediate

operation that is conducted between forgingstages in aluminum alloys. It is frequentlynecessary to repair the forgings, by milling,grinding, and so forth, to remove surface dis-continuities created by the prior forging step sothat such discontinuities do not affect the integ-rity of the final forging product. The need forrepair is usually a function of part complexityand the extent of the tooling manufactured toproduce the part. There is typically a cost trade-off between increased tooling (or number of diesets) and requirements for intermediate repairthat is unique to each forging configuration.Intermediate repair of aluminum alloys is usuallyaccomplished by hand milling, grinding,machining, and/or chipping techniques.

Cleaning. Aluminum alloy forgings areusually cleaned as soon as possible after beingforged. The following treatment is a standardcleaning process that removes lubricant residueand leaves a good surface with a natural alumi-num color:

1. Etch in a 4 to 8% (by weight) aqueous solu-tion of caustic soda at 70 �C (160 �F) for 0.5to 5 min.

2. Rinse immediately in hot water at 75 �C(170 �F) or higher for 0.5 to 5 min.

3. Desmut by immersion in a 10% (by volume)aqueous solution of nitric acid at 88 �C(190 �F) minimum.

4. Rinse in hot water.

The caustic etch, rinse, and nitric desmut processis a potential source of problematic emissionsand of excessive pitting of the surfaces of theworkpieces. Thus, the process and equipment arecarefully controlled and maintained. Theimmersion time in the first two steps varies,depending on the amount of soil to be removedand the forging configuration. The frequency ofcleaning during the forging process sequencealso depends on the forging configuration, theprocess used to produce it, and customer speci-fications. Some forgings are not cleaned until justbefore final inspection. However, some forgingapplications and/or customers require a muchmore rigorous cleaning protocol that involvescleaning after every forging step, prior to heattreatment and prior to final inspection. Addi-tional information on the cleaning of aluminum

alloys is available in the article “Cleaning andFinishing of Aluminum and Aluminum Alloys”in Surface Engineering, Volume 5 of ASMHandbook, 1994.Heat Treatment. All aluminum alloy for-

gings, except 1xxx, 3xxx, and 5xxx series alloys,are heat treated with solution treatment, quench,and artificial aging processes in order to achievefinal mechanical properties. The furnaces usedto heat treat and age aluminum alloy forgingsare either continuous or batch type, fullymuffled gas-fired, electric, fluidized-bed, orother specially designed equipment. Aluminumalloy forgings are immersion quenched becausethis technique is best suited to the relatively lowproduction volumes of forgings and the widerange of forging shapes produced. Because ofthe shape complexity of aluminum forgings,immersion quench racking procedures are par-ticularly critical to obtaining the uniform andsatisfactory quench rates necessary to achievethe required mechanical properties and to mini-mize quench distortion and residual stresses.Therefore, in addition to control of solutiontreatment and age temperature and time, rackingtechniques for forgings are also the subject ofnecessary heat treatment control processes.Furthermore, immersion quenching techni-

ques for aluminum alloy forgings are also criticalbecause of their configuration and frequentlywidely variant cross-sectional thicknesses withinthe same forging. Depending on the specificaluminum alloy being processed, immersionquench media for forgings include controlled-temperature water from 20 to 100 �C (75 to212 �F), synthetic quenchants, such as polyalk-ylene glycol and others additives in water, and

Fig. 10 Forged and formed aluminum alloy 6061-T6truck wheels


most recently alternate, proprietary quenchtechnologies. All immersion quench media aredesigned to achieve the necessary quench rate inorder to develop the required mechanical prop-erties without excessive distortion or excessiveresidual stresses, which adversely affect finalmachining of the component.State-of-the-art aluminum forging solution

treatment and age furnaces have multiple con-trol/recording systems, microprocessor furnacecontrol and operation systems, and quench bathmonitoring and recording equipment, includingvideo camera systems, that provide very precisecontrol and repeatability of the heat treatmentprocess. These systems are interfaced withcomputer integrated manufacturing systems.Aluminum alloy forgings are often straigh-

tened between solution treatment and quenchand artificial aging. Straightening is typicallyaccomplished cold using either hand (frequentlypress assisted) or die straightening techniques.Many aluminum alloy open- and closed-die

forgings in the 2xxx and 7xxx series are com-pressively stress relieved between solutiontreatment and quench and aging in order toreduce or control residual stresses and reduceobjectionable machining distortion. Dependingon the part configuration, such compressivestress relief is accomplished by cold forging ofthe part with open or closed dies, achieving apermanent set (deformation) of 1 to 5%. Withclosed-die compressive stress relief, dependingon part configuration, cold forging is accom-plished either in the hot finish forging dies(temper designation: Txx54) or in a separate setof specially designed cold-work dies (temperdesignation: Txx52). Some annular and othershapes of aluminum alloy forgings are stressrelieved by cold stretching (temper designation:Txx51).The aluminum forging industry has focused on

improving the machining performance of heattreated aluminum forgings to enhance forgingcompetitiveness with other aluminum productforms, especially plate. Specifically, alternativestate-of-the-art quench media, such as syntheticquenchants and recently developed proprietaryquenchants, are being captured because they actsynergistically with enhanced cold compressivestress relief and achieve superior machiningperformance. Further, state-of-the-art CAD andFEM numerical deformation modeling techni-ques have been captured in cold compressivestress relief die, part and process design, andanalysis. Together these two technologies havedramatically reduced or improved control offorging residual stresses and have enabledequivalent machining performance to plate forclosed-die forged shapes. Additional informa-tion on the heat treatment of aluminum alloys,including forgings, is available in the article“Heat Treating of Aluminum Alloys” in HeatTreating, Volume 4 of ASM Handbook (1991)and in Ref 3, 5, and 6.Inspection of aluminum alloy forgings takes

two forms: in-process inspection and finalinspection. In-process inspection, using techni-

ques such as statistical process control and/orstatistical quality control, is used to determinethat the product being manufactured meets cri-tical characteristics and that the forging pro-cesses are under control. Final inspection,including mechanical property testing, is used toverify that the completed forging product con-forms with all drawing and specification criteria.Typical final inspection procedures used foraluminum alloy forgings include dimensionalchecks, heat treatment verification, and non-destructive evaluation.Dimensional Inspection. All final forgings

are subjected to dimensional verification. Foropen-die forgings, final dimensional inspectionmay include verification of all required dimen-sions on each forging or the use of statisticalsampling plans for groups or lots of forgings. Forclosed-die forgings, conformance of the diecavities to the drawing requirements, a criticalelement in dimensional control, is accomplishedprior to placing the dies in service by using lay-out inspection of plaster or plastic casts of thecavities. With the availability of CAD databaseson forgings, such layout inspections can beaccomplished more expediently with CAM-driven equipment, such as coordinate-measuringmachines or other automated inspectiontechniques. With verification of die cavitydimensions prior to use, final part dimensionalinspection may be limited to verifying thecritical dimensions controlled by the process(such as die closure) and monitoring the changesin the die cavity. Further, with high-definitionand precision aluminum forgings, CAD data-bases and automated inspection equipment, suchas coordinate-measuring machines and two-dimensional (2-D) and three-dimensional (3-D)fiber optics, can be used in many cases for actualpart dimensional verification.Heat Treatment Verification. Proper heat

treatment of aluminum alloy forgings is verifiedby hardness measurements and, in the case of7xxx-T7xxx alloys, by eddy-current inspection.In addition to these inspections, mechanicalproperty tests are conducted on forgings to verifyconformance to specifications. Mechanicalproperty tests vary from destruction of forgingsto tests of extensions and/or prolongations forgedintegrally with the parts. Additional informationon hardness and the electrical conductivityinspection and mechanical property testing ofaluminum alloys is available in the article “Heat

Treating of Aluminum Alloys” inHeat Treating,Volume 4 of ASM Handbook, 1991.Nondestructive Evaluation. Aluminum alloy

forgings are frequently subjected to non-destructive evaluation to verify surface orinternal quality. The surface finish of aluminumforgings after forging and caustic cleaning isgenerally good. A surface finish of 125 rms orbetter is considered normal for forged and etchedaluminum alloys. Under closely controlled pro-duction conditions, surfaces smoother than125 rms may be obtained. Selection of non-destructive evaluation requirements depends onthe final application of the forging. Whenrequired, satisfactory surface quality is verifiedby liquid-penetrant, eddy-current, and othertechniques. Aluminum alloy forgings used inaerospace applications are frequently inspectedfor internal quality using high-sensitivity ultra-sonic inspection techniques.

Forging AdvancedAluminum Materials

The preceding discussion of aluminum alloyforging technology is based primarily on exist-ing, commercially available wrought alloys.However, aluminum alloy development con-tinues to provide advanced aluminum materialsdesigned to enhance the capabilities of alumi-num in critical applications, particularly aero-space and automotive components. Alloydevelopment activities pertinent to forgings havebeen focused on development of improvedwrought alloys with superior combinations ofmechanical properties, especially strength inheavy sections, fracture toughness, fatigue crackgrowth resistance, corrosion resistance, reduceddensity, and fatigue resistance.

Advanced Wrought Aluminum Alloys

Wrought aluminum alloy development hasbeen focused on significantly enhancing theperformance capabilities of forgings and otherwrought product forms for key aerospace andautomotive markets that aluminum alloy pro-ducts have dominated for some time. Table 3outlines the nominal compositions of eightrecently developed alloys entering commercial-scale production. Forgings are a strong candidate

Table 3 Nominal compositions of newly developed wrought aluminum alloys

Composition, wt%

Alloy Developer Si Fe Cu Mn Mg Cr Zn Zr Li

2297 McCook 0.10 max 0.10 max 2.8 0.3 0.25 max . . . . . . 0.11 1.46069 NWA 0.9 0.40 max 0.77 0.05 max 1.4 0.18 0.05 max . . . . . .6056 Pechiney 1.01 0.50 max 0.8 0.7 0.9 0.25 max 0.4 . . . . . .7033 Kaiser 0.15 max 0.30 max 1.01 0.10 max 1.75 0.20 max 5.1 0.11 . . .7040 Pechiney 0.10 max 0.13 max 1.9 0.04 max 1.05 0.04 6.2 0.08 . . .7068 Kaiser 0.12 max 0.15 max 2.01 0.10 max 2.6 0.05 max 7.8 0.1 . . .7085 Alcoa 0.06 max 0.06 max 1.65 0.04 max 1.6 0.04 max 7.5 0.11 . . .7449 Pechiney 0.12 max 0.15 max 1.75 0.20 max 2.25 . . . 8.1 0.25 Tiþ Zr . . .


product for capture of these alloys, thus theunique performance characteristics of each ofthese alloys is reviewed. Each of these alloys isreadily fabricated into open- or closed-die for-gings with the processing sequence, equipment,and techniques that have been reviewed.Alloy 2297 is the most commercially

significant aluminum-lithium alloy to haveemerged from extensive research and develop-ment over the last ten years. While forgingapplications to date have been limited, the alloycan be successfully produced via closed-dieforging with significant input material savingsand low buy-to-fly ratios that enable major costreduction. The alloy, whose application is air-frame components, has demonstrated superiorspecific strength and stiffness properties,reduced density (weight savings), and superiorfatigue crack growth resistance to any incumbentaluminum alloy in use.Alloys 6069 and 6056 have excellent combi-

nations of strength and corrosion resistance forautomotive applications, such as suspensioncomponent forgings and in airframe componentapplications. 6069-T6 has strengths about 10%higher than the incumbent alloy 6061-T6 (widelyused in forged automotive components) withcomparable corrosion resistance and superiorfatigue performance. Alloy 6056-T6 has strengthand fatigue capabilities on a par with 2024-T4,but unlike 2024 is fully weldable by all existingtechniques. Also, 6056-T6 has superior corro-sion resistance to the incumbent material in use.Alloys 7033, 7068, and 7449 are all high-

strength or very-high-strength 7xxx series alloysdeveloped to provide significantly enhancedspecific (density compensated) mechanicalproperties for strength and fatigue criticalapplications. Forgings in 7033-T6 are designedto provide significant weight reductions inautomotive structural components over incum-bent 6xxx alloys, but at the same time providegood corrosion resistance. In forgings, 7033 canbe successfully hot and cold forged and stillretain fine grains and excellent microstructures.Alloy 7449 is an alloy intended for aerospaceapplications, in particular advanced wing struc-tures. While the sheet, plate, and extrusions willbe the predominant product forms supplied in7449, wing structures typically contain forgingsas well and thus forgings of this alloy areexpected to be commercially important. Finally,7068-T6 provides among the highest-strengthproperties of any commercially available alu-minum alloy. Forgings in this alloy, whosestrengths approach 690 MPa (100 ksi) but con-currently has good toughness and corrosionresistance, are intended for aerospace applica-tions in competition with existing 7xxx seriesalloys.Alloys 7040 and 7085 are newly developed

high-strength alloys that provide superior com-binations of strength, fracture toughness, andfatigue and stress-corrosion resistance in veryheavy section thicknesses from 150 to 240 mm(6 to 9.5 in.). Forgings, especially large closed-die forgings, are a key product form for capture

of these alloys in critical airframe structure andthereby to reduce weight. Alloy 7040-T7xxx hasbeen demonstrated to provide up to a 10%increase in strength properties in very heavysections when compared to incumbent materialssuch as 7010. 7085-T76xx provides at least a 5%increase in strength in combination with at least a6% increase in fracture toughness when com-pared to incumbent materials such as 7050 andhas demonstrated full mechanical properties insections up to 180 mm (7 in.).

Aluminum-Base DiscontinuousMetal-Matrix Composites

Discontinuous metal-matrix composites areadvanced aluminum materials, where the addi-tion of ceramic particles, or whiskers, to alumi-num-base alloys through the use of either ingotmelting or casting and/or powder metallurgy(P/M) techniques, creates a new class of mate-rials with unique properties. In these materialssystems, the reinforcing material (silicon car-bide, aluminum oxide, boron carbide, or boronnitride) is not continuous, but consists of discreteparticles within the aluminum alloy matrix.Unlike continuous metal-matrix composites,discontinuous metal-matrix composites havebeen found to be workable by all existingmetalworking techniques, including forging.Addition of the reinforcement to the parent alu-minum alloy matrix, typically in volume per-centages from 10 to 40%, modifies the propertiesof the alloy significantly. Typically, compared tothe matrix alloy and temper, these propertymodifications include a significant increase inelastic and dynamic moduli, increase in strength,reduction in ductility and reduction in fracturetoughness, increase in abrasion resistance,increase in elevated-temperature properties, and

no effect on corrosion resistance. A number ofdiscontinuous metal-matrix composite alloysystems are becoming commercially available,based on either 2xxx or 6xxxwrought alloy series.The forging programs with these materials

suggest that reinforcing additions to existingaluminum alloys modify the deformation beha-vior and increase flow stresses. The fabricationhistory of such materials may also be critical totheir deformation behavior in forging and finalmechanical property development. Although therecommended metal temperatures in forgingthese materials remain to be fully defined, cur-rent efforts suggest that temperatures higher thanthose listed in Table 1 for 2xxx and 6xxx matrixalloys are typically necessary. Forging evalua-tions have demonstrated that discontinuousmetal-matrix composites based on existingwrought aluminum alloys in the 2xxx and 6xxxseries can be successfully forged into all forgingtypes, including high-definition and precisionclosed-die forgings. Some evidence suggests thatthese materials are more abusive of closed-dietooling and that die lives in forging these mate-rials may be shorter than is typical of the parentalloys.

Aluminum Alloy Precision Forgings

Precision-forged aluminum alloys are a sig-nificant commercial forging product that hasbeen the subject of significant technologicaldevelopment and capital investment by the for-ging industry. For the purposes of this article, theterm precision aluminum forgings is used toidentify a product that requires no subsequentmachining by the purchaser other than, in somecases, the drilling of attachment holes. Figure 11compares precision aluminum forging designcharacteristics with those of a conventional

Flash extensionin the planeof the metal

Vertical PL

PL

PL

PL

PL

Best metalflow pattern

Thin flange

Metal flowpattern

Machinestock

plus draft

Flashextension

Horizontal

Machineoutline

Narrow ribs

0° to ½°–0° draft

125

Thin web

(a) (b)

Forged undercutor backdraft

surface

surfacefinish

Fig. 11 Cross sections of precision (a) and conventional (b) forgings


aluminum closed-die forging. Precision alumi-num forgings are produced with very thin ribsandwebs; sharp corner and fillet radii; undercuts,backdraft, and/or contours; and, frequently,multiple parting planes that may optimize grainflow characteristics.Design and tolerance criteria for precision

aluminum forgings have been established toprovide a finished product suitable for assemblyor further fabrication. Precision aluminum for-gings do not necessarily conform to the toler-ances provided by machining of other productforms; however, as outlined in Table 4, designand tolerance criteria are highly refined in com-parison with other aluminum alloy forging typesand are suitable for the intended application ofthe product without subsequent machining by thepurchaser. If the standard design and/or tolerancecriteria for precision aluminum forgings are notsufficient, the forging producer frequently com-bines forging and machining to achieve the mostcost-effective method of fabricating the neces-sary tolerances on the finished aluminum part.Tooling and Design. Precision aluminum

forging uses several tooling concepts to achievethe desired design shape, and selection of thespecific tooling concept is based on the designfeatures of the precision-forged part. The threemajor tooling systems used are illustrated inFig. 12. A two-piece upper and lower die system

(Fig. 12a) is typically employed to precisionforge shapes that can be produced with essen-tially horizontal parting lines. This system is verysimilar to the die concepts used for the fabrica-tion of the aluminum alloy blocker, conven-tional, and high-definition closed-die forgingsdiscussed previously. The three-piece (orthrough-die) die system, (Fig. 12b) consists of anupper die, a lower die (through-die), and aknockout/die insert. This system is typicallyemployed for parts without undercuts and withvertical parting lines. The final and most com-plex aluminum precision-forging tooling con-cept is the holder (or wrap-die) system, whichconsists of an upper die, a lower die (or holder),and multiple, movable inserts, or wraps(Fig. 12c). The multiple-insert holder/wrap-diesystem is used to produce the most sophisticatedaluminum precision-forged shapes, includingthose with complex contours, undercuts, andreverse drafts.

The through-die and the holder/wrapmultiple-insert die systems for aluminum alloy precisionforgings are critical elements in the sophistica-tion of the precision-forging parts that can beproduced. Figure 13 provides more insight intothe components comprising these two die sys-tems. These tooling concepts emerged in theearly 1960s with the development of aluminumalloy precision-forging technology and havesince been further refined and developed toprovide increases in the size of precision partmanufactured (see below).

Because the through-die and holder/wrap-diesystems are based on the commonality of sig-nificant portions of the tooling to a range of partsor to families of parts, the fabrication of dies forgiven precision forging is typically restricted tothat necessary to produce the inserts. Thus, thecost of die manufacture for precision forgings isreduced when compared to that necessary toproduce individual dies for each precision shape.However, aluminum precision forging dies/inserts are usually two to four times moreexpensive than dies for other forging types forthe same part.

The holder/wrap multiple-insert die concept isa highly engineered die system that can use twoto six movable segments. Extraction of the part isachieved by lateral opening of the segments(wraps) once they have cleared the bottom dieholder. Figure 14 illustrates the components of

the wrap-die system first when the part has beenforged (Fig. 14a) and then during extraction ofthe completed forging (Fig. 14b).Aluminum alloy precision-forging part and

tooling design are engineering-intensive activ-ities that draw heavily on the experience of for-ging engineers and require interchange betweenproducer and user to define the optimal preci-sion-forging design for utilization, producibility,and cost control. As discussed in the section “DieMaterials, Design, and Manufacture” in thisarticle, CAD, CAM, and CAE technologies havebeen found to be particularly effective in designand tooling manufacture activities for precision

Table 4 Design and tolerance criteria foraluminum precision forgings

Characteristic Tolerance

Draft outside 0� þ 300, � 0

Draft inside 1� þ 300, � 0

Corner radii 1.5+0.75 mm (0.060 0.030 in.)Fillet radii 3.3+0.75 mm (0.130 0.030 in.)Contour +0.38 mm (+0.015 in.)Straightness 0.4 mm in 254 mm (0.016 in. in 10 in.)Minimum webthickness(a)

2.3 mm (0.090 in.)

Minimum ribthickness

2.3 mm (0.090 in.)

Length/widthtolerance

þ0.5 mm, �0.25 mm (þ0.020 in.,�0.010 in.)

Die closuretolerance

þ0.75, �0.25 mm (þ0.030,�0.010 in.)

Mismatchtolerance

0.38 mm (0.015 in.)

Flash extension 0.75 mm (0.030 in.)

(a)Web thicknesses as small as 1.5 mm (0.060 in.) have been produced incertain forging designs

Upperdie

Lower die

(a) (b) (c)

Upperdie Upper

die

Segmenteddie inserts

Lower die

Lower die

Die insert

Fig. 12 Tooling concepts used in the manufacture of precision aluminum forgings. (a) Two-piece die system. (b)Three-piece die (through-die) system. (c) Multipiece (wrap) die system. See also Fig. 13

(a)

(b)

Top punch

Toppunch

Knockout

Knockout

Fig. 13 Components of a three-piece (through-die)system (a) and a multipiece (wrap) die system

(b) used for aluminum precision forgings. Source: Ref 7


forgings to improve the design process, to assistin necessary forging process definition, and toreduce the costs of tooling manufacture.The die materials used in dies, holders, and

inserts for precision aluminum forgings aretypically of the ASM 6F2 and 6G types. In somecases, inserts for high-volume precision alumi-num alloy forgings are produced from hot-workgrades, such as H12 and H13. Tooling for pre-cision aluminum alloy forgings is produced byusing the same techniques described previouslyfor other aluminum alloy forging types; how-ever, CNC direct die sinking or electrical dis-charge machining has been found to beparticularly effective for the manufacture of theclose-tolerance tooling demanded by the designand tolerance criteria for precision aluminumalloy forgings.Processing. Precision aluminum forgings

can be produced from wrought stock, preformedshapes, or blocker shapes, depending on thecomplexity of the part, the tooling system beingused, and cost considerations. Precision alumi-num forgings are usually produced with multipleoperations in finish dies; trimming, etching, andrepair are conducted between operations.Precision aluminum forgings are typically

produced on hydraulic presses, although in somecasesmechanical and/or screw presses have beeneffectively employed. Until recently, most pre-cision aluminum forgings were produced onsmall to intermediate hydraulic presses withcapacities in the range of 9 to 70 MN (1000 to8000 tonf); however, as the size of precisionparts demanded by users has increased, largehydraulic presses in the range of 90 to 310 MN(10 to 35,000 tonf) have been added or upgradedto produce this product. Forging process criteriafor precision aluminum forgings are similar tothose described previously for other aluminumalloy forging types, although the metal and dietemperatures used are usually controlled to nearthe upper limits of the temperature ranges out-lined in Tables 1 and 2 to enhance producibilityand to minimize forging pressures. The three-diesystems described previously are heated withstate-of-the-art die heating techniques. As withother aluminum forging processes, die lubrica-tion is a critical element in precision aluminumforging, and the die lubricants employed,although of the same generic graphite-mineraloil/mineral spirits or graphite-water formula-tions used for other aluminum forging processes,frequently use other organic and inorganiccompounds tailored to the process demands.Because of the design sophistication of pre-

cision aluminum forgings, this aluminum for-ging product is not supplied in mechanicallystress-relieved tempers. However, because of thethin sections and the design complexity of thisproduct, controlled quench rates followingsolution treatment, using such techniques assynthetic and proprietary quenchants, are routi-nely employed to reduce residual stresses in thefinal product and/or to reduce distortion andnecessary straightening to meet dimensionaltolerances. In-process and final inspection for

precision aluminum forgings are the same asdescribed previously for other forging products,including extensive use of automated inspec-tion equipment, such as coordinate-measuringmachines.

Precision aluminum forgings are frequentlysupplied as a completely finished product that isready for assembly. In such cases, the producermay use both conventional and nonconventionalmachining techniques, such as chemical milling,along with forging to achieve the most cost-

effective finished shape. Further, the forgingproducer may apply a wide variety of surface-finishing and coating processes to this product asspecified by the purchaser.Technology Development and Cost Effec-

tiveness. Table 5 presents a summary of thestate-of-the-art in the size of aluminum precisionforging producible. The size of precision alu-minum forging that can be fabricated to thedesign and tolerance criteria listed in Table 4has nearly doubled from 1775 cm2 (275 in.2)

Top die

Top punch

Bottom insertand knock-out

Bottom insertand knock-out

Bottom die

Bottom dieKnock-out stem

Knock-out stem

Bottominsert

(b)(a)

Guide pin

Top punch

Fig. 14 Multipiece (wrap) die system. (a) During forging. (b) After forging, the die system opens to allow extraction ofthe completed part

Table 5 Capabilities of the precision aluminum forging process based on part size

Maximum size that can be processed

Forging type Feature Past Present

T or U section Plan view area 2580 cm2 (400 in.2) 3870 cm2 (600 in.2)Length 1015 mm (40 in.) 1525 mm (60 in.)

H section Plan view area 1775 cm2 (275 in.2) 2580 cm2 (400 in.2)Length 610 mm (24 in.) 1015 mm (40 in.)

280 mm

1016 mm

51 mm

Fig. 15 Very large aluminum alloy 7075-T73 H section precision forging. Plan view area: 2840 cm2 (440 in.2); ribs 2to 2.5 mm (0.080 to 0. 100 in.) thick, 51 mm (2 in.) deep; webs typically 3 mm (0.120 in.), 2 mm (0.080 in.)

in selected areas; finished weight: 5.6 kg (12.3 lb)


for H cross sections to more than 2580 cm2

(400 in.2) through enhancements in the preci-sion aluminum forging processes and forgingand ancillary equipment by forging producersand especially by capture of key enabling tech-nologies such as computer-assisted design andmanufacture and 2-D and 3-D FEM deformationand process modeling.The precision forging shown in Fig. 15 illus-

trates the very large precision aluminum shapesbeing fabricated commercially wherein this dif-ficult H cross-section forging has a plan view

area of 2840 cm2 (440 in.2). This part incorpo-rates some machining in its manufacturing flowpath in selected regions where standard preci-sion-forging tolerances are insufficient forassembly. Critical elements in achieving thecurrent state-of-the-art for aluminum precisionparts are enhanced precision forging processcontrol, CAD/CAM/CAE technologies, 2-D and3-D FEM numerical deformation, process andthermal modeling techniques, advanced and/orintegrated manufacturing technologies, andadvanced die heating and die lubrication sys-tems. Cost-effective design and fabrication oflarge, state-of-the-art precision aluminum for-gings absolutely demand the capture andexploitation of all of these technologies in orderto ensure that the high-strength aluminum pre-cision forged product remains competitiveagainst other component fabrication techniques.

Selection of precision aluminum forging fromthe candidate methods of achieving a final alu-minum alloy shape is based on value analyses forthe individual shape in question. Figure 16 pre-sents a cost comparison for a channel-type alu-minum alloy part machined from plate, asmachined from a conventional aluminum for-ging, and produced as a precision forging. Costsas a function of production quantity includeapplication of all material, tooling, setup, andfabrication costs. The breakeven point for theprecision-forging method versus a conventionalforging occurs with a quantity of 50 pieces, andwhen compared to the cost of machining the partfrom plate, the precision forging is always lessexpensive. Figure 16 also illustrates the potentialcost advantages of precision aluminum alloyforgings. It has generally been found that preci-sion aluminum forgings are highly cost effectivewhen alternate fabrication techniques includemultiple-axis machining in order to achieve thefinal part.

Recent forging industry and user evaluationshave shown that precision aluminum forgingscan reduce final part costs by up to 80 to 90% incomparison to machined plate and 60 to 70% incomparison to machined conventional forgings.

Machining labor can be reduced by up to 90 to95%. With such possible cost reductions inexisting aluminum alloys and with the advent ofmore costly advanced aluminum materials, it isevident that further growth of precision alumi-num forging use can be anticipated.

REFERENCES

1. S. Wasco et al., Forging Processes andMethods, Forging Handbook, T.G. Byrer,S.L. Semiatin, and D.C. Vollmer, Ed., For-ging Industry Association and AmericanSociety for Metals, 1985, p 164

2. L.J. Ogorzaly, Forging Design Principles andPractices for Aluminum, Forging Handbook,T.G. Byrer, S.L. Semiatin, and D.C. Vollmer,Ed., Forging Industry Association andAmerican Society for Metals, 1985, p 34–69

3. J.R. Douglas, G.W. Kuhlman, and D.R.Shrader, “Forging Manual for Aluminum andTitanium Alloys,” AFRL STTR Phase IIContract, F33615-99-C-5078, Air ForceResearch Laboratories, Wright-Patterson AirForce Base, OH, June 2002

4. Aluminum Forging Design Manual, 2nd ed.,Forgings and Impacts Division of the Alu-minum Association, 1995

5. J.E. Hatch, Ed., Aluminum: Properties andPhysical Metallurgy, American Society forMetals, 1984, p 134–199

6. M. Tiryakioglu and L. Lalli, Ed., Metallur-gical Modeling for Aluminum Alloys, ASMInternational, Oct 2003

7. Document D6-72713, Boeing Company, July1985

SELECTED REFERENCES

� Product Design Guide for Forgings, ForgingIndustry Association, 1997

� G.W. Kuhlman, Forging Aluminum Alloysfor Automotive Applications, Fifth PrecisionForging Conference, Forging Industry Asso-ciation, Oct 1999

Hand forgingConventionalPrecision

1200

1000

800

600

400

20025 50 75 100

Production quantity

Tot

al c

ost/p

iece

,$

125

Fig. 16 Cost comparison for the manufacture of analuminum alloy 7075-T73 component


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Forging of Aluminum Alloys - NIST