Chapter 14

Process Design in Impression Die ForgingT. Altan and M. Shirgaokar, ERC/NSM, NSM Laboratory, Ohio State University

FORGING is a process by which a billet ofsimple cross section is plastically deformed byapplying compressive forces through dies or toolsto obtain a more complex shape. In impressiondie forging, two or more dies are moved towardeach other to form a metal billet that is heated tothe appropriate forging temperature. This processis capable of producing components of high qual-ity at moderate cost. It offers a high strength-to-weight ratio, toughness, and resistance to impactand fatigue. Forged components find applicationin the automobile/automotive industry and in air-craft, railroad, and mining equipment.

Some parts can be forged in a single set ofdies, while others, due to shape complexity andmaterial flow limitations, must be shaped in mul-tiple sets of dies. In a common multistage forgingprocess, the part is first forged in a set of bustingdies, then moved to one or more sets of blockingdies, and finally, forged in finisher dies. Finisherdies are used to enhance geometrical detailswithout significant material flow. The quality ofthe finished part depends greatly on the design ofthe previous stages. If the material has been dis-tributed improperly during the blocking stage,defects may appear in the finishing stage. In agood-quality forging, all sections of the die cav-ity must be filled, and the part must not containflow defects, such as laps, cold shuts, or folds.

Before being used in production, forging diesare tested to verify proper filling of the die cavi-ties. The most commonly used method ofprocess verification is die tryout, in which full-scale dies are manufactured and prototype partsare forged to determine metal flow patterns andthe possible occurrence of defects. This methodoften takes several iterations and is very costlyin terms of time, materials, facilities, and labor.Alternatively, two other methods for modelingmetal flow, namely, physical modeling andprocess simulation using finite-element method(FEM)-based software, can be used to obtain in-formation about the effects of die design andprocess variables on the forging process.

The design of any forging process begins withthe geometry of the finished part (Fig. 1).Consideration is given to the shape of the part, thematerial to be forged, the type of forging equip-

ment to be used, the number of parts to be forged,the application of the part, and the overall economyof the process being designed. The finisher die isthen designed with allowances added for flash,draft, shrinkage, fillet and corner radii, and posi-tioning of the parting line. When using multistageforging, the shapes of the preforms are selected, the

blocker dies are designed, and the initial billetgeometry is determined. In making these selec-tions, the forging designer considers design param-eters such as grain flow, parting line, flash dimen-sions, draft angles, and fillet and corner radii.

The terminology used to describe the flashzone in impression and closed-die forging can

Yes

No

FEM programfor metalforming

Database withdie/materialproperties

Functional requirements

Part geometry (assembly ready)

Part design for process(based on experience/rules)

Preliminary die design(based on experience/rules)

Select process/machine variables

Verify die design and processvariables/simulate metal flow

Die design andprocess variables

acceptable?

Analyze die design for stresses,shrinkage, and process conditions

Prepare drawings and machine dies (CNC)

Install dies,select machine parameters,

start forming process

Modify die/partdesign

Fig. 1 A flow chart illustrating forging process design. CNC, computer numerical control; FEM, finite-element method

be seen in Fig. 2. The flash dimensions and bil-let dimensions influence:

● Flash allowance, that is, the material thatflows into the flash zone

● Forging load● Forging energy● Die life

The overall design of a forging process re-quires the prediction of:

● Shape complexity and volume of the forg-ing

● Number and configurations of the preformsor blockers

● The flash dimensions in the dies and the ad-ditional flash volume required in the stockfor preforming and finishing operations

● The forging load, energy, and center ofloading for each of the forging operations

Forging Process Variables

The interaction of the most significant vari-ables in forging is shown in a simplified mannerin Fig. 3. It is seen that for a given billet materialand part geometry, the ram speed of the forgingmachine influences the strain rate and flowstress. Ram speed, part geometry, and die tem-perature influence the temperature distributionin the forged part. Finally, flow stress, friction,and part geometry determine metal flow, forgingload and forging energy, and, consequently, in-fluence the loading and the design of the dies.Thus, in summary, the following three groups offactors influence the forging process:

● Characteristics of the stock or preform to beforged, flow stress and the workability at var-ious strain rates and deformation conditions,stock temperature, preform shape, and so on

● Variables associated with the tooling and lu-brication: tool materials, temperature, de-sign of drafts and radii, configuration, flashdesign, friction conditions, forging stresses,and so on

● Characteristics of the available equipment:load and energy capacities, single or multi-blow availability, stiffness, ram velocityunder load, production rate, availability ofejectors, and so on

Forging Materials. Table 1 lists differentmetals and alloys in order of their respective

forging difficulty (Ref 2). The forging materialinfluences the design of the forging itself as wellas the details of the entire forging process. Forexample, Fig. 4 shows that, owing to difficultiesin forging, nickel alloys allow for less shape def-inition than do aluminum alloys.

In most practical hot forging operations, thetemperature of the workpiece material is higherthan that of the dies. Metal flow and die fillingare largely determined by:

● Forging material resistance to flow and abilityto flow, that is, its flow stress and forgeability

● Friction and cooling effects at the die-mate-rial interface

● Complexity of the forging shape

For a given metal, both the flow stress andforgeability are influenced by the metallurgicalcharacteristics of the billet material and by thetemperatures, strain, strain rates, and stressesthat occur in the deforming material. The flowstress determines the resistance to deformation,that is, the load, stress, and energy requirements.Forgeability has been used vaguely in the litera-ture to denote a combination of both resistanceto deformation and ability to deform withoutfracture. A diagram illustrating this type of in-formation is presented in Fig. 5.

In general, the forgeabilities of metals in-crease with increasing temperature. However, as

2 / Process Design and Workability

Flash

Gutter

Partingline

External and internal draft angles

RibWebFillet

CornerTrim line

Land

Upper die

Lower die

Flash land

Flash

Fig. 2 Schematic of a die set and the terminology usedin impressed die forging with flash

Ram velocity

Billet/forginggeometry

volume, thickness

Die temperature,cooling

Interface,lubrication

Strain rate

Contact timeunder pressure

Temperaturedistribution in

forging

• Metal flow• Forging load• Forging energy

Flow stress/forgeability

Functionconditions and

coefficient

Data on billet material

Fig. 3 Variables in forging

Table 1 Hot forging temperatures ofdifferent metals and alloys

Approximate range ofMetal or alloy forging temperature, ∞C (∞F)

Aluminum alloys (least difficult) 400–500 (750–930)Magnesium alloys 250–350 (480–660)Copper alloys 600–900 (1110–1650)Carbon and low-alloy steels 850–1150 (1560–2100)Martensitic stainless steels 1100–1250 (2010–2280)Maraging steels 1100–1250 (2010–2280)Austenitic stainless steels 1100–1250 (2010–2280)Nickel alloys 1000–1150 (1830–2100)Semiaustenitic PH stainless steels 1100–1250 (2010–2280)Titanium alloys 700–950 (1290–1740)Iron-base superalloys 1050–1180 (1920–2160)Cobalt-base superalloys 1180–1250 (2160–2280)Niobium alloys 950–1150 (1740–2100)Tantalum alloys 1050–1350 (1920–2460)Molybdenum alloys 1150–1350 (2100–2460)Nickel-base superalloys 1050–1200 (1920–2190)Tungsten alloys (most difficult) 1200–1300 (2190–2370)

PH, precipitation-hardenable. Source: Ref 2

Fig. 4 Comparison of typical design limits for rib-web-type structural forgings of (a) aluminum al-

loys and (b) nickel-base superalloys. All dimensions inmillimeters. Source: Ref 2

(a) (b)

and temperature effects, for the same forgingprocess, different forging loads and energies arerequired by different presses. For the hammer, theforging load is initially higher, due to strain-rateeffects, but the maximum load is lower than for ei-ther hydraulic or screw presses. The reason for thisis that in the presses, the extruded flash cools rap-idly, whereas in the hammer, the flash temperatureremains nearly the same as the initial stock tem-perature. Thus, in hot forming, not only the mate-rial and the formed shape but also the type ofequipment used (rate of deformation and die chill-ing effects) determine the metal flow behavior andthe forming load and energy required for theprocess. Surface tearing and cracking or develop-ment of shear bands on the formed material oftencan be explained by excessive chilling of the sur-face layers of the formed part near the die-materialinterface.

Production Lot Size and Tolerances. As isthe case in all manufacturing operations, thesetwo factors have a significant influence on diedesign in forging. If the production lot size islarge, the main reason for changing the dies isdie wear. In this case, die materials and theirhardnesses are selected to be especially wear re-sistant, even if they are made from somewhat ex-pensive alloys. The preforming and the finishingdies are designed such that relatively little mate-rial movement is allowed in the finisher dies;thus, the finisher dies, which determine the finalpart dimensions, do not wear out easily.

If the production lot size is small, as is thecase in the aerospace forging industry, die wearis not a major problem, but die costs are verysignificant because these costs must be amor-tized over a smaller number of parts. As a result,some of the preforming or blocker dies may beomitted, even if this would cause the use of morebillet material. Also, in this case, the dies mustbe changed more often than in large-scale pro-duction. Therefore, quick die changing and au-tomatic die-holding mechanisms are required foreconomic production.

Forging tolerances are very important in design-

Chapter 14: Process Design in Impression Die Forging / 3

temperature increases, grain growth occurs, andin some alloy systems, forgeability decreaseswith increasing grain size. The forgeabilities ofmetals at various deformation rates and temper-atures can be evaluated by using various tests,such as torsion, tension, and compression tests.In all these tests, the amount of deformationprior to failure of the specimen is an indicationof forgeability at the temperature and deforma-tion rates used during that particular test.

Forging Equipment. In hot and warm forg-ing, the behavior and the characteristics of theforging press influence:

● Contact time between the material and thedies under load. This depends on the ramvelocity and the stiffness of a given press.The contact time is extremely important be-cause it determines the heat transfer be-tween the hot or warm material and thecolder dies. Consequently, the contact timealso influences the temperatures of the forg-ing and that of the dies. When the contacttime is large, the material cools down ex-cessively during deformation, the flowstress increases, and the metal flow and diefilling are reduced. Thus, in conventionalforging operations, that is, non-isothermal,it is desirable to have short contact times.

● Rate of deformation, that is, the strain rate. Incertain cases, for example, in isothermal andhot die forging of titanium and nickel alloys,that are highly rate dependent, the large rateof deformation leads to an increase in flowstress and excessive die stresses.

● Production rate. With increasing stroke rate,the potential production rate increases, pro-vided the machine can be loaded and un-loaded with billet or preforms at these in-creased rates.

● Part tolerances. Hydraulic and screwpresses, for example, operate with kissingdies, that is, the dies have flat surfaces that

contact each other at the end of each work-ing stroke of the forging press. This allowsvery close control of the thickness toler-ances, even if the flow stress and frictionconditions change during a production run.Ram guiding, stiffness of the press frame,and drive also contribute to tolerances thatcan be achieved in forging.

Friction and Lubrication. The flow ofmetal in forging is caused by the pressure trans-mitted from the dies to the deforming material;therefore, the friction conditions at the die-mate-rial interface are extremely important and influ-ence the die stresses and the forging load as wellas the wear of the dies. In order to evaluate theperformances of various lubricants and to beable to predict forming pressures, it is necessaryto express the interface friction quantitatively, interms of a factor or coefficient. In forging, thefrictional shear stress, t, is most commonly ex-pressed as:

(Eq 1)

where t is the frictional shear stress, s– is effec-tive stress, f is the friction factor, and m is theshear friction factor (0 ≤ m ≤ 1).

For various forming conditions, the values ofm vary as follows:

● m = 0.05 to 0.15 in cold forging of steels,aluminum alloys, and copper, using conven-tional phosphate soap lubricants or oils

● m = 0.2 to 0.4 in hot forging of steels, cop-per, and aluminum alloys with graphite-based lubricants

● m = 0.1 to 0.3 in hot forging of titanium andhigh-temperature alloys with glass lubri-cants

● m = 0.7 to 1 when no lubricant is used, forexample, in hot rolling of plates or slabsand in nonlubricated extrusion of aluminumalloys

Heat Transfer and Temperatures. Heattransfer between the forged material and the diesinfluences the lubrication conditions, die life,properties of the forged product, and die fill.Often, temperatures that exist in the materialduring forging are the most significant variablesinfluencing the success and economics of agiven forging operation. In forging, the magni-tudes and distribution of temperatures dependmainly on:

● The initial material and die temperatures● Heat generated due to plastic deformation

and friction at the die-material interface● Heat transfer between the deforming mate-

rial and the dies as well as between the diesand the environment (air, coolant, lubricant)

The effect of contact time on temperatures andforging load is illustrated in Fig. 6, where the load-displacement curves are given for hot forging of asteel part using different types of forging equip-ment. These curves illustrate that, due to strain rate

t s s= =fm

3

Fig. 5 Generalized diagram illustrating the influenceof forgeability and flow stress on die filling.

Source: Ref 2

0 0.2 0.4 0.6 0.8 1.0

0 5 10 15 20 250

25

50

75

100

125

150

175

0

27.5

55

82.5

110

137.5

165

192.5

Drop hammerVpi = 21.1 ft/s,

or 6.4 m/s

Screw pressVpi = 0.96 ft/s,

or 0.29 m/s

Hydraulic pressVpi = 0.33 ft/s,

or 0.1 m/s

H0 – H, in.

Displacement, Ho – H, mm

For

ging

load

, met

ric

tons

U.S

. ton

s

1.18

1.58

Fig. 6 Load vs. displacement curves for the same partforged in three different machines with different

initial velocities, Vpi (dimensions of the part in inches; ini-tial temperature = 1100 ∞C (2012 ∞F): H0, initial height; H,instantaneous height)

ing the die holders and die inserts because they de-pend considerably on the manufacturing tolerancesand elastic deflections of the dies during forging.Precision forging of gears and blades, for example,requires not only very close manufacturing accura-cies on the dies but also close control of die tem-peratures. In addition, it is often necessary to esti-mate the changes in die dimensions under forgingconditions so that corrections can be made whiledesigning and manufacturing these dies. Die di-mensions vary during the forging operation be-cause of thermal expansion, mechanical loadingduring assembling of the dies in a holder, and me-chanical loading during the forging process itself.

Design of Finisher Dies

The most critical information necessary forforging die design is the geometry of the forgingto be produced. The forging geometry, in turn, isobtained from the machined part drawing by mod-ifying this part to facilitate forging. Starting withthe forging geometry, the die designer first designsthe finisher dies by selecting the appropriate dieblock size and the flash dimensions and estimatingthe forging load and stresses to ascertain that thedies are not subjected to excessive loading.

The geometry of the finisher die is essentiallythat of the finish forging augmented by flashconfiguration. In designing finisher dies, the di-mensions of the flash should be optimized. Thedesigner must make a compromise; on the onehand, to fill the die cavity, it is desirable to in-crease the die stresses by restricting the flash di-mensions (thinner and wider flash on the dies),but, on the other hand, the designer should notallow the forging pressure to reach a high value,which may cause die breakage due to mechani-cal fatigue. To analyze stresses, the slab methodof analysis or process simulation using FEM-based computer codes is generally used.

By modifying the flash dimensions, the dieand material temperatures, the press speed, andthe friction factor, the die designer is able toevaluate the influence of these factors on theforging stresses and loads. Thus, conditions thatappear most favorable can be selected. In addi-tion, the calculated forging-stress distributioncan be used for estimating the local die stressesin the dies by means of elastic FEM analysis.After these forging stresses and loads are esti-mated, it is possible to determine the center ofloading for the forging in order to locate the diecavities in the press, such that off-center loadingis reduced.

Flash Design in Closed-Die Forging. Asmentioned earlier, the flash dimensions and thebillet dimensions influence the flash allowance,forging load, forging energy, and the die life.The selection of these variables influences thequality of the forged part and the magnitude offlash allowance, forging load, and the die life.The influence of flash thickness and flash landwidth on the forging pressure is reasonably wellunderstood from a qualitative point of view. Theforging pressure increases with:

● Decreasing flash thickness● Increasing flash land width because of the

combinations of increasing restriction, in-creasing frictional forces, and decreasingmetal temperatures at the flash gap

Flash Dimensions. The variations in the flashdimensions influence the forging load, forging en-ergy, and the flash allowance used to determinethe initial material (billet) volume. The dimen-sions of the flash can be varied in three ways:

● Changing the flash width with constantthickness

● Changing the flash thickness with constantwidth

● Changing the flash width and thickness withconstant width-thickness ratio

Choosing the Flash Width and Thickness.Several factors influence the choice of a goodflash thickness. The choice of the flash thicknessis influenced by the part weight as well as theshape complexity (Fig. 7). Based on the complex-ity, the majority of forging parts are classified into:

● Compact shape, spherical and cubical (class1)

● Disc shape (class 2)● Oblong shape (class 3)

The first group of compact shapes has thethree major dimension, namely, the length (l),breadth or width (w), and the height (h), approx-imately equal. The number of parts that fall intothis group is rather small.

The second group consists of disk shapes forwhich two of the three dimensions (length and

4 / Process Design and Workability

h

Shape class 1,compact shape

Shape class 2,disc shape

Parts with circular, square,

and similarcontours;

cross piece withshort arms;

upset heads; and

long shapes(flanges, valves,

etc.)

Sub-group

Sub-group

Shapegroup

Sub-group

Shapegroup

Nosubsidiaryelements

Nosubsidiaryelements

No subsidiaryelements

Unilateralsubsidiaryelements

Unilateralsubsidiaryelements

Rotationalsubsidiaryelements

101 102 103 104

With hub

With huband hole

Withrim

With rimand hub

Discshapewith

unilateralelement

Discshapewith

bilateralelement

Spherical and cubical

21 211 212 213 214 215

22 222 223 224 225

l

h w l = w = h

l = w > h

l > w ≥ h

l

h

l

h w

Subsidiaryelementsparallel

to axis of principalshape

With twoor more

subsidiary elementsof similar

size

With open or closed

forkelement

Withsubsidiaryelements

asymmetricalto axis of

principal shape

Shape class 3,oblong shape

Principalshape

elementwith

straightaxis

Longit.axis ofprincialshape

elementcurved inone plane

Long. axisof princial

shapeelementcurved inseveral planes

31131 312 313 314 315

32132

33

322 323 324 325

331 332 333 334 335

Parts with pronouncedlongit. axis

length groups:

1. Short partsl < 3w

2. Avg. lengthl = 3w to 8w

3. Long partsl = 8w to 16w

4. Very long partsl > 16w

Length groupnumbers added

behind bar,e.g., 334/2

w

≈>

…

Fig. 7 Classification of forging shapes. l, length; w, width; h, height. Source: Ref 3

width) are approximately equal and are largerthan the height. All the round forgings belong tothis group, which includes approximately 30%of all the commonly used forgings.

The third group of forgings consists of longshapes, which have one dimension significantlylarger than the other two (l > w ≥ h).

These three basic groups are further subdi-vided into subgroups, depending on the presenceand type of elements subsidiary to the basicshape. This classification is useful for practicalpurposes, such as estimating costs and predict-ing preforming steps. This method, however, isnot entirely quantitative and requires some sub-jective evaluation based on past experience.Depending on the shape complexity of the partthat the user desires to produce, a range ofgraphs can be selected for each group.

Figure 8 shows a graph for selecting the flashthickness based on the weight, Q, of the forgingfor a particular group of forgings. This graphshows the relationship between the flash width-thickness (w/t) ratio and the forging weight.Thus, knowing the weight of the part to beforged, it is possible to find the correspondingflash thickness and w/t ratio. Thus, the user canobtain the flash dimensions based on the weightof the forging.

There is no unique choice of the flash dimen-sions for a forging operation. The choice is vari-able within a range of values where the flash al-lowance and the forging load are not too high.There has to be a compromise between these two.

Empirical Formulae. There are differentsets of formulae, based on billet dimensions, to

determine the flash dimensions. These dimen-sions are used to obtain little flash allowance andto minimize the forging energy.

For round forgings, Eq 2 and 3 predict flashdimensions that are a good compromise betweenflash allowance and forging load (Ref 4):

(Eq 2)

(Eq 3)

where w is the flash width, (mm). t is the flashthickness, (mm). H is the height of the ribs orshaft, D is the outside diameter of the forging,and Rh is the radial distance of the center of a ribfrom the axis of symmetry of the forging.

Preform (Blocker) Design inImpression-Die Forging

One of the most important aspects of closed-die forging is the design of preforms or blockersto achieve adequate metal distribution. The de-termination of the preform configuration is anespecially difficult task and an art by itself, re-quiring skills achieved only by years of exten-sive experience.

In preforming, round or round-corneredsquare stock with constant cross section is de-formed in such a manner that a desired volume

w

tD

D

H R Dh

=

+ ◊+

ÊËÁ

ˆ¯̃

È

ÎÍÍ

˘

˚˙˙

30

122

2

3( )

t DD

= ◊ ++

È

ÎÍ

˘

˚˙[ . ]0 017

1

5

Chapter 14: Process Design in Impression Die Forging / 5

distribution is achieved prior to impression dieforging. In blocking, the preform is forged in ablocker cavity prior to finish forging. Designinga correct preform allows the control of the vol-ume distribution of the part during forging aswell as control over the material flow. The ob-jectives of preform design are:

● Ensure defect-free metal flow and adequatedie filling

● Minimize the amount of material lost asflash

● Minimize die wear in the finish-forging cav-ity by reducing the metal movement in theoperation

● Achieve desired grain flow and control me-chanical properties

Basic Rules of Preform Design. In forgingsteel parts, a correct preform can be designed byusing the following three general design rules(these rules do not apply to forging nonferrousmaterials):

● The area of cross section of the preformequals the area of cross section of the fin-ished product plus the flash allowance(metal flowing into flash). Thus, the initialstock distribution is obtained by determin-ing the areas of cross sections along themain axis of the forging.

● All the concave radii, including the filletradii, on the preform must be greater thanthe corresponding radii on the finished part.

● In the forging direction, the thickness of thepreform should be greater than that of the fin-ished part so that the metal flow is mostly byupsetting rather than extrusion. During thefinishing stage, the material is then squeezedlaterally toward the die cavity without addi-tional shear at the die-material interface. Suchconditions minimize friction and forging loadand reduce wear along the die surfaces.

In attempting to develop quantitative or ob-jective engineering guidelines for preform de-sign, a thorough understanding of metal flow isessential. Metal flow during forging occurs intwo basic modes:

● Parallel to the motion of the dies, that is, ex-trusion

● Perpendicular to the motion of the dies, thatis, upsetting

Conventionally, blocker dies were designedusing some guidelines, which are summarized asfollows. Prior to the advent of computer-aideddesign methods, blocker dies and preforms weredesigned by tryouts. The guidelines used dependon the material and the forging machines used:

● The blocker is slightly narrower than thefinisher in the top view by approximately0.5 to 1.0 mm (0.02 to 0.04 in.) and haslarger fillet and corner radii. This helps en-hanced metal distribution.

● The areas of the various blocker cross sec-tions are augmented from those of the fin-isher by the flash allowance.

5.0

4.0

3.0

2.0

1.0

0

5.0

6.0

7.0

8.0

9.0

10.0

4.0

3.0

2.0

1.0

0

0.39

0.31

0.24

0.16

0.08

in. mm.

Fla

sh th

ickn

ess,

t

w/t

ratio

0.1 0.5 1 5 10 50 100 kg2.2 22 110 220 lb

Forging weight, Q

Fig. 8 Variation in flash thickness (t) and width-thickness (w/t) ratio for carbon and alloy steel forgings of differentweights. Source: Ref 3

● To forge high ribs in the finisher, those inthe blocker are, at times, shorter. Addition-ally, the web thickness in the blocker islarger than that in the finisher.

● To enhance the metal flow toward the ribs,an opening taper may be useful from thecenter of the web toward the ribs.

● In the case of steel forgings, whenever pos-sible, the ribs in the blocker sections shouldbe narrower but slightly higher than those inthe finisher sections to reduce the die wear.

The common practice in preform design is toconsider planes of metal flow, that is, selectedcross sections of the forging (Fig. 9).Understanding the principles of the materialflow during the forging operation can help attaina better understanding of the design rules. Anycomplex shape can be divided into axisymmetricor plane-strain flow regions, depending on thegeometry in order to simplify the analysis.

The example steel forging presented in Fig. 10illustrates the various preforming operations nec-essary to forge the part shown. The round barfrom rolled stock is rolled in a special machinecalled a reducer roller for volume distribution,bent in a die to provide the appropriate shape,blocked in a blocker die cavity, and finish forged.

In determining the forging steps for any part, itis first necessary to obtain the volume of the forg-ing based on the areas of successive cross sec-tions throughout the forging. The volume distri-bution can be obtained in the following manner:

1. Lay out a dimensioned drawing of the finishconfiguration, complete with flash.

2. Construct a baseline for area determinationparallel to the centerline of the part.

3. Determine the maximum and minimumcross-sectional areas perpendicular to thecenterline of the part.

4. Plot these area values at proportional dis-tances from the baseline.

5. Connect these points with a smooth curve.6. Above this curve, add the approximate area

of the flash at each cross section, giving con-sideration to those sections where the flashshould be widest. The flash is generally ofconstant thickness but is widest at the nar-rower sections and smallest at the wider sec-tions.

7. Convert the minimum and maximum areavalues to rounds or rectangular shapes havingthe same cross-sectional area.

Figure 11 shows two examples of obtaining avolume distribution through the previously men-tioned procedure.

The applications of the design rules for pre-forming are illustrated by examples shown inFig. 12. Figure 13 shows some suggestedblocker and finish cross sections for varioussteel forgings.

The preform is the shape of the billet beforethe finish operation. In certain cases, dependingon the ratio of the height of the preform to itswidth, there might be more than one preform op-eration involved.

Preform design guidelines differ from mate-rial to material. They are basically categorizedinto the following three categories:

● Carbon and low-alloy steel parts● Aluminum alloy web-rib-type parts● Titanium alloy web-rib-type parts

Guidelines for Carbon and Low-AlloySteels. In hammer forging of carbon or low-alloy steels, the preform usually does not haveflash. The blend-in radius of the preform (RP) atthe parting line is influenced by the adjacentcavity depth (C) (Table 2). In the preform, thefillet radius (RPF) between the web to a rib islarger than that in the finish forging (RFF), espe-cially when the height of the rib over the web islarger than the rib width, that is, DF > wF (Fig.14).

Guidelines for Aluminum Parts. For rib-web-type aluminum alloy parts, the recom-mended preform dimensions fall into the rangesgiven in Table 3. The preform is usually de-signed to have the same draft angles as the finishpart. However, when very deep cavities are pres-ent in the finisher die, larger draft angles are pro-vided in the preform. A greater web thickness inthe preform is selected when the web area is rel-atively small and when the height of the adjoin-ing ribs is very large. A comparison of the pre-form and the finished part is illustrated in Fig.15.

Guidelines for Titanium Alloys. The guide-lines for designing titanium alloy preforms (Table4) are similar to those for aluminum alloys.

Prediction of Forging Stresses andLoads

In designing forging dies, the forging stressesand load must be estimated in order to predictwhether the dies may break under load or notand to select the forging machine with adequateload and energy capacity. In most multistageforging operations, the finish-forging operationrequires the highest load because in the finisherdie, the thickness of the forging and all the filletand corner radii are reduced to obtain the finalpart geometry.

Prediction of the forging load and pressure inclosed- or impression-die forging is difficult dueto the nonsteady state of the process, that is, vari-ables affecting the process, such as temperature,stresses, and so on. In addition, forgings comprisean enormously large number of geometricalshapes and materials that require different, eventhough similar, techniques of engineering analy-sis. The following methods are generally used fordetermination of the forging stresses and loads:

● By empirical formulae, based on past expe-rience

6 / Process Design and Workability

Fig. 9 Planes and direction of metal flow during forging of two simple shapes (left) and a complex shape (right). (a)Planes of flow. (b) Finish-forged shape. (c) Directions of flow

Fig. 10 Preforming, blocking, and finish-forging oper-ations for an example steel forging. Source:

Ref 5

● By performing approximate calculationsthrough one of the well-known methods ofplasticity, such as slab, upper bound, slipline, or FEM.

Load-Stroke Curves. A typical load-ver-sus-stroke curve for a closed-die forging opera-tion indicates that loads are relatively low untilthe more difficult details are partly filled and themetal reaches the flash opening (Fig. 16, 17).This stage corresponds to point P1 in Fig. 17.For successful forging, two conditions must befulfilled when this point is reached (Ref 8):

● A sufficient volume of metal must betrapped within the confines of the die to fillthe remaining cavities.

● The extrusion of metal through the narrow-ing gap of the flash opening must be moredifficult than the filling of the more intricatedetail in the die.

As the dies continue to close, the load in-creases sharply to point P2 (Fig. 17), the stage atwhich the cavity is filled completely. Ideally, atthis point, the cavity pressure provided by theflash geometry is just sufficient to fill the entirecavity, and the forging is completed. However,

Chapter 14: Process Design in Impression Die Forging / 7

P3 represents the final load reached in normalpractice for ensuring that the cavity is com-pletely filled and that the forging has the properdimensions. During the stroke from P2 to P3, allthe metal flow occurs near or in the flash gap,which in turn becomes more restrictive as thedies close. Thus, the detail most difficult to filldetermines the minimum forging load requiredto produce a fully filled forging.

The dimensions of the flash determine thefinal load required to close the dies. The forma-tion of flash, however, is greatly influenced bythe amount of excess material available in thecavity because that amount determines the in-stantaneous height of the extruded flash andtherefore the die stresses.

Studies have revealed that it is possible to filla cavity with various flash geometries, providedthere is always a sufficient supply of material inthe die. Thus, it is possible to fill a die cavityusing a less restrictive flash, that is, a thickerflash, and to do this at a lower total forging loadif the necessary excess material is available or ifthe workpiece is properly preformed. In the for-mer, the advantages of low forging load and cav-ity stress are offset by increased scrap loss. Inthe latter, low stresses and material losses areobtained by extra preforming (Ref 8).

Empirical Methods for Estimation ofForging Pressure and Load. In estimatingthe forging load empirically, the surface area ofthe forging, including the flash zone, is multi-plied by an average forging pressure knownfrom experience. The forging pressures encoun-tered in practice vary from 275 to 950 MPa (20to 70 tons/in.2), depending on the material andthe geometry of the part. Forging experimentswere conducted (Ref 9) with various carbonsteels (up to 0.6% C) and with low-alloy steelsusing flash ratios, w/t (where w is flash-landwidth, and t is the flash thickness), from 2 to 4(Fig. 18). It was found that the variable that mostinfluences the forging pressure, Pa, is the aver-age height, Ha, of the forging. The lower curverelates to relatively simple parts, whereas theupper curve relates to slightly difficult ones (Ref9).

Fig. 11 Preform designs for two example parts. Inboth examples: (a) Forging. (b) Cross-

sectional area vs. length. (c) and (d) Ideal preform. VE andqE, volume and cross section of the finish forging, respec-tively; VG and qG, volume and cross section of the flash,respectively. Source: Ref 5

Upset stock

Preform

Finish

h = b h = 2b h = 3b

None

Trimmed

h hbb

h

b

Fig. 12 Preforms for different H-shaped forgings. h, height; b, breadth. Source: Ref 6

P E P E P E

P E

Fig. 13 The blocker and finish cross sections for var-ious shapes. P, preform; E, end. Source: Ref 6

Table 2 Preform dimensions for carbon orlow-alloy steels

Dimensions of the finish forgings Preform dimensions

Flash No flashBlend-in radii (RF) RP @ RF + C or

HR/6 < RP < HR/4Fillet radii (RFF) RPF @ 1.2 RFF + 3.18 mm

(0.125 in.)Depth of cavity (HR), When HR is less than 10 mm

mm (in.) and depth of (0.4 in.), then C = 2 adjacent cavity (C), mm (0.08 in.)mm (in.)

When HR is between 10 mm and 25 mm (1. in.), then C = 3 mm (0.12 in.)

When HR is between 25 mm and 50 mm (2 in.), then C = 4 mm (0.16 in.)

When HR is greater than 50 mm, then C = 5 mm (0.2 in.)

RP, blend-in radii of perform; RPF, fillet radii of preform; HR, cavitydepth; C, adjacent cavity depth. Source: Ref 7

Fig. 18 Forging pressure versus average forgingheight, Ha, for forging of carbon and low-

alloy steels at flash ratios, w/t, from 2 to 4. Source: Ref 9

180

160

140

120

100

80

60

40

20

228.0

170.1

114.0

57.0

28.5

0

For

ging

pre

ssur

e, 1

000

psi

kg/m

m2

0 5 10 15 20 25 30 35 40 mmin.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Average height of forging Ha

wF

F

P

wP

RPC

RFC

RPC

RPF

tP tF

Preform

Finish

Fig. 15 Comparison of the preform and finished partfor a quarter of an “H” cross section. aF, draft

angle of forging; aP, draft angle of preform; wF, width offorging; wP, width of preform; RFC, corner radius of forg-ing; RPC, corner radius of preform; RPF, fillet radius of pre-form; tP, thickness of preform; tF, thickness of forging.Source: Ref 7

Most empirical methods, summarized interms of simple formulae or nomograms, are notsufficiently general to predict forging loads for avariety of parts and materials. Lacking a suitableempirical formula, one may use suitable analyt-ical techniques of varying degrees of complexityfor calculating forging load and stresses. Amongthese techniques, the relatively simple slabmethod has been proved to be very practical forpredicting forging loads.

Simplified Slab Method to Estimate Forg-ing Load. The slab method has been used suc-cessfully for predicting forging loads andstresses with acceptable engineering accuracy.For this purpose, a forging is divided into vari-ous plane-strain and axisymmetric sections, andthen, simplified equations are used to predict theaverage pressure and load for each section be-fore all these load components are added to-gether. This method, used in the practical pre-diction of forging loads, is shown in Fig. 19 (Ref8). In this analysis, it is assumed that the cavityhas a rectangular shape and the flash geometryillustrated in Fig. 19. In actual practice, where

the cavity is not rectangular, the cross section issimplified to conform to this model.

As seen in Fig. 19, the cavity height is de-noted by H, the radius (or half-width of the cav-ity) by r, the flash thickness by t, and the flashwidth by w. The stresses at various locations ofthe cross section and hence the load acting onthe cross section can be estimated according tothe following equations.

With the flow stress in the flash region de-noted by s0f and the frictional shear factor by m,the stress at the entrance from the cavity into theflash of an axisymmetric cross section, sea, isgiven by:

(Eq 4)

Because of rapid chilling and a high deforma-tion rate, the flow stress in the flash region isconsidered to be different from the flow stress inthe cavity. Hence, two different flow stresses are

s sea = +ÊËÁ

ˆ¯̃

2

31 0m

w

t f

8 / Process Design and Workability

RP

RPF DF

HR

wF

RF

RFF

(a) (b)

Fig. 14 (a) Preform. (b) Finish shape. RPF, fillet radius of preform; RP, blend-in radius of preform; DF, height of forg-ing; RFF, finish-forging radius; wF, width of forging; HR, depth of cavity; RF blend-in radii of forging. Source:

Ref 7

Table 3 Preform dimensions for aluminumalloys

Dimensions of the finish forgings Dimensions of the preforms

Web thickness (tF) tP @ (1–1.5) * tFFillet radii (RFF) RPF @ (1.2–2) * RFFCorner radii (RFC) RPC @ (1.2–2) * RFCDraft angle (aF) aP @ aF + (2–5∞)Width of the rib (wF) wP @ wF -0.8 mm (1/32 in.)

tP, web thickness of preform; RPF, fillet radii of preform; RPC, cornerradii of preform; aP, draft angle of preform; wP, width of preform rib.Source: Ref 7

Table 4 Preform dimensions for titaniumalloys

Dimensions of the finish forgings Dimensions of the preforms

Web thickness (tF) tP @ (15–2.2) * tFFillet radii (RFF) RPF @ (3–3) * RFFCorner radii (RFC) RPC @ (2) * RFCDraft angle (aF) aP @ aF + (3–5∞)Width of the rib (wF) wP @ wF -1.6 to 3.2 mm

(1/16 to 1/8 in.)

tP, thickness of preform; RPF, fillet radii of preform; RPC, corner radii ofpreform; aP, draft angle of preform; wP, width of preform rib. Source Ref7

Fig. 16 Metal flow and the corresponding load-strokecurve. (a) Upsetting. (b) Filling. (c) End. (d)

Load-stroke curve. Source: Ref 8

Extra load requiredto close dies

Cavity fills completely

Forging energy

Flash beginsto form

Dies contactworkpiece

Forgingcomplete

Forging stroke

Incr

easi

ng fo

rgin

g lo

ad

P3

P2

P1

Fig. 17 Typical load-stroke curve for closed-die forgingshowing three distinct stages. Source: Ref 8

Flowstress σe σe σf

r

Metal flow

Diemotion

w

H

t

Fig. 19 Schematic of a simple closed-die forging andforging stress distribution. H, cavity height; r,

radius; t, flash thickness; w, flash width; sf, flow stress inflash region; sc, flow stress in cavity, se flow stress atedge., Source: Ref 8

used for the flash and cavity regions. The totalload, Pta, on the cross section is the summationof the load acting on the flash region and theload acting on the die cavity:

(Eq 5)

where R = r + w, s0f is the flow stress in the flashregion, and sc is the flow stress in the cavity.

For the plane-strain cross sections, the equa-tions corresponding to Eq 4 and 5 are:

(Eq 6)

(Eq 7)

where L is the cavity width, that is, L = 2r in Fig19. The previous equations are relatively simpleand can be programmed for practical use. Thefollowing information is required to performthese calculations:

● Geometry of the part● Flow stresses in the cavity and the flash dur-

ing the final stages of the forging operation● Friction at the die-forging interface

Process Simulation to Predict MetalFlow and Forging Stresses

One of the major concerns in the research ofmanufacturing processes is to find the optimalproduction conditions in order to reduce produc-tion costs and lead time. In order to optimize aprocess, the effect of the most important processparameters has to be investigated. Conductingexperiments, as stated earlier, can be very time-consuming and expensive. Therefore, various

P wmw

t

L

H

mLftp ep c= +Ê

ˈ¯ + +

ÊËÁ

ˆ¯̃

2

32

2 30s s s

s sep = +ÊË

ˆ¯

2

310 f m

w

t

Pm

tR r

m R

t

R r

rm r

H

fta

c ea

= - -È

ÎÍ

+ +ÊËÁ

ˆ¯̃

-ÊËÁ

ˆ¯̃

˘

˚˙˙

+ +È

ÎÍ

˘

˚˙

22

3 3

1

1 23 2

23 3 2

03 3

2 2

2

ps

ps s

( )

computational methods have been developedand used to reduce the number of necessary ex-periments. One of these methods, FEM, hasproved to be the most powerful analysis tool.With the increasing use of computers in indus-try, FEM has steadily gained importance in thesimulation of metal-forming processes.

Investigation of Defect Formation in RingGear Forging. The process analyzed was theforging of an automotive ring gear blank (Ref 10).In production, the part is hot forged fromAmerican Iron and Steel Institute (AISI) 4320steel in three sets of dies. The dies were of H11steel, lubricated with a graphite-and-water mixtureand maintained at approximately 150 ∞C (300 ∞F).

The first step in the manufacturing process in-volves cold shearing the billets from stock andinduction heating them to 1200 ∞C (2200 ∞F).Next, a billet is placed in the busting dies andupset (Fig. 20a). It is then transferred to a blockerdie and forged (Fig. 20b) and finally transferredto and forged in a finisher die (Fig. 20c). Duringinitial forging trials, buckling flow in the blockerdies caused a lap to be formed intermittentlyaround the circumference of the part (Fig. 20d).As the finish dies filled, the lap worsened.Because of this defect, the part was rejected, andhence, a new blocker die design was required.

The following observations were made duringsimulation of the process:

● The sharp corner radius and steep angle ofthe inside wall on the upper die resulted inthe formation of a gap between the insidedie wall and the workpiece.

● As the workpiece contacted the uppermostsurface of the top die and began upsetting, theinside surface of the blocker began to buckle.

● The radial flow from the web region forcedthe buckle out toward the outer die walls,and as the upsetting and radial flow com-bined, the buckling became more severe.

To counter the previously stated problem, thefollowing modification was made to the originalblocker design:

● The corner radius (region “A” of Fig. 21)was increased by a factor of 2, to aid themetal flow around the corner.

● The angle of the top surface of the upper die(region “B” of Fig. 21) was decreased untilit was horizontal, to increase the height ofthe blocker.

● The outer wall of the lower die (region “C”of Fig. 21) was modified so that upsettingflow from the top die would fill voids in theupper die cavity instead of voids in thelower die cavity.

Figure 22 shows the die fill in the simulationrun with the new blocker design. At the start ofthe working stroke, the workpiece followed thewalls of the upper and lower die. With further de-formation, the workpiece contacted the upper-most wall of the top die, and a gap formed be-tween the inside wall of the top die and theworkpiece. At the final stroke position, a smallgap remained along the inside wall of the upperdie, but no buckle was formed. Figures 22 (a–c)show the finish die operation with the modified

Chapter 14: Process Design in Impression Die Forging / 9

(a)

(c)

(b)

(d)

D

BA

E

C

F

GH

Fig. 20 Investigation of defects in ring gear forging using finite-element modeling. (a) Busting dies. (b) Blocker dies.(c) Finisher dies. (d) Deformed mesh showing lap formation. Source: Ref 10

(a)

(b)

(c)

Fig. 22 Deformed mesh of the finishing simulationwith the modified blocker design. Source: Ref

10

blocker output. On deformation, the upper diepushes the workpiece down until contact is madewith the outer wall of the lower die. With furtherreduction, the workpiece contacts the outer webregion of the upper die. As the stroke continues,the inside corner fills up without any indicationof defective flow patterns. With further upsettingof the workpiece, the uppermost fillet of the topdie and the outside fillet of the bottom die con-tinue to fill, and the die cavity fills up completely.Hence, the result from the finisher simulation in-dicates that the modified blocker workpiece fillsthe finisher die without defects.

Investigation of Tool Failure. Hot forgingis a widely used manufacturing process in theautomotive industry. High production rates re-sult in severe thermomechanical stresses in thedies. Either thermal cracking or wear governsthe life of the dies. In the forging industry, thetooling cost alone can constitute up to 20% ofthe total cost of the component.

This example deals with the investigation of theeffect of thermomechanical stresses on the tool

life in the hot extrusion of the automotive compo-nent shown in Fig. 23 (Ref 11). The resultingstresses in this process are a combination of thepurely mechanical stresses due to forging and thethermomechanical stresses as a result of thermalcycling of the punch surface due to the alternatinghot forging and waiting periods. The stresses dueto thermal cycling were found to comprise ap-proximately 75% of the total stress field. This cy-cling causes tool damage known as heat checking.Originally, the punch had to be changed approxi-mately every 500 cycles, due to cracking as a re-sult of thermal cycling (Fig. 24). It is a commonlyknown fact that geometry changes are not the bestway to reduce the stress level with regard to ther-mal stresses. From this study, it was determinedthat increased tool life could be achieved by mod-ifying the hot forging process parameters, such asbillet temperature and the forging rate.

Finite-element modeling simulation and ex-perimental work were used to conduct a para-metric study to determine the optimal processparameters to achieve higher life expectancy ofthe tools. This combined numerical and experi-mental approach can be summarized as:

● A two-step numerical simulation:a. Process simulation to determine the

purely mechanical stresses, forgingloads, and thermal boundary conditionsfor the punch

b. Thermoelastic simulation for thermal-stress analysis of the punch

● A two-step experimental stage:a. Metallurgical validation of the constitu-

tive laws of the workpiece material

b. Industrial forging tests for validation ofthe thermal boundary conditions for thepunch

The surface temperatures on the punch are afactor of the heat-transfer coefficient at the tool-workpiece interface. This coefficient is a func-tion of various factors, such as surface topogra-phy, contact pressures, temperature difference,and duration of contact (Ref 12). Forging testswere conducted on an industrial press using atest punch with five thermocouples. Several nu-merical iterations (FEM simulations) were per-formed by using different heat-transfer coeffi-cients until the calculated temperaturedistribution was in agreement with that from theexperiments.

In order to reduce the thermal stresses, a re-duction of the thermal gradient during forgingmust be obtained. There are two options: modi-fication of process parameters to decrease thetemperature (reduction of the punch speed, thusreducing the flow stress, or decreasing work-piece temperature, resulting in an increase inflow stress) or use of lubricating/insulatingproducts during forging to reduce the heat trans-fer, which is an empirical approach. The first op-tion was selected, because the available presscould handle increased forging loads as a resultof increased flow stress.

A parametric study was conducted to investi-gate the influence of forging speed and initialworkpiece temperature on the final thermome-chanical stresses. The optimal process parame-ters were thus determined, resulting in a 30% de-crease in the stresses. Thus, a combination ofprocess simulation and experimental verificationresulted in an increase in the tool life for thepunch in this hot forging process.

Multistage Forging Simulations of AircraftComponents. Multistage forging simulationsof two aircraft components (a titanium fittingand an aluminum wheel) were run to studymetal flow, temperature distribution, die filling,and die stresses (Ref 13). The commercial FEMcode “DEFORM-3D” (Scientific FormingTechnol-ogies Corporation, Columbus, Ohio)was used for these simulations. The two com-ponents considered for this study are producedby closed-die forging with flash. Because theparts are forged at elevated temperatures, it wasnecessary to run nonisothermal simulations.Flash removal between the forging stages alsohad to be considered for the simulations in orderto ensure appropriate material volume in thedies for the subsequent forging stage. Each ofthe components was forged in three stages,namely, two blocker stages followed by a fin-isher stage. Figures 25 and 26 show the forgingsequence of the titanium fitting and the alu-minum wheel, respectively. The results obtainedat the end of the simulations were the effectivestress distribution, die filling, metal flow duringforging, temperature distribution, and strain dis-tribution.

Flash removal between the forging stages alsohad to be considered because the amount of

10 / Process Design and Workability

A

B

C

Fig. 21 Modified blocker design (broken lines) posi-tioned in the open finisher dies (solid lines).

Source: Ref 10

Fig. 23 Automotive component formed by forward/backward hot forging process. Source: Ref 11

Fig. 24 Cracks formed as a result of thermal cycling.Source: Ref 11

flash to be removed influences the volume ofmaterial available for the subsequent forgingstage and thus, the die filling and the diestresses. The simulation strategy adopted for thetwo components was to remove the flash in be-tween stages. This was done by using theBoolean capability of DEFORM, that is, volumemanipulation. Die filling was checked by exam-ining various cross sections along the length ofthe forging (Fig. 27, 28).

The simulations were stopped when die filling

was achieved, and it was this stage of the simu-lation, that was used to determine the stresses inthe dies. In order to reduce computational time,the dies were kept rigid throughout the simula-tion. At the last step, they were changed to elas-tic, and the stresses from the workpiece were in-terpolated onto the dies. The results obtainedfrom the die stress analysis simulations were theeffective stress, the maximum principal stress,and the temperature distribution. Using these re-sults, an effective die design was established.

Cold and Warm Forging

The current chapter is devoted to impression-die forging, which is essentially a hot forgingprocess with flash, where the workpiece mate-rial is at a higher temperature than the dies. Thedies are designed to provide for flash that en-sures proper filling of the die cavity and the flowof excess material outside the die cavity.However, it is appropriate to discuss, verybriefly, cold and warm forging processes thatuse die designs without flash.

Cold forging is a process wherein metal atroom temperature is forced to flow plasticallyunder compressive force into a variety of shapes.These shapes are usually axisymmetric, with rel-atively small, nonsymmetrical features, and theygenerally do not generate flash. The terms coldforging and cold extrusion are often used inter-changeably and refer to well-known forming op-erations, such as extrusion, upsetting or heading,coining, ironing, and swaging. Through a com-bination of these techniques (Fig. 29), a verylarge number of parts can be produced.

Due to the limitations with regard to load onthe tooling and the formability of certain materi-als at room temperature, the extrusion process isalso carried out above room temperatures andbelow hot forging temperatures. This process isclassified as warm extrusion. For low-carbonsteel, warm extrusion is carried out between 400and 800 ∞C (750 and 1475 ∞F. The tooling usedfor this process is similar to that used for coldextrusion. Because the flow stress of steel islower at higher temperatures, larger reduction inarea is achievable, allowing a subsequent reduc-tion in the number of stages required to manu-facture a part. There is also a possibility forusing combined cold and warm extrusion.

Cold and warm forging are extremely importantand economical processes, especially for produc-ing round or nearly round parts in large quantities.Some of the advantages of these processes are:

● High production rates● Excellent dimensional tolerances and sur-

face finish● Significant savings in material and machining● Higher tensile strengths in the forged part

than in the original material because ofstrain hardening

● Favorable grain flow to improve strength

The most common extrusion processes encoun-tered are forward rod and backward cup extrusion(Fig. 29). Several combinations of extrusionprocesses are possible, and other operations, suchas upsetting, heading, coining, embossing, andironing, can be used in conjunction with extrusion.Figure 30 gives an overview of the production se-quence for cold forging of a gear blank. The oper-ations consist of combined forward rod and back-ward cup extrusion followed by the simultaneousupsetting and coining of the shoulder.

The main advantages of the cold extrusionprocess are the large material savings comparedwith processes such as machining and other metal-

Chapter 14: Process Design in Impression Die Forging / 11

Fig. 25 Forging sequence of the titanium fitting. Courtesy of Weber Metals Inc.

Fig. 26 Forging sequence of the aircraft wheel. Courtesy of Weber Metals Inc. Source: Ref 13

Fig. 27 Sections taken along the fitting to check for die filling at the blocker stage. Source: Ref 13

A final advantage is the option of using steel oflower strength for machine and construction ele-ments without the need for further hardening. Incold forging, the forming load and stresses are rel-atively higher than in hot forging, up to 2070 MPa(300 ksi). Therefore, the tool or die design is quiteelaborate, and cold forging dies are relatively ex-pensive. Thus, production of a large quantity ofparts is required to amortize the tool costs.

These advantages, combined with the widepossibilities of shapes that can be manufacturedby extrusion, have increased the popularity ofthis process in the industry.

REFERENCES

1. V. Vasquez and T. Altan, New Concepts inDie Design—Physical and ComputerModeling Applications, J. Mater. Process.Technol., Vol 98, 2000, p 212–223

2. A.M. Sabroff et al., Forging Materials andPractices, Reinhold, 1968.

3. K. Spies, “Preforming in Forging andPreparation of Reducer Rolling,” Ph.D. dis-sertation, University of Hannover, 1959

4. K. Vieregge, “Contribution to Flash Designin Closed-Die Forging,” Ph.D. dissertation,Technical University of Hannover, 1969

5. H.W. Haller, Handbook of Forging, CarlHanser Verlag, 1971 (in German)

6. K. Lange and H. Meyer-Nolkemper, Closed-DieForging, Springer-Verlag, 1977 (in German)

7. T. Altan, F.W. Boulger, J.R. Becker, N.Akgerman, and H.J. Henning, ForgingEquipment, Materials and Practices,Batelle Columbus, 1973

8. T. Altan, S.-I. Oh, and H.L. Gegel, MetalForming Fundamentals and Applications,American Society for Metals, 1983

9. F. Neuberger and S. Pannasch, MaterialConsumption in Die Forging of Steel, Vol12, Fertiegungstechnik und Betrieb, 1962, p775–779 (in German)

10. B.L. Jenkins, S.-I. Oh, and T. Altan,Investigation of Defect Formation in a 3-Station Closed-Die Forging Operation,CIRP Ann., Vol 381, 1989, p 243

11. O. Brucelle and G. Bernhart, Methodologyfor Service Life Increase of Hot ForgingTools, J. Mater. Process. Technol., Vol 87,1999, p 237

12. B. Snaith, S.D. Probert, and P.W. O’Callag-han, Thermal Resistances of Pressed Contacts,Appl. Energy, Vol 22, 1986, p 31–84

13. M. Shirgaokar, G. Ngaile, and T. Altan,“Multistage Forging Simulations of AircraftComponents,” Engineering Research Centerfor Net Shape Manufacturing, ERC/NSM-02-R-84, 2002

14. H.D. Feldman, Cold Extrusion of Steel,Merkblatt 201, 1977 (in German)

15. F. Sagemuller, Cold Impact Extrusion ofLarge Formed Parts, Wire, No. 95, June1968, p 2

12 / Process Design and Workability

Top die

Diecavity

Flash cavity

Bottom die

Flash

Fig. 28 Section A-A of the fitting seen in Fig. 27 after the first blocker operation. Source: Ref 13

Fig. 29 Example of a component produced using forward rod and backward extrusion. Source: Ref 15

Fig. 29 Common cold extrusion processes. (a) Forward rod extrusion. (b) Forward cup extrusion. (c) Backward cupextrusion. (d) Combined forward rod and backward cup extrusion. (e) Combined forward and backward cup

extrusion. P, punch; W, workpiece; C, container; E, ejector. Source: Ref 14

forming processes, such as hot forging; and thehigher productivity, that is, parts produced perhour, in comparison with other processes. Other

advantages include good dimensional and formerror tolerances, good surface finish, and im-proved mechanical properties of the workpieces.