Introduction to Bulk-Forming Processes

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Introduction to Bulk-Forming ProcessesS.L. Semiatin, Air Force Research Laboratory, Materials and Manufacturing Directorate

METALWORKING consists of deformationprocesses in which a metal billet or blank isshaped bytools or dies. The design and control of such processes depend on the characteristicsof the workpiece material, the conditions at thetool/workpiece interface, the mechanics of

plastic deformation (metal flow), the equipmentused, and the finished-product requirements.These factors influence the selection of toolgeometry and material as well as processingconditions (for example, workpiece and dietemperatures and lubrication). Because of thecomplexity of many metalworking operations,models of various types, such as analytical,physical, or numerical models, are often reliedupon to design such processes.

This Volume presents the state-of-the-art inbulk-metalworking processes. A companionvolume ( ASM Handbook , Volume 14B, Metal-working: Sheet Forming) describes the state-of-the-art in sheet-forming processes. Variousmajor sections of this Volume deal with

descriptions of specific processes, selection of equipment and die materials, forming practicefor specific alloys, and various aspects of processdesign and control. This article provides a brief historical perspective, a classification of metal-working processes and equipment, and a sum-mary of some of themore recentdevelopments inthe field.

Historical Perspective

Metalworking is one of three major technol-ogies used to fabricate metal products; the othersare casting and powder metallurgy. However,

metalworking is perhaps the oldest and mostmature of the three. The earliest records of metalworking describe the simple hammering of gold and copper in various regions of the MiddleEast around 8000 B.C. The forming of thesemetals was crude because the art of refining bysmelting was unknown and because the ability towork the material was limited by impurities thatremained after themetal hadbeen separatedfromthe ore. With the advent of copper smeltingaround 4000 B.C., a useful method becameavailable for purifying metals through chemicalreactions in the liquid state. Later, in the CopperAge, it was found that the hammering of metal

brought about desirable increases in strength(a phenomenon now known as strain hardening).The quest for strength spurred a search for alloysthat were inherently strong and led to the utili-zation of alloys of copper and tin (the BronzeAge) and iron and carbon (the Iron Age). The

Iron Age, which can be dated as beginningaround 1200 B.C., followed the beginning of the Bronze Age by some 1300 years. The reasonfor the delay was the absence of methods forachieving the high temperatures needed to meltand to refine iron ore.

Most metalworking was done by hand untilthe 13th century. At this time, the tilt hammerwas developed and used primarily for forgingbars and plates. The machine used water powerto raise a lever arm that had a hammering toolat one end; it was called a tilt hammer becausethe arm tilted as the hammering tool was raised.After raising the hammer, the blacksmith letit fall under the force of gravity, thus generat-ing the forging blow. This relatively simple

device remained in service for a number of centuries.

The development of rolling mills followedthat of forging equipment. Leonardo da Vinci’snotebook includes a sketch of a machinedesigned in 1480 for the rolling of lead forstained glass windows. In 1495, da Vinci isreported to have rolled flat sheets of preciousmetal on a hand-operated two-roll mill for coin-making purposes. In the following years, severaldesigns for rolling mills were utilized inGermany, Italy, France, and England. However,the development of large mills capable of hotrolling ferrous materials took almost 200 years.This relatively slow progress was primarily due

to the limited supply of iron. Early millsemployed flat rolls for making sheet and plate,and until the middle of the 18th century, thesemills were driven by water wheels.

During the Industrial Revolution at the end of the 18th century, processes were devised formaking iron and steel in large quantities tosatisfy the demand for metal products. A needarose for forging equipment with larger capacity.This need was answered with the invention of thehigh-speed steam hammer, in which the hammeris raised by steam power,and thehydraulic press,in which the force is supplied by hydraulicpressure. From such equipment came products

ranging from firearms to locomotive Similarly, the steam engine spurred dements in rolling, and, in the 19th cea variety of steel products were rolsignificant quantities.

The past 100 years have seen the develo

of new types of metalworking equipmenew materials with special propertieapplications. The new types of equipmenincluded mechanical and screw pressehigh-speed tandem rolling mills. The mathat have benefited from such developmeequipment range from the low-carbon steadvanced high-strength steels used inmobiles and appliances to specialty alumititanium-, and nickel-base alloys used aerospace and other industries. In the apmately 20 years since this Volume waupdated, methods for the bulk formingnumber of new materials, such as intermalloys and composites, have been deveFurthermore, the advent of user-friendly

puter codes andinexpensivecomputers haa revolution in the application of nummethods for the design and control of a plof bulk-forming processes, thus leadihigher-quality products and increased effiin the metalworking industry.

Classification of MetalworkingProcesses

In metalworking, an initially simple piece—a billet or a blanked sheet,for examis plastically deformed between tools (or d

obtain the desired final configuration. Mforming processes are usually classified aing to two broad categories:

Bulk, or massive, forming operations Sheet-forming operations (Sheet form

also referred to as forming. In thebroademost accepted sense, however, theforming is used to describe bulk- as wsheet-forming processes).

In both types of processes, the surfaces deforming metal and the tools are in contacfriction between them may have a majorence on material flow. In bulk formin

© 2005 ASM International. All Rights Reserved. ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)

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input material is in billet, rod, or slab form,and the surface-to-volume ratio in the formedpart increases considerably under the action of largely compressive loading. In sheet forming,on the other hand, a piece of sheet metal isplastically deformed by tensile loads into athree-dimensional shape, often without signifi-

cant changes in sheet thickness or surfacecharacteristic.

Processes that fall under the category of bulk forming have the following distinguishing fea-tures (Ref 1, 2):

The deforming material, or workpiece,undergoes large plastic ( permanent) defor-mation, resulting in an appreciable change inshape or cross section.

The portion of the workpiece undergoingplastic deformation is generally much largerthan the portion undergoing elastic deforma-tion; therefore, elastic recovery after defor-mation is negligible.

Examples of generic bulk-forming processesare extrusion, forging, rolling, and drawing.

Specific bulk-forming processes are listed inTable 1.

Types of Metalworking Equipment

The various forming processes discussedpreviously are associated with a large variety of forming machines or equipment, including thefollowing (Ref 1, 2):

Rolling mills for plate, strip, and shapes Machines for profile rolling from strip Ring-rolling machines Thread-rolling and surface-rolling machines Magnetic and explosive forming machines Draw benches for tube and rod; wire- and rod-

drawing machines Machines for pressing-type operations

(presses)

Among those listed, pressing-type machinesare the most widely used and are applied toboth bulk- and sheet-forming processes. Thesemachines can be classified into three types:load-restricted machines (hydraulic presses),stroke-restricted machines (crank and eccentric,or mechanical, presses), and energy-restrictedmachines (hammers and screw presses). Thesignificant characteristics of pressing-typemachines comprise all machine design and per-formance data that are pertinent to the econom-ical use of the machine. These characteristicsinclude:

Characteristics for load and energy: Avail-able load, available energy, and efficiencyfactor (which equals the energy available forworkpiece deformation/energy supplied to the

machine) Time-related characteristics: Number of

strokes per minute, contact time under pres-sure, and velocity under pressure

Characteristics for accuracy: For example,deflection of the ram and frame, particularlyunder off-center loading, and press stiffness

Recent Developments inBulk Forming

Since the publication in 1988 of the pre-vious edition of the ASM Handbook on Formingand Forging, metalworking practice has seen anumber of notable advances with regard to the

development of new processes; new materials,the increased control of microstructure via spe-cialized thermomechanical processes, and thedevelopment of advanced tools for predictingmicrostructure and texture evolution; and theapplication of sophisticated process simulationand design tools. Some of these technologicaladvances are summarized in the following sec-tions of this article.

New Processes

A number of novel processes have recentlybeen introduced and/or investigated. In the

bulk-forming area, these include advroll-forming methods, equal-channel aextrusion, incremental forging, and forming.

Advanced roll-forming methods havedeveloped for making axisymmetric compwith very complex cross sections. Shap

conducted using opposed rollers or a comtion of rollers and a mandrel acting on a roworkpiece (Fig. 1). Unlike former simplforming processes used to make long cones, and so forth, newer roll-forming mrely on the simultaneous control of radiaxial metal flow. For example, the internfile of complex shapes can be generatcombined radial-axial roll forming that typmakes use of a sophisticated internal mconsisting of several angular segmentdevices to quickly lock or unlock the segSuch roll-forming operations can be condunder either cold- or hot-working conditiospecific temperature depends on the ductili

strength of the workpiece material ancomplexity of the shape to be made. Thenique has been used to make aircraft enginand cases, automotive wheels, and otherRecently, the feasibility of radial roll fohas been demonstrated for titanium allo6Al-4V, Ti-6Al-2Sn-4Zr-2Mo, and Ti-6A4Zr-6Mo and nickel-base alloys 718, WasRené 95, René 88, and Merl 76 (Ref The forming of such alloys is enhanced development of an ultrafine grain structthe preform material. Advanced roll formiprovide near-net shapes at lower cost comto forging and ring rolling because of theination of dies andthe ability to utilize a giof rollers for multiple geometries. Because

generally high deformation that is imposethe entire part cross section, microstructurformity also tends to be excellent. More deinformation on the technology can be fouthe article “Roll Forming of Axially SymComponents” in this Volume.

Equal-channel angular extrusion (ECAan emerging metal-processing technique oped by Segal in the former Soviet Union1970s, but not widely known in the Westhe 1990s (Ref 5). In ECAE, metal flowprises deformation through two interschannels of equal cross-sectional area (FThe imparted strain is a function priof the angle between the two channels, 2

the angle of the curved outer corner, y2w=90 and y=0, for example, the effstrain is approximately equal to 1.15. By pthe workpiece through the tooling many very large deformations can thus be imHence, the process has been investigatemeans to refine microstructure and to ccrystallographic texture; in many casemicrocrystalline grain structures are deveThe majority of work to date has been perfon aluminum and aluminum alloys, coppernickel, and titanium. Although the gattention has been focused on square or billet products, ECAE has also been u

Table 1 Classification of bulk (massive)forming processes

Forging

Closed-die forging with flashClosed-die forgingwithout flash

Coining

Electro-upsettingForward extrusion forging

Backward extrusion forging

HobbingIsothermal forging

Nosing

Open-die forging

Rotary (orbital) forgingPrecision forgingMetal powder forging

Radial forging

UpsettingIncremental forging

Rolling

Sheet rolling

Shape rollingTube rolling

Ring rolling

Rotary tube piercingGear rolling

Roll forging

Cross rollingSurface rolling

Shear forming

Tube reducing

Radial roll forming

Extrusion

Nonlubricated hot extrusion

Lubricated direct hot extrusionHydrostatic extrusion

Co-extrusion

Equal channel angular extrusion

Drawing

DrawingDrawing with rolls

Ironing

Tube sinkingCo-drawing

Source: Ref 1

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produce ultrafine-grain plate materials for use asis or as preforms for subsequent sheet rolling.Furthermore, work has been performed tomodify the ECAE concept to allow continuous,rather than batch, processing; such efforts havebeen limited to thin cross-section productssuch as wire, rod, and sheet. More detailedinformation on ECAE can be found in the article“Equal-Channel Angular Extrusion” in thisVolume.

Incremental forging is a closed-die forgingprocess in which only a portion of the workpieceis shaped during each of a series of press strokes.The process is analogous to open-die forging

(cogging) of ingots, billets, thick plates, andshafts. In contrast to such operations, however,impression dies (not flator V-shaped tooling) areutilized. The primary applications of the tech-nique are very large plan-area components of high-temperature alloys for which die pressurescan easily equal or exceed 10 to 20 tsi. In suchinstances, part plan area is limited to approxi-mately several thousand square inches for thelargest presses (50,000 tons) currently availablein the United States. By forging only a portion of the part at a time, however, press requirementsare reduced. Applications of the techniqueinclude large, axisymmetric components forland-based gas turbines made from nickel-base

alloy 706 (Wyman-Gordon Company and AlcoaForged Products) and various Ti-6Al-4V (rib-web) structural components for F-18 aircraft(manufactured at Alcoa Forged Products). Thelatter parts had plan areas of the order of 5000 in.

2. Needless to say, part symmetry and

forging design (e.g., forging envelope) play acritical role in the design of incremental-forgingprocesses. Forging design, representing a keyfeature of incremental forging, is often highlyspecialized and proprietary in nature.

Microforming is a technology generallydefined as the production of parts or structureswith at least two dimensions in the submillimeter

range.Most of thedevelopments in this arebeen driven by the needs of the elecindustry for mass-produced miniature parsummarized by Geiger et al. (Ref 6), challenges in microforming fall into one obroad categories: workpiece material, toequipment, and process control. For ex

the flow and failure behavior of a worwith only one or several grains across the ssubjected to large strains can be very diffrom that of its polycrystalline counused in macro bulk-forming processes. Mforming operations include cold headinextrusion of wire. For example, Geigerdescribed the bulk forming of copper piforward rod extrusion and backward extrusion to produce a shaft diameter of 0(0.03 mils) and a wall thickness of 1(5 mils). For this and similar micro-operchallenges include handling of small premanufacture of tooling with complex geometry, tooling alignment, and the o

precision of the forming equipment.

Materials-Related Developments

Recent materials-related developmenclude breakthroughs in the bulk forming omaterials, increased control of microstrdevelopment using specialized thermomecal processes, and the development of advtools for predicting microstructure and tevolution.

New materials for which substantiagress has been made over the last 20include structural-intermetallic alloys an

continuously reinforced metal-matrix csites (MMCs). For intermetallic bulk-forming approaches have been dramatic for aluminide-based materials (RBulk forming on a commercial scale haused for MMCs with aluminum-alloy anlesser extent, titanium-alloy matrices.

Iron-aluminide alloys based on the compound are probably the structural metallic materials that have been producedlargest quantities to date. These materials eexcellent oxidation and sulfidation resiand potentially lower cost than the ststeels with which they compete. As sunumber of potential applications for thes

aluminides have been identified. These inmetalworking dies, heat shields, furnace fiand heating elements, and a variety ofmotive components. Sikka and his colleagOak Ridge National Laboratory have headedthe development of techniques for textrusion, forging, and rolling of ingot-lurgy Fe3Al-base alloys at temperatures range of 900 to 1200 C (1650 to 21(Ref 8). The wrought product can then berolled to plate or sheet at temperatures be500 and 600 C (930 and 1110 F) to mfacture material with room-temperature ductility of 15 to 20%. Wrought Fe3Al

Ram

Workpiece

2φ

A

Die

ψ A'

Fig. 2 Equal-channel-angular extrusion

(a)

(b) Roll-formed

shapeSonic

shape

Fig. 1 Radial roll forming. (a) Schematic of the process. (b) Complex-shape titanium-alloy component fabricatedvia radial roll forming. The sketch in (b) shows the outline of the roll-formed part relative to the sonic shape.

Source: Ref 3, 4

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do not possess adequate workability for coldrolling or cold drawing, however.

Titanium-aluminide alloys based on the face-centered tetragonal (fct) gamma phase (TiAl)represent a second type of aluminide material forwhich significant progress has been made towardcommercialization. The gamma-titanium alu-

minide alloys have a number of applications aslightweight replacements for superalloys in thehot section of aircraft engines and as thermalprotection systems in hypersonic vehicles.Spurred by substantial efforts at the Air ForceResearch Laboratory, Battelle Memorial Insti-tute, Ladish Company, and Wyman-Gordon, avariety of metalworking techniques for bothingot-metallurgy (I/M) and powder-metallurgy(P/M) materials have been developed for thesematerials, which were once thought to beunworkable (Ref 7, 9). For instance, isothermalforging and canned hot-extrusion techniqueshave been demonstrated for the breakdown of medium- to large-scale ingots. Novel can designs

and the use of a controlled dwell time betweenbillet removal from the preheat furnace anddeformation have greatly enhanced the feasi-bility of hot extrusion. In particular, the dwelltime is chosen in order to develop a temperaturedifference between the sacrificial can materialand the titanium aluminide preform; by thismeans the flow stresses of the two components issimilar, thereby promoting uniform co-extru-sion. Secondary processing of parts has beenmost often conducted via isothermal closed-dieforging (Fig. 3a, b). Careful can design andunderstanding of temperature transients havealso enabled the hot pack rolling of gammatitanium-aluminide sheet and foil products usedin subsequent superplastic-forming operations

(Fig. 3c). A key to the success of each of theseprocesses has been the development of a detailedunderstanding of the pertinent phase equilibria/ phase transformations and the effects of micro-structure, strain rate, and temperature on failuremodes during processing. More detailed infor-mation on the bulk processing of intermetallicalloys can be found in the article “Bulk Formingof Intermetallic Alloys” in this Volume.

Discontinuously reinforced aluminum metal-matrix composites (DRA MMCs) have beensynthesized and subsequently bulk formed by avariety of techniques. The majority of MMCshave been based on aluminum matrices withsilicon carbide particulate or whisker reinforce-

ments synthesized in tonnage quantities by I/Mor P/M approaches. In the I/M method, which ismost often used forautomotive parts,the ceramicparticles are introduced and suspended in theliquid aluminum alloy prior to casting using ahigh-energy mixing process. In the P/Mapproach, most often used for aerospace mate-rials, the matrix and ceramic powders are blen-ded in a high-shear mixer prior to canning andoutgassing. Following the casting/canningoperations, conventional extrusion, forging, androlling processes are used to make billet andplate products. Secondary processing mayinclude extrusion (e.g., automotive driveshafts,

fan exit guide vanes in commercial jet engines,bicycle-frame tubing), rolling (to make sheet),and closed-die forging (e.g., helicopter rotor-blade sleeves, automobile engine pistons andconnectingrods). Cast-and-extruded DRA MMCdriveshafts with a 6061 aluminum matrix andalumina reinforcements have also been intro-

duced for pickup trucks and sports cars. Simi-larly, extrusion and upsetting of pressed-andsintered Ti-6Al-4V reinforced with TiB2 parti-culate have been used to mass produce auto-mobile and motorcycle engine valves. Moredetailed information on the bulk processing of metal-matrix composites can be found in thearticle “Forging of Discontinuously ReinforcedAluminum Composites” in this Volume andin the article “Processing of Metal-MatrixComposites” in composites, Volume 21, of ASM

Handbook .Thermomechanical processing (TMP)

refers to the design and control of metalworkingand heat treatment steps in an overall manu-

facturing process in order to enhance finalmicrostructure and properties. Thermo-mechanical processing was developed initiallyas a method for producing high-strength or high-toughness microalloyed steels via (ferrite) grainrefinement and controlled precipitation. Currenttrends in the TMP of ferrous alloys are focusingon the development of carbide-free steels withbainitic microstructures to obtain yet higherstrength levels. Further information on the state-of-the-art of ferrous TMP is contained in thearticle “Thermomechanical Processes for Fer-rous Alloys” in this Volume.

Thermomechanical processing is now alsobeing used routinely for nickel- and titanium-base alloys. Two examples of recent advances in

the TMP of nickel-base superalloys, intended toimprove damage tolerance or creep resistance inservice, comprise techniques to produce a uni-form intermediate grain size (ASTM ~6) or agraded microstructure. The former technique isespecially useful for the manufacture of P/Msuperalloys such as René 88, N18, and alloy 720.In this instance, TMP consists of isothermalforging of consolidated-powder preforms fol-lowed by supersolvus heat treatment. To achievethe desired final grain size after supersolvus heattreatment, however, forging must be performedin a very tightly controlled strain, strain-rate, andtemperature window for each specific material(Ref 10). Lack of control during deformation

may result in uncontrolled (abnormal) graingrowth during the subsequent supersolvus heattreatment, leading to isolated grains or groups of grains that are several orders of magnitude largerthan the average grain size. Processes to developgraded microstructures in superalloys consist of local heating above the solvus temperature todissolve the grain-boundary pinning phase (e.g.,gamma prime) and thus facilitate grain growth inthese regions while other portions of the com-ponent are cooled (Ref 11).

Thermomechanical processes for titaniumalloys include processing to produce ultrafinegrain billet, “through-transus” forging, and final

heat treatment to obtain graded microstrucMethods to obtain ultrafine billet microstruin alpha/beta titanium alloys such as Ti-6ATi-6Al-2Sn-4Zr-2Mo, and Ti-17 rely on sforging practices for partially converted containing an initial transformed-beta (cbasketweave alpha) microstructure. In

approach, multistep hot forging alongorthogonal directions is conducted at rates of the order of 10 3 s1 and a sertemperatures in the alpha/beta phase(Ref 12). By this means, an alpha grain s4 to 8 mm (0.16 to 0.31 mils) with goodsonic inspectability is obtained. In a sapproach, warm working, involving verystrains and somewhat lower temperaturesto 700 C, or 1020 to 1290 F), has beento yield a submicrocrystalline alpha grai(Ref 13).

Fig. 3 Wrought gamma titanium products. (apressor blades. (b) Subscale isothermall

disk. (a) and (b) Source: D.U. Furrer, Ladish CompLarge, conventionally (pack) rolled sheet. Source:Memorial Institute, Air Force Research Laboratory

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Through-transus forging of alloys such as Ti-6Al-4Sn-4Zr-6Mo is a TMP process that com-bines aspects of beta and alpha-beta forging inorder to develop a microstructure with both highstrength and good fracture toughness/fatigueresistance. By working through the transus, thedevelopment of a continuous (and deleterious)

layer of alpha along the beta grain boundaries isavoided (Ref 14). Instead, a transformed betamatrix microstructure with equiaxed alpha par-ticles on the beta grain boundaries (“snow on theboundaries”) is produced. If forging is conductedto temperatures that are too low, however,undesirable equiaxed, primary alpha is nucleatedwithin the matrix. To help meet the tight limitson temperature for the process, therefore, hot-dieforging coupled with finite-element modeling forprocess design have been utilized.

As with nickel-base superalloys, special heattreatments have been developed to provide dual(and graded) microstructures in alpha-beta tita-nium alloys (Ref 15) (Fig. 4). Most of these

methods comprise local heating of selectedregions of a part above the beta-transus tem-perature followed by controlled cooling. Infor-mation on beta annealing under continuous-heating conditions and the effect of textureevolution on beta grain growth is invaluable forthe selection of heating rates and peak tempera-

tures for such TMP routes (Ref 16–18). Inaddition, because the decomposition of themetastable beta is very sensitive to cooling rate,dual-microstructure TMP processes may be usedto produce components with a gradation of microstructure morphologies.

More detailed information on the TMP of nickel-base and titanium alloys can be found inthe article “Thermomechanical Processes forNonferrous Alloys” in this Volume.

Microstructure-evolution models fall intotwo broad categories: phenomenological andmechanistic. Phenomenological microstructure-evolution models have been developed to cor-relate measured microstructural features to

imposed processing conditions and artypically valid only within the specific rathe observations (Ref 19). For examplevolution of recrystallized volume frand recrystallized grain size that evolve dhot deformation (due to “dynamic” recryzation) can be described as a function

imposed strain, strain rate, and tempeSimilar models treat the evolution ofstructure during annealing following coldworking as a function of time due to “srecrystallization. In both cases, the recrystvolume fraction typically follows a sigm(“Avrami”) dependence on strain or timenomenological models of dynamic andrecrystallization have been developed variety of steels, aluminum alloys, and nbase alloys. Grain growth during heatment of single-phase alloys without oa dispersion of second-phase particlealso be quantified using phenomenolequations such as that based on a paraboli

observations for a very wide range of metaalloys.Mechanism-based approaches have also

investigated during the last 20 years to microstructure evolution during hot workiannealing. These models incorporate denistic and statistical aspects to varying deand seek to quantify the specific mechunderlying microstructure changes. Most models incorporate physics-based ruleevents such as nucleation andgrowth durinrecrystallization and grain growth. The effstored work, concurrent hot working, clographic texture, grain-boundary energmobility, second-phase particles, and so fomicrostructure evolution can thus be des

by these approaches. As such, accurate mothis type can delineate microstructure evoover a broader range of processing condthan phenomenological models and are alsuseful for processes involving strain ratemperature transients. In addition, the mcan provide insight into the source of obsdeviations from classical Avrami behavioing recrystallization or nonparabolic growth (e.g., Fig. 5). Two principal tymechanism-based approaches are those bacellular automaton (primarily used for rtallization problems) and the Monte-Carloformalism (used for both recrystallizatiograin-growth problems). Both of these for

tions seek to describe phenomena at the(grain) scale. Challenges with regard validation and industrial application of mechanism-based models still remain, howlargely because of the dearthof reliable maproperty data.

More detailed information on phenological and mechanism-based modemicrostructure evolution can be found article “Models for Predicting MicrostrEvolution” in this Volume and in Ref 21.

Texture-evolution models fall intomain categories, those principally foprediction of either deformation textu

Fig. 4 Graded microstructure obtained in a 75 mm (3 in.) diam Ti-6Al-4V bar via localized induction heating.(a) Macrostructure. (b) Microstructure in core. (c) Microstructure in surface layer. Source: Ref 15



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recrystallization/transformation textures. Thedevelopment of such modeling techniques hasgreatly accelerated in recent years due to theready availability of powerful computer resour-ces. Deformation texture modeling is moreadvanced compared to efforts for predictingrecrystallization/transformation textures. Defor-

mation texture modeling treats the slip andtwinning processes and the associated crystalrotations to predict anisotropic plastic flow andtexture evolution. Models of this sort includelower- and upper-bound approaches in whicheither stress or strain compatibility is enforcedamong the grains in a polycrystalline aggregate,respectively. Upper-bound models give reason-able estimates of deformation texture evolutionin many cases. However, more-detailed approa-ches, which incorporate strain variations fromgrain to grain (so-called self-consistent models)as well as within each grain (crystal-plasticityFEM techniques, or CPFEM), offer the promiseof even more accurate predictions. The latter

(CPFEM) approach may also be useful for thedetermination of local conditions that may giverise to cavitation, spheroidization, and so forthprovided that the physics associated with such

processes can be quantified in terms of the fieldvariables used in these codes. More detailedinformation on deformation texture modelingcan be found in the article “Polycrystal Model-ing, Plastic Forming, and Deformation Textures”in this Volume.

A relatively recent development in texture

modeling is that associated with the recrystalli-zation or transformation phenomena during orfollowing hot deformation. For example, thetextures that evolve during hot working are aresult of both dislocation glide and dynamicrecrystallization. Texture evolution can bequantified by mechanisms such as orientednucleation and selective growth. In the formermechanism, recrystallization nuclei are formedin those grains that have suffered the least shearstrain (i.e., dislocation glide). Selective (i.e.,faster) growthis then assumed to occur for nucleiof particular misorientations with respect to thematrix. More detailed information on the mod-eling of the evolution of recrystallization and

transformation textures can be found in thearticle “Transformation and RecrystallizationTextures Associated with Steel Processing” inthis Volume.

Process Simulation and Design

With the advent of powerful and inexpcomputer hardware and software, a verevolution in the design of bulk-formincesses using advanced modeling and optition techniques has occurred in the last 20

Advances in process simulationbeen spurred primarily by the developmgeneral-purpose, finite-element-method (codes such as DEFORM, ABAQUS, Fand MSC.Marc. The speed, accuracy, andfriendliness of FEM codes has been facilby optimization of the element type uthe program; the development of automeshing and remeshing routines; the intrtion of advanced solvers; and the incorpoof advanced graphics-user interfaces ((Ref 22).

In the 1980s, early FEM codes were apppredict metal flow in simple two-dimennon-steady-state problems (e.g., closed-d

ging). Since the early 1990s, two-dimenapplications have grown significantly. Intion, increasingly powerful FEM codesbeen applied to simulate a number of dimensional (3-D) forging problems. RFEM applications include the design of tfor forgings that require multiple die imsions, the simulation of open-die forgincesses,and various steady-stateproblemssextrusion; drawing; flat, shape, and pack roand ring rolling (Ref 22–24). The simulatopen-die forging processes for billet pr(e.g., cogging, radial forging) is particchallenging because of the size of theworkthe large number of forging blows, and piece rotations between blows, among

factors. Other complex 3-D problems, have been analyzed using FEM, include oforging, forging of crankshafts, extrusishapes, and helical-gear extrusion.

In addition to predictions of metal flowafill (and associated metal-flow defects slaps, folds, pipe), FEM is also being useularly to analyze the evolution of microstrand defects within the workpiece, die strtooling failure, and so forth. The predictdefects due to cavitation and ductile fractuexample, usually relies on continuum c(e.g., the Cockcroft and Latham maximusile work criterion) and FEM model prediof stresses and strains. Similarly, in the a

microstructure evolution, variations of frrecrystallized and recrystallized graindeveloped within a workpiece during hot ing are typically estimated using phenological models and FEM predictioimposed strain, strain rate, and temperatur24, 25). Such approaches have been relasuccessful for non-steady-state processes sclosed-die press and hammer forging as wfor cogging of steels and superalloys (see Rand the article “Practical Aspects of ConvIngot to Billet” in this Volume).

FEM-based models have also been devefor the prediction of microstructure evo

(a) (b)

0 100 200 300 400 500 600 700 800 900 1000

10

20

30

40

50

60

70

Case C

Case B

Case A

N o r m a l i z e d G

r a i n A r e a ( A v e r a g e )

Time, MCS

(c) (d)

Fig. 5 Monte Carlo (three-dimensional) model predictions of (a, b, and c) grain structure (two-dimensional) sectionsafter 1000 Monte-Carlo Steps) and (d) grain-growth behavior for materials with various starting textures and

assumed grain-boundary properties. (a) Case A, isotropic starting texture and isotropic boundary properties (normal grain-growth case). (b) Case B, initial, single component texture, weakly anisotropic grain-boundary properties. (c) Case C,initial, single component texture, strongly anisotropic grain-boundary properties. Source: Ref 20

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during steady-state processes such as the hotrolling of steel (Ref 26, 27). For instance, in thework of Pauskar (Ref 27), metal flow andmicrostructure evolution during multistandshape rolling were modeled using the integratedsystem ROLPAS-M, which consists of threemain modules. The main module (ROLPAS)

comprises a 3-D nonisothermal FEM code. Thesecond module, MICON, uses the computedthermomechanical history from ROLPAS tomodel the deformation, retained work, dynamic/ static recrystallization, and grain growth of aus-tenite during the rolling process itself. As withmany multistage steel rolling processes, micro-structure changes are controlled primarily bystatic recrystallization and grain growth betweenrolling stands. The fraction recrystallized andretained work are used to estimate the flow stressof the material as it enters each successive rollstand. Last, the module AUSTRANS uses thetemperature history after rolling and cooling-transformation curves to model the decomposi-

tion of austenite during cool-down. More detailson microstructure modeling during multipass hotrolling of steel can be found in the article “Flat,Bar, and Shape Rolling” in this Volume.

Recently, an FEM modeling procedurewas developed within the framework of thecommercial code DEFORM to predict residualstresses that develop during heat treatmentand distortion during subsequent machiningprocesses for ferrous and nickel-base superalloyparts (Ref 28). The FEM model for heat treat-ment assumes that the residual stresses devel-oped during the forging and cool-downoperations arerelieved early during solution heattreatment. Thus, residual stresses are inducedprimarily during quenching and continue to

evolveduring theremainder of theheat treatmentprocess. The rapid cooling during quenchingproduces severe temperature gradients within thepart and gives rise to nonuniform strains. Thedevelopment of residual stresses is thus handledusing a standard elastoplastic constitutive for-mulation in the FEM code. The effect of phasetransformations during cooling (as in steels) onresidual stresses is also treated in the newestFEM heat treatment codes. For this purpose,phase-transformation data are incorporated inorder to quantify the volume changes associatedspecific transformation products that are formedin different areas of a part. To model subsequentstress-relief operations, creep models (e.g., Bai-

ley-Norton, Soderburg) are incorporated into thecode.

Additional information on process simulationmethods for bulk forming is contained in thearticle “Finite Element Method (FEM) Appli-cations in Bulk Forming” in this Volume.

Process design and optimization techni-ques represent the latest and perhaps mostimportant methodology in the development of computer-aided applications for bulk-formingprocesses. The advent of advanced processsimulation tools has replaced former methodsinvolving costly and time-consuming machiningand tryout of dies. However, the selection of

preform designs and processing conditions todetermine optimal die fill, microstructure evo-lution, die life, and so forth may still entailsubstantial trial and error and thus multiplesimulations when computer-modeling techni-ques alone are used. Hence, integrated systemsare now being developed to automate the opti-

mization process. For bulk-forming processes,such systems include an FEM metal-formingcode, a solid-geometry module or program, andan optimization routine or program. Althoughthe specifics of each problem vary, the overallapproach typically comprises three elements:choice of an objective function and constraints,calculation of the objective function (as may bedone by the FEM simulation code), and a searchfor the combination of design parameters thatprovide a minimum or maximum for the objec-tive function. In bulk forming, objective func-tions may include forging weight (minimumusually is best), die fill (minimum underfill isbest), uniformity of strain or strain rate (max-

imum uniformity is best), and so forth. Con-straints may include maximum or minimumallowable strain, strain rate, or temperature toprevent metallurgical defects, the specificationof maximum die stresses or press loading, and soforth.

Several examples of the application of opti-mization to bulk forming are described in Ref24,29, and 30 as well as the article “Design Opti-mization for Dies and Preforms” in this Volume.For example, optimal preform design for two-dimensional (axisymmetric) forgings has beensummarized by Ohet al. (Ref 29). In the examplecited in this work, the objective was to minimizeunderfill via optimization of the preform shape,which was represented as a series of B-spline

curves described through a collection of shape-control parameters. FEM simulations were runto determine the rate at which die fill changedwith respect to changes in the shape-controlparameters. At the end of each FEM forgingsimulation (using DEFORM), the values of theobjective function (and constraint functions) andtheir gradients (i.e., changes with respect to thechange in each shape-control parameter) weredetermined in order to pick a new search direc-tion in shape-control-parameter space. Srivatsaperformed similar analysis to minimize theweight of superalloy engine disks usingDEFORM (for FEM modeling),Unigraphics (forsolid modeling), and iSIGHT (for the optimiza-

tion code) (Ref 30).Initial work has also been conducted to auto-

mate the optimization of preform design for 3-Dforgings. For example, Oh et al. and Walterset al. (Ref 24, 29) describe a two-step process inwhich the final forging geometry is first “fil-tered” to obtain an initial guess for the preformshape (Fig. 6). This is done using a Fouriertransform technique in which sharp corners,edges, and small surface details are smoothed.The boundary of the preform is then “trimmed”to obtain a realistic preform for input to the FEMsimulation used to determine regions of underfilland inadequate/excessive flash formation. Based

on the FEM metal-flow predictions, addimaterial is then added or removed to theprthe new preform is filtered and trimmeadditional simulations are run in an itefashion until the desired result is obtained

Conclusions and Future Outlook

Recent advances in the bulk forming of have focused on the development of a numnew processes, the increasing utilizatithermomechanical processing (TMP) foferrous and nonferrous alloys, and the spread application of computer-based pmodels. Although relatively few new mahave reached the level of mass productionthe last decade, the processing of a numbercalled conventional alloys has undergonnificant improvements due to novel sequences, thus leading to less strikin

nonetheless important, improvements in sperformance. Further improvements in mproperties are likely as the quantitative ustanding and modeling of the evolutimicrostructure and texture expands aapplied in industry. The integration of pmodels with models of microstructure evo

(a)

(b)

(c)

Fig. 6 Application of finite-element-based otion to determine preform shape for a

dimensional forging. (a) Final forging. (b) “preform geometry. (c) “Trimmed” preform geSource: Ref 24, 29



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and defect formation will help refine allowableprocessing windows and needed process con-trols. Such integration will thus form a veryimportant part of overall process optimization.

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American Society for Metals, 19832. T. Altan, G. Ngaile, and G. Shen, Cold and

Hot Forging: Fundamentals and Applica-tions, ASM International, 2004

3. B.P. Bewlay, M.F.X. Gigliotti, F.Z. Utya-shev, and O.A. Kaibyshev, Superplastic RollForming of Ti Alloys, Mater. Des., Vol 21,2000, p 287–295

4. B.P. Bewlay, M.F.X. Gigliotti, C.U. Hard-wicke, O.A. Kaibyshev, F.Z. Utyashev, andG.A. Salishchev, Net-Shape Manufacturingof Aircraft Engine Disks by Roll Forming

and Hot Die Forging, J. Mater. Proc. Tech-nol., Vol 135, 2003, p 324–3295. V.M. Segal, Equal Channel Angular Extru-

sion: From Macromechanics to StructureFormation, Mater. Sci. Eng. A, Vol A271,1999, p 322–333

6. M. Geiger, M. Kleiner, R. Eckstein,N. Tiesler, and U. Engel, Microforming,

Ann. CIRP, Vol 50 (No. 2), 2001, p 445–4627. S.L. Semiatin, J.C. Chestnutt, C. Austin, and

V. Seetharaman, Processing of IntermetallicAlloys, StructuralIntermetallics 1997, M.V.Nathal, R. Darolia, C.T. Liu, P.L. Martin,D.B. Miracle, R. Wagner, and M. Yama-guchi, Ed., TMS, 1997, p 263–277

8. V.K. Sikka, Melting, Casting, and Proces-

sing of Nickel and Iron Aluminides, HighTemperature Ordered Intermetallic AlloysVI, J. Horton, I. Baker, S. Hanada, R.D.Noebe, and D.S. Schwartz, Ed., MaterialsResearch Society, 1995, p 873–878

9. S.L. Semiatin, Wrought Processing of IngotMetallurgy Gamma Titanium AluminideAlloys, Gamma Titanium Aluminides,Y-W. Kim, R. Wagner, and M. Yamaguchi,Ed., TMS, 1995, p 509–524

10. D.D. Krueger, R.D. Kissinger, and R.G.Menzies, Development and Introduction of a Damage Tolerant High TemperatureNickel-Base Disk Alloy, René 88DT,Superalloys 1992, S.D. Antolovich et al.,

Ed., TMS, 1992, p 277–28611. D. Furrer and J. Gayda,Dual-Microstructure

Heat Treat Processing of Turbine Engine

Disks, Adv. Mater. Process., July 2003,p 36–39

12. M.F.X. Gigliotti, R.S. Gilmore, J.N. Bar-shinger, B.P. Bewlay, C.U. Hardwicke,G.A.Salishchev, R.M. Baleyev, and O.R.Valiakhmetov, Titanium Alloy BilletProcessing for Low Ultrasonic Noise,

Ti-2003: Science and Technology, G. Luet- jering and J. Albrecht, Ed., Wiley-VCHVerlag GmbH, Weinheim, Germany, 2004,p 297–304

13. S.V. Zherebstov, G.A. Salishchev, R.M.Galeyev, O.R. Valiakhmetov, and S.L.Semiatin, Formation of SubmicrocrystallineStructure in Large-Scale Ti-6Al-4V BilletDuring Warm Severe Plastic Deformation,Proc. Second International Conference on

Nanomaterials by Severe Plastic Deforma-tion: Fundamentals-Processing-Applica-tions, M.J. Zehetbauer and R.Z. Valiev, Ed.,Wiley-VCH, Weinheim, Germany, 2004,p 835–840

14. J. Williams, Thermo-Mechanical Proces-sing of High-Performance Ti Alloys: RecentProgress and Future Needs, J. Mater. Proc.Technol., Vol 117, 2001, p 370–373

15. S.L. Semiatin and I.M. Sukonnik, RapidHeat Treatment of Titanium Alloys, Proc.Seventh International Symposium on Phy-

sical Simulation of Casting, Hot Rolling and Welding, H.G. Suzuki, T. Sakai, and F.Matsuda, Ed., Dynamic Systems, Inc., 1997,p 395–405

16. O.M. Ivasishin, S.L. Semiatin, P.E. Mar-kovsky, S.V. Shevchenko, and S.V. Ulshin,Grain Growth and Texture Evolution inTi-6Al-4V During Beta Annealing underContinuous Heating Conditions, Mater. Sci.

Eng. A, Vol A337, 2002, p 88–9617. O.M. Ivasishin, S.V. Shevchenko, N.L.

Vasiliev, and S.L. Semiatin, 3-D Monte-Carlo Simulation of Texture Evolution andGrain Growth During Annealing, Acta

Mater., Vol 51, 2003, p 1019–103418. O.M. Ivasishin, S.V. Shevchenko, P.E.

Markovsky, and S.L. Semiatin, Experi-mental Investigation and 3-D Monte-Carlo Simulation of Texture-ControlledGrain Growth in Titanium Alloys, Ti-2003:Science and Technology, G. Luetjeringand J. Albrecht, Ed., Wiley-VCH VerlagGmbH, Weinheim,Germany, 2004, p 1307–1314

19. F.J. Humphreys and M. Hatherly, Recrys-tallization and Related Phenomena, Else-vier, Oxford, U.K., 1995

20. O.M. Ivasishin, S.V. Shevchenko,Vasiliev, and S.L. Semiatin, 3-D MCarlo Simulation of Texture EvolutioGrain Growth During Annealing, Met

Adv. Technol., Vol 23, 2001, p 1569–21. S.L. Semiatin, Evolution of Microstr

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ability and Process Design, G.E. DH.A. Kuhn, and S.L. Semiatin, Ed.,International, 2003, Chap. 3

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and J. Gayda, Modeling Machining Dtion of Aircraft-Engine Disk For

NUMIFORM 2004, S. Ghosh, J.M. Cand J.K. Lee, Ed., American InstitPhysics, 2004, p 400–405

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8 / Introduction


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Introduction to Bulk-Forming Processes

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