Introduction to Bulk-Forming Processes

Home , Coining (metalworking), Ductility, Explosive forming, Extrusion, Forming (metalworking), Forming processes

S.L. Semiatin, Air Force Research Laboratory, Materials and Manufacturing Directorate

METALWORKING consists of deformation brought about desirable increases in strength ranging from firearms to locomotive parts. processes in which a metal billet or blank is (a phenomenon now known as strain hardening). Similarly, the steam engine spurred develop- shaped by tools or dies. The design and control of The quest for strength spurred a search for alloys ments in rolling, and, in the 19th century, such processes depend on the characteristics that were inherently strong and led to the utili- a variety of steel products were rolled in of the workpiece material, the conditions at the zation of alloys of copper and tin (the Bronze significant quantities. tool/workpiece interface, the mechanics of Age) and iron and carbon (the Iron Age). The The past 100 years have seen the development plastic deformation (metal flow), the equipment Iron Age, which can be dated as beginning of new types of metalworking equipment and used, and the finished-product requirements. around 1200 B.C., followed the beginning of new materials with special properties and These factors influence the selection of tool the Bronze Age by some 1300 years. The reason applications. The new types of equipment have geometry and material as well as processing for the delay was the absence of methods for included mechanical and screw presses and conditions (for example, workpiece and die achieving the high temperatures needed to melt high-speed tandem rolling mills. The materials temperatures and lubrication). Because of the and to refine iron ore. that have benefited from such developments in complexity of many metalworking operations, Most metalworking was done by hand until equipment range from the low-carbon steel and models of various types, such as analytical, the 13th century. At this time, the tilt hammer advanced high-strength steels used in auto- physical, or numerical models, are often relied was developed and used primarily for forging mobiles and appliances to specialty aluminum-, upon to design such processes. bars and plates. The machine used water power titanium-, and nickel-base alloys used in the This Volume presents the state-of-the-art in to raise a lever arm that had a hammering tool aerospace and other industries. In the approxi- bulk-metalworking processes. A companion at one end; it was called a tilt hammer because mately 20 years since this Volume was last volume (ASM Handbook, Volume 14B, Metal- the arm tilted as the hammering tool was raised. updated, methods for the bulk forming of a working: Sheet Forming) describes the state-of- After raising the hammer, the blacksmith let number of new materials, such as intermetallic the-art in sheet-forming processes. Various it fall under the force of gravity, thus generat- alloys and composites, have been developed. major sections of this Volume deal with ing the forging blow. This relatively simple Furthermore, the advent of user-friendly com- descriptions of specific processes, selection of device remained in service for a number of puter codes and inexpensive computers has led to equipment and die materials, forming practice centuries. a revolution in the application of numerical for specific alloys, and various aspects of process The development of rolling mills followed methods for the design and control of a plethora design and control. This article provides a brief that of forging equipment. Leonardo da Vinci’s of bulk-forming processes, thus leading to historical perspective, a classification of metal- notebook includes a sketch of a machine higher-quality products and increased efficiency working processes and equipment, and a sum- designed in 1480 for the rolling of lead for in the metalworking industry. mary of some of the more recent developments in stained glass windows. In 1495, da Vinci is the field. reported to have rolled flat sheets of precious metal on a hand-operated two-roll mill for coin- Classification of Metalworking making purposes. In the following years, several Historical Perspective designs for rolling mills were utilized in Processes Germany, Italy, France, and England. However, Metalworking is one of three major technol- the development of large mills capable of hot In metalworking, an initially simple work- ogies used to fabricate metal products; the others rolling ferrous materials took almost 200 years. piece—a billet or a blanked sheet, for example— are casting and powder metallurgy. However, This relatively slow progress was primarily due is plastically deformed between tools (or dies) to metalworking is perhaps the oldest and most to the limited supply of iron. Early mills obtain the desired final configuration. Metal- mature of the three. The earliest records of employed flat rolls for making sheet and plate, forming processes are usually classified accord- metalworking describe the simple hammering of and until the middle of the 18th century, these ing to two broad categories: gold and copper in various regions of the Middle mills were driven by water wheels. Bulk, or massive, forming operations East around 8000 B.C. The forming of these During the Industrial Revolution at the end of Sheet-forming operations (Sheet forming is metals was crude because the art of refining by the 18th century, processes were devised for also referred to as forming. In the broadest and smelting was unknown and because the ability to making iron and steel in large quantities to most accepted sense, however, the term work the material was limited by impurities that satisfy the demand for metal products. A need forming is used to describe bulk- as well as remained after the metal had been separated from arose for forging equipment with larger capacity. sheet-forming processes). the ore. With the advent of copper smelting This need was answered with the invention of the around 4000 B.C., a useful method became high-speed steam hammer, in which the hammer In both types of processes, the surfaces of the available for purifying metals through chemical is raised by steam power, and the hydraulic press, deforming metal and the tools are in contact, and reactions in the liquid state. Later, in the Copper in which the force is supplied by hydraulic friction between them may have a major influ- Age, it was found that the hammering of metal pressure. From such equipment came products ence on material flow. In bulk forming, the © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)

2 / Introduction input material is in billet, rod, or slab form, Specific bulk-forming processes are listed in bulk-forming area, these include advanced and the surface-to-volume ratio in the formed Table 1. roll-forming methods, equal-channel angular part increases considerably under the action of extrusion, incremental forging, and micro- largely compressive loading. In sheet forming, Types of Metalworking Equipment forming. on the other hand, a piece of sheet metal is Advanced roll-forming methods have been plastically deformed by tensile loads into a developed for making axisymmetric components three-dimensional shape, often without signifi- The various forming processes discussed with very complex cross sections. Shaping is cant changes in sheet thickness or surface previously are associated with a large variety of conducted using opposed rollers or a combina- characteristic. forming machines or equipment, including the tion of rollers and a mandrel acting on a rotating Processes that fall under the category of bulk following (Ref 1, 2): workpiece (Fig. 1). Unlike former simple roll- forming have the following distinguishing fea- Rolling mills for plate, strip, and shapes forming processes used to make long tubes, tures (Ref 1, 2): Machines for profile rolling from strip cones, and so forth, newer roll-forming methods Ring-rolling machines rely on the simultaneous control of radial and The deforming material, or workpiece, axial metal flow. For example, the internal pro- undergoes large plastic (permanent) defor- Thread-rolling and surface-rolling machines Magnetic and explosive forming machines file of complex shapes can be generated by mation, resulting in an appreciable change in combined radial-axial roll forming that typically shape or cross section. Draw benches for tube and rod; wire- and rod- drawing machines makes use of a sophisticated internal mandrel The portion of the workpiece undergoing consisting of several angular segments and plastic deformation is generally much larger Machines for pressing-type operations (presses) devices to quickly lock or unlock the segments. than the portion undergoing elastic deforma- Such roll-forming operations can be conducted tion; therefore, elastic recovery after defor- Among those listed, pressing-type machines under either cold- or hot-working conditions; the mation is negligible. are the most widely used and are applied to specific temperature depends on the ductility and Examples of generic bulk-forming processes both bulk- and sheet-forming processes. These strength of the workpiece material and the are extrusion, forging, rolling, and drawing. machines can be classified into three types: complexity of the shape to be made. The tech- load-restricted machines (hydraulic presses), nique has been used to make aircraft engine disks stroke-restricted machines (crank and eccentric, and cases, automotive wheels, and other parts. Table 1 Classification of bulk (massive) or mechanical, presses), and energy-restricted Recently, the feasibility of radial roll forming forming processes machines (hammers and screw presses). The has been demonstrated for titanium alloys Ti- significant characteristics of pressing-type 6Al-4V, Ti-6Al-2Sn-4Zr-2Mo, and Ti-6Al-2Sn- Forging machines comprise all machine design and per- 4Zr-6Mo and nickel-base alloys 718, Waspaloy, Closed-die forging with flash formance data that are pertinent to the econom- Rene´ 95, Rene´ 88, and Merl 76 (Ref 3, 4). Closed-die forging without flash ical use of the machine. These characteristics The forming of such alloys is enhanced by the Coining include: Electro-upsetting development of an ultrafine grain structure in Forward extrusion forging Characteristics for load and energy: Avail- the preform material. Advanced roll forming can Backward extrusion forging able load, available energy, and efficiency provide near-net shapes at lower cost compared Hobbing to forging and ring rolling because of the elim- Isothermal forging factor (which equals the energy available for Nosing workpiece deformation/energy supplied to the ination of dies and the ability to utilize a given set Open-die forging machine) of rollers for multiple geometries. Because of the Rotary (orbital) forging Time-related characteristics: Number of generally high deformation that is imposed over Precision forging the entire part cross section, microstructure uni- Metal powder forging strokes per minute, contact time under pres- Radial forging sure, and velocity under pressure formity also tends to be excellent. More detailed Upsetting Characteristics for accuracy: For example, information on the technology can be found in Incremental forging deflection of the ram and frame, particularly the article “Roll Forming of Axially Symmetric Rolling under off-center loading, and press stiffness Components” in this Volume. Sheet rolling Equal-channel angular extrusion (ECAE) is Shape rolling an emerging metal-processing technique devel- Tube rolling Recent Developments in oped by Segal in the former Soviet Union in the Ring rolling 1970s, but not widely known in the West until Rotary tube piercing Bulk Forming Gear rolling the 1990s (Ref 5). In ECAE, metal flow com- Roll forging Since the publication in 1988 of the pre- prises deformation through two intersecting Cross rolling vious edition of the ASM Handbook on Forming channels of equal cross-sectional area (Fig. 2). Surface rolling and Forging, metalworking practice has seen a The imparted strain is a function primarily Shear forming w Tube reducing number of notable advances with regard to the of the angle between the two channels, 2 , and Radial roll forming development of new processes; new materials, the angle of the curved outer corner, y. For w= y= Extrusion the increased control of microstructure via spe- 2 90 and 0, for example, the effective strain is approximately equal to 1.15. By passing Nonlubricated hot extrusion cialized thermomechanical processes, and the Lubricated direct hot extrusion development of advanced tools for predicting the workpiece through the tooling many times, Hydrostatic extrusion microstructure and texture evolution; and the very large deformations can thus be imposed. Co-extrusion application of sophisticated process simulation Hence, the process has been investigated as a Equal channel angular extrusion and design tools. Some of these technological means to refine microstructure and to control Drawing advances are summarized in the following sec- crystallographic texture; in many cases sub- Drawing tions of this article. microcrystalline grain structures are developed. Drawing with rolls The majority of work to date has been performed Ironing on aluminum and aluminum alloys, copper, iron, Tube sinking New Processes Co-drawing nickel, and titanium. Although the greatest A number of novel processes have recently attention has been focused on square or round Source: Ref 1 been introduced and/or investigated. In the billet products, ECAE has also been used to © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)

Introduction to Bulk-Forming Processes / 3

range. Most of the developments in this area have been driven by the needs of the electronics industry for mass-produced miniature parts. As summarized by Geiger et al. (Ref 6), major challenges in microforming fall into one of four broad categories: workpiece material, tooling, equipment, and process control. For example, the ﬂow and failure behavior of a workpiece with only one or several grains across the section subjected to large strains can be very different from that of its polycrystalline counterpart used in macro bulk-forming processes. Micro- (a) forming operations include cold heading and extrusion of wire. For example, Geiger et al. described the bulk forming of copper pins via forward rod extrusion and backward (can) extrusion to produce a shaft diameter of 0.8 mm (0.03 mils) and a wall thickness of 125 mm (5 mils). For this and similar micro-operations, challenges include handling of small preforms, manufacture of tooling with complex inner geometry, tooling alignment, and the overall precision of the forming equipment.

(b) Materials-Related Developments Roll-formed Sonic shape shape Recent materials-related developments in- Fig. 1 Radial roll forming. (a) Schematic of the process. (b) Complex-shape titanium-alloy component fabricated clude breakthroughs in the bulk forming of new via radial roll forming. The sketch in (b) shows the outline of the roll-formed part relative to the sonic shape. materials, increased control of microstructure Source: Ref 3, 4 development using specialized thermomechanical processes, and the development of advanced tools for predicting microstructure and texture Incremental forging is a closed-die forging evolution. process in which only a portion of the workpiece New materials for which substantial pro- is shaped during each of a series of press strokes. gress has been made over the last 20 years Ram The process is analogous to open-die forging include structural-intermetallic alloys and dis- (cogging) of ingots, billets, thick plates, and continuously reinforced metal-matrix compo- shafts. In contrast to such operations, however, sites (MMCs). For intermetallic alloys, impression dies (not flat or V-shaped tooling) are bulk-forming approaches have been most utilized. The primary applications of the tech- dramatic for aluminide-based materials (Ref 7). φ 2 nique are very large plan-area components of Bulk forming on a commercial scale has been Workpiece high-temperature alloys for which die pressures used for MMCs with aluminum-alloy and, to a A can easily equal or exceed 10 to 20 tsi. In such lesser extent, titanium-alloy matrices. instances, part plan area is limited to approxi- Iron-aluminide alloys based on the Fe3Al mately several thousand square inches for the compound are probably the structural inter- largest presses (50,000 tons) currently available metallic materials that have been produced in the in the United States. By forging only a portion of largest quantities to date. These materials exhibit A' ψ the part at a time, however, press requirements excellent oxidation and sulfidation resistance are reduced. Applications of the technique Die and potentially lower cost than the stainless include large, axisymmetric components for steels with which they compete. As such, a land-based gas turbines made from nickel-base number of potential applications for these iron Fig. 2 Equal-channel-angular extrusion alloy 706 (Wyman-Gordon Company and Alcoa aluminides have been identified. These include Forged Products) and various Ti-6Al-4V (rib- metalworking dies, heat shields, furnace fixtures web) structural components for F-18 aircraft and heating elements, and a variety of auto- (manufactured at Alcoa Forged Products). The motive components. Sikka and his colleagues at produce ultrafine-grain plate materials for use as latter parts had plan areas of the order of Oak Ridge National Laboratory have spear- is or as preforms for subsequent sheet rolling. 5000 in.2. Needless to say, part symmetry and headed the development of techniques for the hot Furthermore, work has been performed to forging design (e.g., forging envelope) play a extrusion, forging, and rolling of ingot-metal- modify the ECAE concept to allow continuous, critical role in the design of incremental-forging lurgy Fe3Al-base alloys at temperatures in the rather than batch, processing; such efforts have processes. Forging design, representing a key range of 900 to 1200 C (1650 to 2190 F) been limited to thin cross-section products feature of incremental forging, is often highly (Ref 8). The wrought product can then be warm such as wire, rod, and sheet. More detailed specialized and proprietary in nature. rolled to plate or sheet at temperatures between information on ECAE can be found in the article Microforming is a technology generally 500 and 600 C (930 and 1110 F) to manu- “Equal-Channel Angular Extrusion” in this defined as the production of parts or structures facture material with room-temperature tensile Volume. with at least two dimensions in the submillimeter ductility of 15 to 20%. Wrought Fe3Al alloys © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)

4 / Introduction do not possess adequate workability for cold fan exit guide vanes in commercial jet engines, heat treatment to obtain graded microstructures. rolling or cold drawing, however. bicycle-frame tubing), rolling (to make sheet), Methods to obtain ultrafine billet microstructures Titanium-aluminide alloys based on the face- and closed-die forging (e.g., helicopter rotor- in alpha/beta titanium alloys such as Ti-6Al-4V, centered tetragonal (fct) gamma phase (TiAl) blade sleeves, automobile engine pistons and Ti-6Al-2Sn-4Zr-2Mo, and Ti-17 rely on special represent a second type of aluminide material for connecting rods). Cast-and-extruded DRA MMC forging practices for partially converted ingots which significant progress has been made toward driveshafts with a 6061 aluminum matrix and containing an initial transformed-beta (colony/ commercialization. The gamma-titanium alu- alumina reinforcements have also been intro- basketweave alpha) microstructure. In one minide alloys have a number of applications as duced for pickup trucks and sports cars. Simi- approach, multistep hot forging along three lightweight replacements for superalloys in the larly, extrusion and upsetting of pressed-and orthogonal directions is conducted at strain À 3 À1 hot section of aircraft engines and as thermal sintered Ti-6Al-4V reinforced with TiB2 parti- rates of the order of 10 s and a series of protection systems in hypersonic vehicles. culate have been used to mass produce auto- temperatures in the alpha/beta phase field Spurred by substantial efforts at the Air Force mobile and motorcycle engine valves. More (Ref 12). By this means, an alpha grain size of Research Laboratory, Battelle Memorial Insti- detailed information on the bulk processing of 4to8mm (0.16 to 0.31 mils) with good ultra- tute, Ladish Company, and Wyman-Gordon, a metal-matrix composites can be found in the sonic inspectability is obtained. In a similar variety of metalworking techniques for both article “Forging of Discontinuously Reinforced approach, warm working, involving very high ingot-metallurgy (I/M) and powder-metallurgy Aluminum Composites” in this Volume and strains and somewhat lower temperatures (~550 (P/M) materials have been developed for these in the article “Processing of Metal-Matrix to 700 C, or 1020 to 1290 F), has been found materials, which were once thought to be Composites” in composites, Volume 21, of ASM to yield a submicrocrystalline alpha grain size unworkable (Ref 7, 9). For instance, isothermal Handbook. (Ref 13). forging and canned hot-extrusion techniques Thermomechanical processing (TMP) have been demonstrated for the breakdown of refers to the design and control of metalworking medium- to large-scale ingots. Novel can designs and heat treatment steps in an overall manu- and the use of a controlled dwell time between facturing process in order to enhance final billet removal from the preheat furnace and microstructure and properties. Thermo- deformation have greatly enhanced the feasi- mechanical processing was developed initially bility of hot extrusion. In particular, the dwell as a method for producing high-strength or high- time is chosen in order to develop a temperature toughness microalloyed steels via (ferrite) grain difference between the sacrificial can material refinement and controlled precipitation. Current and the titanium aluminide preform; by this trends in the TMP of ferrous alloys are focusing means the flow stresses of the two components is on the development of carbide-free steels with similar, thereby promoting uniform co-extru- bainitic microstructures to obtain yet higher sion. Secondary processing of parts has been strength levels. Further information on the state- most often conducted via isothermal closed-die of-the-art of ferrous TMP is contained in the forging (Fig. 3a, b). Careful can design and article “Thermomechanical Processes for Fer- understanding of temperature transients have rous Alloys” in this Volume. also enabled the hot pack rolling of gamma Thermomechanical processing is now also titanium-aluminide sheet and foil products used being used routinely for nickel- and titanium- in subsequent superplastic-forming operations base alloys. Two examples of recent advances in (Fig. 3c). A key to the success of each of these the TMP of nickel-base superalloys, intended to processes has been the development of a detailed improve damage tolerance or creep resistance in understanding of the pertinent phase equilibria/ service, comprise techniques to produce a uni- phase transformations and the effects of micro- form intermediate grain size (ASTM ~6) or a structure, strain rate, and temperature on failure graded microstructure. The former technique is modes during processing. More detailed infor- especially useful for the manufacture of P/M mation on the bulk processing of intermetallic superalloys such as Rene´ 88, N18, and alloy 720. alloys can be found in the article “Bulk Forming In this instance, TMP consists of isothermal of Intermetallic Alloys” in this Volume. forging of consolidated-powder preforms fol- Discontinuously reinforced aluminum metal- lowed by supersolvus heat treatment. To achieve matrix composites (DRA MMCs) have been the desired final grain size after supersolvus heat synthesized and subsequently bulk formed by a treatment, however, forging must be performed variety of techniques. The majority of MMCs in a very tightly controlled strain, strain-rate, and have been based on aluminum matrices with temperature window for each specific material silicon carbide particulate or whisker reinforce- (Ref 10). Lack of control during deformation ments synthesized in tonnage quantities by I/M may result in uncontrolled (abnormal) grain or P/M approaches. In the I/M method, which is growth during the subsequent supersolvus heat most often used for automotive parts, the ceramic treatment, leading to isolated grains or groups of particles are introduced and suspended in the grains that are several orders of magnitude larger liquid aluminum alloy prior to casting using a than the average grain size. Processes to develop high-energy mixing process. In the P/M graded microstructures in superalloys consist of approach, most often used for aerospace mate- local heating above the solvus temperature to rials, the matrix and ceramic powders are blen- dissolve the grain-boundary pinning phase (e.g., ded in a high-shear mixer prior to canning and gamma prime) and thus facilitate grain growth in outgassing. Following the casting/canning these regions while other portions of the com- operations, conventional extrusion, forging, and ponent are cooled (Ref 11). Fig. 3 Wrought gamma titanium products. (a) Com- rolling processes are used to make billet and Thermomechanical processes for titanium pressor blades. (b) Subscale isothermally forged disk. (a) and (b) Source: D.U. Furrer, Ladish Company. (c) plate products. Secondary processing may alloys include processing to produce ultrafine Large, conventionally (pack) rolled sheet. Source: Battelle include extrusion (e.g., automotive driveshafts, grain billet, “through-transus” forging, and final Memorial Institute, Air Force Research Laboratory © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)

Introduction to Bulk-Forming Processes / 5

Through-transus forging of alloys such as Ti- methods comprise local heating of selected imposed processing conditions and are thus 6Al-4Sn-4Zr-6Mo is a TMP process that com- regions of a part above the beta-transus tem- typically valid only within the specific range of bines aspects of beta and alpha-beta forging in perature followed by controlled cooling. Infor- the observations (Ref 19). For example, the order to develop a microstructure with both high mation on beta annealing under continuous- evolution of recrystallized volume fraction strength and good fracture toughness/fatigue heating conditions and the effect of texture and recrystallized grain size that evolve during resistance. By working through the transus, the evolution on beta grain growth is invaluable for hot deformation (due to “dynamic” recrystalli- development of a continuous (and deleterious) the selection of heating rates and peak tempera- zation) can be described as a function of the layer of alpha along the beta grain boundaries is tures for such TMP routes (Ref 16–18). In imposed strain, strain rate, and temperature. avoided (Ref 14). Instead, a transformed beta addition, because the decomposition of the Similar models treat the evolution of grain matrix microstructure with equiaxed alpha par- metastable beta is very sensitive to cooling rate, structure during annealing following cold or hot ticles on the beta grain boundaries (“snow on the dual-microstructure TMP processes may be used working as a function of time due to “static” boundaries”) is produced. If forging is conducted to produce components with a gradation of recrystallization. In both cases, the recrystallized to temperatures that are too low, however, microstructure morphologies. volume fraction typically follows a sigmoidal undesirable equiaxed, primary alpha is nucleated More detailed information on the TMP of (“Avrami”) dependence on strain or time. Phe- within the matrix. To help meet the tight limits nickel-base and titanium alloys can be found in nomenological models of dynamic and static on temperature for the process, therefore, hot-die the article “Thermomechanical Processes for recrystallization have been developed for a forging coupled with finite-element modeling for Nonferrous Alloys” in this Volume. variety of steels, aluminum alloys, and nickel- process design have been utilized. Microstructure-evolution models fall into base alloys. Grain growth during heat treat- As with nickel-base superalloys, special heat two broad categories: phenomenological and ment of single-phase alloys without or with treatments have been developed to provide dual mechanistic. Phenomenological microstructure- a dispersion of second-phase particles can (and graded) microstructures in alpha-beta tita- evolution models have been developed to cor- also be quantified using phenomenological nium alloys (Ref 15) (Fig. 4). Most of these relate measured microstructural features to equations such as that based on a parabolic fit of observations for a very wide range of metals and alloys. Mechanism-based approaches have also been investigated during the last 20 years to model microstructure evolution during hot working and annealing. These models incorporate determi- nistic and statistical aspects to varying degrees and seek to quantify the specific mechanism underlying microstructure changes. Most of the models incorporate physics-based rules for events such as nucleation and growth during both recrystallization and grain growth. The effects of stored work, concurrent hot working, crystallographic texture, grain-boundary energy and mobility, second-phase particles, and so forth on microstructure evolution can thus be described by these approaches. As such, accurate models of this type can delineate microstructure evolution over a broader range of processing conditions than phenomenological models and are also very useful for processes involving strain rate and temperature transients. In addition, the models can provide insight into the source of observed deviations from classical Avrami behavior during recrystallization or nonparabolic grain growth (e.g., Fig. 5). Two principal types of mechanism-based approaches are those based on cellular automaton (primarily used for recrystallization problems) and the Monte-Carlo/Potts formalism (used for both recrystallization and grain-growth problems). Both of these formula- tions seek to describe phenomena at the meso (grain) scale. Challenges with regard to the validation and industrial application of many mechanism-based models still remain, however, largely because of the dearth of reliable material- property data. More detailed information on phenomenological and mechanism-based models of microstructure evolution can be found in the article “Models for Predicting Microstructure Evolution” in this Volume and in Ref 21. Texture-evolution models fall into two Fig. 4 Graded microstructure obtained in a 75 mm (3 in.) diam Ti-6Al-4V bar via localized induction heating. main categories, those principally for the (a) Macrostructure. (b) Microstructure in core. (c) Microstructure in surface layer. Source: Ref 15 prediction of either deformation textures or © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)

6 / Introduction recrystallization/transformation textures. The processes can be quantified in terms of the field Process Simulation and Design development of such modeling techniques has variables used in these codes. More detailed greatly accelerated in recent years due to the information on deformation texture modeling With the advent of powerful and inexpensive ready availability of powerful computer resour- can be found in the article “Polycrystal Model- computer hardware and software, a veritable ces. Deformation texture modeling is more ing, Plastic Forming, and Deformation Textures” revolution in the design of bulk-forming pro- advanced compared to efforts for predicting in this Volume. cesses using advanced modeling and optimiza- recrystallization/transformation textures. Defor- A relatively recent development in texture tion techniques has occurred in the last 20 years. mation texture modeling treats the slip and modeling is that associated with the recrystalli- Advances in process simulation have twinning processes and the associated crystal zation or transformation phenomena during or been spurred primarily by the development of rotations to predict anisotropic plastic flow and following hot deformation. For example, the general-purpose, finite-element-method (FEM) texture evolution. Models of this sort include textures that evolve during hot working are a codes such as DEFORM, ABAQUS, Forge3, lower- and upper-bound approaches in which result of both dislocation glide and dynamic and MSC.Marc. The speed, accuracy, and user- either stress or strain compatibility is enforced recrystallization. Texture evolution can be friendliness of FEM codes has been facilitated among the grains in a polycrystalline aggregate, quantified by mechanisms such as oriented by optimization of the element type used in respectively. Upper-bound models give reason- nucleation and selective growth. In the former the program; the development of automatic able estimates of deformation texture evolution mechanism, recrystallization nuclei are formed meshing and remeshing routines; the introduc- in many cases. However, more-detailed approa- in those grains that have suffered the least shear tion of advanced solvers; and the incorporation ches, which incorporate strain variations from strain (i.e., dislocation glide). Selective (i.e., of advanced graphics-user interfaces (GUIs) grain to grain (so-called self-consistent models) faster) growth is then assumed to occur for nuclei (Ref 22). as well as within each grain (crystal-plasticity of particular misorientations with respect to the In the 1980s, early FEM codes were applied to FEM techniques, or CPFEM), offer the promise matrix. More detailed information on the mod- predict metal flow in simple two-dimensional, of even more accurate predictions. The latter eling of the evolution of recrystallization and non-steady-state problems (e.g., closed-die for- (CPFEM) approach may also be useful for the transformation textures can be found in the ging). Since the early 1990s, two-dimensional determination of local conditions that may give article “Transformation and Recrystallization applications have grown significantly. In addi- rise to cavitation, spheroidization, and so forth Textures Associated with Steel Processing” in tion, increasingly powerful FEM codes have provided that the physics associated with such this Volume. been applied to simulate a number of three- dimensional (3-D) forging problems. Recent FEM applications include the design of tooling for forgings that require multiple die impres- sions, the simulation of open-die forging processes, and various steady-state problems such as extrusion; drawing; flat, shape, and pack rolling; and ring rolling (Ref 22–24). The simulation of open-die forging processes for billet products (e.g., cogging, radial forging) is particularly challenging because of the size of the workpiece, the large number of forging blows, and workpiece rotations between blows, among other factors. Other complex 3-D problems, which have been analyzed using FEM, include orbital forging, forging of crankshafts, extrusion of shapes, and helical-gear extrusion. In addition to predictions of metal flow and die fill (and associated metal-flow defects such as (a) (b) laps, folds, pipe), FEM is also being used reg- ularly to analyze the evolution of microstructure and defects within the workpiece, die stresses/ 70 tooling failure, and so forth. The prediction of Case A 60 defects due to cavitation and ductile fracture, for example, usually relies on continuum criteria 50 (e.g., the Cockcroft and Latham maximum ten-

40 sile work criterion) and FEM model predictions of stresses and strains. Similarly, in the area of 30 Case B microstructure evolution, variations of fraction 20 recrystallized and recrystallized grain size Case C developed within a workpiece during hot work- 10 ing are typically estimated using phenomen- Normalized Grain Area (Average) ological models and FEM predictions of 0 100 200 300 400 500 600 700 800 900 1000 imposed strain, strain rate, and temperature (Ref Time, MCS 24, 25). Such approaches have been relatively successful for non-steady-state processes such as (c) (d) closed-die press and hammer forging as well as for cogging of steels and superalloys (see Ref 24 Fig. 5 Monte Carlo (three-dimensional) model predictions of (a, b, and c) grain structure (two-dimensional) sections and the article “Practical Aspects of Converting after 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- Ingot to Billet” in this Volume). growth case). (b) Case B, initial, single component texture, weakly anisotropic grain-boundary properties. (c) Case C, FEM-based models have also been developed initial, single component texture, strongly anisotropic grain-boundary properties. Source: Ref 20 for the prediction of microstructure evolution © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)

Introduction to Bulk-Forming Processes / 7 during steady-state processes such as the hot preform designs and processing conditions to on the FEM metal-flow predictions, additional rolling of steel (Ref 26, 27). For instance, in the determine optimal die fill, microstructure evo- material is then added or removed to the preform, work of Pauskar (Ref 27), metal flow and lution, die life, and so forth may still entail the new preform is filtered and trimmed, and microstructure evolution during multistand substantial trial and error and thus multiple additional simulations are run in an iterative shape rolling were modeled using the integrated simulations when computer-modeling techni- fashion until the desired result is obtained. system ROLPAS-M, which consists of three ques alone are used. Hence, integrated systems main modules. The main module (ROLPAS) are now being developed to automate the opti- comprises a 3-D nonisothermal FEM code. The mization process. For bulk-forming processes, second module, MICON, uses the computed such systems include an FEM metal-forming Conclusions and Future Outlook thermomechanical history from ROLPAS to code, a solid-geometry module or program, and model the deformation, retained work, dynamic/ an optimization routine or program. Although Recent advances in the bulk forming of metals static recrystallization, and grain growth of aus- the specifics of each problem vary, the overall have focused on the development of a number of tenite during the rolling process itself. As with approach typically comprises three elements: new processes, the increasing utilization of many multistage steel rolling processes, micro- choice of an objective function and constraints, thermomechanical processing (TMP) for both structure changes are controlled primarily by calculation of the objective function (as may be ferrous and nonferrous alloys, and the wide- static recrystallization and grain growth between done by the FEM simulation code), and a search spread application of computer-based process rolling stands. The fraction recrystallized and for the combination of design parameters that models. Although relatively few new materials retained work are used to estimate the flow stress provide a minimum or maximum for the objec- have reached the level of mass production during of the material as it enters each successive roll tive function. In bulk forming, objective func- the last decade, the processing of a number of so- stand. Last, the module AUSTRANS uses the tions may include forging weight (minimum called conventional alloys has undergone sig- temperature history after rolling and cooling- usually is best), die fill (minimum underfill is nificant improvements due to novel TMP transformation curves to model the decomposi- best), uniformity of strain or strain rate (max- sequences, thus leading to less striking, but tion of austenite during cool-down. More details imum uniformity is best), and so forth. Con- nonetheless important, improvements in service on microstructure modeling during multipass hot straints may include maximum or minimum performance. Further improvements in material rolling of steel can be found in the article “Flat, allowable strain, strain rate, or temperature to properties are likely as the quantitative under- Bar, and Shape Rolling” in this Volume. prevent metallurgical defects, the specification standing and modeling of the evolution of Recently, an FEM modeling procedure of maximum die stresses or press loading, and so microstructure and texture expands and is was developed within the framework of the forth. applied in industry. The integration of process commercial code DEFORM to predict residual Several examples of the application of opti- models with models of microstructure evolution stresses that develop during heat treatment mization to bulk forming are described in Ref 24, and distortion during subsequent machining 29, and 30 as well as the article “Design Opti- processes for ferrous and nickel-base superalloy mization for Dies and Preforms” in this Volume. parts (Ref 28). The FEM model for heat treat- For example, optimal preform design for two- ment assumes that the residual stresses devel- dimensional (axisymmetric) forgings has been oped during the forging and cool-down summarized by Oh et al. (Ref 29). In the example operations are relieved early during solution heat cited in this work, the objective was to minimize treatment. Thus, residual stresses are induced underfill via optimization of the preform shape, primarily during quenching and continue to which was represented as a series of B-spline evolve during the remainder of the heat treatment curves described through a collection of shape- process. The rapid cooling during quenching control parameters. FEM simulations were run produces severe temperature gradients within the to determine the rate at which die fill changed (a) part and gives rise to nonuniform strains. The with respect to changes in the shape-control development of residual stresses is thus handled parameters. At the end of each FEM forging using a standard elastoplastic constitutive for- simulation (using DEFORM), the values of the mulation in the FEM code. The effect of phase objective function (and constraint functions) and transformations during cooling (as in steels) on their gradients (i.e., changes with respect to the residual stresses is also treated in the newest change in each shape-control parameter) were FEM heat treatment codes. For this purpose, determined in order to pick a new search direc- phase-transformation data are incorporated in tion in shape-control-parameter space. Srivatsa order to quantify the volume changes associated performed similar analysis to minimize the specific transformation products that are formed weight of superalloy engine disks using in different areas of a part. To model subsequent DEFORM (for FEM modeling), Unigraphics (for (b) stress-relief operations, creep models (e.g., Bai- solid modeling), and iSIGHT (for the optimiza- ley-Norton, Soderburg) are incorporated into the tion code) (Ref 30). code. Initial work has also been conducted to auto- Additional information on process simulation mate the optimization of preform design for 3-D methods for bulk forming is contained in the forgings. For example, Oh et al. and Walters article “Finite Element Method (FEM) Appli- et al. (Ref 24, 29) describe a two-step process in cations in Bulk Forming” in this Volume. which the final forging geometry is first “fil- Process design and optimization techni- tered” to obtain an initial guess for the preform ques represent the latest and perhaps most shape (Fig. 6). This is done using a Fourier important methodology in the development of transform technique in which sharp corners, (c) computer-aided applications for bulk-forming edges, and small surface details are smoothed. processes. The advent of advanced process The boundary of the preform is then “trimmed” Fig. 6 Application of finite-element-based optimiza- simulation tools has replaced former methods to obtain a realistic preform for input to the FEM tion to determine preform shape for a three- dimensional forging. (a) Final forging. (b) “Filtered” involving costly and time-consuming machining simulation used to determine regions of underfill preform geometry. (c) “Trimmed” preform geometry. and tryout of dies. However, the selection of and inadequate/excessive flash formation. Based Source: Ref 24, 29 © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)

8 / Introduction and defect formation will help reﬁne allowable Disks, Adv. Mater. Process., July 2003, 20. O.M. Ivasishin, S.V. Shevchenko, N.L. processing windows and needed process con- p 36–39 Vasiliev, and S.L. Semiatin, 3-D Monte- trols. Such integration will thus form a very 12. M.F.X. Gigliotti, R.S. Gilmore, J.N. Bar- Carlo Simulation of Texture Evolution and important part of overall process optimization. shinger, B.P. Bewlay, C.U. Hardwicke, G.A. Grain Growth During Annealing, Met. Phys. Salishchev, R.M. Baleyev, and O.R. Adv. Technol., Vol 23, 2001, p 1569–1587 Valiakhmetov, Titanium Alloy Billet 21. S.L. Semiatin, Evolution of Microstructure REFERENCES Processing for Low Ultrasonic Noise, During Hot Working, Handbook of Work- Ti-2003: Science and Technology, G. Luet- ability and Process Design, G.E. Dieter, 1. T. Altan, S.I. Oh, and H.L. Gegel, Metal jering and J. Albrecht, Ed., Wiley-VCH H.A. Kuhn, and S.L. 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