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Semiatin2005-BulkForming.pdf
© 2005 ASM International. All Rights Reserved.
ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)
www.asminternational.org
Introduction to Bulk-Forming Processes
S.L. Semiatin, Air Force Research Laboratory, Materials and Manufacturing Directorate
METALWORKING consists of deformation
processes in which a metal billet or blank is
shaped by tools or dies. The design and control of
such processes depend on the characteristics
of the workpiece material, the conditions at the
tool/workpiece interface, the mechanics of
plastic deformation (metal flow), the equipment
used, and the finished-product requirements.
These factors influence the selection of tool
geometry and material as well as processing
conditions (for example, workpiece and die
temperatures and lubrication). Because of the
complexity of many metalworking operations,
models of various types, such as analytical,
physical, or numerical models, are often relied
upon to design such processes.
This Volume presents the state-of-the-art in
bulk-metalworking processes. A companion
volume (ASM Handbook, Volume 14B, Metalworking: Sheet Forming) describes the state-ofthe-art in sheet-forming processes. Various
major sections of this Volume deal with
descriptions of specific processes, selection of
equipment and die materials, forming practice
for specific alloys, and various aspects of process
design and control. This article provides a brief
historical perspective, a classification of metalworking processes and equipment, and a summary of some of the more recent developments in
the field.
Historical Perspective
Metalworking is one of three major technologies used to fabricate metal products; the others
are casting and powder metallurgy. However,
metalworking is perhaps the oldest and most
mature of the three. The earliest records of
metalworking describe the simple hammering of
gold and copper in various regions of the Middle
East around 8000 B.C. The forming of these
metals was crude because the art of refining by
smelting was unknown and because the ability to
work the material was limited by impurities that
remained after the metal had been separated from
the ore. With the advent of copper smelting
around 4000 B.C., a useful method became
available for purifying metals through chemical
reactions in the liquid state. Later, in the Copper
Age, 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 alloys
that were inherently strong and led to the utilization of alloys of copper and tin (the Bronze
Age) and iron and carbon (the Iron Age). The
Iron Age, which can be dated as beginning
around 1200 B.C., followed the beginning of
the Bronze Age by some 1300 years. The reason
for the delay was the absence of methods for
achieving the high temperatures needed to melt
and to refine iron ore.
Most metalworking was done by hand until
the 13th century. At this time, the tilt hammer
was developed and used primarily for forging
bars and plates. The machine used water power
to raise a lever arm that had a hammering tool
at one end; it was called a tilt hammer because
the arm tilted as the hammering tool was raised.
After raising the hammer, the blacksmith let
it fall under the force of gravity, thus generating the forging blow. This relatively simple
device remained in service for a number of
centuries.
The development of rolling mills followed
that of forging equipment. Leonardo da Vinci’s
notebook includes a sketch of a machine
designed in 1480 for the rolling of lead for
stained glass windows. In 1495, da Vinci is
reported to have rolled flat sheets of precious
metal on a hand-operated two-roll mill for coinmaking purposes. In the following years, several
designs for rolling mills were utilized in
Germany, Italy, France, and England. However,
the development of large mills capable of hot
rolling ferrous materials took almost 200 years.
This relatively slow progress was primarily due
to the limited supply of iron. Early mills
employed flat rolls for making sheet and plate,
and until the middle of the 18th century, these
mills were driven by water wheels.
During the Industrial Revolution at the end of
the 18th century, processes were devised for
making iron and steel in large quantities to
satisfy the demand for metal products. A need
arose for forging equipment with larger capacity.
This need was answered with the invention of the
high-speed steam hammer, in which the hammer
is raised by steam power, and the hydraulic press,
in which the force is supplied by hydraulic
pressure. From such equipment came products
ranging from firearms to locomotive parts.
Similarly, the steam engine spurred developments in rolling, and, in the 19th century,
a variety of steel products were rolled in
significant quantities.
The past 100 years have seen the development
of new types of metalworking equipment and
new materials with special properties and
applications. The new types of equipment have
included mechanical and screw presses and
high-speed tandem rolling mills. The materials
that have benefited from such developments in
equipment range from the low-carbon steel and
advanced high-strength steels used in automobiles and appliances to specialty aluminum-,
titanium-, and nickel-base alloys used in the
aerospace and other industries. In the approximately 20 years since this Volume was last
updated, methods for the bulk forming of a
number of new materials, such as intermetallic
alloys and composites, have been developed.
Furthermore, the advent of user-friendly computer codes and inexpensive computers has led to
a revolution in the application of numerical
methods for the design and control of a plethora
of bulk-forming processes, thus leading to
higher-quality products and increased efficiency
in the metalworking industry.
Classification of Metalworking
Processes
In metalworking, an initially simple workpiece—a billet or a blanked sheet, for example—
is plastically deformed between tools (or dies) to
obtain the desired final configuration. Metalforming processes are usually classified according to two broad categories:
Bulk, or massive, forming operations
Sheet-forming operations (Sheet forming is
also referred to as forming. In the broadest and
most accepted sense, however, the term
forming is used to describe bulk- as well as
sheet-forming processes).
In both types of processes, the surfaces of the
deforming metal and the tools are in contact, and
friction between them may have a major influence on material flow. In bulk forming, the
© 2005 ASM International. All Rights Reserved.
ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)
www.asminternational.org
2 / Introduction
input material is in billet, rod, or slab form,
and the surface-to-volume ratio in the formed
part increases considerably under the action of
largely compressive loading. In sheet forming,
on the other hand, a piece of sheet metal is
plastically deformed by tensile loads into a
three-dimensional shape, often without significant changes in sheet thickness or surface
characteristic.
Processes that fall under the category of bulk
forming have the following distinguishing features (Ref 1, 2):
The deforming material, or workpiece,
undergoes large plastic (permanent) deformation, resulting in an appreciable change in
shape or cross section.
The portion of the workpiece undergoing
plastic deformation is generally much larger
than the portion undergoing elastic deformation; therefore, elastic recovery after deformation is negligible.
Examples of generic bulk-forming processes
are extrusion, forging, rolling, and drawing.
Table 1 Classification of bulk (massive)
forming processes
Forging
Closed-die forging with flash
Closed-die forging without flash
Coining
Electro-upsetting
Forward extrusion forging
Backward extrusion forging
Hobbing
Isothermal forging
Nosing
Open-die forging
Rotary (orbital) forging
Precision forging
Metal powder forging
Radial forging
Upsetting
Incremental forging
Rolling
Sheet rolling
Shape rolling
Tube rolling
Ring rolling
Rotary tube piercing
Gear rolling
Roll forging
Cross rolling
Surface rolling
Shear forming
Tube reducing
Radial roll forming
Extrusion
Nonlubricated hot extrusion
Lubricated direct hot extrusion
Hydrostatic extrusion
Co-extrusion
Equal channel angular extrusion
Drawing
Drawing
Drawing with rolls
Ironing
Tube sinking
Co-drawing
Source: Ref 1
Specific bulk-forming processes are listed in
Table 1.
Types of Metalworking Equipment
The various forming processes discussed
previously are associated with a large variety of
forming machines or equipment, including the
following (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 roddrawing machines
Machines for pressing-type operations
(presses)
Among those listed, pressing-type machines
are the most widely used and are applied to
both bulk- and sheet-forming processes. These
machines can be classified into three types:
load-restricted machines (hydraulic presses),
stroke-restricted machines (crank and eccentric,
or mechanical, presses), and energy-restricted
machines (hammers and screw presses). The
significant characteristics of pressing-type
machines comprise all machine design and performance data that are pertinent to the economical use of the machine. These characteristics
include:
Characteristics for load and energy: Available load, available energy, and efficiency
factor (which equals the energy available for
workpiece deformation/energy supplied to the
machine)
Time-related characteristics: Number of
strokes per minute, contact time under pressure, and velocity under pressure
Characteristics for accuracy: For example,
deflection of the ram and frame, particularly
under off-center loading, and press stiffness
Recent Developments in
Bulk Forming
Since the publication in 1988 of the previous edition of the ASM Handbook on Forming
and Forging, metalworking practice has seen a
number of notable advances with regard to the
development of new processes; new materials,
the increased control of microstructure via specialized thermomechanical processes, and the
development of advanced tools for predicting
microstructure and texture evolution; and the
application of sophisticated process simulation
and design tools. Some of these technological
advances are summarized in the following sections of this article.
New Processes
A number of novel processes have recently
been introduced and/or investigated. In the
bulk-forming area, these include advanced
roll-forming methods, equal-channel angular
extrusion, incremental forging, and microforming.
Advanced roll-forming methods have been
developed for making axisymmetric components
with very complex cross sections. Shaping is
conducted using opposed rollers or a combination of rollers and a mandrel acting on a rotating
workpiece (Fig. 1). Unlike former simple rollforming processes used to make long tubes,
cones, and so forth, newer roll-forming methods
rely on the simultaneous control of radial and
axial metal flow. For example, the internal profile of complex shapes can be generated by
combined radial-axial roll forming that typically
makes use of a sophisticated internal mandrel
consisting of several angular segments and
devices to quickly lock or unlock the segments.
Such roll-forming operations can be conducted
under either cold- or hot-working conditions; the
specific temperature depends on the ductility and
strength of the workpiece material and the
complexity of the shape to be made. The technique has been used to make aircraft engine disks
and cases, automotive wheels, and other parts.
Recently, the feasibility of radial roll forming
has been demonstrated for titanium alloys Ti6Al-4V, Ti-6Al-2Sn-4Zr-2Mo, and Ti-6Al-2Sn4Zr-6Mo and nickel-base alloys 718, Waspaloy,
René 95, René 88, and Merl 76 (Ref 3, 4).
The forming of such alloys is enhanced by the
development of an ultrafine grain structure in
the preform material. Advanced roll forming can
provide near-net shapes at lower cost compared
to forging and ring rolling because of the elimination of dies and the ability to utilize a given set
of rollers for multiple geometries. Because of the
generally high deformation that is imposed over
the entire part cross section, microstructure uniformity also tends to be excellent. More detailed
information on the technology can be found in
the article “Roll Forming of Axially Symmetric
Components” in this Volume.
Equal-channel angular extrusion (ECAE) is
an emerging metal-processing technique developed by Segal in the former Soviet Union in the
1970s, but not widely known in the West until
the 1990s (Ref 5). In ECAE, metal flow comprises deformation through two intersecting
channels of equal cross-sectional area (Fig. 2).
The imparted strain is a function primarily
of the angle between the two channels, 2w, and
the angle of the curved outer corner, y. For
2w = 90 and y = 0, for example, the effective
strain is approximately equal to 1.15. By passing
the workpiece through the tooling many times,
very large deformations can thus be imposed.
Hence, the process has been investigated as a
means to refine microstructure and to control
crystallographic texture; in many cases submicrocrystalline grain structures are developed.
The majority of work to date has been performed
on aluminum and aluminum alloys, copper, iron,
nickel, and titanium. Although the greatest
attention has been focused on square or round
billet products, ECAE has also been used to
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© 2005 ASM International. All Rights Reserved.
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. Microforming 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.
(a)
(b)
Roll-formed
shape
Sonic
shape
Fig. 1 Radial roll forming. (a) Schematic of the process. (b) Complex-shape titanium-alloy component fabricated
via radial roll forming. The sketch in (b) shows the outline of the roll-formed part relative to the sonic shape.
Source: Ref 3, 4
Ram
2φ
Workpiece
A
A'
ψ
Die
Fig. 2
Equal-channel-angular extrusion
produce ultrafine-grain plate materials for use as
is or as preforms for subsequent sheet rolling.
Furthermore, work has been performed to
modify the ECAE concept to allow continuous,
rather than batch, processing; such efforts have
been limited to thin cross-section products
such as wire, rod, and sheet. More detailed
information on ECAE can be found in the article
“Equal-Channel Angular Extrusion” in this
Volume.
Incremental forging is a closed-die forging
process in which only a portion of the workpiece
is shaped during each of a series of press strokes.
The process is analogous to open-die forging
(cogging) of ingots, billets, thick plates, and
shafts. In contrast to such operations, however,
impression dies (not flat or V-shaped tooling) are
utilized. The primary applications of the technique are very large plan-area components of
high-temperature alloys for which die pressures
can easily equal or exceed 10 to 20 tsi. In such
instances, part plan area is limited to approximately several thousand square inches for the
largest presses (50,000 tons) currently available
in the United States. By forging only a portion of
the part at a time, however, press requirements
are reduced. Applications of the technique
include large, axisymmetric components for
land-based gas turbines made from nickel-base
alloy 706 (Wyman-Gordon Company and Alcoa
Forged Products) and various Ti-6Al-4V (ribweb) structural components for F-18 aircraft
(manufactured at Alcoa Forged Products). The
latter parts had plan areas of the order of
5000 in.2. Needless to say, part symmetry and
forging design (e.g., forging envelope) play a
critical role in the design of incremental-forging
processes. Forging design, representing a key
feature of incremental forging, is often highly
specialized and proprietary in nature.
Microforming is a technology generally
defined as the production of parts or structures
with at least two dimensions in the submillimeter
Materials-Related Developments
Recent materials-related developments include breakthroughs in the bulk forming of new
materials, increased control of microstructure
development using specialized thermomechanical processes, and the development of advanced
tools for predicting microstructure and texture
evolution.
New materials for which substantial progress has been made over the last 20 years
include structural-intermetallic alloys and discontinuously reinforced metal-matrix composites (MMCs). For intermetallic alloys,
bulk-forming approaches have been most
dramatic for aluminide-based materials (Ref 7).
Bulk forming on a commercial scale has been
used for MMCs with aluminum-alloy and, to a
lesser extent, titanium-alloy matrices.
Iron-aluminide alloys based on the Fe3Al
compound are probably the structural intermetallic materials that have been produced in the
largest quantities to date. These materials exhibit
excellent oxidation and sulfidation resistance
and potentially lower cost than the stainless
steels with which they compete. As such, a
number of potential applications for these iron
aluminides have been identified. These include
metalworking dies, heat shields, furnace fixtures
and heating elements, and a variety of automotive components. Sikka and his colleagues at
Oak Ridge National Laboratory have spearheaded the development of techniques for the hot
extrusion, forging, and rolling of ingot-metallurgy Fe3Al-base alloys at temperatures in the
range of 900 to 1200 C (1650 to 2190 F)
(Ref 8). The wrought product can then be warm
rolled to plate or sheet at temperatures between
500 and 600 C (930 and 1110 F) to manufacture material with room-temperature tensile
ductility of 15 to 20%. Wrought Fe3Al alloys
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© 2005 ASM International. All Rights Reserved.
ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)
4 / Introduction
do not possess adequate workability for cold
rolling or cold drawing, however.
Titanium-aluminide alloys based on the facecentered tetragonal (fct) gamma phase (TiAl)
represent a second type of aluminide material for
which significant progress has been made toward
commercialization. The gamma-titanium aluminide alloys have a number of applications as
lightweight replacements for superalloys in the
hot section of aircraft engines and as thermal
protection systems in hypersonic vehicles.
Spurred by substantial efforts at the Air Force
Research Laboratory, Battelle Memorial Institute, Ladish Company, and Wyman-Gordon, a
variety of metalworking techniques for both
ingot-metallurgy (I/M) and powder-metallurgy
(P/M) materials have been developed for these
materials, which were once thought to be
unworkable (Ref 7, 9). For instance, isothermal
forging and canned hot-extrusion techniques
have been demonstrated for the breakdown of
medium- to large-scale ingots. Novel can designs
and the use of a controlled dwell time between
billet removal from the preheat furnace and
deformation have greatly enhanced the feasibility of hot extrusion. In particular, the dwell
time is chosen in order to develop a temperature
difference between the sacrificial can material
and the titanium aluminide preform; by this
means the flow stresses of the two components is
similar, thereby promoting uniform co-extrusion. Secondary processing of parts has been
most often conducted via isothermal closed-die
forging (Fig. 3a, b). Careful can design and
understanding of temperature transients have
also enabled the hot pack rolling of gamma
titanium-aluminide sheet and foil products used
in subsequent superplastic-forming operations
(Fig. 3c). A key to the success of each of these
processes has been the development of a detailed
understanding of the pertinent phase equilibria/
phase transformations and the effects of microstructure, strain rate, and temperature on failure
modes during processing. More detailed information on the bulk processing of intermetallic
alloys can be found in the article “Bulk Forming
of Intermetallic Alloys” in this Volume.
Discontinuously reinforced aluminum metalmatrix composites (DRA MMCs) have been
synthesized and subsequently bulk formed by a
variety of techniques. The majority of MMCs
have been based on aluminum matrices with
silicon carbide particulate or whisker reinforcements synthesized in tonnage quantities by I/M
or P/M approaches. In the I/M method, which is
most often used for automotive parts, the ceramic
particles are introduced and suspended in the
liquid aluminum alloy prior to casting using a
high-energy mixing process. In the P/M
approach, most often used for aerospace materials, the matrix and ceramic powders are blended in a high-shear mixer prior to canning and
outgassing. Following the casting/canning
operations, conventional extrusion, forging, and
rolling processes are used to make billet and
plate products. Secondary processing may
include 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 rotorblade sleeves, automobile engine pistons and
connecting rods). Cast-and-extruded DRA MMC
driveshafts with a 6061 aluminum matrix and
alumina reinforcements have also been introduced for pickup trucks and sports cars. Similarly, extrusion and upsetting of pressed-and
sintered Ti-6Al-4V reinforced with TiB2 particulate have been used to mass produce automobile and motorcycle engine valves. More
detailed information on the bulk processing of
metal-matrix composites can be found in the
article “Forging of Discontinuously Reinforced
Aluminum Composites” in this Volume and
in the article “Processing of Metal-Matrix
Composites” in composites, Volume 21, of ASM
Handbook.
Thermomechanical processing (TMP)
refers to the design and control of metalworking
and heat treatment steps in an overall manufacturing process in order to enhance final
microstructure and properties. Thermomechanical processing was developed initially
as a method for producing high-strength or hightoughness microalloyed steels via (ferrite) grain
refinement and controlled precipitation. Current
trends in the TMP of ferrous alloys are focusing
on the development of carbide-free steels with
bainitic microstructures to obtain yet higher
strength levels. Further information on the stateof-the-art of ferrous TMP is contained in the
article “Thermomechanical Processes for Ferrous Alloys” in this Volume.
Thermomechanical processing is now also
being used routinely for nickel- and titaniumbase alloys. Two examples of recent advances in
the TMP of nickel-base superalloys, intended to
improve damage tolerance or creep resistance in
service, comprise techniques to produce a uniform intermediate grain size (ASTM ~6) or a
graded microstructure. The former technique is
especially useful for the manufacture of P/M
superalloys such as René 88, N18, and alloy 720.
In this instance, TMP consists of isothermal
forging of consolidated-powder preforms followed by supersolvus heat treatment. To achieve
the desired final grain size after supersolvus heat
treatment, however, forging must be performed
in a very tightly controlled strain, strain-rate, and
temperature window for each specific material
(Ref 10). Lack of control during deformation
may result in uncontrolled (abnormal) grain
growth during the subsequent supersolvus heat
treatment, leading to isolated grains or groups of
grains that are several orders of magnitude larger
than the average grain size. Processes to develop
graded microstructures in superalloys consist of
local heating above the solvus temperature to
dissolve the grain-boundary pinning phase (e.g.,
gamma prime) and thus facilitate grain growth in
these regions while other portions of the component are cooled (Ref 11).
Thermomechanical processes for titanium
alloys include processing to produce ultrafine
grain billet, “through-transus” forging, and final
heat treatment to obtain graded microstructures.
Methods to obtain ultrafine billet microstructures
in alpha/beta titanium alloys such as Ti-6Al-4V,
Ti-6Al-2Sn-4Zr-2Mo, and Ti-17 rely on special
forging practices for partially converted ingots
containing an initial transformed-beta (colony/
basketweave alpha) microstructure. In one
approach, multistep hot forging along three
orthogonal directions is conducted at strain
rates of the order of 10 3 s 1 and a series of
temperatures in the alpha/beta phase field
(Ref 12). By this means, an alpha grain size of
4 to 8 mm (0.16 to 0.31 mils) with good ultrasonic inspectability is obtained. In a similar
approach, warm working, involving very high
strains and somewhat lower temperatures (~550
to 700 C, or 1020 to 1290 F), has been found
to yield a submicrocrystalline alpha grain size
(Ref 13).
Fig. 3
Wrought gamma titanium products. (a) Compressor blades. (b) Subscale isothermally forged
disk. (a) and (b) Source: D.U. Furrer, Ladish Company. (c)
Large, conventionally ( pack) rolled sheet. Source: Battelle
Memorial Institute, Air Force Research Laboratory
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ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)
Introduction to Bulk-Forming Processes / 5
Through-transus forging of alloys such as Ti6Al-4Sn-4Zr-6Mo is a TMP process that combines aspects of beta and alpha-beta forging in
order to develop a microstructure with both high
strength and good fracture toughness/fatigue
resistance. By working through the transus, the
development of a continuous (and deleterious)
layer of alpha along the beta grain boundaries is
avoided (Ref 14). Instead, a transformed beta
matrix microstructure with equiaxed alpha particles on the beta grain boundaries (“snow on the
boundaries”) is produced. If forging is conducted
to temperatures that are too low, however,
undesirable equiaxed, primary alpha is nucleated
within the matrix. To help meet the tight limits
on temperature for the process, therefore, hot-die
forging coupled with finite-element modeling for
process design have been utilized.
As with nickel-base superalloys, special heat
treatments have been developed to provide dual
(and graded) microstructures in alpha-beta titanium alloys (Ref 15) (Fig. 4). Most of these
Fig. 4
methods comprise local heating of selected
regions of a part above the beta-transus temperature followed by controlled cooling. Information on beta annealing under continuousheating conditions and the effect of texture
evolution on beta grain growth is invaluable for
the selection of heating rates and peak temperatures for such TMP routes (Ref 16–18). In
addition, because the decomposition of the
metastable beta is very sensitive to cooling rate,
dual-microstructure TMP processes may be used
to produce components with a gradation of
microstructure morphologies.
More detailed information on the TMP of
nickel-base and titanium alloys can be found in
the article “Thermomechanical Processes for
Nonferrous Alloys” in this Volume.
Microstructure-evolution models fall into
two broad categories: phenomenological and
mechanistic. Phenomenological microstructureevolution models have been developed to correlate measured microstructural features to
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
imposed processing conditions and are thus
typically valid only within the specific range of
the observations (Ref 19). For example, the
evolution of recrystallized volume fraction
and recrystallized grain size that evolve during
hot deformation (due to “dynamic” recrystallization) can be described as a function of the
imposed strain, strain rate, and temperature.
Similar models treat the evolution of grain
structure during annealing following cold or hot
working as a function of time due to “static”
recrystallization. In both cases, the recrystallized
volume fraction typically follows a sigmoidal
(“Avrami”) dependence on strain or time. Phenomenological models of dynamic and static
recrystallization have been developed for a
variety of steels, aluminum alloys, and nickelbase alloys. Grain growth during heat treatment of single-phase alloys without or with
a dispersion of second-phase particles can
also be quantified using phenomenological
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 deterministic 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 formulations 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 materialproperty 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
main categories, those principally for the
prediction of either deformation textures or
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ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)
6 / Introduction
recrystallization/transformation textures. The
development of such modeling techniques has
greatly accelerated in recent years due to the
ready availability of powerful computer resources. Deformation texture modeling is more
advanced compared to efforts for predicting
recrystallization/transformation textures. Deformation texture modeling treats the slip and
twinning processes and the associated crystal
rotations to predict anisotropic plastic flow and
texture evolution. Models of this sort include
lower- and upper-bound approaches in which
either stress or strain compatibility is enforced
among the grains in a polycrystalline aggregate,
respectively. Upper-bound models give reasonable estimates of deformation texture evolution
in many cases. However, more-detailed approaches, which incorporate strain variations from
grain to grain (so-called self-consistent models)
as well as within each grain (crystal-plasticity
FEM techniques, or CPFEM), offer the promise
of even more accurate predictions. The latter
(CPFEM) approach may also be useful for the
determination of local conditions that may give
rise to cavitation, spheroidization, and so forth
provided that the physics associated with such
(a)
processes can be quantified in terms of the field
variables used in these codes. More detailed
information on deformation texture modeling
can be found in the article “Polycrystal Modeling, Plastic Forming, and Deformation Textures”
in this Volume.
A relatively recent development in texture
modeling is that associated with the recrystallization or transformation phenomena during or
following hot deformation. For example, the
textures that evolve during hot working are a
result of both dislocation glide and dynamic
recrystallization. Texture evolution can be
quantified by mechanisms such as oriented
nucleation and selective growth. In the former
mechanism, recrystallization nuclei are formed
in those grains that have suffered the least shear
strain (i.e., dislocation glide). Selective (i.e.,
faster) growth is then assumed to occur for nuclei
of particular misorientations with respect to the
matrix. More detailed information on the modeling of the evolution of recrystallization and
transformation textures can be found in the
article “Transformation and Recrystallization
Textures Associated with Steel Processing” in
this Volume.
(b)
Normalized Grain Area (Average)
70
Case A
60
50
40
30
Case B
20
Case C
10
0
100 200 300 400 500 600 700 800 900 1000
Time, MCS
(c)
Fig. 5
(d)
Monte Carlo (three-dimensional) model predictions of (a, b, and c) grain structure (two-dimensional) sections
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 graingrowth 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
Process Simulation and Design
With the advent of powerful and inexpensive
computer hardware and software, a veritable
revolution in the design of bulk-forming processes using advanced modeling and optimization techniques has occurred in the last 20 years.
Advances in process simulation have
been spurred primarily by the development of
general-purpose, finite-element-method (FEM)
codes such as DEFORM, ABAQUS, Forge3,
and MSC.Marc. The speed, accuracy, and userfriendliness of FEM codes has been facilitated
by optimization of the element type used in
the program; the development of automatic
meshing and remeshing routines; the introduction of advanced solvers; and the incorporation
of advanced graphics-user interfaces (GUIs)
(Ref 22).
In the 1980s, early FEM codes were applied to
predict metal flow in simple two-dimensional,
non-steady-state problems (e.g., closed-die forging). Since the early 1990s, two-dimensional
applications have grown significantly. In addition, increasingly powerful FEM codes have
been applied to simulate a number of threedimensional (3-D) forging problems. Recent
FEM applications include the design of tooling
for forgings that require multiple die impressions, 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
laps, folds, pipe), FEM is also being used regularly to analyze the evolution of microstructure
and defects within the workpiece, die stresses/
tooling failure, and so forth. The prediction of
defects due to cavitation and ductile fracture, for
example, usually relies on continuum criteria
(e.g., the Cockcroft and Latham maximum tensile work criterion) and FEM model predictions
of stresses and strains. Similarly, in the area of
microstructure evolution, variations of fraction
recrystallized and recrystallized grain size
developed within a workpiece during hot working are typically estimated using phenomenological models and FEM predictions of
imposed strain, strain rate, and temperature (Ref
24, 25). Such approaches have been relatively
successful for non-steady-state processes such as
closed-die press and hammer forging as well as
for cogging of steels and superalloys (see Ref 24
and the article “Practical Aspects of Converting
Ingot to Billet” in this Volume).
FEM-based models have also been developed
for the prediction of microstructure evolution
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© 2005 ASM International. All Rights Reserved.
ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)
Introduction to Bulk-Forming Processes / 7
during steady-state processes such as the hot
rolling of steel (Ref 26, 27). For instance, in the
work of Pauskar (Ref 27), metal flow and
microstructure evolution during multistand
shape rolling were modeled using the integrated
system ROLPAS-M, which consists of three
main modules. The main module (ROLPAS)
comprises a 3-D nonisothermal FEM code. The
second module, MICON, uses the computed
thermomechanical history from ROLPAS to
model the deformation, retained work, dynamic/
static recrystallization, and grain growth of austenite during the rolling process itself. As with
many multistage steel rolling processes, microstructure changes are controlled primarily by
static recrystallization and grain growth between
rolling stands. The fraction recrystallized and
retained work are used to estimate the flow stress
of the material as it enters each successive roll
stand. Last, the module AUSTRANS uses the
temperature history after rolling and coolingtransformation curves to model the decomposition of austenite during cool-down. More details
on microstructure modeling during multipass hot
rolling of steel can be found in the article “Flat,
Bar, and Shape Rolling” in this Volume.
Recently, an FEM modeling procedure
was developed within the framework of the
commercial code DEFORM to predict residual
stresses that develop during heat treatment
and distortion during subsequent machining
processes for ferrous and nickel-base superalloy
parts (Ref 28). The FEM model for heat treatment assumes that the residual stresses developed during the forging and cool-down
operations are relieved early during solution heat
treatment. Thus, residual stresses are induced
primarily during quenching and continue to
evolve during the remainder of the heat treatment
process. The rapid cooling during quenching
produces severe temperature gradients within the
part and gives rise to nonuniform strains. The
development of residual stresses is thus handled
using a standard elastoplastic constitutive formulation in the FEM code. The effect of phase
transformations during cooling (as in steels) on
residual stresses is also treated in the newest
FEM heat treatment codes. For this purpose,
phase-transformation data are incorporated in
order to quantify the volume changes associated
specific transformation products that are formed
in different areas of a part. To model subsequent
stress-relief operations, creep models (e.g., Bailey-Norton, Soderburg) are incorporated into the
code.
Additional information on process simulation
methods for bulk forming is contained in the
article “Finite Element Method (FEM) Applications in Bulk Forming” in this Volume.
Process design and optimization techniques represent the latest and perhaps most
important methodology in the development of
computer-aided applications for bulk-forming
processes. The advent of advanced process
simulation tools has replaced former methods
involving costly and time-consuming machining
and tryout of dies. However, the selection of
preform designs and processing conditions to
determine optimal die fill, microstructure evolution, die life, and so forth may still entail
substantial trial and error and thus multiple
simulations when computer-modeling techniques alone are used. Hence, integrated systems
are now being developed to automate the optimization process. For bulk-forming processes,
such systems include an FEM metal-forming
code, a solid-geometry module or program, and
an optimization routine or program. Although
the specifics of each problem vary, the overall
approach typically comprises three elements:
choice of an objective function and constraints,
calculation of the objective function (as may be
done by the FEM simulation code), and a search
for the combination of design parameters that
provide a minimum or maximum for the objective function. In bulk forming, objective functions may include forging weight (minimum
usually is best), die fill (minimum underfill is
best), uniformity of strain or strain rate (maximum uniformity is best), and so forth. Constraints may include maximum or minimum
allowable strain, strain rate, or temperature to
prevent metallurgical defects, the specification
of maximum die stresses or press loading, and so
forth.
Several examples of the application of optimization to bulk forming are described in Ref 24,
29, and 30 as well as the article “Design Optimization for Dies and Preforms” in this Volume.
For example, optimal preform design for twodimensional (axisymmetric) forgings has been
summarized by Oh et al. (Ref 29). In the example
cited in this work, the objective was to minimize
underfill via optimization of the preform shape,
which was represented as a series of B-spline
curves described through a collection of shapecontrol parameters. FEM simulations were run
to determine the rate at which die fill changed
with respect to changes in the shape-control
parameters. At the end of each FEM forging
simulation (using DEFORM), the values of the
objective function (and constraint functions) and
their gradients (i.e., changes with respect to the
change in each shape-control parameter) were
determined in order to pick a new search direction in shape-control-parameter space. Srivatsa
performed similar analysis to minimize the
weight of superalloy engine disks using
DEFORM (for FEM modeling), Unigraphics (for
solid modeling), and iSIGHT (for the optimization code) (Ref 30).
Initial work has also been conducted to automate the optimization of preform design for 3-D
forgings. For example, Oh et al. and Walters
et al. (Ref 24, 29) describe a two-step process in
which the final forging geometry is first “filtered” to obtain an initial guess for the preform
shape (Fig. 6). This is done using a Fourier
transform 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 FEM
simulation used to determine regions of underfill
and inadequate/excessive flash formation. Based
on the FEM metal-flow predictions, additional
material is then added or removed to the preform,
the new preform is filtered and trimmed, and
additional simulations are run in an iterative
fashion until the desired result is obtained.
Conclusions and Future Outlook
Recent advances in the bulk forming of metals
have focused on the development of a number of
new processes, the increasing utilization of
thermomechanical processing (TMP) for both
ferrous and nonferrous alloys, and the widespread application of computer-based process
models. Although relatively few new materials
have reached the level of mass production during
the last decade, the processing of a number of socalled conventional alloys has undergone significant improvements due to novel TMP
sequences, thus leading to less striking, but
nonetheless important, improvements in service
performance. Further improvements in material
properties are likely as the quantitative understanding and modeling of the evolution of
microstructure and texture expands and is
applied in industry. The integration of process
models with models of microstructure evolution
(a)
(b)
(c)
Fig. 6
Application of finite-element-based optimization to determine preform shape for a threedimensional forging. (a) Final forging. (b) “Filtered”
preform geometry. (c) “Trimmed” preform geometry.
Source: Ref 24, 29
© 2005 ASM International. All Rights Reserved.
ASM Handbook, Volume 14A, Metalworking: Bulk Forming (#06957G)
www.asminternational.org
8 / Introduction
and defect formation will help refine allowable
processing windows and needed process controls. Such integration will thus form a very
important part of overall process optimization.
12.
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