Betzalel Avitzur Lehigh University
to Understanding Metal Forming
Drawing: Metal forming process whereby the workpiece
is a shaped longitudinal prism that undergoes a reduction and change in
its cross section area and shape while being pulled through a shaped converging
Extrusion: Metal forming process whereby the workpiece is placed in
a chamber with an opening and is forced to escape through the opening,
usually being pushed out by a mandrel.
Forging: Metal forming process whereby the workpiece is placed between
an anvil and a hammer and subjected to compressive force between them.
Friction: Resistance to sliding motion along the interface between two
Lubrication: Supply of substance on the inter-face between two sliding
solids aimed at re-ducing the friction and/or the wear on the interface.
Metal-forming processes: Processes that cause changes in the shape of
solid metal articles via plastic (permanent) deformations.
Modeling: Procedure to present a physical re-ality by other means.
Perfectly plastic materials: Idealization of the characteristics of
metals undergoing plastic deformations.
Pressure-induced ductility: Increase in the ability of metals to
undergo plastic deformations without fracturing, this ability being enhanced
by high environmental pressure.
Rolling: Metal forming process whereby the workpiece is a longitudinal
prism, which is placed between two opposing circular rolls that rotate
in opposite directions, drag the workpiece along, and force it to reduce
in cross section.
Simulation procedures: Procedures that provide representation of
a physical reality through a mathematical or physical model.
Metallic components can be shaped in a manner similar to the molding
of pottery. The raw material of a fundamental simpler shape is provided
by a primary process like casting, powder consolidation, earlier forming
processes, or even by electric deposition. Metals deform very much like
soft clay or wax. Even in the solid state, permanent changes in shape can
be forced upon them by displacement of relative positions between neighboring
material points. To enforce these changes, external forces are applied.
While soft plasticine can be molded by tiny toddler’s fingers, for metal
forming, specially constructed tooling, usually of hard materials, are
manipulated, sometimes by colossal machinery.
A variety of processes, the equipment and tooling, and the concepts
involved will be discussed in this article. This will provide an understanding
of the state of the art in metal forming, typical processes (not all),
and basic phenomena and concepts involved.
PRIMARY AND SECONDARY
FORMING < FONT SIZE=2>PROCESSES
The ingots of a relatively large volume, coming as
cast billets through solidification of molten metal, are usually shaped
through plastic deformations into intermediate shapes. This primary shaping
provides profiles that are closer to the profile of the final product and
also causes a refinement of the crystal structure of the cast ingot. This
refining of the structure, called recrystallization, occurs at elevated
temperatures. Furthermore, metals are softer and more ductile at elevated
temperatures. Thus, primary forming is done at elevated temperatures.
In the process of extrusion (Fig. 1), a billet is placed
into a chamber with a shaped opening (called a die) on one end and a ram
on the other. As the ram is forced into the chamber, the workpiece is forced
out through the die. The extrudate, a long product (i.e., a rod), emerges
through the die duplicating its cross sectional shape. The flow lines indicate
that a dead metal zone forms in the corner on the exit side of the chamber
where the separated ring of a triangular cross section remains stagnant.
The process of rolling, whereby the ingot is gripped by
two rolls and squeezed between them is described by Fig. 2. The rolls are
identical and they are rotating in opposite directions so that they grab
the ingot and drag it by friction into the narrowing gap between them.
The product may become thinner while passing through the rolls. Flat products
are produced by cylindrical rolls, while profiles are provided by grooved
The process of forging is performed on a press or a hammer.
Basically, the ingot is placed between two platens that are forced one
against each other, squeezing the ingot between them (see Fig. 3). A variety
of shapes can be produced between flat platens by manipulation of the ingot
while the platens squeeze and release the
FIG. 1. Extrusion (a) and an assortment of extrudates
FIG. 2. Rolling. Friction forces: F1,
driving force; F2, opposing force. Net driving force
= F1- F2; v0 < ?
< v f and v f / v0 = t0
/ t f.
Alternately, the platens may be shaped with a cavity that imparts its shape
on the product.
FIG. 3. Forging: open die (a) and closed die
AND THE WORKPIECE
A typical system for a metal-forming process
is presented here through forging (see Fig. 4). The platens are manipulated
by a hydraulic cylinder. The force applied to the workpiece through the
piston and platens is contained by the frame. The resultant force on the
system is zero. However, the frame must be strong enough to contain the
forming forces. While the largest forging press during World War II was
a 5,000 ton press available only in Germany in limited numbers, there are
throughout the world today a few production presses of 50,000 to 80,000
tons. These presses are huge and expensive. The power supply needed for
a press this size is an impressive system by itself. Not so long ago, the
control and manipulation of the workpiece and tools were manual. Today’s
modern presses are automated. The following description is the state of
the art in several of the most advanced designs (Lange, 1985).
The shape of the product, together with other information
about the feed stock is given as the input to an online computer that activates
the press and its accessories. The entire workpiece, tooling, and press
manipulations schedules are calculated by the computer. Workpiece after
workpiece is automatically fed to the press from its storage. An assortment
of tools is stored on a rack at the press, and automatic selection of the
desired tools at the proper portion of the cycle is affected. The tools
and workpiece are manipulated in synchronization to shape the workpiece
to the proper design by repeated forging actions. When forming of one workpiece
is completed, the workpiece is removed to make room for the next one. On-the-spot
automatic inspection is, on occasion, affected with possible
FIG. 4. Schematic of a hydraulic "C" clamp press.
closed loop feedback capabilities for automatic corrective measures.
Almost all of the disciplines of engineering at their most advanced
stage interact in providing the present-day metal-forming system. Starting
with the workpiece, knowledge of metallurgy and mechanics combine to provide
an insight to its behavior. Sliding occurs on the interface between the
workpiece and the tool, friction is manifested, and lubrication is exercised.
Thus, tribology (i.e., the study of friction and wear) is necessary. The
tools are made from hard metals and nonmetals as well, and therefore the
latest advances in material science are immediately applied. Furthermore,
to optimize tool wear, the latest in surface treatments, by coating, ion
implantation, and laser beam surface hardening, are all practiced. In the
design of the machine tool itself, all disciplines combine. To mention
a few, the frame is made of any material from cast iron to plastic, which
will reduce weight and noise. Hydraulics and electronics with robotics
combine to provide motion inspection and vibration control. [See
MANUFACTURING PROCESSES TECHNOLOGY; METALLURGY, MECHANICAL;
PLASTICITY (ENGINEERING); TRIBOLOGY.]
Metal forming is normally performed after the primary processes
of extraction, casting, and powder compaction and before the finishing
processes of metal cutting, grinding, polishing, painting, and assembly.
With few exceptions, the bulk of the products of the metal fabrication
industry are shaped by forming or a combination of forming and other processes
like metal cutting or joining. Forming operations are classified as those
processes where the desired shape is achieved by imparting plastic deformation
to the workpiece in the solid state. Classification by (1) product, (2)
material, (3) forming temperature, and (4) nature of deformation (sheet
metal versus bulk deformation) can also be helpful. However, the boundaries
between categories are not perfectly defined. For example, impact extrusion
can be classified as a forging process or as an extrusion. It is needless
to say that any specific product can be made from a number of materials,
by a variety of processes, and at a range of temperatures.
OF THE METAL-FORMING