FUNDAMENTALS OF METAL FORMING
•
There are four basic production processes for producing desired shape of a
product.
These are casting, machining, joining (welding, mechanical fastners, epoxy, etc.), and
deformation processes.
Casting process exploit the fluidity of a metal in liquid state as it takes shape and
solidifies in a mold.
Machining processes provide desired shape with good accuracy and precision but
tend to waste material in the generation of removed portions.
Joining processes permit complex shapes to be constructed from simpler components
and have a wide domain of applications.
Deformation processes exploit a remarkable property of metals, which is their ability
to flow plastically in the solid state without deterioration of their properties. With the
application of suitable pressures, the material is moved to obtain the desired shape
with almost no wastage.
The required pressures are generally high and the tools and equipment needed are
quite expensive. Large production quantities are often necessary to justify the process.
• To deform the metal permanently, the stress must exceed
the elastic limit. At room temperature, the metal is in a
more rigid state than when at higher temperature. Thus, to
deform the metal, greater pressures are needed when it is
in cold state than when in hot state.
• When metal is deformed in cold state, severe stresses
known as residual stresses are set up in the material. These
stresses are often undesirable, and to remove them the
metal is heated to some temperature below the
recrystalline range temperature. In this temperature range,
the stresses are rendered ineffective without appreciable
change in physical properties or grain structure.
A forming operation is one in which the shape of a
metal sample is altered by plastic deformation.
Forming processes include stamping, rolling, extrusion
and forging, where deformation is induced by external
compressive forces or stresses exceeding the yield stress of
the material.
Drawing is a fundamentally different process in that
the external forces are tensile in nature and hence the yield
stress of the material cannot be exceeded.
Metals or alloys used in forming processes require a
moderate level of ductility to enable plastic deformation with
no fracture.
Forming can be divided into two categories:
Hot working and Cold working
Hot working
• Deformation is carried out at a temperature high enough
for fast recrystallisation to occur.
• Deformation energy requirements for hot working are less
than that of cold working.
• At hot working temperatures, a metal remains ductile
through dynamic reforming of its grain structure, so
repeated, large deformations are possible. The strain rates
of many metal-working processes are so high that there is
insufficient time for the metal to recrystallise as it deforms.
• Hot working achieves both the mechanical purpose of
obtaining the desired shape and also the purpose of
improving the physical properties of the material by
destroying its original cast structure.
• Cold Working
• Plastic deformation which is carried out
in a temperature region and over a time
interval such that the strain hardening is
not relieved is called cold working.
• Cold working produces additional
dislocations within the metal structure.
When compared to hot working, cold-working processes have certain
distinct advantages:
• No heating required
• Better surface finish obtained
• Superior dimension control
• Better reproducibility and interchangeability of parts
• Improved strength properties
• Directional properties can be minimized
Some disadvantages associated with cold-working processes include:
• Higher forces required for deformation
• Heavier and more powerful equipment required
• Less ductility available
• Metal surfaces must be clean and scale-free
• Strain hardening occurs (may require intermediate anneals)
• Imparted directional properties may be detrimental
• May produce undesirable residual stresses
The major cold-working operations can be classified basically under the
headings of squeezing, bending, shearing and drawing, as follows.
• Rolling is the most widely used deformation
process. It consists of passing metal between two
rollers, which exert compressive stresses,
reducing the metal thickness. Where simple
shapes are to be made in large quantity, rolling is
the most economical process. Rolled products
include sheets, structural shapes and rails as well
as intermediate shapes for wire drawing or
forging. Circular shapes, ‘I’ beams and railway
tracks are manufactured using grooved rolls
• In this operation, a single piece of metal,
normally hot, is deformed mechanically by the
application of successive blows or by
continuous squeezing. Forged articles range in
size from nuts and bolts, hip replacement
prostheses and crankshafts to (traditionally)
gun barrels.
• In extrusion, a bar or metal is forced from an enclosed
cavity via a die orifice by a compressive force applied
by a ram. Since there are no tensile forces, high
deformations are possible without the risk of fracture
of the extruded material. The extruded article has the
desired, reduced cross-sectional area, and also has a
good surface finish so that further machining is not
needed. Extrusion products include rods and tubes
with varying degrees of complexity in cross-section.
• Examples of metals that can be extruded include lead,
tin, aluminium alloys, copper, brass and steel
• Stamping
• Stamping is used to make high volume parts such
as aviation or car panels or electronic
components. Mechanical or hydraulic powered
presses stamp out parts from continuous sheets
of metal or individual blanks. The upper die is
attached to the ram and the lower die is fixed.
Whereas mechanical machinery transfers all
energy as a rapid punch, hydraulic machinery
delivers a constant, controlled force.
• Deep Drawing
• For deep drawing, the starting sheet of metal is larger
than the area of the punch. A pressure plate, fixed to
the machine, prevents wrinkling of the edges as the
plug is drawn into a top die cavity. The outer parts of
the sheet are drawn in towards the die as the
operation proceeds. The process is limited by the
possibility of fracture occurring during drawing; the
maximum sheet width is rarely more that twice the die
diameter.
• Many shapes are possible including cups, pans,
cylinders and irregular shaped products.
• Pressing
• A sheet of metal is deformed between two
suitably shaped dies usually to produce a cup
or dish shaped component. A thick pad of
rubber may replace one of the dies, giving
reduced tooling costs and allowing larger
deformations to be imposed.
Squeezing Processes
• Most of the cold-working squeezing processes have identical hot-working
counterparts or are extensions of them. The primary reasons for
deforming cold rather than hot are to obtain better dimensional accuracy
and surface finish. In many cases, the equipment is basically the same,
except that it must be more powerful.
• Cold working is the plastic deformation of
metals below the recrystallization
temperature. In most cases, such cold forming
is done at room temperature.
• The major cold-working operations can be
classified basically as squeezing, bending,
shearing and drawing.
• Initially the dislocations can move
through the metal structure.
• As the working continues, however, the
movement of the dislocations becomes
more difficult.
• The metal becomes less malleable and
ductile.
Why Cold Working?
• Good dimensional control
• Good surface finish of the component.
• Strength and hardness of the metal are
increased.
• An ideal method for increasing hardness
of those metals which do not respond to
the heat treatment.
Why not?
• Only ductile metals can be shaped
through cold working.
• Over-working of metal.
• Subsequent heat treatment is mostly
needed
How to Calculate Cold
Working
• Ao -the original area
• Ad -the area after deformation
• %CW is a measure of degree of plastic
deformation
What Does Cold Working Effect?
• The following properties are affected by cold
work significantly:
• Tensile Strength
• Hardness
• Yield Strength
• Ductility
Effect of Cold Working on Yield
and Tensile Strength
Cold Working Methods
• cold rolling
• drawing
• deep drawing
• pressing
Drawing of a metal