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Metal forming refers to a metal working
process of creating metallic products
through mechanical deformation.
The work piece is reshaped without adding
or removing material.
The tool, (a die) applies stresses that
exceed the yield strength of the metal.
The metal takes a shape determined by the
geometry of the die.
13 December 2022
A
die is a specialized tool used in
manufacturing industries to cut or
shape materials mostly using a pressing
machine.
 A pressing machine is a machine tool
that changes the shape of a work piece
by the application of pressure.
13 December 2022
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Stresses to plastically deform the metal
are usually compressive stresses.
 Examples: rolling, forging, extrusion,
…...
However, some forming processes
 Stretch the metal (Tensile stresses)
 Others bend the metal (Tensile and
Compressive)
 Still others apply shear the metal
(Shearing stress)
13 December 2022
 Suitable
material properties:
Low yield strength
High ductility
 These properties are affected by temperature:
Ductility increases and yield strength
decreases when work temperature is raised
 Other factors:
Strain rate and friction
13 December 2022
A yield strength or yield point is the material
property defined as the stress at which a
material begins to deform plastically. Prior to
the yield point the material will deform
elastically and will return to its original shape
when the applied stress is removed.
 Elasticity is simply the ability of a material to
return to its original shape and dimension
when the applied load/stress is removed
 Ductility is when a material loaded under
tensile stress deform easily and occupy the
needed geometry easily.
5
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1.
2.
Bulk deformation
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Rolling processes
Forging processes
Extrusion processes
Wire and bar drawing
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Bending operations
Deep or cup drawing
Shearing processes
Sheet metal
working
Products of Bulk Deformation
Processes
Spring 2005
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Deformation process in which work
thickness is reduced by compressive
forces exerted by two opposing rolls.
13 December 2022
The Rolls
The rotating rolls perform two main
functions:
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Pull the work into the gap between them
by friction between work-part and rolls.

Simultaneously squeeze the work to
reduce cross section.
Spring 2005
Types of Rolling

By geometry of work:
 Flat rolling - used to reduce thickness of a
rectangular cross-section.
 Shape
rolling - a square cross-section is
formed into a shape such as an I-beam.
 By temperature of work:
 Hot Rolling – most common due to the large
amount of deformation required.
 Cold rolling – produces finished sheet and
plate stock.
Spring 2005
Rolled Steel Products
Spring 2005
Flat Rolling
 It involves the rolling of slabs, strips, sheets, and
plates, work-parts of rectangular cross section in
which the width is greater than the thickness.
Spring 2005
Flat Rolling Analysis
Draft: The work is squeezed between two rolls so that
its thickness is reduced by an amount called the draft.
where
d t o t f
d = draft; to = starting thickness; and tf = final thickness
Reduction: The draft is sometimes expressed as a fraction
of the starting stock thickness, called the reduction
Reduction =
d
r 
to
When a series of rolling operations is used, reduction is taken as
the sum of the drafts divided by the original thickness
Spring 2005
Flat Rolling Analysis
Spreading: In addition to thickness reduction, rolling
usually increases work width. This is called spreading
 Conservation of material is preserved, so the volume of metal
exiting the rolls equals the volume entering:
t0 w0 L0  t f w f L f
where
w0 and wf are the before and after work widths (mm)
L0 and Lf are the before and after work lengths (mm)
 Similarly, before and after volume rates of materials flow
must be the same, so the before and after velocities can be
related:
t0 w0 v0  t f w f v f
where v0 and vf are the entering and exiting velocities
of the work
Spring 2005
Flat Rolling Analysis
Maximum Draft: There is a limit to the maximum
possible draft that can be accomplished in flat rolling
with a given coefficient of friction
d max   R
2
where;
dmax = maximum draft (mm).
μ = coefficient of friction
R= roll radius (mm)
Spring 2005
Flat Rolling Analysis
Rolling Force: The rolling force can be calculated
based on the average flow stress experienced by the
work material in the roll gap
where;
Y
F  Y wL
= average flow stress (MPa)
wL = the roll-work contact area (mm2)
 Contact length can be approximated by
L  R (t0  t f )
Spring 2005
Flat Rolling Analysis
The torque in rolling can be estimated by assuming that
the roll force is centered on the work as it passes the
rolls and that it acts with a moment arm of one-half
the contact length L.
T  0.5FL
The power required to drive each roll is the product of
torque and angular velocity
where;
P  2NFL
P = power (J/s)
F = rolling force (N)
N = rotational Speed (rev/min)
L = contact length (m)
Spring 2005
Shape Rolling

Work is deformed into a contoured cross-section
rather than flat (rectangular).
 Accomplished by passing work through rolls that have the
reverse of desired shape.
Products include:
 Construction shapes such as I-beams, L-beams,
and U-channels
 Rails for railroad tracks
 Round and square bars and rods
Spring 2005
Rolling Mill Configuration
 Two-high – two opposing large diameter rolls
Spring 2005
Rolling Mill Configuration
 Three-high – work passes through both directions
Spring 2005
Rolling Mill Configuration
Four-high – backing rolls support smaller work rolls 
Spring 2005
Rolling Mill Configuration
 Cluster mill – multiple backing rolls on smaller rolls
Spring 2005
Rolling Mill Configuration
Tandem rolling mill – sequence of two-high mills 
Spring 2005
Thread Rolling
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Bulk deformation process used to form threads on cylindrical parts
by rolling them between two dies.
Most important commercial process for mass producing bolts and
screws.
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Performed by cold working in thread rolling machines.
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Advantages over thread cutting (machining):
 Higher production rates
 Better material utilization
 Stronger threads due to work hardening
Spring 2005
Ring Rolling
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Deformation process in which a thick-walled ring of smaller
diameter is rolled into a thin-walled ring of larger diameter.
As thick-walled ring is compressed, deformed metal elongates,
causing diameter of ring to be enlarged.
Hot working process for large rings and cold working process for
smaller rings.
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Applications: ball and roller bearing races, steel tires for railroad
wheels, and rings for pipes, pressure vessels, and rotating.
machinery
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Advantages: material savings, strengthening through cold working
Spring 2005
Forging
Forging is a deformation process in which work is
compressed between two dies.
 Oldest of the metal forming operations, dating from about 5000
BC
 Components: engine crankshafts, connecting rods, gears,
aircraft structural components, jet engine turbine parts.
 In addition, basic metals industries use forging to
establish basic form of large components that are
subsequently machined to final shape and size.
Spring 2005
Classification of Forging
Processes
Cold vs. hot forging:
Hot or warm forging – most common, due to
the significant deformation, and the need to
reduce strength and increase ductility of work
metal.
Cold forging - advantage is increased strength
that results from strain hardening.
Impact vs. press forging:
Forge hammer - applies an impact load.
Forge press - applies gradual pressure.
Spring 2005
Classification of Forging
Processes
Another
difference
among
forging
operations is the degree to which the flow
of the work metal is constrained by the
dies.
1- Open-die forging operation.
2- Impression-die forging operation.
3- Flashless forging operation.
Spring 2005
Open-die Forging Operation
The work is compressed between two flat dies,
thus allowing the metal to flow without
constraint in a lateral direction relative to the
die surfaces.
Spring 2005
Impression-die Forging
Operation
The die surfaces contain a cavity or impression
that is imparted to work-part, thus constraining
metal flow - flash is created.
Flash is excess metal that must be trimmed off later.
Spring 2005
Flashless Forging Operation
The work is completely
constrained within the
die and no excess
flash is produced.
Spring 2005
Open-die Forging Operation
with No Friction
If no friction occurs between work and die surfaces, then
homogeneous deformation occurs, so that radial flow is
uniform throughout work-part height
(1) start of process with workpiece at its original length
and diameter, (2) partialSpring
compression,
and (3) final size
2005
Open-die Forging Operation
with Friction
Friction between work and die surfaces constrains
lateral flow of work, resulting in barreling effect
(1) start of process, (2) partial deformation, and (3) final
shape
Spring 2005
Open-die Forging Analysis
Under ideal conditions, the true strain
experienced by the work during the process is
determined by:
ho
  ln
h
where
ho= starting height; and
h = height at some point during compression
• At h = final value hf, true strain is maximum value
Spring 2005
Open-die Forging Analysis
The forging force required to continue compression
at any height h during the process is obtained by
F  Yf A
where
F = Upsetting (Forging) force (N).
A = Cross sectional area of the part (mm2)
Yf = Flow stress corresponding to the true strain
(MPa)
Spring 2005
Open-die Forging Analysis
 In actual forging process, the force is obtained by,
F  K f Yf A
where Kf = The Forging Shape Factor defined as:
where
0.4D
K f  1
h
μ = Coefficient of friction.
h = Work height (mm).
D = Work-part diameter or other dimension representing
contact length with die surface (mm)
Spring 2005
Impression-die Forging
Operation
Compression of work-part by dies with inverse of
desired part shape.
 Flash is formed by metal that flows beyond die cavity
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into small gap between die plates.
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Flash must be later trimmed from part, but it serves
an important function during compression:
 As flash forms, friction resists continued metal flow
into gap, constraining material to fill die cavity.
 In hot forging, metal flow is further restricted by
cooling against die plates.
Spring 2005
Impression-die Forging
Operation
o Several forming steps often required, with separate die
cavities for each step.
o Beginning steps redistribute metal for more uniform
deformation.
o Final steps bring the part to its final geometry.
Sequence in impression-die forging:
(1) just prior to initial contact with raw work-piece,
(2) partial compression, and
(3) final die closure, causing flash
to form in gap between die plates.
Spring 2005
Impression-die Forging Operation
Advantages and Limitations
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Advantages compared to machining from solid
stock:
 Higher production rates.
 Conservation of metal (less waste).
 Greater strength.
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Limitations:
 Not capable of close tolerances
 Machining often required to achieve
accuracies and features needed, such as
holes, threads, and mating surfaces that fit
with other components
Spring 2005
Flashless Forging Operation
Compression of work in punch and die tooling whose
cavity does allow for flash.
Starting work-part volume must equal die cavity
volume within very close tolerance.
Best suited to part geometries that are simple and
symmetrical
Often classified as a
precision forging process
(1) just before initial contact
with work-piece,
(2) partial compression, and
(3) final punch and die closure
Spring 2005
Forging Hammers (Drop
Hammers)
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Apply an impact load against
work-part - two types:
 Gravity drop hammers impact energy from falling
weight of a heavy ram
 Power drop hammers accelerate the ram by
pressurized air or steam

Disadvantage: impact energy
transmitted through anvil into
floor of building
Most commonly used for
impression-die forgingSpring 2005
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Forging Presses
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Apply gradual pressure to accomplish
compression operation
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types:
 Mechanical presses - converts rotation
of drive motor into linear motion of ram
 Hydraulic presses - hydraulic piston
actuates ram
 Screw presses - screw mechanism drives
ram
Spring 2005
Upsetting and Heading
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Forging process used to form heads on nails,
bolts, and similar hardware products.
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More parts produced by upsetting than any
other forging operation.
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Performed cold, warm, or hot on machines
called headers or formers.
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Wire or bar stock is fed into machine, end is
headed, then piece is cut to length.
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For bolts and screws, thread rolling is then
used to form threads.
Spring 2005
Upsetting and Heading
An upset forging operation to form a head on a bolt or similar
hardware item The cycle consists of:
(1) wire stock is fed to the stop
(2) gripping dies close on the stock and the stop is retracted
(3) punch moves forward
(4) bottoms to form the head
Spring 2005
Upsetting and Heading
Examples of heading (upset forging) operations:
(a) heading a nail using open dies
(b) round head formed by punch
(c) and (d) two common head styles for screws formed by die
(e) carriage bolt head formed by punch and die
Forming and related operations performed on
metal sheets, strips, and coils.
 High surface area-to-volume ratio of starting
metal, which distinguishes these from bulk
deformation.
 Often called press-working because presses
perform these operations.
 Parts are called stampings.
 Usual tooling: punch and die.
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Sheet Metal Bending
Basic sheet metalworking operations: bending
Deep Drawing
Basic sheet metalworking operations: drawing
Shearing of Sheet Metal
Basic sheet metalworking operations: shearing
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Plastic region of stress-strain curve
is primary interest because material
is plastically deformed
In plastic region, metal's behavior is
expressed by the flow curve:
Y f  K n
where K = strength coefficient;
and n = strain hardening exponent
 Flow curve based on true stress
and true strain
 For
most metals at room
temperature, strength increases
when deformed due to strain
hardening
 Flow stress = instantaneous value
of stress required to continue
deforming the material
Y f  K n
where Yf = flow stress, i.e., the yield strength as a
function of strain
 Determined
by integrating the flow curve
equation between zero and the final
strain value defining the range of interest
_
Yf
K n

1 n
where Y = average flow stress; and  =
maximum strain during deformation
process. n = strain hardening exponent
_
f

For any metal, K and n in the flow curve depend
on temperature.
 Both strength (K), and strain hardening (n) are
reduced at higher temperatures.
 In addition, ductility is increased at higher
temperatures.
o
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Any deformation operation can be:
accomplished with lower forces, and power
at elevated temperature.
Three temperature ranges in metal forming:
 Cold working
 Warm working
 Hot working
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Performed at room temperature or slightly
above.
Many cold forming processes are important
mass production operations.
Minimum or no machining usually required.
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Better accuracy, closer tolerances
Better surface finish
Strain hardening increases strength
and hardness
No heating of work required
Higher forces and power required in the deformation
operation
 Ductility and strain hardening limit the amount of
forming that can be done
 In some cases, metal must be annealed to allow
further deformation
 In other cases, metal is simply not ductile enough
to be cold worked.

As Cold work is increased
• Yield strength (sy) increases.
• Tensile strength (TS) increases.
• Ductility (%EL or %AR) decreases.
low carbon steel
• What are the values of yield strength, tensile
strength & ductility after cold working Cu?
Copper
Cold
Work
Do = 15.2 mm
%CW =
Dd = 12.2 mm
pDo2 pDd2
4 x 100
%CW = 4
pDo2
4
Do2 - Dd2
=
x 100
Do2
(15.2 mm) 2 - (12.2 mm) 2
(15.2 mm) 2
x 100 = 35.6%
500
300
300 MPa
100
0
20
40
Cu
% Cold Work
60
sy = 300 MPa
60
800
600
400 340 MPa
Cu
200
0
20
40
60
% Cold Work
TS = 340 MPa
ductility (%EL)
700
tensile strength (MPa)
yield strength (MPa)
• What are the values of yield strength, tensile strength &
ductility for Cu for %CW = 35.6%?
40
20
Cu
7%
00
20
40
60
% Cold Work
%EL = 7%
• 1 hour treatment at Tanneal...
decreases TS and increases %EL.
• Effects of cold work are nullified!
100 200 300 400 500 600 700
600
60
tensile strength
50
500
40
400
30
ductility
300
20
ductility (%EL)
tensile strength (MPa)
annealing temperature (ºC)
• Three Annealing stages:
1. Recovery
2. Recrystallization
3. Grain Growth
 During recovery, some of the stored
strain energy is relieved. In addition,
properties such as electrical and
conductivities are recovered to their
worked states.
internal
physical
thermal
precold-
 New grains are formed that:
-- have low dislocation densities
-- are small in size
-- consume and replace parent cold-worked grains.
0.6 mm
33% cold
worked
brass
0.6 mm
New crystals
nucleate after
3 sec. at 580C.
• All cold-worked grains are eventually consumed/replaced.
0.6 mm
After 4
seconds
0.6 mm
After 8
seconds
 All cold-worked grains are eventually consumed/replaced.
0.6 mm
After 4
seconds
0.6 mm
After 8
seconds
 Can be induced by rolling a polycrystalline metal
- before rolling
- after rolling
rolling direction
235 m
- isotropic
since grains are
equiaxed &
randomly oriented.
- anisotropic
since rolling affects grain
orientation and shape.
• At longer times, average grain size increases.
-- Small grains shrink (and ultimately disappear)
-- Large grains continue to grow
0.6 mm
After 8 s,
580ºC
0.6 mm
After 15 min,
580ºC
• Empirical Relation:
exponent typ. ~ 2
grain diam.
n
d
at time t.
- don = Kt
coefficient dependent
on material and T.
elapsed time
TR = recrystallization
temperature
TR
º
TR = recrystallization temperature = temperature at
which recrystallization just reaches completion in 1 h.
0.3Tm < TR < 0.6Tm
For a specific metal/alloy, TR depends on:
• %CW -- TR decreases with increasing %CW
• Purity of metal -- TR decreases with increasing purity.


Performed at temperatures above room
temperature but below recrystallization
temperature
Dividing line between cold working and
warm working often expressed in terms of
melting point:
 0.3Tm, where Tm = melting point (absolute temperature) for
metal.
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Lower forces and power than in cold
working
More intricate work geometries
possible
Need for annealing may be reduced
or eliminated
Low spring back
Disadvantage:
1. Scaling of part surface
Deformation at temperatures above the
recrystallization temperature
 Recrystallization temperature = about one-half of
melting point on absolute scale
 In practice, hot working usually performed
somewhat above 0.6Tm
 Metal continues to soften as temperature
increases above 0.6Tm, enhancing advantage of
hot working above this level

Capability for substantial plastic deformation of the
metal - far more than possible with cold working or
warm working.
 Why?
 Strength coefficient (K) is substantially less than at
room temperature.
 Strain hardening exponent (n) is zero (theoretically)
 Ductility is significantly increased.
 Work-part shape
can be significantly altered
 Lower forces and power required
 Metals that usually fracture in cold working
can be hot formed
 Strength properties of product are generally
isotropic
 No work hardening occurs during forming.

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Lower dimensional accuracy in case of bulk
forming
Higher total energy required (due to the
thermal energy to heat the work-piece)
Work surface oxidation (scale), poorer
surface finish
Shorter tool life