METAL FORMING

PROCESSES

J. J. Ellard, PhD

MEFP-520 : 2024/2025
1
Overview of Metal
Forming

2
 Metal Forming
➢ Large group of manufacturing processes in which
plastic deformation is used to change the shape of
metal workpieces.
➢ The tool, usually called a die, applies stresses that
exceed the yield strength of the metal.
➢ The metal takes a shape determined by the geometry
of the die.
➢ Stresses to plastically deform the metal are usually
compressive
• Mention three examples
3
 Metal Forming
➢ However, some forming processes apply:
• Tensile stresses. Give an example
• Tensile and compressive stresses. Give an example
• Shear stresses. Give an example

➢ Desirable material properties include:

• Low yield strength
• High ductility

4
 Metal Forming
➢ These properties are affected by temperature:
• Ductility increases and yield strength decreases
when work temperature is raised.
➢ Other factors:
• Strain rate and friction

5
Fundamentals of Metal
Forming

Part A

6
 Contents
1. Classification of Forming Processes
2. Flow Stress Determination
3. Effect of Temperature, Strain Rate and
Metallurgical Structure on Metal Working

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1. Classification of Forming Processes

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1. Classification of Forming Processes
BULK DEFORMATION PROCESSES
➢Characterized by significant deformations and
massive shape changes.
➢"Bulk" refers to work parts with relatively low surface
area-to-volume ratios
➢Starting work shapes are usually simple geometries.
Examples:
• Cylindrical billets
• Rectangular bars
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 1. Classification of Forming Processes
BULK DEFORMATION PROCESSES

Rolling Forging

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 1. Classification of Forming Processes
BULK DEFORMATION PROCESSES

Extrusion Wire and Bar Drawing

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1. Classification of Forming Processes
SHEET METALWORKING
➢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 pressworking because these operations
are performed on presses
• Parts are called stampings
• Usual tooling: punch and die
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1. Classification of Forming Processes
SHEET METALWORKING

Bending Deep Drawing

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1. Classification of Forming Processes
SHEET METALWORKING

Shearing

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 2. Flow Stress Determination
➢Flow curve indicates whether metal is readily deformed at
given conditions, i.e., strain rate, temperature.
➢Flow curve is strongly dependent on strain rate and
temperature.

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 2. Flow Stress Determination
➢Hook’s law is followed up to the yield point, 𝜎𝑜 .
➢Beyond 𝜎𝑜 , metal deforms plastically (strain
hardening).
➢The curve is given by:
𝜎 = 𝐾𝜀 𝑛
where 𝜎 = True Stress,
𝜀 = True Strain
𝐾 =Strength Coefficient
𝑛 = Work Hardening Exponent
(Valid from beginning of plastic flow to the
maximum load at which the specimen begins to Ductile metal under
neck down) uniaxial tensile loading

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 2. Flow Stress Determination

➢Unloading from A immediately

decreases the strain from 𝜀1 to
𝜎
𝜀2 = the strain decrease 𝜀1 − 𝜀2
𝐸
is the recoverable elastic strain.

Ductile metal under

uniaxial tensile loading

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 2. Flow Stress Determination
➢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

Yf = K  n

where Yf = flow stress (yield strength as a function of strain)

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2. Flow Stress Determination

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 2. Flow Stress Determination
➢In certain forging operations, the instantaneous force
during compression can be determined from the flow
stress value.
➢Maximum force can be calculated based on the flow
stress that results from the final strain at the end of the
forging stroke.
➢ In extrusion, the analysis is based on the average
stresses and strains that occur during deformation.

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 2. Flow Stress Determination
Average Flow Stress
➢Determined by integrating the flow curve
equation between zero and the final
strain value defining the range of interest.
_
K n
Yf =
1+ n
_
where Yf = Average flow stress
 = Maximum strain during deformation
process

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 2. Flow Stress Determination
Average Flow Stress

➢Average flow stress Yf

in relation to
• Flow stress Yf
• Yield strength Y

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming
➢Three temperature ranges in metal forming:
• Hot working
• Warm Working
• Cold Working

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
➢Hot working involves deformation at temperatures
where recrystallisation can take place simultaneously
with the deformation (0.6-0.8𝑇𝑚 ).
➢Examples: Rolling, Forging, Extrusion
➢Metal continues to soften as temperature increases
above 0.5 𝑇𝑚 , enhancing advantage of hot working
above this level.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recrystallisation during hot working

➢The minimum temperature at which reformation of the

crystals occurs is called Recrystallisation Temperature
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recrystallisation during hot working
➢Above the recrystallisation temperature the kinetic
energy of atoms increases and therefore they are able
to attach themselves to the newly formed nuclei
which in turn begin to grow into crystals.
➢The process continues until all the distorted crystals
have been transformed.
➢Hot working results in grain refining
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recrystallisation during hot working
➢Recrystallisation takes place at higher temperatures
than recovery which leads to a new formation of
grains.
➢The process includes
• Primary recrystallisation
• Secondary recrystallisation and grain growth
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recrystallisation during hot working

➢ It occurs at the beginning of the new grain formation

process.
➢ Recrystallisation temperature does not depend on
the metal alone, but on a whole number of variables
temperature, strain and minimum dislocation density
available (amount of deformation)
➢ Small impurities in pure metals can considerably
increase RT

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recrystallisation during hot working

➢At higher temperature and longer annealing time, further

grain growth processes take place in the primary
recrystallisation structure.
➢The driving force energy is from the energy gained by
lowering the ratio of the grain boundary area to the
enclosed volume.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recrystallisation during hot working

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recovery
➢It is a thermally activated process, which results in lower
density of dislocations or rearrangement of dislocation
structure (as a consequence of strain hardening during
deformation process).
➢It includes:
• Polygonisation of dislocation
• Annihilation of dislocation
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recovery

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recovery
➢Recovery involve re-arrangement and annihilation of
dislocations introduced during plastic deformation.
➢When metals are plastically deformed, some fraction of the
deformation energy (~ 5%) is retained internally; the
remainder is dissipated as heat.
➢The major portion of this stored energy is as strain energy
associated with dislocations.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recovery
➢Thus increase in dislocation density during
deformation.
➢Dislocations can re-arrange so that the strain fields
partially cancel.
➢Dislocations of opposite ‘sign’ meet and annihilate.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recovery
➢The two main fundamental processes which allow
dislocation lines the freedom to move so that they can form
lower energy structures and/or annihilate are:
1. CLIMB
• Dislocations can move out of their slip plane by either
emitting or absorbing vacancies at the line.
• Depends on the thermally activated movement of
vacancies.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recovery
2. CROSS-SLIP
• Occurs with pure screw-character dislocation line
segments.
• Such segments can move on other slip planes.
• Thermal activation.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recovery
➢Recovery will give changes in microstructure:
• Formation of a 'cellular' structure

Almost dislocation free cells + cell walls with high dislocation density.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recovery
• Condensation to give subgrains

Planar low-angle boundaries between subgrains, which were cells.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Recovery
• Continuous subgrain growth

Growth of subgrains occurs by boundary migration, reducing the amount

of condensed boundaries.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Effect of Recovery annealing on stress-strain curve
➢Recovery process depends strongly on temperature.
➢Increasing temperature (𝑇 ≥ 0.5𝑇𝑚 ) during step tensile tests reduces the
yield stress (b), due to the rearrangement and reactions of dislocations
during recovery.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Effect of Recovery annealing on stress-strain curve

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Static and Dynamic changes of structure during hot
forming
➢During plastic deformation, new dislocations and vacancies
are produced continuously, which leads to a new state of
equilibrium through dynamic recrystallisation and dynamic
recovery.
➢These two processes take place in the forming zone during
plastic deformation at corresponding stresses and strain rates.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Static and Dynamic changes of structure during hot
forming

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Static and Dynamic changes of structure during hot
forming
➢Dynamic and static recovery are strongly encouraged in
metals with high stacking fault energy (easy for climb and
cross slip) such as:
• Aluminium
• 𝛼-Fe
• Ferritic alloys

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Static and Dynamic changes of structure during hot
forming
➢Hot flow curve with a constant or
slightly drop of yield stress are
typical for Dynamic recovery.
➢On the contrary, the flow curves
with Dynamic recrystallisation (after
initial hardening) show a sudden
drop in yield stress.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
➢Dynamic recovery is recovery taking place during
deformation.
➢This is very significant in determining the work hardening
behaviour of metals.
➢To approximately fit experimental data to work hardening
behaviour:
𝜎 = 𝑘𝜀 𝑛 ; σ = 𝜎𝑜 + 𝑘𝜀 𝑛
But… no basis in microstructural processes.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
➢Work hardening as a combination hardening and softening
(dynamic recovery) processes is given by:
𝜕𝜎
Ɵ= = Ɵ+ − Ɵ−
𝜕𝜀𝑃
Ɵ+ : Hardening term (‘athermal hardening’)
the chance trapping of dislocations during plastic
deformation varies weakly with temperature. It scales with
the elastic moduli: Ɵ+ /G would be approximately constant.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
Ɵ− : Softening term
depend on the current levels of stress, temperature and strain
rate. The effect of temperature on this term is much more
significant than on Ɵ+ .

➢The more difficult dynamic recovery is, the higher will be

the work hardening.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
➢At constant temperature and strain rate, the softening term is
proportional to stress:
Ɵ− = η𝜎
so that: Ɵ = Ɵ+ − η𝜎
σ = 𝜎𝑠𝑠 − 𝜎𝑠𝑠 − 𝜎𝑜 𝑒𝑥𝑝 −η𝜀 Voce hardening law
Proposed as a curve fitting equation by Voce in 1948
Ɵ− depends on 𝜎 → the work hardening rate decreases as the
𝜎 → 𝜎𝑠𝑠 . Saturation of steady state stress (𝜎𝑠𝑠 ): Ɵ+ = Ɵ−
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
Voce hardening law This saturation is analogous to a vessel with a
hole in it being filled from a water tap

Flow in = ‘athermal’ hardening

Flow out= ‘recovery’

This simple form works quite well, but often has to be modified to give good fitting.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
The effect of Temperature and strain rate

Ɵ+ : not much change (slow decrease

with T
Ɵ− : increases with T, decreases with
strain rate

➢Increasing temperature and/or decreasing strain rate will lead to a lower

steady- state stress.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
The effect of Temperature and strain rate
➢The effects of temperature and strain rate can be
combined:

➢The activation energy is characteristic of the ‘softening’

mechanism
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
The effect of Temperature and strain rate
➢Temperature dominates:
• Consider aluminium where 𝑄 ≈ 150 𝑘𝐽 𝑚𝑜𝑙 −1 .
• Deformation at 400 ˚C with a strain rate of 0.1 𝑠 −1 gives
Z = 4.4 × 1010 𝑠 −1 .
• To give the same Z at 200 ˚C would require a strain rate of
about 10−6 𝑠 −1 .
• At room temperature, the strain rate giving that Z value
would be ≈ 10−16 𝑠 −1 : a geological rate (1% stretch in 3 million years)!
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
➢The overall effect of Z via its effect on dynamic
recovery, can be shown by its effect on 𝜎𝑠𝑠 :

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
➢The dimensionality of Z as a strain rate means it has a
very wide range, and using a log. scale makes it vary as
inverse temperature, 𝑇 −1 .
➢The range of stress will also be large, hence a log. scale
for that as well.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
➢Sellars and McTegart proposed a correlation for hot
deformation data:
𝑚
𝑍 = 𝐴 sinh 𝛼𝜎
(hyperbolic sinusoidal Arrhenius-type equation)
The values of 𝐴, 𝛼 and 𝑚 are characteristics of the
material.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
➢At low stresses, the equation reduces to:
𝑍 = 𝐴1 𝜎 𝑚1
(Power Arrhenius-type equation)
➢At high stresses, the equation reduces to:
𝑍 = 𝐴2 exp 𝛽𝜎
(Exponential Arrhenius-type equation)
𝛽
𝛼=
𝑚
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery
➢It has been found experimentally that, in simple ductile
metals, stress can be related to Z using:

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery - Exercise
An alloy has a strain hardening behaviour that can be fitted to a Voce
equation with η = 5.0, over a range of temperatures from 20 ˚C to
500 ˚C. In this range, the following expression works for the steady-
state stress:

where 𝜎𝑠𝑠 is in MPa and Z in 𝑠 −1 . The material is being

deformed in uniaxial compression.

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recovery - Exercise
a. Calculate the value of the steady-state stress in the following
cases:
i. at 200 ℃ with 𝜀ሶ = 0.001 𝑠 −1 , and
ii. at 400 ℃ with 𝜀ሶ = 10 𝑠 −1
The activation energy takes the value of 260 kJ mol−1 . Gas
constant R = 8.314 J mol−1 K −1 .
b. What is the work hardening rate at a stress of 50 MPa when the
deformation takes place at 300 ℃ with a strain rate of 0.1 𝑠 −1
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recrystallisation
➢In some materials it is possible to
build up sufficient stored energy
(related to the dislocation content)
at temperatures sufficiently high for
rapid high-angle boundary
migration to occur.
➢This leads to recrystallisation
during deformation
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working
Dynamic Recrystallisation
➢The HAB movement softens the material
by ‘sweeping out’ dislocations, and small
grains can actually be softer at these
temperatures.
➢Eventually a population of grains with a
size characteristic of the deformation
conditions (Z) for a particular alloy is
formed.
In some cases, high angle boundaries can migrate during deformation without
forming ‘new’ grains.
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Hot working

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Warm Working
➢It is performed at temperatures above room
temperature but below recrystalisation 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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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Warm Working
Advantages
➢Lower forces and power than in cold working
➢More intricate work geometries possible
➢Need for annealing may be reduced or eliminated
Disadvantage
➢Work-piece must be heated

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Cold Working
➢Normally performed at room temperature but in general < 0.3𝑇𝑚
where recovery is limited and recrystallisation does not occur.
➢Work hardening occurs (strength and hardness increase but
ductility decreases).
➢The extent of deformation is rather limited if cracks are to be
avoided, therefore intermediate anneals that enable
recrystallisation are frequently used afterwards.
➢The materials suitable for cold working should have a relatively
low yield stress and a relatively high work hardening rate
(determined primarily by its tensile properties).
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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming – Cold Working

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3. Effect of Temperature, Strain Rate and Metallurgical
Structure on Metal Forming
Effect of Temperature on Metal Forming

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