Metal Forming Part - I

Dr.R.Narayanasamy B.E.,M.Tech.,M.Engg.,Ph.D.,(D.Sc.)
Professor, Department of Production Engineering,
National Institute of Technology,
Tiruchirappalli – 620 015, Tamil Nadu, India.

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 Strain Hardening
• Strain hardening occurs when the metals are
deformed at lower temperature.
• Flow stress increases with increasing strain.
• This applies to forming temperatures that are
low, that the thermally activated process plays
no significant role.
• Strain hardening results in higher forming
force and forming work, thus enhancing the
load stresses acting on the tool.
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 Strain Hardening cont…
• As a result of strain hardening (in many cases)
heat treatment is required.
• After every forming operation to improve
formability for obtaining the required
deformation heat treatment is required.
Alternative way:
• Metal forming at high (elevated) temperatures.
• Dimensional accuracy and surface quality is poor
in hot forming operation.

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 Strain Hardening cont…
• Strain hardening can be used in all metals and
alloys to increase strength and hardness.
• The increase in flow stress and tensile
strength permits the use of materials with
lower initial strength compared to machined
components.
• In many cases heat treatment becomes
unnecessary because of strain hardening.

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 Work hardening
• Atoms are arrested in a certain geometric fashion
(called as crystal structure).
• Impossible to have perfect arrangement of atoms
(certain type of defects will be in crystal
structure).
• The important type of defect is dislocation – line
defect.
• To cause plastic deformation – external forces are
applied. Dislocations move till they are arrested
by barriers (grain boundaries, inclusions etc.).
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 Work hardening cont…
• During the process, the density of the dislocation
increases by Frank Reed sources.
• Dislocation density for annealed metal is 106 − 108 /
𝑐𝑚2 .
• Dislocation density for cold worked metal is 1012 /𝑐𝑚2 .
• As the dislocation density increases, the movement of
dislocation becomes more difficult. (because mean free
path of dislocation reduces and this is called back
stress).
• This process increases the strength of metal called
work hardening.
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 Annealing
Annealing: when the metal is subjected to cold
working.
• Strength increases.
• Hardness increases.
• Percentage elongation decreases.
• Percentage area reduction in cross section
decreases.

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 Annealing

𝑻𝒎
𝝉𝒆𝒓 ≅
𝟐
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 Annealing cont…
• If the cold work is severe/excessive, the metal
will fracture before reaching the desired shape
or size.
• So, the cold working is carried out with several
steps with intermediate annealing.
• Intermediate annealing- softens the metal and
restores ductility
• This repeated sequence (cold work +
annealing) is called cold work – anneal cycle.
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 Annealing cont…

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 Annealing cont…
• Cold worked metals are annealed in inert gas
atmosphere. Otherwise, the metal will react with
atmospheric oxygen and get oxidized.
• Manufacturing cost is increased.
• After annealing metals have:
a. Low yield stress.
b. Low hardness.
c. High elongation.
d. Low strain hardening exponent.

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 Annealing of cold worked metal
• Annealing cycles/techniques used for cold
working metals:

a.
•Full annealing

b.
•Sub critical annealing

c.
•Spheroidize annealing
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 Annealing of cold worked metal cont…
• Cold worked metal has higher internal energy
than un deformed metal.
• When the metal is heated, the cold worked
state becomes more unstable.
• The metal softens to a strain - free condition.
• The overall process by which this occurs is
known as Annealing.
• Annealing restores the ductility of the metal.
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 Annealing of cold worked metal cont…
Three stages of Annealing process

• Recovery
a.

• Recrystallization
b.

• Grain growth
c.
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 Annealing of cold worked metal cont…

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 Annealing of cold worked metal cont…
Recovery:
• Restores physical properties. (Ex. Electrical
conductivity).
• Reduces dislocation density.
• No change in microstructure and Mechanical
properties.

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 Annealing of cold worked metal cont…
Recrystallization: Replacement of cold worked
microstructure with new set of strain- free
grains.
• It reduces strength and hardness.
• Increases the ductility.
The stored energy of cold worked metal is the
driving force for both recovery and
recrystallization.

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 Annealing of cold worked metal cont…

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 Annealing of cold worked metal cont…
Main variables of recrystallization are:
1. Amount of prior deformation.
2. Temperature.
3. Time.
4. Initial grain size.
5. Composition.
6. Amount of recovery.
𝑴𝒆𝒕𝒂𝒍 𝒎𝒆𝒍𝒕𝒊𝒏𝒈 𝒕𝒆𝒎𝒑𝒆𝒓𝒂𝒕𝒖𝒓𝒆
𝑹𝒆𝒄𝒓𝒚𝒔𝒕𝒂𝒍𝒍𝒊𝒛𝒂𝒕𝒊𝒐𝒏 𝒕𝒆𝒎𝒑𝒆𝒓𝒂𝒕𝒖𝒓𝒆 =
𝟐
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 Annealing of cold worked metal cont…
Relationship of variables:
• Smaller the degree of deformation, higher temperature
is required for recrystallization.
• A minimum amount of deformation is required to
cause recrystallization.
• Increasing the annealing time, decreases the
recrystallization temperature.
• Greater the degree of deformation and lower the
annealing temperature (recrystallized grain size will be
small).
• Crystallization temperature decreases with increasing
purity of metals. Solid solution alloying additions raise
the recrystallization temperature.
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 Cold Working
Cold working is the process of deforming the
metal under conditions of temperature and
strain rate, such that the strain hardening
produced by working is not relieved.

• Cold working is carried out below the

recrystallization temperature (𝑇𝑅𝐶 )of metals.

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 Cold Working cont…

Before working After working

Structural changes due to cold working

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 Cold Working cont…
Advantages Dis advantages
• No scaling and oxidation occur, • Higher capacity machines are
because of low deformation required because the strength of
temperature. metal increases at low
• Good surface finish. temperature.
• Close dimensional tolerance can • The formability of metals is poor
be obtained. at low temperatures. Hence,
• Automation is possible because costly annealing operations are
of low working temperature. required.
• Thin gauge sheets can be
manufactured. • Cold working introduces residual
stresses. Hence, stress relieving
• Strength can be controlled by
controlling formability annealing operation is required.
parameters. • Costly tools are required to take
care of high forming stresses.

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 Hot Working
• Hot working is a process of plastically deforming a
metal under conditions of temperature and strain rate.
• Recrystallization occurs during or immediately after
deformation.
• No work hardening occurs in metals.
• Microstructure elongates in the direction of the
applied load.
• Strain – free grains are formed because of high
temperature.
• Size of grains depends on following parameters:
a. Temperature.
b. Percentage amount of work.
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 Hot Working cont…

(a) (b)

Property changes due to cold working

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 Hot Working cont…

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 Hot Working cont…
Advantages Dis advantages
• Lower capacity machines • Heating facilities are
are sufficient because of required which increases
lower forming stresses. the investment cost.
• Number of forming stages • Scaling and oxidation occurs
required to obtain particular
shape is less (because of during hot working.
good formability at higher • Material losses due to
temperatures). scaling.
• No work hardening occurs – • Poor dimensional
higher deformation is tolerances.
possible.

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 Hot Working cont…
Advantages Dis advantages
• Stress relieving is not • Thin gauge material cannot
necessary because of higher be manufactured.
temperature. • Automation is difficult.
• Reduces chemical in • Surface decarburization
homogeneities because of reduces the strength of
rapid diffusion at high surface.
temperature. • Due to non uniform
deformation, structure and
• Blow holes and porosities properties are not same
can be eliminated at high over the cross section.
temperature. (Normalizing is needed).

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 Warm Working
• Warm working is a process of plastically
deforming a metal under conditions of
temperature and strain rate.
• The drawbacks of both cold working and hot
working are eliminated with their combined
advantages.

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 Warm Working cont…
Selection of warm working temperature is based
on the following factors:
1. Yield or flow strength of material.
2. Ductility or Formability of the material.
3. Dimensional tolerances.
4. Scaling and oxidation losses.

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 Effect of strain rate on Formability
Strain rate/ deformation velocity has three
principal effects in metal working.
a. Flow stress of the metal increases with strain
rate.
b. The temperature of work piece increases
because of adiabatic heating.
c. Lubrication at the tool-metal interface.

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 Effect of strain rate on Formability
cont…
The strain rate for a cylindrical upset in compression
is:
𝒅𝜺 𝟏 𝒅𝒉 𝒗
ἐ𝒕 = = =
𝒅𝒕 𝒉 𝒅𝒕 𝒉
Where h- instantaneous height (varies with axial
distance – converging dies)
V- the deformation velocity.
It is convenient to define mean strain rate by: ἐ̅𝒙 =
𝑳
𝟏/𝑳 𝟎 ἐ𝒕 𝒅𝒙
Where L- length of contact b/w tool & work piece.
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 Effect of strain rate on Formability
cont…
It is more usual to evaluate the mean strain rate in
terms of the time for an element to travel through
the die
𝒕𝒇
𝟏
ἐ̅𝒙 = ἐ𝒕 𝒅𝒕
𝒕𝒇
𝟎
For hot extrusion root mean power strain rate is
𝟏
𝟏 𝒍𝒏𝑹 𝒎
ἐ𝒓𝒎𝒑 = [ 𝟎
(ἐ) 𝒅𝜺] 𝒎
𝒍𝒏𝑹
(for strain rate sensitive material at large
deformation)
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 Effect of strain rate on Formability
cont…
• Forming velocity of equipment is
faster than the cross head velocity of
UTM m/c.
• The values of flow stress obtained
using tension/compression test are
not directly applicable for forming
load calculation.
In practical cold work – increase in flow
stress due to increase in speed is not
considered.
• For flow stress determination
camplastometer can be used.
• When the forming zone is very small,
it is possible to produce very high Camplastometer
local strain rates.
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 Effect of strain rate on Formability
cont…
Examples:
• Fine-wire drawing at a speed of 120ft/s results
in a strain rate in the order of 105 𝑆𝑒𝑐 −1 .
• In rolling of thin sheet, the strain r is in the
order of 2𝑋10−3 𝑆𝑒𝑐 −1 .
• When the deformation zone is narrow, it
produces a very high particle velocity.
• In high energy rate forming, the deformation
is delivered at a much higher rate.
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 Typical Values of Velocity and Forming
Operation
Operation Velocity (m/s)

Tension test 2 x 10^-6 to 2x 10^-2

Hydraulic Extrusion Press 0.01 to 10

Mechanical Press 0.5 to 5

Forging Hammer 10 to 20

Explosive Forming 100 to 400

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 Effect of Strain Rate cont…
• For many materials the elongation to fracture
increases with strain rate until a critical strain
rate is reached .
• Once the strain rate reaches a critical value,
the ductility falls off sharply.
• Explosive forming hardens the metal to a quite
high value with no grain distortion.
• Other extreme forming is super plastic
forming .

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 Effect of Strain Rate cont…
• Material having high strain rate sensitivity
(0.3 ≤ m ≤ 1.0) exhibit pronounced resistance to
necking.
• Generally this occurs with a very fine grain size,
of the order of 1 µm and at deformation
temperature above 0.4 Tm .
• For any super plastic material, there is a limit
strain rate above which it is no longer super
plastic.

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 Effect of Microstructure on
Forming

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 Effect of Temperature on Metal
Working
• Forming processes are commonly classified into
hot-working and cold-working operations.
• Hot working is defined as deformation under
conditions of temperature and strain rate such
that recovery and recrystallization of processes
takes place simultaneously with the deformation.
• On the other hand, cold working is deformation
carried out under condition where strain
hardening is dominant.
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 Hot Working Cold Working

1. No strain hardening. 1. Strain hardening exists.

2. Distorted grains are replaced 2. Grains get distorted.
by new strain free grains.
3. Recovery, recrystallization 3. No such reaction.
and grain growth take place.
4. Very large deformation is 4. No large deformation is
possible. possible.
5. At constant flow stress hot 5. Flow stress increases
working occurs because flow with deformation.
stress decreases with
temperature.
6. Energy required is less. 6. Energy required is more.

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 Hot Working
• It is an initial step in mechanical working of
most metal and alloys.
• The rapid diffusion at hot working
temperature aids in decreasing the chemical
inhomogenities.
• Blow holes and porosity are eliminated by
the welding together of these cavities.

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 Hot Working cont…
• Coarse columnar grains of the casting are
broken and refined into smaller equiaxed
recrystallized grains.
• These changes in structure result in increase
in ductility and toughness over the cast state.

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 Hot Working cont…
Disadvantages :-
i. Surface oxidization.
ii. Scale formation takes place.
iii. Embrittlement of metal by O2 is possible.
iv. Surface decarburization.
v. Poor surface finish.

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 Hot Working cont…
Disadvantages :-
vi. More allowance must be made for thermal
expansion and contraction in design.
vii. Structure and properties are not uniform
because the deformation is always greater in
the surface than core. This results in fine
grains at the surface and coarse grain at the
core.

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 Hot Working cont…
• The lower temperature limit for the hot-working
of metals is the lowest temperature at which the
rate of recrystallization is rapid enough to
eliminate strain hardening in the time when the
metal is at temperature of recrystallization.

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 Hot Working cont…
Hot working temperature depends on
amount of deformation and the time that the
metal is at temperature.
the amount of deformation Tre
• Metal which is rapidly deformed and cooled
rapidly from temperature will require a higher
hot working temperature for the same degree
of deformation than will metal slowly
deformed and slowly cooled.
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 Hot Working cont…
• The upper limit for working is determined by the
temperature at which melting or excessive
oxidization takes place.
• Maximum temperature = Tmelting – 100° F
• Whenever the metal contains low melting
constituents along the grain boundaries, they
melt and make the material crumble into pieces
during hot-working.
• Such condition is known as hot shortness or grain
boundary burning.

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 Hot Working cont…
• Finishing temperature is
always just above Tre
(in order to get fine
recrystallized grains)
compared to initial and Cold short

Deformation (%)
intermediate forming
stages.

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 Hot Working cont…
• Hot shortness is a limitation.
• As the strain rate of deformation increases,
more heat is retained in the workpiece.
• The temperature is to be reduced to keep its
final temperature from reaching the hot
shortness temperature.

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 Mechanics of Metal Forming
• The primary objective is to express Stress and
Strain involved in metal forming operation in
some mathematical form.
• These are required to produce desired shape.
• Siebel and other German worker proposed
that the maximum shear stress law was the
proper criterion for describing stresses during
forming.

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Mechanics of Metal Forming cont…
• Subsequent work showed that Von-Mises flow
criterion provided better agreement with
experimental data.

• Von-Mises criterion is preferred and most

widely used in the analysis of forming
operations.
• These two criteria differ by only 15%.

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Mechanics of Metal Forming cont…
• Maximum shear stress law will be used
wherever it needs simplification to the
analysis.
• In metal forming, the volume is assumed to be
constant .

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 Work of Plastic Deformation

• WT = Total Work
• Wτ = Redundant Work
• Wd = work of deformation (or)
work required for homogenous deformation
• Wf = Work done against friction

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Work of Plastic Deformation cont…
• Redundant work is energy expended on
deforming the body which is not involved in a
pure change in shape.

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 Formability Test
• It is necessary to predict the success of forming
operation on the basis of simple laboratory tests.
• Tension test : stretch forming
• Compression test : Forging, Rolling
• Torsion test : Extrusion, Rolling
• Shift cup test : Deep Drawing
• Erichson test : Stretch forming
• Bent test : Bending
Formability of metal can be studied using the
above tests for forming behaviour.
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 Thank You

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