mech

Recrystallised grain siz Recrystallised grain size, mm

10

5

Critical
deformation

5 10

Plast
The greater the driving force
the greater the number of nu
be the final grain size.

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ze and prior plastic strain

Aluminium sheet

15 20 25

tic deformation, %

e (greater prior plastic deformation),
uclei that will form and the finer will

Tapany Udomphol Jan-Mar 2007

Effects of grain size
recrystallisation tem

Schematic of recrystallisatio

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and strain on
mperature

on diagram Jan-Mar 2007

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PropertiesEffects of grain size on

Gra

• Small grains make disloc
• More slip plane, therefor

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n properties

Ductility

Strength

ain size

cations more difficult to move
re, greater ductility

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Effects of strain rate a

Flow stress of aluminium as a function of
strain at different temperature

Temp Flow stress

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and temperature

Flow curves of Cu Zn28

Strain rate Flow stress

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Secondary recrystallisation

• At higher temperature and lon
growth processes take place i
structure.

• The driving force energy from
the ratio of the grain boundary a

Secondary Only individua
recrystallisation preferentially,
very large gra
Grain growth near the prim
recystallised g

Result in an in
average grain

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and grain growth

nger annealing time, further grain
in the primary recrystallisation

m the energy gained by lowering
area to the enclosed volume.

al grains grow Mechanical
, resulting in property
ains present deterioration
marily
grains. Ductility

ncrease in Formability
n diameter.

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Recovery

• Recovery is a thermally acti
lower density of dislocations o
structure (as a consequence o
deformation process).

• Recovery process includes
annihilation of dislocation,
polygonization of dislocation,
dislocation climb.

• Certain amount of stored
energy is released during
annealing without an obvious
change in optical microstructure.

Polygonization www.

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differ

ivated process, which results in
or rearrangement of dislocation
of strain hardening during

.esrf.fr

overy of 38% cold-rolled aluminium showing

rent sizes of subgrains.

Tapany Udomphol Jan-Mar 2007

Effect of recovery an
stress-strain diagram

• Recovery process depends str
• Increasing temperature (T ≥ 0.
(b) reduces the yield stress, due
reactions of dislocations during

Effect of recovery annealing

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nnealing on
m

rongly on temperature.

.5Tm) during step tensile tests fig.
e to the rearrangement and
recovery.

g on stress-strain diagram Jan-Mar 2007

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Static and dynamic
structure during hot

• During plastic deformation, ne
produced continuously, which l
through dynamic recrystallisa

• These two processes take pla
plastic deformation at correspo

Dynamic recrystallisation
Dynamic recovery

Hot working (rolling,
forging, extrusion, etc.)

• During forming,

Note: recrystallisation a
• During cooling o

static recrystallisa

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changes of
forming

ew dislocations and vacancies are
leads to a new state of equilibrium
ation and dynamic recovery.

ace in the forming zone during
onding stresses and strain rates.

Static recrystallisation
Static recovery

During cooling

, structure changes through dynamic

and dynamic recovery.

or heating, structure changes through

ation and static recovery.

Tapany Udomphol Jan-Mar 2007

Static and dynamic
structure during hot

• Dynamic and static recovery
are strongly encouraged in meta
with high stacking fault energ
(easy for climb and cross slip) s
as aluminium , α –Fe, ferritic
alloys.
• Hot flow curve with a constant
slightly drop of yield stress are
typical for dynamic recovery.
• On the contrary, the flow curve
with dynamic recrystallisation
(after initial hardening) show a
sudden drop in yield stress.

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changes of
forming

y
als
gy
such

t or

es
n

Schematic form of hot flow curves by
(a) dynamic recovery alone
(b) both dynamic recovery and
recrystallisation.

Tapany Udomphol Jan-Mar 2007

Advantages and disa
hot working

Advantages
• Higher ductility – more deformatio
• Lower flow stress – less mechani
• Pores seal up.
• Smaller grain size.
• Microsegregation is much reduce

which is higher at high temperatu
• Stronger, tougher and more ducti

down and refinement of coarse co

Disdvantages

• Surface reactions between the m
oxidation (oxide scales), decabur

• Hot shortness, when the working
temperature of constituent at grai

• Dimension tolerance is poor due
temperatures.

• Handling is more difficult (from fu

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advantages of

on without cracking.
ical energy required for deformation.

ed or removed due to atomic diffusion,
ures.
ile than as-cast metals due to breaking
olumnar grains in the cast ingot.

metal and the furnace atmosphere, i.e.,
risation in steels.

temperature exceeds the melting
in boundaries such as FeS.
to thermal expansion at high

urnace to machine).

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Cold working

• Normally performed at room te
where recovery is limited and re

• Work hardening occurs (stre
ductility decreases).

• The extent of deformation is ra
avoid, therefore intermediate a
are frequently used afterwards

• The materials suitable for cold
low yield stress and a relative

(determined primarily by its tens

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emperature but in general < 0.3Tm,
ecrystallisation does not occur.

ength and hardness increase but

ather limited if cracks are to be
anneals that enable recrystallisation
s.

d working should have a relatively
ely high work hardening rate
sile properties).

Tapany Udomphol Jan-Mar 2007

Advantages and disa
cold working

Advantages
• Provide work hardening, material
• Provide fine grain size and good
• Dimension tolerance is better tha
• Easier handling (low operating te

Disdvantages
• Use high amount of deformation d

therefore, require soft materials.
• Equipment (rolls, dies, presses) i
• Reduced ductility, therefore, requ

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advantages of

ls are stronger.
surface finish.
an in hot working.
emperatures).

due to low operating temperatures,
is big and expensive.
uire subsequent annealing treatments.

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Properties of steels (C10

Mechanical H
properties

Ultimate tensile

strength, σTS

(MPa)

Yield stress, σy

(MPa)

Brinell hardness
(HB)

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0) after hot-cold working

Hot rolled Cold rolled
427 558
220 345
94 174

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Increasing reductionEffects of metall
on working proc

Deformation
bands
associated
with plastic
instability.

Schematic representation of shear band
formation in compression of a cylinder.

•The presence of preferred orie
mechanical properties, espec
• The development of texture is
bands or shear bands, which a
portion of grains have rotated to
accommodate the applied strain

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lurgical structure
cesses

Rolling direction

Fibrous texture in rolled plate.

entation causes anisotropy of
cially in rolled sheets.

the formation of deformation
are regions of distortion where a
owards another orientation to
n.

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Example: Plastic workin

The plastic working characteristic
microscopic distribution of the tw

• A high Vf of hard uniformly disp
and makes working difficult.

• Hard and massive particles te
softer matrix.

• Second phase particles or inclu
working direction (fibrous struct

• Precipitation hardening during
and lowered ductility.

Working
direction

microcrack

Cracked particle in softer ma
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ng in two-phase alloys

cs of two-phase alloys depends on the
wo phases.
persed particles increases the flow stress

end to fracture on deformation with

usions will be distorted in the principal
ture)- affect mechanical properties.
g hot working results in high flow stress

Second phase particles

atrix. Alignment of second phase
particles along the principal
working direction.

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Effect of principal stre

• When there is no shear stresses acti
the planes giving the maximum nor
stress acting on the planes.

• These planes are called the principa
planes, and stresses normal to these

are the principal stresses σ1, σ2 and

in general do not coincide with the car
coordinate axes x, y, z. Directions of
stresses are 1, 2 and 3.

Biaxial-plane stress condition

• Two principal stresses, σ1 and σ2.

Triaxial-plane strain condition

• Three principal stresses, σ1 , σ2
and σ3 , where σ1 > σ2 > σ3.

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esses in metal working

ing on
rmal

al
planes

σ3 which

rtesian-
f principal

Biaxial stresses Triaxial stresses

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Effects of speed o

High deformation speed • High
(high strain rate) • Incre
• impr

Flow stress dependence on strain rate and temperatu

Note: • If the speed of deformation is
• Can cause plastic instability
• Can cause hot shortness in

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of deformation

h flow stress.
eased the temperature of the workpiece.
roved lubrication at the tool-metal interface.

ure

too high, metal cracking is possible.
y in cold working
hot working

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Effects of friction

Friction at tool-workpiece in
the tooling and the geometry o
nature of metal, speed of defo

Die-workpiece interface (a) on th

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and lubrication

nterface depends on geometry of
of the deformation, temperature,
ormation.

he macroscale, (b) on the microscale.

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Effects of friction

When two surfaces are brought
will come into contact.

(a) Contact at asperities (b) overlap of
deformation zones to produce subsurface
shear zone.

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and lubrication

into contact, the high spot (asperities)

• As we increase the load, the metal
at the asperities deform plastically
and produce sub-shear zone.
• The coefficient of friction is given by

µ = F = τ i Ar = τ i

P pAr p …Eq.15

Where

µ = frictional coefficient
τ = the shearing stress at the interface

P = the load normal to the interface

F = the shearing force

Ar= summation of asperity areas in contact
p = the stress normal to the interface

Tapany Udomphol Jan-Mar 2007

Example : homogeneous com

Assumption: no barrelling an
conditions on the top and botto
by a constant coefficient of Co

σ

µ=τ

p

σ

Where

µ = frictional coefficient
τ = the shearing stress at the interface

p = the stress normal to the interface

Defo
of u

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mpression of a flat circular disk

nd small thickness, then the frictional
om faces of the disk are described
oulomb friction;

ormation pressure in compression as a function
u and a/h.

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Example: friction in forging

X=0 X=a

P

F

h

P

Functions of a metal wor
- Reduces deforma
- Increases limit of
- Controls surface
- Minimises metal
- Minimises tool we
- Thermally insulat
- Cools the workpi

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Friction hill Pmax

-a σo

aX

rking lubricant
ation load
f deformation before fracture
finish
pickup on tools
ear
tes the workpiece and the tools
iece and/or tools

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Effect of residual s

• Residual stresses is genera
deformation when external s
• Ex: in rolling process, the su
deformed and tend to elongat
are unaffected.

(a) Inhomogeneous deformation in rolling
sheet, (b) resulting distribution of longitu
residual stress over thickness of sheet.

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stresses

ated by non-uniform plastic
tresses are removed.

urface grains in the sheet are
te, while the grain in the centre

• Due to continuity of the
sheet, the central fibres tend to
restrain the surface fibres from
elongating while the surface
fibres tend to stretch the
central fibres.

g of • Residual stress pattern
udinal consisting of high
compressive stress at
the surface and tensile
stress in the centre.

Tapany Udomphol Jan-Mar 2007

Effect of residual s

• Residual stresses are only e
value which a residual stress c
the material.
• Residual stresses can be con
applied stresses.
• Compressive residual stress
the applied tensile stresses.
• Metals containing residual str
by heating to a temperature wh
material is the same or lower t
stress such that the material c
• However slow cooling is req
can again develop during cool

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stresses

elastic stresses. The maximum
can reach is the yield stress of

nsidered the same as ordinary

can effectively subtracts from

resses can be stress relieved
here the yield strength of the
than the value of the residual
can deform and release stress.
quired otherwise residual stress
ling.

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Workability

• Workability is concerned with
can be deformed in a specific m
the formation of cracks.
• Cracks which occur in metal
working processes can be
grouped into three broad
categories:

Dependence of forming limit of
mean normal stress σm.
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the extent to which a material
metal working process without

1. Cracks at a free surface
2. Cracks that develop in a surface

where interface friction is high
3. Internal cracks.

Examples of cracks in metalworking (a) free

surface crack (b) surface crack from heavy die

friction in extrusion, (c) centre burst or chevron

cracks in a drawn rod.

Tapany Udomphol Jan-Mar 2007

References

• Dieter, G.E., Mechanical m
McGraw-Hill, ISBN 0-07-100
• Edwards, L. and Endean, M
1990, Butterworth Heineman
• Lange, K., Handbook of m
Sons Company, ISBN 0-07-
• Lecture notes, Birmingham
• Metal forming processes, P

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metallurgy, 1988, SI metric edition,
0406-8.

M., Manufacturing with materials,
nn, ISBN 0-7506-2754-9.

metal forming, 1985, R.R Donnelly &
-036285-8.

m, UK, 2003

Prof Manus

Tapany Udomphol Jan-Mar 2007