Upload
akshay-meena
View
360
Download
1
Embed Size (px)
Citation preview
Effect of temperature on flow stress
Hot working
• Above a certain temperature, strain
hardening is nullified by softening
mechanisms like recovery and
recrystallisation that occur simultaneously
with deformation.
• Flow stress decreases with temperature and
above recrystallisation temperature, flow
stress remains almost constant as the
deformation proceeds.
Strain rate
For example, in a uniaxial tension test, rate
of elongation can be controlled and it is
called cross head velocity (v).
So,
Initial strain rate is given by
Typical strain rates used in metal forming
• Superplastic forming 10-8 to 10-5 /sec
• Lab tests 10-5 to 10-3 /sec
• Hot deformation 10-3 to 10 /sec
• Cold working 10 to 103 /sec
• Impact tests 103 to 106 /sec
• HERF > 106 /sec
Effect of strain rate and temperature
on flow stress
m is strain rate sensitivity index and C is a coefficient.
Superplasticity
• It is the phenomenon by which some alloys
exhibit very large elongations (more than
1000%) under controlled conditions of
tensile deformation.
• Examples are some Al based and Ti based
alloys.
• Used in aerospace applications.
Superplasticity
Optimum conditions for
superplasticity
• Very low strain rate (10-8 to 10-5 /sec)
• Uniform and equiaxed grain structure
• Very fine grain size (less than 10 microns)
• Sufficiently high temperature (T > Trecryst)
• High value of strain rate sensitivity index
(m > 0.5)
Applications of SPF
Friction in cold working
• In cold working under well lubricated
conditions, the frictional conditions at the
tool-work piece interface are such that there
is a sliding motion between the two. The
work piece slides over the tool surface.
• It is called sliding friction or slipping
friction.
Friction in cold working
• If sliding friction conditions exist, friction is
described by Coulomb’s law of friction:
τ = μp
where τ is frictional shear stress at the
interface
p is normal stress and
μ is coefficient of friction (0.1-0.3)
Friction in hot working
• An alternative approach to treat friction in metal
working operations is to consider the friction to be
“sticking”.
• Sticking friction conditions exist particularly in
hot working processes and in those situations
where lubrication is inadequate.
• This approach considers that the work piece in
contact with the tools can be represented as a
material of constant shear strength.
Friction in hot working• This interface shear strength can be expressed as a
fraction of yield strength of the material in pure
shear (k):
τ = m k
where m is the interface friction factor.
Values of m vary from 0 to 1.
m = 0 indicates perfect sliding.
m = 1 indicates prefect sticking.
Since accurate determination of m value in hot
working is difficult, m is taken to be 1 in the
analysis.
Hot working Cold working
1.Temp of working above
Trecryst.
2. At high temp, ductility
is high, so large strains
can be given.
Temp of working below
Trecryst.
At low temp, ductility is
low, large strains can not
be given. For large
dimensional changes,
intermediate annealing is
necessary.
Hot working Cold working
3. At high temp, flow
stress is low, required
forming loads are low. No
need for presses with high
capacity.
4. Need for high temp
facilities increases cost.
For reactive metals inert
atmosphere is required.
Flow stress is high,
required forming loads
are high. Need for high
capacity presses.
Need for intermediate
annealing facilities for
large strains.
Hot working Cold working
5. Automation is difficult.
6. Surface oxidation problem
leads to poor surface finish,
wastage of material. Defects
like rolled in oxides occur.
7. Poor dimensional accuracy
due to large strains and
thermal
expansion/contractions.
8. Structure and properties are
not uniform.
Automation is easy.
No oxidation problems, good
surface finish, less wastage of
material.
Better dimensional accuracy
due to small strains and good
control of the process.
Better uniformity of
properties.