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.