Forgeability

• The forgeability of a metal can be defined as its capability to undergo deformation by forging without cracking

• Metal which can be formed easily without cracking, with low force has good forgeability.

Tests to determine forgeability

• Upsetting test: cracks while upsetting cylindrical specimen– Various temperatures and strain rates

– Just provides guidelines

• Hot-twist test– Metal rod is twisted at various temperatures.

– Forgeability can be determined for different materials using this method.

– Used for steel.

Extrusion

• In the basic extrusion process, a round billet is placed in a chamber and forced through a die opening by a ram.

• Methods:– Direct extrusion– Indirect extrusion– Hydrostatic extrusion – Impact extrusion

• Cladding can be done by coaxial billet.

• Flow stresses should be same for two metals

Metal flow in extrusion

• Substantial reduction in the cross sectional area

• Metal flow is important

Types of flow• Homogenous flow pattern

– No friction between billet and die– Continuous– Good lubrication

• Friction – leads to formation of dead metal zone

• High wall/billet friction – Outer wall cools down while central part is still

hot.– Leads to defects

Mechanics of extrusion

Extrusion ratio (R) = Ao/Af

True strain:

Where Lf=extruded product length

L0=billet length

RL

L

A

A f

f

lnlnln0

01

• Energy dissipated per unit volume

Where Y = yield stress

Total work done on the billet:

Ram force F which travels L0

p=extrusion pressure

 

 

 

Y

RYup

LpAFLWork

uLAWork

Yu

ln000

00

1

Average flow stress

 

 

Ideal Formation and friction

• When friction is included

• If die angle is 45o and Yield Stress is:

• Then:

1tan

1 cot

RYpav

kY

2RYp ln7.1

When friction along the container wall is considered

• Total pressure:

• As extrusion proceeds, L reduces thus p reduces.

0

2

D

LYp f

0

2ln7.1

D

LRYptotal

Actual Force

• If we take into account friction, die angle, etc., we can use empirical formula:

• a=0.8, b=1.2-1.5 for strain hardening material

)ln( RbaYp

YY

Optimum Die Angle

• The ideal work should be independent

• Friction work increases with decreasing die angle

• Redundant work caused by inhomogeneous deformation – increases with increasing die angle

Die angle and Force

b = ideal

a = total

d = friction

c = redundant

Forces in hot extrusion

• Velocity effects metal with strain rate sensitivity

• For high extrusion ratios ando45fDD 0

RD

Vln

6

0

0

RDD

DV

f

lntan6

330

200

As V0 increases, pressure increases

As temperature becomes hot, pressure reduces

As V0 rate of work done on the billet also increases, thus temperature increases

This can cause melting and “speed crack” on the surface.

Problem• Copper billet 5” in diameter to be reduced

to 2” in diameter at speed of 10 in/sec at 1500oF Initial Length =10

s/2225.6ln5

106

psiC m 870,22)22)(000,19( 06.0

Y

5

)10(2)83.1(7.1)870,22(

2ln7.1

0D

LRYp

R=52/22=6.25

Assume c=19000psi, m=0.06

assume

psi630,162

.102.34

)5()630,162( 6

2

0 lbpAF

Extrusion Processes

• Cold Extrusion– Room temperature or a few hundred degrees– Advantages

• Close control of tolerance

• Improved surface finish

• Strain hardening ca give some desirable properties

• No oxide layer formation

– High stresses on dies– Lubrication is very critical (phosphate, wax,

etc.)

Impact Extrusion• Punch descends at high speed and strikes a blank• Used to make thin tubular sections• thickness of the tube to diameter of the tube

=0.005

Hydrostatic extrusion• Pressure applied by fluid medium

• Reduces friction

Defects

• Surface Cracking– Speed cracking (high speed, high friction)

• Intergranular cracks– Occurs with Al, Mg, Zn, molybdenum alloys

• Can also be caused by metals sticking to die surfaces

Extrusion Defects

• Surface defects may extrude into the center of the extruded parts

• Oxides, impurities usually caused due to inhomogeneous flow of metal

http://www.transvalor.com/

Internal Cracking

• Center of the extrusion can have cracks. Known as center crack – chevron crack

• Depends on contact length, angle, die opening, ratio of extrusion.