1. 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.

2. 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.

3. 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

5. Cladding can be done by coaxial billet.
Flow stresses should be same for two metals

6. Metal flow in extrusion
Substantial reduction in the cross sectional area
Metal flow is important

7. Types of flow Homogenous flow pattern Friction
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

8. Mechanics of extrusion
Extrusion ratio (R) = Ao/Af
True strain:
Where Lf=extruded product length
L0=billet length

9. Energy dissipated per unit volume
Where Y = yield stress
Total work done on the billet:
Ram force F which travels L0
p=extrusion pressure
Average flow stress

10. Ideal Formation and friction
When friction is included
If die angle is 45o and Yield Stress is:
Then:

11. When friction along the container wall is considered
Total pressure:
As extrusion proceeds, L reduces thus p reduces.

12. Actual Force
If we take into account friction, die angle, etc., we can use empirical formula:
a=0.8, b= for strain hardening material

13. 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

14. Die angle and Force
a = total
c = redundant
b = ideal
d = friction

15. Forces in hot extrusion
Velocity effects metal with strain rate sensitivity
For high extrusion ratios and

16. 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.

17. Problem
Copper billet 5” in diameter to be reduced to 2” in diameter at speed of 10 in/sec at 1500oF Initial Length =10
R=52/22=6.25
Assume c=19000psi, m=0.06
assume

18. 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.)

19. 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

21. Hydrostatic extrusion
Pressure applied by fluid medium
Reduces friction

22. Defects Surface Cracking Intergranular cracks
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

23. Extrusion Defects
Surface defects may extrude into the center of the extruded parts
Oxides, impurities usually caused due to inhomogeneous flow of metal

25. 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.