Forgeability

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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.

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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.

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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

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

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Metal flow in extrusion

• Substantial reduction in the cross sectional area
• Metal flow is important

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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

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Mechanics of extrusion

• Extrusion ratio (R) Ao/Af
• True strain
• Where Lfextruded product length
• L0billet length

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• Energy dissipated per unit volume

Where Y yield stress
Total work done on the billet
 
Ram force F which travels L0
 
pextrusion pressure
 
Average flow stress
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Ideal Formation and friction
 

• When friction is included
• If die angle is 45o and Yield Stress is
• Then

 
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When friction along the container wall is
considered

• Total pressure
• As extrusion proceeds, L reduces thus p reduces.

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Actual Force

• If we take into account friction, die angle,
  etc., we can use empirical formula
• a0.8, b1.2-1.5 for strain hardening material

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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

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Die angle and Force
a total
c redundant
b ideal
d friction
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Forces in hot extrusion

• Velocity effects metal with strain rate
  sensitivity
• For high extrusion ratios and

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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.
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Problem

• Copper billet 5 in diameter to be reduced to 2
  in diameter at speed of 10 in/sec at 1500oF
  Initial Length 10

R52/226.25
Assume c19000psi, m0.06
assume
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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.)

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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

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Hydrostatic extrusion

• Pressure applied by fluid medium
• Reduces friction

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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

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Extrusion Defects

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

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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.