Transcript POWDER METALLURGY Lecture 12 5/20/2016 1

POWDER METALLURGY
Lecture 12
5/20/2016
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FUNDAMENTALS OF
COMPACTION
Px = P exp (- 4 u z x / D)
a single action pressing
Where
Px = pressure at any position x
; P = external pressure
u = coefficient friction between the powder and die wall
Z = a constant (the axial to radial pressure distribution)
X = distance below the punch
D = compact diameter
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Double Acting Compaction
It has a simultaneous
pressure profile from
both top and bottom
punches.
The equation of
pressure is valid, but
the distance x is now
the distance to the
nearest punch.
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It results in a more
homogeneous
pressure distribution
in the compact.
For homogeneous
compact, small height
to diameter ratios H/D
are desirable.
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The Average Compaction Stress
For a single-ended pressing
 = P [1 – 2 u z (H/D)]
For a double-ended pressing
 = P [1 – u z (H/D)]
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Particle Bonding in the Green State
The interparticle bonds that form due to compaction
provide the green strength.
The interparticle bonds are improved by:
- High initial packing density
- Clean powder surface
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Particle Bonding in the Green State
Mechanical interlocking makes the major
contribution to the green strength.
Rounded but irregular particle shape often
results in the optimal green strength.
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Goals in Compaction
To achieve compact properties with minimal wall
friction.
H/D ratio is important to create uniform compact
properties.
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Density Lines after Compacted Powder
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An increase in H/D ratio results in greater
density gradients and a lower green density.
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SINTERING
Sintering is the bonding
together of particles
at high temperatures.
T < Tm by solid-state
atomic transport
(solid-state sintering)
Involves formation of
a liquid phase (liquidstate sintering)
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Sintering Theory
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Pore Structures in Sintering
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Liquid Phase Sintering
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Practical Sintering Operations
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SECONDARY OPERATIONS
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