CORROSION - User Area

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Transcript CORROSION - User Area

POWDER METALLURGY
Manufacturing Process Lecture
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Introduction
Powder metallurgy, or P/M (pronounced
PM), is a process for forming metal parts
by heating compacted metal powders to
just below their melting points.
Powder metallurgy is the study of the
processing of metal powders, including the
fabrication, characterisation, and
conversion of metal powders into useful
engineering components.
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REFERENCES
Kalpakjian, S. and Schmid, S.R.
Manufacturing Engineering and
Technology, 4th ed., Prentice Hall, 2001.
German, R. M., Powder Metallurgy
Science, 2nd ed., MPIF, New Jersey,
1994.
________, Powder Metallurgy, ASM
Handbook vol. 7, E. Klar (editor), 9th ed.,
ASM International, Ohio, 1984.
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What do we learn?
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Introduction to Powder Metallurgy
Attraction of P/M : the
ability to fabricate high
quality, complex parts
to close tolerances in
an economical manner.
Automated operation :
low relative energy
consumption,
high
material utilisation, and
low capital costs.
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POWDER METALLURGY PROCESS
TOPICS COVERED
IN THIS LECTURE
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Advantages of the P/M Process
Eliminates
or
minimises
machining.
Eliminates or minimises scrap
loses.
Maintains close dimensional
tolerances.
Permits a wide variety of alloy
systems.
Produces
good
surface
finishes.
Provides materials which may
be heat-treated for increased
strength or increased wear
resistance.
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Provides controlled porosity for
self-lubrication or filtration.
Facilitates manufacture of
complex or unique shapes
which would be impractical or
impossible
with
other
metalworking processes.
Suited to moderate-to high
volume component production
requirements.
Offers long-term performance
reliability
in
critical
applications.
Cost effective.
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Applications for Metal Powders
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POWDER CHARACTERISTICS
P/M deals with particles that
are larger than smoke
(0.01 to 1 mm), but
smaller than sand (0.1 to
3 mm). Many metal
powders are similar in
size to the diameter of a
human hair (25-200mm).
What is a particle ?
Collecting
a
representative sample of
powder for analysis can
be difficult. Sampling
errors can be avoided
using some simple rules.
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Particle size and
distribution.
Particle shape and its
variation with particle size.
Surface area
Interparticle friction
Flow and packing
The internal particle
structure
Composition, homogeneity,
and contamination.
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Particle Size
Particle size is a
determination of the
dimensions
of
a
particle.
Most particle size
analysers use one
geometric parameter
and
make
the
assumption
of
a
spherical
particle
shape.
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Particle Size Distribution
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Particle Shape
Particle shape influences
packing,
flow,
and
compressibility,
provides information on
the powder fabrication
route, and helps explain
many
processing
characteristics.
Particle shape varies with
size and manufacturing
technique.*
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Variations in Shape
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*Water Atomisation Process
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Powder Structure
A magnified cross section
through a powder reveals
artifacts that indicate
Fabrication conditions
Possible processing
problems
Internal pores,
segregation, inclusions,
and oxides.
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Powder Chemistries
There are 3 groups of powder:
1. The
elemental powders are
relatively high purity materials
where chemical analysis focuses
on the impurity concentration.
2. Premixed powders are
combinations of two or more
distinct powders that will
diffusionally alloy in sintering
(concerns: impurity levels and
proper compounding of the
mixture).
3. The prealloyed powders constitute
micro-castings with multiple
elements in a predetermined ratio
(concerns: alloy composition and
impurity concentrations).
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Surface Conditions:
- Oxides
- Adsorbed organic films
- The presence of surface
coatings like silica to
determine appropriate
cleaning treatments
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Powder Adjustment to Improve Packing and Flow
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Packing Structures
The packing density
dictates the die fill, binder
content, and shrinkage in
sintering.
The greater the surface
roughness or the more
irregular the particle
shape, the lower the
packing density.
For common P/M
powders the packing
density ranges from 3065% of theoretical.
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Mixing and Blending
Mixing and blending are
necessary :
to prepare unique particle
size distribution.
Combine powders to
generate new alloys
during sintering.
Add lubricants for
compaction
To prepare a powderbinder mixture for
shaping.
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Shaping and Compaction
Injection Molding
Slurry Techniques :
Slip casting and tape
casting
Freezing technique
Extrusion
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Injection Molding
Slurry Techniques
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COMPACTION
To achieve greater
densities requires an
external pressure.
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Stages of Compaction
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Conventional Compaction
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Density Lines after Compacted Powder
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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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