POWDER METALLURGY

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POWDER METALLURGY

• Lecture 10

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POWDER HANDLING STEPS

• BEFORE SHAPING OR COMPACTION

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Precompaction Stageare aimed at adjusting the
powder for easier processing.

• Classification
• Milling powder deagglomeration.
• Blending and Mixing.
• Coating, Bonding, and Agglomeration of Powders.
• Lubrication

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CLASSIFICATION OF A POWDER

• In certain powders, the main impurities are
  concentrated in a narrow particle size range.

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Safety and Health Considerations

• Powder handling requires safety precautions and
  cleanliness.
• Certain powders can have harmful effects on
  workers exposed to them.
• Inhaled powders are a health concern and can
  cause disease or lung dysfunction.

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Safety and Health Considerations

• The smaller the particle size the greater the
  potential health hazard 0.11.0 mm.
• Toxic metals As, Te, Be, Cd, co, Pb, Ni, and Cr.
• Another hazard thermal instability in the
  presence of O2 (pyrophoric burn in air).
• e.g. Al, Zr, Ta, Th, Ti, Mg can ignite in air at
  concentrations of 40 g/m3. Combustion pressure up
  to 2 MPa and can occur at 200-700oC.
• Moderate hazard Fe, Zn, Sn, and Cu.

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Safety and Health Considerations

• Mixture of Ti and graphite ? TiC exothermic
  reaction even in the inert conditions
  (temperature rise can be 1000-2000oC).
• Many similar sytems Ni-Al, Ni-Si, Ti-Al,
• Ti-B, Pt-Zr, and Fe-Al.

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AIR CLASSIFICATION

• Using screen or air classifiers
• To remove selective size fractions.
• To remove contaminants such as inclusions or
  crucible materials.
• - To size powders for the production of
  controlled pore size filters and flow restrictors.

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• Differences in the deceleration of particles
  exiting from the disk are based on the particle
  diameter and mass.
• Control of the disk rotational speed and air flow
  velocity provides the means of altering the
  particle size separation.
• Size range 1-150 mm.

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POWDER DEAGGLOMERATION

• Agglomeration occurs because of
• A high surface area.
• The action of one of the weak forces
• van der Waals attraction
• Electrostatic charges
• Chemical bonding
• Capillary liquid forces
• Magnetic forces

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POWDER DEAGGLOMERATION

• Milling, attritioning.
• Surface Treatments
• Thin coatings of polar molecules.
• Organic lubricants.

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Powder Adjustment to Improve Packing and Flow
The tumbling action minimises particle
attritioning, but is effective in deagglomerating
and smoothing the powder surface.
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Surface Removal

• Many impurities are segregated to the surface of
  a metal powder during solidifi- cation and
  adsorption of contaminants during handling.
• Surface removal can be done using
• surface chemical treatments.
• Oxide can be reduced by heating in H2, NH3, or CO
  during annealing. Addition chlorine or fluorine
  gas is effective in removing surface films.
• ultrasonic treatments.

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PARTICLE PACKING MODIFICATIONS

• Packing Structures
• Particle packing is important in most forming
  processes. It dictates
• Die fill
• Binder content
• Shrinkage in sintering.

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PARTICLE PACKING MODIFICATIONS

• Common P/M powders the packing density ranges
  from 30-65 of theoretical.

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Particle Packing Modifications

• As the particle shape becomes more rounded
  (spherical) the packing density increases.
• The packing of fibers provides an illustration of
  a decreasing packing density as the particles
  have a larger L/D ratio.

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Packing Structures

• The key to improved packing rests with the
  particle size ratio.
• Small particles are selected to fit the
  interstices between large particles without
  forcing them apart (Figure 5.10).

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• The optimal composition in terms of the weight
  fraction of large particles depends on the amount
  of void space between large particles . Read
  textbook page 169.

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Mixing and Blending

• They both combine powders into a homogeneous
  mass.
• What is the difference between blending and
  mixing ?

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• E.g., to form a bronze component, the pressing
  can be made from a prealloyed powders or from
  mixed elemental powders of copper (Cu) and tin
  (Sn).
• What are the advantages and the disadvantages of
  those two starting materials?

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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 powder-binder mixture for shaping.

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The variables involved in blending or mixing
powders

• Material
• Particle sizes
• Mixer type
• Mizer Size
• Relative Powder Volume in the Mixer
• Speed of mixing
• Shear
• Mixing time

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Some simple rules to reduce the problems

• Reblend a dry powder after transport.
• Do not vibrate a dry powder.
• Do not feed a dry powder through a free-fall
  where sizes can segregate.
• Minimise unnecessary shear for a powder-binder
  mixture.

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MIXTURE HOMOGENEITY
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POWDER LUBRICATION
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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 lt Tm by solid-state atomic transport
  (solid-state sintering)
• Involves formation of a liquid phase
  (liquid-state 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