Powder Methods of Change of Form

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Powder Methods of Change of Form

• Chapter 10

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Introduction To Powder Metallurgy

• Powder metallurgy is similar to casting in that a
  shape can be created by the use of a mold.
• The powdered solid material can be made to flow
  in much the same way as a liquid.
• Powder metallurgy is versatile and widely used in
  industry.

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Powder Metallurgy/Powder Forming

• Involves the compaction of finely powdered solids
  into a mold and then fusing the particles
  together.
• Can be formed into a large variety of shapes to
  meet industry needs.

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5 Processes To Powder Metallurgy

1. Preparation of the powdered material
2. Blending of the powdered materials
3. Filling the die with the powder and compaction
4. Sintering (the process of bonding particles)
5. Postsintering treatment to produce the final
   dimensions and properties

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Preparation and Blending

• Many types of metals and materials may be used
• Preparation of powder depends on physical
  properties of material and type of final product

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Brittle materials can be crushed to powder
Ceramics Glasses Bismuth
Titanium Antimony Carbide
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Other methods for making powder include

• Grinding
• Plating it onto temporary surface and scraping
  off
• Melting and atomizing the metal
• Chemical reduction from an oxide
• Chemical precipitation from solution
• Solidification from the vapor stage

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Making Powder

• Can be pulverized in a ball mill (10-1)
• Ground under water spray in inert gas
• Electroplating from a solution
• Melted and sprayed into inert gas
• Chemical precipitation (rust)
• Powder may need to be annealed to change physical
  properties

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Dry Lubricants

• Powders are often blended with dry lubricants for
  even compaction
• Stearic acid
• Lithium stearate
• Graphite
• Flammable metal powder must be mixed wet

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Compaction

• Die for part is filled with prepared powder
• Then compressed from both bottom and top to
  prevent compaction at pressure points
• Process is called briquetting (10-2)
• Amount of pressure controls porosity and size of
  final product
• Part formed is called briquette or green compact

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Compaction

• Powder can be compacted between two rollers
  (10-3)
• Forced through mild steel tube (extruding)(10-4)
• Explosive compacting (10-5)
• Compaction is economically feasible

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Sintering

• Material is not melted
• Material is compacted under great pressure and
  then heated to 60-80 of melting point
• Sintering is the process of fusing the individual
  faces of each particle atomically together (10-6)
• Forms a true weld

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Postsintering

• Three of the most common operations of
  postsintering are
• Infiltration
• Impregnation
• Sizing

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Postsintering Infiltration

• Powder formed parts can be porous
• Other material can be placed in voids to enhance
  the properties of the product
• Iron based powder form voids can be filled with
  bronze or copper
• Increases the strength 70 to 100
• Absorbed by capillary action
• Also increases the density and hardness of part

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PostsinteringImpregnation

• Most common postsintering process
• Used for bushings, bearings, cams, gears, pump
  parts and many parts with wear surfaces
• Permanently lubricates parts
• Oil fills the voids or pores of parts
• Can be impregnated under pressure, vacuum or
  merely soaking

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Sizing

• Powder formed and sintered parts can be sized by
  machining, grinding and other methods
• Another method is to force part through a die or
  have a mandrel forced through part
• Metal is not removed by this method but material
  is further compacted
• Sintered parts require less work to meet
  tolerances

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Applications

• The process of powder forming and powder
  metallurgy is increasing in popularity
• Has a vast area of uses in industry
• Processes continue to improve and grow in
  application possibilities

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Products Made by Powder Forming

• Wide variety of permanently lubed parts
• Used in many kitchen appliances, video recorders,
  speedometer cables
• Brushes on alternators, brake pads
• Powerful magnets, abrasive metals, dies
• Cutting tools end mills, thread taps, carbide
  inserts
• Metallurgy with coarse grain can be used as
  filters, fuel elements for nuclear reactors, also
  being extensively used in superconductor
  technology

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New Technology

• High velocity oxygen fuel (HVOF)
• Uses an internal combustion jet to generate
  supersonic gas velocities
• Molten powder sprayed at 67mph
• Surfaces of parts sprayed with considerable
  impact to form a hard coat

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Spray Forming

• Molten powder is sprayed on to build up shape
• Wide variety of materials can be used

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Conclusion

• Powder metallurgy refers to those change-of-form
  processes in which the material is made to flow
  by powdering it. Powder metallurgy allows parts
  to be made from materials that have very high
  strengths, high melting temperatures, high
  hardness, or are very brittle.