IE 21: Powder Metallurgy

• Powder Metallurgy - highly developed method of manufacturing precision metal parts.

  • Basically a chip-less process since PM uses roughly 97% of the starting material in the finished part
  • Made by mixing elemental or alloy powders then compacting the mixture in a die
• The vast majority of PM application is the consolidation of powder onto dense parts and shapes.
• The resulting shape is sintered in an atmosphere-controlled furnace to convert mechanical bonds into metallurgical bonds.
• Repacking occurs with the elimination of particle bridges. With higher compaction pressures, particle deformation is the dominant mode of densification.
• Basic steps of PM

  1. Powder Production
  2. Mixing
  3. Forming/Compaction
  4. Sintering
  5. Optional Operations
  6. Finished Products
• Methods of metal-powder production

  1. Atomization
  2. Reduction - use of gas, such as hydrogen and carbon monoxide, to reduce metal oxide to its metallic state
  3. Electrolytic deposition – utilizes either aqueous solution or fused salts. Powders produced are among the purest.
  4. Carbonyls – letting iron or nickel react with carbon monoxide to form iron and nickel carbonyls
  5. Mechanical Comminution – involves crushing, milling in a ball mill or grinding brittle metals into small particles
• Mixing - the second step in powder metallurgy
• Purposes of Mixing:

  1. Obtain Uniformity in materials
  2. To impart special physical and mechanical properties to the PM product, and
  3. Lubricants can be mixed to improve flow characteristics (reduced friction between metal particles thus longer die life).

  Metal powders are explosive (particularly Al, Mg, Ti, Zr, Th). Precautions include grounding equipment, avoiding dust clouds, open flames, and chemical reactions; and preventing sparks.
• Bowl Geometries

  
• Compaction - step wherein the blended powders are pressed into shapes in dies. It usually occurs at room temperature, at a pressure range of 25-50 tons per sq. in
• Compacting the loose powder produces a green compact, which has the size and shape of the finished part
• Typical compaction techniques use rigid dies, set into mechanical or hydraulic presses.
• Density of the green compact depends on

  • the pressure applied (high compacting pressure, density approaches the bulk metal form),
  • Size of particles (same size will always result to porosity), and
  • Friction between the particles and the die walls and punches.
• The higher the density, the higher the strength and elastic modulus of the part

  • Since higher metal amount in the volume
• Compaction Cycle

  1. Cycle Start
  2. Charge die w/powder
  3. Compaction begins
  4. Compaction complete
  5. Ejection of compact
  6. Recharging of die
• Density variation can be minimized through proper punch and die design and by control of friction.
• It is necessary to use multiple punches with separate movements in order to ensure that the density is more nearly uniform throughout the part.
• Sintering - The process whereby green compacts are heated in a controlled-atmosphere furnace to a temperature within 70% to 90% of the melting point of metal or alloy.
• Sintering temperature must be sufficiently high to allow bonding of individual particles but not too high to melt the metals.
• Sintering:

  • Transforms compacted mechanical bonds between powder particles into metallurgical bonds.
  • Particles bind together
  • Part shrinks in size
  • Density increases, up to 95%
  • Strength ~ Density
• Sintering times range from a minimum of about 10 mins for iron and copper alloys to as much as 8 hours for W and Ta.
• Mechanical Properties:

  • Porosity cannot be avoided.
  • Voids remain after compaction
  • gases evolve during sintering
  • If the material density is 80% only of theoretical, pores are possibly interconnected.
  • Further heat treatment after sintering increases material strength.
• Design Considerations - Compact shape must be as simple and as uniform as possible.
• Design Considerations:

  • Provision must be made for ejecting the green compact from the die without damaging the compact.
  • P/M parts should be made with the widest dimensional tolerances consistent with their applications.
  • To increase tool and die life and reduce production costs.
• PM vs Casting:

  • Mass produce small steel parts, net-shape
  • Less waste
  • Unusual “alloys”
  • Range of densities
  • Less energy use
  • But: Smaller parts and less complexity