Powder Metallurgy

• Powder Metallurgy


  Highly developed method of manufacturing precision metal parts. Basically a "chip-less" process, PM uses roughly 97% of the starting material in the finished part and is made by mixing elemental or alloy powders then compacting the mixture in a die.
• Powder Metallurgy


  • The vast majority of PM application is the consolidation of powder onto dense parts and shapes and 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 in Powder Metallurgy

  • Powder Production
  • Mixing
  • Forming/Compaction
  • Sintering
  • Optional Operations
  • Finished Products
• Methods of Powder Production

  • Atomization (gas, water, rotating consumable electrode, centrifugal with a spinning disk or cup)
  • Reduction
  • Electrolytic deposition
  • Carbonyls
  • Mechanical Comminution
• Mixing


  The second step in powder metallurgy and has the following purposes: 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 in Blending Metal Powders

  Since metal powders are abrasive, mixers rely on the rotation or tumbling of enclosed geometries as opposed to using aggressive agitators
• Compaction


  This is the 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.
• Green compact


  Compacting the loose powder produces a "green compact" which, with conventional pressing techniques, has the size and shape of the finished part when ejected from the press. Green compacts have sufficient strength for in-process handling.
• Density of the green compact

  • Depends on: 1) the pressure applied (high compacting pressure, density approaches the bulk metal form), 2) Size of particles (same size will always result to porosity), and 3) Friction between the particles and the die walls and punches
• Compaction Cycle


  1. Cycle start

  2. Charge die w/powder

  3. Compaction begins

  4. Compaction complete

  5. Ejection of compact

  6. Recharging of die
• 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. Avoid sharp edges, thin sections, thickness variations, high length-to-diameter ratios, and sharp changes in contour.
  • 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.
• Reduction. The use of gas, such as hydrogen and carbon monoxide, to reduce metal oxide to its metallic state.
• Electrolytic deposition. This utilizes either aqueous solution or fused salts. Powders produced are among the purest
• Carbonyls. Letting iron or nickel react with carbon monoxide to form iron and nickel carbonyls.
• Mechanical Comminution. This involves crushing, milling in a ball mill or grinding brittle metals into small particles.
• The higher the density, the higher the strength and elastic modulus of the part since higher metal amounts in the volume.
• 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
• It is necessary to use multiple punches with separate movements in order to ensure that the density is more nearly uniform throughout the part