Introduction to Powder Metallurgy

Introduction to Powder Metallurgy

• Powder metallurgy (PM) transforms metallic powders into parts through compression and heating.
• Compression, or pressing, shapes the powders using specialized tool designs, often resulting in high tooling costs.
• Sintering occurs at temperatures below the metal's melting point, bonding the particles into a solid mass.

Advantages of Powder Metallurgy

• Enables mass production of parts to net shape or near net shape, minimizing additional processing.
• High material efficiency: Approximately 97% of starting powders convert to final product.
• Produces parts with controlled porosity for applications like filters and oil-impregnated bearings.
• Capable of fabricating hard-to-shape metals like tungsten, used in incandescent bulbs.
• Effective for unique metal alloy combinations and cermets that other methods cannot produce.
• Dominates casting processes in terms of dimensional accuracy, with tolerances routinely at 0.13 mm (0.005 in).
• PM processes can be automated, enhancing production economics.

Disadvantages and Limitations of Powder Metallurgy

• High initial costs for tooling and equipment.
• Metallic powders can be expensive and pose storage and handling issues.
• Metal powders may degrade over time, and some have fire hazards.
• Design limitations exist due to poor lateral flow of powders in the die, requiring ejector allowances.
• Challenges with material density uniformity, especially in complex geometries.
• Most PM components typically weigh less than 2.2 kg (5 lb).

Characterization of Engineering Powders

• Powders can be categorized by:
  • Particle size and distribution
  • Shape and internal structure
  • Surface area
  • Interparticle friction and flow characteristics
  • Packing, density, and porosity
  • Chemistry and surface films

Production Methods for Metallic Powders

• Methods for creating metallic powders involve energy input to increase the surface area.
• Principal methods include:
  • Atomization
  • Chemical
  • Electrolytic
• Mechanical methods are usually linked with ceramic powder production.

Atomization Process

• Atomization converts molten metal into droplets that solidify as powders, widely used for all metals.
• Gas atomization employs a high-velocity gas stream to atomize molten metal through a nozzle.
• Gravity-assisted methods atomize molten metal with air jets, creating spherical powders.
• Water atomization, favored for metals melting below 1600°C, leads to irregular powder shapes due to rapid cooling but risks oxidation.
• Synthetic oil is a recent alternative to water in atomization to mitigate oxidation.
• Particle size in atomization processes varies inversely with fluid stream velocity.