Introduction to Powder Metallurgy

Study Notes

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.

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.

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).

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 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.