Advanced Materials in Powder Metallurgy
A wide variety of specialty material systems are manufactured using advanced processes such as hot and cold isostatic pressing, metal injection molding, and spray forming. In addition, certain alloy systems are produced nearly exclusively using powder metallurgy (PM) techniques.
The use of superalloys is most common in the production of near-net shapes and forging preforms for aircraft turbine engines. Economic benefits have been the prime driving force in the use of PM for the manufacture of these high-cost alloys.
These superalloys are processed from highly controlled, metallurgically clean powders produced by inert-gas atomization methods that minimize surface oxidation during processing. After powder screening and cleaning, the powder is compacted using hot isostatic processing. Thermochemical processing may be performed to enhance mechanical properties or microstructure. Hot extrusion of the atomized powder is an alternate consolidation method.
Powder metallurgy processing offers the advantages of a homogeneous microstructure and near-net-shape configuration for a lower cost. As a result, over 10,000,000 pounds of PM superalloy components are now flying in both military and commercial aircraft engines.
High-performance tool steels and high-speed steels are produced in mill shapes using PM process techniques.
Powder metallurgy tool steels offer these advantages:
- Finer grain structures
- Improved homogeneity and distribution of secondary and carbide phases
- Elimination of secondary stringers
- Less distortion in heat treatment
- Improved grindability
- Greater wear resistance
- Improved toughness and fracture strength
Hot isostatic pressing (HIP) has also been used in the production of PM tool steels to near-net tool shapes, such as hobs and shaper cutters. Tool steel parts can also be made by conventional cold compacting combined with high-temperature sintering.
Refractory metals, alloys, and composites are produced using non-fusion powder metallurgy techniques.
- tungsten lamp filaments,
- tantalum capacitors,
- molybdenum heating elements,
- tungsten-copper composite heat sinks, and
- tungsten-silver composite circuit breaker contacts.
The tungsten-based “heavy alloys” (tungsten plus liquid-phase sintering aids) are used for radiation absorption, gyroscope weights, and kinetic energy penetrators.
The PM processing of beryllium starts with cold isostatic pressing (CIP) followed by hot pressing or a hot forging or extrusion step. Sometimes hot isostatic pressing is substituted for the hot pressing step. Due to its light weight, beryllium is used mainly for aircraft and aerospace products.
The use of PM titanium metal and its alloys continues to increase since PM provides a near-net-shape product as compared to more expensive cast, forged, and machined processing. Depending on the application and production quantities, titanium PM products are processed either by conventional press-and-sinter techniques or hot isostatic pressing. The latter exceeds the minimum wrought alloy specifications. Product applications include aerospace/aircraft components, sporting goods, chemical processing equipment, and biomedical systems.
Metal Matrix Composites
Powder metal processing has been successfully applied to variety of metal matrix composites, the most popular being aluminum-silicon carbide particulate reinforced materials. These composites offer high stiffness (modulus), reduced thermal coefficient of expansion with little effect on thermal conductivity or density. These Al-SiC composites are being used for their structural stiffness and strength in aircraft and sporting goods products, as well as in electronic thermal management applications such as heat sinks. Manufacturing techniques require cold isostatic pressing (CIP) to consolidate the powders, followed by hot extrusion or hot forging to full density.