Zinc–aluminium alloys (ZA)

Composition and grades

The principal variable in ZA alloys is the aluminium percentage. Designations such as ZA8 indicate roughly 8% aluminium by weight; other common grades include ZA12 and ZA27, where increasing aluminium generally raises hardness and tensile strength. Minor amounts of other elements are sometimes present to refine grain structure and improve castability.

Properties and characteristics

• Good castability: ZA alloys fill molds readily and reproduce fine detail, making them suitable for die casting and gravity casting.
• Range of mechanical behavior: lower-aluminium grades are more ductile while higher-aluminium grades offer higher strength and wear resistance.
• Bearing performance: ZA alloys are used for plain bearings and wear surfaces because they can provide a conformable, load-bearing interface similar in service role to Babbitt metal, though their chemistry and failure modes differ.
• Thermal and corrosion limits: they perform well at ordinary service temperatures but are not suited for sustained high-temperature use; corrosion resistance varies with environment and mating metals.

Manufacture and treatment

ZA alloys are typically cast into shape by gravity die casting, pressure die casting or sand casting. Casting yields a fine microstructure that can be further modified by simple heat treatments or aging to improve strength and hardness. Their relatively low melting range compared with many ferrous alloys simplifies casting operations and tooling.

Applications and examples

Common uses include bearings, bushings, gears, pump components, valve bodies and decorative hardware. The combination of good dimensional accuracy and bearing-compatible surface behavior makes ZA alloys an economical choice where high-end bearing alloys are unnecessary. For information on typical engineering uses see sources about bearings and material selection guides (overview).

Comparisons and notable facts

Compared with tin-based bearing alloys such as Babbitt, ZA alloys are generally harder and less costly, but they are also stiffer and may behave differently under conforming or boundary-lubricated conditions. The naming convention that uses a number to indicate aluminium content is practical and widely used; for a discussion of this convention see percentage-based grade naming. ZA alloys thus occupy a useful niche between soft bearing metals and harder cast irons or bronzes.