ZINC ALLOY INGOT

A predecessor of the more widely used ZA 2, it has the highest strength and hardness in the Zinc – 4% aluminium die casting alloy family. Because of its relatively high copper content, it is about 25% stronger as cast than ZA 3, and almost 10% stronger than ZA 5, with higher hardness than either. The high copper content, however necessitates segregation of trim scrap.

After aging, the alloy retains tensile strength better than the other alloys in the group and becomes more dimensionally stable, but it gives up impact strength and loses hardness. Because of these limitations, ZA 2 is used only when the strength or hardness of ZA 3 or ZA 5 are not sufficient for the end use. This alloy is recommended only where casting design cannot provide enough strength with ZA 5 or where increased hardness is required.

Of all the Zinc casting alloys, ZA 3 is the most widely used, accounting for some 85% of all zinc casting tonnage. Its excellent physical and mechanical properties, excellent castibility and long-term dimensional stability provide the basis for its broad usage. The ease with which it can be electroplated adds to the popularity of this alloy. Where properties comparable to or better than those of aluminum alloys are required, Alloy ZA-8, ZA-12, or ZA-27 should be considered.

Most often, through casting design procedures, an ZA 3 pressure die-casting can be made to meet service or functional requirements. When this is not the case, especially where strength is concerned, ZA 5 is the logical choice. Except for a nominal 1% copper addition the chemistry of ZA 5 is comparable to that of ZA 3. This composition modification results in higher tensile strength and increased hardness, with a sacrifice in elongation. ZA 5 also has significantly better creep resistance than the other alloys in the conventional group.

The third most popular alloy is a high purity, low magnesium alloy called No. 7. This alloy has improved fluidity (because of the lower magnesium content) allowing it to be cast at lower temperatures for better as-cast surface finish and higher production rates. ZA 7 is mostly used in decorative hardware applications. Since it possesses slightly more ductility than other zinc alloys. No. 7 is also used where castings require severe deformation in assembly operations.

Of all the zinc alloys that can be cast in hot-chamber machines, ZA-8 is the most creep resistant (three times that of ZA 3) and is the strongest and hardest. It is used principally in structural or highly stressed applications. In spite of its relatively high aluminium content, it can be electroplated using conventional 0′ plating techniques.

Excellent castibility in cold-chamber die casting machines, coupled with outstanding properties, including low density and superior creep strength, makes ZA-12 a high performance material that is suitable for use with all casting processes. Its superior wear properties make it possible, in some situations, to eliminate costly inserts in die-castings which otherwise are required to provide bearing or wear surfaces.

The alloy can be electroplated. Intensification (applying a high pressure to overcome sluggish feeding behavior after the die is filled) can be beneficial, but is not considered necessary.

The strongest, hardest and lightest of all the zinc pressure die casting alloys, ZA-27 also has the best wear properties and is among the most creep resistant. However, it is the most difficult to electroplate. Sand cast parts can be heat-treated to increase ductility. Because of its higher melting range compared with ZA-8 and ZA-12. It requires longer cycle times. As with ZA-12, ZA-27, is specified for high-performance applications when conventional zinc or aluminium alloys are inadequate. In many cases, both of these ZA alloys may be substituted directly for aluminium using existing tooling. They also can be continuously and centrifugally cast for bearings and machine parts.