Machine Design


Zinc, a crystalline metal with moderate strength and ductility, is seldom used alone except as a coating. In addition to its metal and alloy forms, zinc also extends the life of other materials such as steel (by hot dipping or electrogalvanizing), rubber and plastics (as an aging inhibitor), and wood (in paints). Zinc is also used to make brass, bronze, and die-casting alloys in plate, strip, and coil; foundry alloys; superplastic zinc; and activators and stabilizers for plastics.

Casting alloys: Zinc-casting alloys can be grouped into two general categories: standard zinc die-casting alloys, and the newer ZA (zinc-aluminum) casting alloys.

Standard die-casting alloys: For pressure die casting, the established zinc alloys are the No. 3, 5, and 7 Zamak alloys. As die castings, they have good general-purpose tensile properties and can be cast in thin sections and with good dimensional accuracy. The alloys are often selected for plated or highly decorative applications because of their excellent finishing characteristics. Three major end-use areas for zinc die-cast components are automotive, building hardware, and electrical.

Zamak alloys contain approximately 4% aluminum with low percentages of magnesium, copper, and sometimes nickel. Impurities such as tin, lead, and cadmium are carefully controlled. These alloys are not recommended for gravity casting. They are cast by the hot-chamber die-casting process, which is different from, and more efficient than, the cold-chamber die-casting process commonly used for aluminum. In addition, a specialized process is used for efficient production of miniature die-cast components, using these alloys as well as ZA-8.

Typical tolerances of zinc die-cast parts are ±0.0015 in./in. for the first inch with an additional ±0.002 in./in. for larger parts. New zinc-casting technology allows for thin walls down to 0.025 in., improved internal soundness, and surface finishes that range typically from 32 to 64 rms.

Part dimensions change slightly when zinc die castings are aged. Zamak alloys No. 3 and 7 can shrink about 0.0007 in./in. after several weeks at room temperature. Alloy No. 5 responds similarly, but total shrinkage can be 0.0009 to 0.0024 in./in., followed by expansion of 0.0020 in./in. over a period of years. When it is necessary to bring these changes to completion within a short time after casting, a stabilizing treatment of 3 to 6 hr at 212°F is recommended.

ZA casting alloys: Designated as ZA-8, ZA-12, and ZA-27 (the numerical suffix represents the approximate percent by weight of aluminum), the high-aluminum alloys differ radically from the standard Zamak alloys in composition, properties, and castability. Although the ZA alloys were first introduced for gravity casting (sand and permanent mold), they have expanded into pressure die castings as well as the new, precision graphite-mold process. Important: Alloys ZA-12 and ZA-27 must be cold-chamber die cast; alloy ZA-8 is hot-chamber castable.

Gravity casting into low-cost graphite permanent molds provides high-quality ZA castings with excellent precision, eliminating much machining. It is particularly competitive for production quantities of 500 to 15,000 parts/year, where die casting or plastic injection molding would be prohibitive because of tooling costs.

ZA alloys combine high strength and hardness (up to 60,000 psi and 120 Bhn), good machinability with good bearing properties, and wear resistance often superior to standard bronze alloys. ZA castings are now competing with cast iron, bronze, and aluminum because of various property and processing advantages.

Of the three alloys, ZA-12 is preferred for most applications, and particularly for gravity casting. However, ZA-27 offers the highest mechanical properties regardless of casting method. Both are excellent bearing materials. ZA-8, on the other hand, gives the best plating characteristics. Because of its hot-chamber die castability and high mechanical properties, ZA-8 is also used for high-performance applications where standard zinc alloys may be marginal. All ZA alloys offer superior creep resistance and performance at elevated temperatures compared to the Zamak alloys.

Wrought alloys: Wrought-zinc alloys are available in rolled sheet, strip, foil, and as drawn rod or wire. With controlled rolling, zinc alloys can be tailored to meet a wide range of hardness, luster, and ductility requirements. Rolled zinc can be worked by common fabricating methods, and then polished, lacquered, painted, or plated.

When zinc alloys are formed in progressive presses, as in battery-shell manufacture, they are self annealing. After successive forming operations in nonprogressive presses, however, the alloys work harden and break. This can be overcome, in copper-free alloys, by intermediate annealing for 5 min in boiling water to which 20% glycerine has been added. Copper-bearing alloys should be heated 5 to 10 min at 350°F. The copper and titanium-containing alloy should be held at 390°F for about 15 min to bring about crystallization. Excessive exposure to higher temperatures should be avoided, however, to prevent grain growth, cleavage cracks, and property deterioration.

Highly workable and highly forgeable wrought-zinc alloys containing Ti, Al, Pb, Cd, Cu, or Fe in various quantities are easily machined. Forged or extruded parts are free from porosity and have good detail.

TAGS: Materials
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