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May 3, 2008

Plating on Zinc Alloy Die Castings

Composition and Structure of Zinc Alloy Die Castings


Die casting is the most versatile method of fabricating complex metal shapes with close dimensional tolerances at a low cost. Because of their susceptibility to corrosion, zinc alloys often require a protective coating. Electroplated coatings are popular because they reliably provide good corrosion protection.

The most popular zinc alloy used for die casting is ASTM alloy made with special high grade zinc alloyed with about 4 % aluminum, 0.04% magnesium, and maximum of 0.25% copper. Impurities such as iron, lead, cadmium, and tin are controlled at levels below 0.1, 0.005, 0.004, and 0.003%, respectively, to inhibit intergranular corrosion during exposure to corrosive environments.

A similar alloy, which contains 0.75 to 1.25 % copper, is slightly stronger, but more costly. A newer alloy, contains 1.0 to 1.5 % copper, 0.25 to 0.3% aluminum, and 0.01 to 0.03% magnesium. This alloy resist creep, another alloy which is used for gravity casting, contains 11 to 13% aluminum, 0.5 to 1.25% copper, and 0.01 to 0.03% magnesium.

Faulty die design or casting techniques result in surface layer defects such as cresives (cold shut), skin blisters, hemispherical pores (gas holes), or cracks. Blistering that occurs during plating or soon after plating is the result of hydrogen gas generated by the reaction between zinc and corrosive solutions entrapped in pits and crevices. Heating electroplated casting accelerates blister formation traceable to the entrapment of corrosive chemicals and volatilizes any entrapped oil or grease. Casting are usually contaminated with parting compound applied at frequent intervals to die surfaces to facilitate the ejection of the casting from the die.

Blistering especially during after plating, has also been attributed to zinc corrosion products formed when castings are quenched in water emulsions of oils to lower their temperature quickly. The removal of these corrosion products partly engulfed in crevices or pores must be thorough.

Burrs, fins, and flash are frequently left at parting lines where the mating surfaces of two diw haves meet, even after castings are trimmed in a punch press. Close die trimming can reduce or even eliminate parting line smoothing. Porosity along the parting lines is usually proportional to the thickness of the flash.