Die Casting Alloys
Die casting alloys are normally non-ferrous, and there is a large number available with a wide range of physical and mechanical properties covering almost every conceivable application a designer might require.
Aluminum and zinc alloys are the most widely used, and are followed by magnesium, zinc-aluminum (ZA) alloys, copper, tin and lead.
Zinc, lead and tin based alloys are classified as low melting point metals, all melting at less than 725°F (385°C). Zinc-aluminum (ZA) alloys have a slightly higher melting range of 800°F to 900°F (426°C to 482°C). Aluminum and magnesium alloys are considered to be moderate melting point alloys, being cast in the 1150°F to 1300°F (621°C to 704°C) range. Copper alloys are considered to be high melting point alloys, over 1650°F (899°C). Low melting point alloys are cast in hot chamber machines. Intermediate and high melting point alloys are cast in cold chamber machines. In recent years, specially designed hot chamber machines for die casting magnesium alloys have come into use.
Aluminum die casting alloys (Table 1) are lightweight, offer good corrosion resistance, ease of casting, good mechanical properties and dimensional stability.
Although a variety of aluminum alloys made from primary or recycled metal can be die cast, most designers select standard alloys listed below:
360 -- Selected for best corrosion resistance. Special alloys for special applications are available, but their use usually entails significant cost premiums.
380 -- An alloy which provides the best combination of utility and cost.
383 & 384 -- These alloys are a modification of 380. Both provide better die filling, but with a moderate sacrifice in mechanical properties, such as toughness.
390 -- Selected for special applications where high strength, fluidity and wear-resistance/bearing properties are required.
413 (A13) -- Used for maximum pressure tightness and fluidity.
COMPOSITION AND PROPERTIES OF ALUMINUM DIE CASTING ALLOYS
Zinc-based alloys (Table 2) are the easiest to die cast. Ductility is high and impact strength is excellent, making these alloys suitable for a wide range of products. Zinc alloys can be cast with thin walls and excellent surface smoothness making preparation for plating and painting relatively easy.
It is essential that only high purity (99.99 + 0/0) zinc metal be used in the formulation of alloys. Low limits on lead, tin and cadmium ensure the long-term integrity of the alloyís strength and dimensional stability.
COMPOSITION AND PROPERTIES OF ZINC DIE CASTING ALLOYS
ZA alloys represent a new family of zinc-based die casting materials which contain higher aluminum content than standard zinc alloys. These alloys provide high strength characteristics plus high hardness and good bearing properties (Table 2). Thin wall castability characteristics and die life are similar to zinc alloys. ZA-8 is recommended for hot chamber die casting, whereas ZA-12 and ZA-27 must be cast by the cold chamber die casting process. All ZA alloys offer similar creep properties and are superior to standard zinc alloys.
ZA-8 -- Provides strength, hardness and creep properties.
ZA-12 -- Provides excellent bearing properties with strength and hardness characteristics between ZA-8 and ZA-27, plus good dimensional stability properties and somewhat better castability than ZA-27.
ZA-27 -- Offers the highest mechanical properties of the ZA family and is, therefore, recommended when maximum performance is required.
Magnesium alloys (Table 3) are noted for low weight, high strength to weight ratio, exceptional damping capacity, and ease of machining. Casting temperatures are about the same as aluminum, and both hot chamber and cold chamber machines are used to produce castings.
Casting rates for magnesium are high because of its low heat content which produces rapid solidification. For the same reason, less energy is required to heat the metal to casting temperature.
AZ91HP (high purity) alloy has been developed for die casting parts subject to corrosive environments. Because of lower levels of nickel, iron, copper and silicon versus AZ91D, this alloy is finding applications in automobiles, computers and peripheral equipment, and in other applications where paint or coatings are either undesirable or expensive.
Although magnesium die castings are used uncoated, they can be finished in a variety of ways to give increased protection against corrosion, wear and abrasion resistance, and to improve appearance. Common inorganic treatments include chemical dips, anodizing and plating. Organic coatings -- oil, wax, resin or paint -- are usually applied over chemical treatments or anodizing to seal the surface, increase corrosion protection and provide an attractive appearance.
COMPOSITION AND PROPERTIES OF MAGNESIUM ALLOY FOR DIE CASTING
Copper-based alloys (Table 4) are generally known as brass or bronze. They provide the highest mechanical properties of any of the normally die cast metals. Brasses have high strength and toughness, good wear resistance, and excellent corrosion resistance. Dimensional stability is excellent. Of the various brass casting alloys available, alloy Z30A is used for the majority of die casting applications.
COMPOSITION AND PROPERTIES OF COPPER ALLOY FOR DIE CASTING
Lead and tin alloys enjoy only minor use in die casting since their strengths are low. Lead die castings find applications where high density (wheel weights) or corrosion resistance (plumbing goods) is needed. Tin may be found in small parts where extremely close dimensional tolerances are needed, or where contact with food or certain chemicals may be encountered.