Pot metal

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Pot metal refers to an alloy of inexpensive, low-melting point metals used to make fast, inexpensive castings for toys, tool parts, phonograph and gramophone components, and automotive parts and accessories. There is no scientific metallurgical standard for pot metal (which is a slang term), but it is also known as white metal, die-cast zinc and often derisively as monkey metal.

A typical zinc diecasting alloy, Zinc#3, has the following composition (manufactured by North American Die Casting):-

Aluminum 3.5-4.3%
Copper 0.25%
Magnesium 0.02-0.05%
Iron 0.100%
Lead 0.005%
Cadmium 0.004%
Tin 0.003%
Zinc ca. 95% (balance).

This alloy has an ultimate tensile strength of 40 ksi, a density of 0.24 lb/in3, and a melting range of 718-728 deg F. The manufacturers data sheet claims that it has relatively good "strength, hardness and creep properties".

Pot metal is known for its instability over time, as it has a tendency to bend, distort, crack, shatter, and pit with age. The low boiling point of zinc and the fast cooling of the newly-cast part often allow air bubbles and zinc oxide to remain within the cast part, weakening the metal. Many of the components of ‘pot metal’ are susceptible to corrosion from airborne acids and other contaminants, and the internal corrosion of the metal often causes the decorative plating to flake off.

The primary component of pot metal is zinc, but often the caster adds other metals to the mix in an effort to strengthen the part, improve the flow of the molten metal, or to reduce cost. With a low melting point of 419° C (786° F), zinc is often alloyed with other metals including lead (melting point 327° C), tin (232° C), aluminum (610° C) and copper (1084° C). Alloying metals often can completely alter the individual attributes of a single metal. But zinc also has an extremely low boiling point of 907° C (1664° F), lower than the melting point of many of the metals with which it is alloyed. The boiling zinc produces zinc oxide which is often responsible for the porosity of the die cast part.

Because of the internal degradation of the metals within the alloy, gluing is often not satisfactory due to lack of strength, and the low melting point precludes traditional welding or brazing. Low-temperature soldering has not proved satisfactory because of the incompatibility of flux to zinc and the inability to prevent oxidation, which inhibits the solder's ability to flow and bond.