OVERVIEW

Aluminium is one of the most versatile of the common foundry metals and the ratio of cast to wrought aluminium alloy products is increasing primarily because of the larger amounts of castings being used for automotive applications. This ratio varies from country to country and in 2004 it was approximately 1:2 in North America. A wide range of cast aluminium alloys is available for commercial use and, for example, nearly 300 compositions were registered with the United States Aluminium Association in 2005. The most widely used are those based on the Al Si, Al Si Mg and Al Si Cu systems. In general, alloys are classed as primary if prepared from new metals and secondary if recycled materials are used. Secondary alloys usually contain more undesirable impurity elements that complicate their metallurgy and often lead to properties inferior to those of the equivalent primary alloys.

The most commonly used processes are sand casting, permanent mould (gravity die) casting, cold chamber and hot chamber pressure die casting. Sand moulds are fed with molten metal by gravity. The metal moulds used in permanent mould casting are fed either by gravity or by using low-pressure air or other gas to force the metal up the sprue and into the mould. In pressure diecasting, molten aluminium is forced into a steel die through a narrow orifice (or gate) at high speeds ranging from 20 to 100m/s (Fig. 5.7). This is achieved by means of a piston and cylinder (or hot sleeve) where the piston is driven by a...