The Modification of Al-Si Casting Alloys: Important Practical and Theoretical Aspects

Abstract

A review is given of the modification of aluminum-silicon casting alloys. The mechanisms involved and important practical aspects are described. While a number of IA and IIA elements and several lanthanides produce a modified eutectic, only strontium and sodium have found significant commercial application. They react with and remove aluminum phosphide (AlP), an effective nucleant for silicon, and produce a fine fibrous eutectic. The resulting as-cast structure has improved ductility. In heat treated castings it is possible to reduce the solution time when a modifier is employed. The result is a significant cost saving—usually ten or more times the cost of the modifier addition. Modification may change the relative formation of porosity and shrinkage in a casting. This subject has been the subject of a great deal of controversy and confusion in the past, but recent research in Australia has clarified the mechanisms involved. This work is summarized and practical guidelines are offered for the foundryman. The modifiers strontium and sodium are poisoned by phosphorus, antimony and bismuth. Consequently, the levels of these impurities should be monitored carefully in secondary alloys.