Effect of processing variables on structure and tensile properties of investment cast Al–Si–Mg casting alloy

Abstract

A research programme was conducted to study the effects of grain refinement, eutectic silicon modification, filtering, pouring and shell preheat temperatures, and heat treatment on the structure and tensile properties of an investment cast Al–Si–Mg alloy, LM25 (BS 1490 : 1988). The principal findings of the research were that: an increase in shell preheat temperature adversely affects the structure and, hence, the tensile properties; grain refinement was enhanced as the titanium content was increased to about 0·28% but the tensile properties were not affected; a modified eutectic silicon structure was achieved with strontium additions in the range 0·01–0·02%, with the optimum addition, based on tensile properties, being 0·01%; and, as would be expected, heat treatment improved the tensile properties. On the basis of the interrelationships between process variables, structural changes, and tensile properties observed, an optimum processing route was identified. The optimum tensile properties were obtained in fully heat treated specimens that had been both grain refined and modified and produced in moulds poured at ambient temperature.