Microstructure-mechanical property relationships in a 356 alloy. Part II: Mechanical properties

Abstract

The effect of chemical modification by addition of strontium or sodium on the heat treatment response of 356 alloy has been investigated on both gravity-diecast (permanent mould) and sand cast samples. Solution heat treatment time and artificial aging time were the two variables examined. The mechanical property improvement was evaluated through tensile properties, and the influence of the modifier amount was also studied. The beneficial effects of modification are much more important in castings which have coarse structures in the unmodified state, such as in sand casting. Tensile property results were analysed using a rating system which helps in the evaluation of the five alloys (with 0%, 0.002% Sr, 0.02% Sr, 0.08% Sr and 0.01% Na). Porosity caused by modification can negate many of the microstructural benefits by decreasing tensile strength and percent elongation. The possibility of using HIP treatment to eliminate these flaws was investigated. Both the healing of the voids and Si spheroidisation due to the thermal treatment of HIP process increase the ductility of the alloy. Microstructure-mechanical property relationships have been established.