Microstructure—mechanical property relationships in a 356 alloy. Part I: Microstructure

Abstract

The effects of chemical modification and/or heat treatment of 356 alloy have been investigated on both gravity-diecast (permanent mould) and sand cast samples. Sodium (0.01%) and strontium (0.02%) were used to produce well modified microstructures. Undermodification and overmodification were also examined in castings with 0.002% Sr and 0.08% Sr. The use of image analysis coupled with SEM allowed a complete assessment of the microstructural changes occurring in the alloy. Modification changes both the size and the shape of the Si particles. The casting process (solidification rate) affects the size of the eutectic silicon more than the shape. The as-cast microstructure has an influence on the response of 356 alloy to heat treatment. The coarser the initial as-cast microstructure, the larger the microstructural differences between heat treated unmodified and modified alloys. As a result, the benefits of modification are much more important in sand casting than in gravity diecasting.