Impact of Section Thickness on the Microstructure and Mechanical Properties of Semi-Solid Castings

Slower cooling and solidification rates tend to increase both the grain size and dendrite arm spacings (DAS) of conventional castings. Both grain size and DAS impact strength and ductility, and so heavy section castings generally have inferior mechanical properties than thin-walled castings. However, the primary phase of semi-solid castings is not produced during solidification in the mold, but instead is developed in the feed material. Therefore, the size of the primary phase is essentially independent of the solidification rate, and so the mechanical properties of semi-solid castings should be less sensitive to the wall thickness of the casting. This paper describes the results of a study to examine the impact of wall thickness on the microstructure, hardness and mechanical properties of semi-solid castings. Castings were produced at three thicknesses — 7.5 mm, 15 mm and 50 mm. Results are compared to literature data for conventional fully-liquid castings produced at similar section thicknesses.