The effect of processing on the mechanical and fatigue properties of semi-solid formed A357 aluminum.

Abstract

The fundamental relationship between semi-solid processing and microstructure and their effect on the flow characteristics of semisolid metals have been studied for several years. However, how the process related microstructure influences mechanical and fatigue properties has not been given the same attention. This study examines the influence of process-related microstructures on the mechanical and fatigue properties of semi-solid formed A357 alloys. Low solid fraction (<40% solid) and high solid fraction (>50% solid) semi-solid A357 aluminum were formed by two different processes, rheocasting and thixocasting. Solid fraction, globule size, globule shape factor, globule density, and the eutectic particle size and aspect ratio after T6 heat treatment were evaluated to determine their effect on the as-cast, T5, and T6 properties. The mechanical properties of low solid fraction (LSF) and high solid fraction (HSF) semi-solid formed A357 vary considerably with solid fraction, microstructure, chemistry, and heat treatment. In spite of these differences, common traits were identified that influence the mechanical properties, regardless of the process or the heat treatment condition. Increasing globule size, porosity, and iron content have a detrimental effect on strength and ductility in the as-cast, T5, and T6 conditions. Low solid fraction semi-solid formed A357 alloys apparently have lower strength in the as-cast and T5 conditions than high solid fraction semi-solid formed A357 alloys. This is attributed to the higher processing temperature and its adverse affect on the solid solubility of magnesium in the primary alpha-aluminum globules. Fatigue life was found to be a function of material strength, increasing with increasing ultimate tensile strength. Extrinsic fatigue initiation features, such as pores, were found to reduce the axial fatigue life by 25% or more, as compared to fatigue initiation features associated with the microstructure. Linear elastic fracture mechanics (LEFM) was used to quantitatively show that the fatigue crack growth rates are relatively insensitive to microstructure and heat treatment.