Formation of spherical primary silicon crystals during semi-solid processing of hypereutectic Al-15.5wt%Si alloy

Abstract

Semi-solid state processing is considered as an attractive processing technique because of it offers several potential advantages such as energy saving for forming, reduction of microsegregation and porosity, especially for near net shape manufacturing of components. This technique is frequently applied to make sound casting products of hypoeutectic Al-Si alloys. On the other hand, hypereutectic Al-Si alloys have limited applications because of their inherent problems associated with poor machinability and mechanical properties, although the commercial potential for their outstanding resistance to wear and corrosion, higher hardness and low coefficient of thermal expansion is well recognized. To exploit the full potential of hypereutectic Al-Si alloys, semi-solid processing is utilized in an attempt to improve the mechanical properties through the spheroidization of primary Si crystals. The purpose of the present work is to provide the experimental evidence of the fragmentation of primary Si crystals in hypereutectic Al-15.5wt%Si alloy during semi-solid processing. Microstructural evolution during the isothermal shearing above the eutectic temperature of this alloy as a function of processing time is also reported.