Effect of melt superheating on convection-free precipitation and sedimentation of primary ?-Fe phase in liquid Al-11.5Si-0.4Mg alloy

Abstract

The effect of melts superheated at 760, 850 and 900 °C on the precipitation and sedimentation of primary α-Fe phase has been investigated at 600 °C for 4 hours in convection-free conditions. Compared with normal melting at 760 °C, an intermediate superheating temperature of 850 °C may facilitate the nucleation of primary °-Fe phase and/or hinder its growth, causing clear increases in precipitated particle weight and number, and a decrease in particle size. No significant variations in the precipitated weight, number and size of primary α-Fe particles were found at a superheating temperature of 900 °C. The profound influence of superheating temperatures on the crystallisation behaviour of primary α-Fe particles might be related to the transformation of oxides from α or η- to α-Al₂O₃. The possibility of α-Fe phase (primary or script type) being nucleated on oxide films was assessed by planar disregistry. Good matching planes were found between the α-Fe phase and main oxide films in cast Al-Si-Mg alloys. It seems that α- or η-Al₂O₃, α-Al₂O₃, MgO.Al₂O₃ and MgO films could all be good substrates for the nucleation and growth of α-Fe phase. After sedimentation processing at 600 °C, approximately 50% Fe and 70% Mn has been removed from the nominal Al-11.5Si-0.4Mg-1.2Fe-1.1Mn alloy.