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

The effect of melts superheated at 760, 850 and 900degreesC on the precipitation and sedimentation of primary alpha-Fe phase has been investigated at 600degreesC for 4 hours in convection-free conditions. Compared with normal melting at 760degreesC, an intermediate superheating temperature of 850degreesC may facilitate the nucleation of primary a-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 alpha-Fe particles were found at a superheating temperature of 900degreesC. The profound influence of superheating temperatures on the crystallisation behaviour of primary a-Fe particles might be related to the transformation of oxides from gamma- or eta- to alpha-Al2O3. The possibility of a-Fe phase (primary or script type) being nucleated on oxide films was assessed by planar disregistry. Good matching planes were found between the a-Fe phase and main oxide films in cast Al-Si-Mg alloys. It seems that gamma- or eta-Al2O3, alpha-Al2O3, MgO.Al2O3 and MgO films could all be good substrates for the nucleation and growth of a-Fe phase. After sedimentation processing at 600degreesC, approximately 50% Fe and 70% Mn has been removed from the nominal Al-11.5Si-0.4Mg1.2Fe-1.1Mn alloy.