The effects of magnesium additions on the structure and properties of Al-7 Si-10 SiC_p composites

Abstract

The additions of magnesium to an aluminium alloy matrix, which contains insufficient magnesium, was found to be essential during the synthesis of composites by the stir-casting technique. Magnesium promotes interfacial wetting between the dispersoid surface and the matrix. Dispersion of SiC_p in Al-7 Si-0.3 Mg (356) alloy matrix without agglomeration and rejection was not possible. Hence, the addition of up to 3 wt% Mg was made to the alloy matrix during the dispersion of 10 wt% SiC_p (34 Μm), and the microstructure and mechanical properties of the composites were investigated with a view to optimize the magnesium content. With a magnesium content less than 1 wt% in the matrix, the SiC_p particles were essentially in agglomerated form. The highest UTS of 280–300 MPa was obtained with 1 wt% Mg content and SiC_p was uniformly distributed in the matrix. A higher magnesium content (>1.0 wt%) did not further improve the uniformity in the dispersion of SiC_p but the ultimate tensile strength properties deteriorated. This decrease in strength was attributed to the observed coarseness of the Mg₂Si phase, the precipitation of Mg₅Al₈ phase and the presence of a higher amount of porosity in the composites in the heat-treated condition. The aspect ratio (length/width) of precipitates changed from 1–3 for 1% Mg to 3–9 for 3.2% Mg in the matrix. Corresponding values for per cent porosity were 2% and 6%, respectively.