THE EFFECTS OF MAGNESIUM ADDITIONS ON THE STRUCTURE AND PROPERTIES OFAL-7 SI-10 SICP COMPOSITES

The additions of magnesium to an aluminium alloy matrix, which contain s insufficient magnesium, was found to be essential during the synthes is of composites by the stir-casting technique. Magnesium promotes int erfacial wetting between the dispersoid surface and the matrix. Disper sion of SiCp in Al-7 Si-0.3 Mg (356) alloy matrix without agglomeratio n 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 % SiCp (34 mu m), and the microstructure and mechanical properties of the com posites were investigated with a view to optimize the magnesium conten t. With a magnesium content less than 1 wt % in the matrix, the SiCp p articles were essentially in agglomerated form. The highest UTS of 280 -300 MPa was obtained with 1 wt % Mg content and SiCp was uniformly di stributed in the matrix. A higher magnesium content (> 1.0 wt %) did n ot further improve the uniformity in the dispersion of SiCp but the ul timate tensile strength properties deteriorated. This decrease in stre ngth 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 r atio (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.