Synergy Effect of Bi Addition, Cooling Condition and Artificial Aging ‎on ‎Performance of Al-8Si-0.3Mg Cast Alloy‎

In this paper, synergy effect of various bismuth contents and cooling rates on performance of ‎Al-8Si-0.3Mg cast alloy after applying T6 heat treatment was evaluated. Cooling rate was ‎measured based on analyzing cooling curve and its second derivative curve. A step mould with ‎different casting thicknesses was designed to obtain four different cooling rates (0.55 to 6 ‎‎°C/s). Microstructures of sample were evaluated by optical microscopy and scanning electron ‎microscopy (SEM). Results showed that sample modified with Bi and solidified at higher ‎cooling rate required less time for annealing and spherodising of eutectic silicon. Moreover, ‎tensile test results indicated that the highest yield strength (142.2 MPa), ultimate tensile ‎strength (235 MPa) and elongation percentage (6.1%) obtained for the sample containing 0.5 ‎wt% Bi and solidified at the highest cooling rate (6 °C/s). Fracture surface of samples cooled at ‎low cooling rate comprised of cleavage indicating brittle fracture mode resulted in low ‎ductility. Likewise, fracture surface of samples solidified at highest cooling rate exhibits ‎dimples and ductile fracture mode. The counter graph of quality index used to predict the ‎quality of heat-treated samples with different levels of Bi and solidified at various cooling rates ‎showed that the highest index (352.8 MPa) obtained for specimen containing 0.5wt% Bi ‎solidified at 6 °C/s cooling rate.‎