Effect of Bi addition on microstructure and mechanical properties of hypereutectic Al-17.6Si alloy

Al-17.6Si-XBi (X = 0.0, 0.5, 1.0, 1.5 and 2.0 wt%) alloys have been synthesized using gravity casting technique to study the influence of Bi concentration on the microstructure and mechanical properties of hypereutectic Al-17.6Si alloy. The as-cast unmodified hypereutectic Al-17.6Si alloy consists of irregular polygonal and plate-like primary Si (Si P ) particles and irregular plate and rod-like shape, coarse eutectic Si (Si E ). The Bi addition modifies and refines the Si P particles into small pentagonal shape particle with blunt corners. Volume fraction, average equivalent diameter and aspect ratio of the Si P particle decrease with increase in Bi concentration in the alloy. The bulk hardness of the alloys and micro-hardness of Si P particles are increased with increase in Bi concentration as the Si P particles became compact and finer. The microstructure modification also has a significant effect on the mechanical properties of the alloy. The ultimate tensile strength (UTS), yield strength (YS) and elongation (%El) increase with the increase in Bi concentration in the alloy. Further, fractographs reveal that the brittle mode of fracture decreases and ductile fracture with dimples formation increases due to the formation of finer Si P and Si E with round corners.