The effect of mischmetal and heat treatment on the microstructure and tensile properties of A357 Al-Si casting alloy

The effects of La-based mischmetal (MM) and heat treatment on the microstructure and tensile properties of two different sections of A357 Al-Si casting alloy have been investigated in this study. Different concentrations (0-1 wt%) of the MM were added to the molten alloy directly. The tensile testing was employed to measure the quality index (Q.I.=UTS+150×log (elongation)) for evaluating the modification efficiency of the alloy with different MM contents. T6 heat treatment was applied before tensile testing. The results showed that the optimum levels of added mischmetal are 0.1 wt% and 0.3 wt% for thin and thick section castings respectively. A new AlSiLa intermetallic was detected through the microstructural studies at higher MM levels. Further results demonstrated that T6 heat treatment improves tensile properties of the modified alloy significantly. The differences observed in the fracture behavior were attributed to the microstructural changes as well as morphological aspects of Si particles.