Effects of grain refining and modification on mechanical properties and microstructures of Al–7.5Si–4Cu cast alloy

Abstract

Al–7.5Si–4Cu cast alloy melt modified by Al–5Ti–B, RE and Al–10Sr master alloys were poured in the chromite sand moulds, to investigate comparatively the effects of individual or combined additions of grain refiners and modifiers on the mechanical properties, microstructures, grain refining and modification, and intermetallic compounds of the alloy. The results show that the mechanical properties and the microstructures of Al–7.5Si–4Cu cast alloys are improved immensely by combining addition of 0.8%Al–5Ti–B, 0.1%RE and 0.1%Al–10Sr grain refiners and modifiers compared with the individual addition and cast conditions. For individual addition condition, addition of 0.8% Al–5Ti–B master alloy can obtain superior tensile strength, Brinell hardness and finer equiaxed α(Al) dendrites. The alloy with 0.1% RE master alloy shows the highest improvement in ductility because the rare earth can purify the molten metal and change the shape of intermetallic compounds. While the alloy with 0.1% Al–10Sr modifier shows only good improvement in yield strength, and the improvement of other performance is unsatisfactory. The Al–10Sr modifier has a significant metamorphism for the eutectic silicon, but will make the gas content in the aluminum alloy melt increase to form serious columnar grain structures. The effects of grain refining and modification on mean area and aspect ratio have the same conclusions obtained in the mechanical properties and the microstructures analyses.