Impact of grain size on mechanical properties of AlSi7Mg0.3 alloy

The standard grain refiner, Al–5Ti–1B, and Al–3B were employed to investigate the potential of B additions to grain refine the popular AlSi7Mg0.3 alloy with a focus on the impact of the latter on the mechanical properties. The average grain size of the AlSi7Mg0.3 alloys grain refined with the Al–5Ti–1B and Al–3B grain refiners were 465±102 μm and 156±29 μm, respectively, while their secondary dendrite arm spacing (DAS) values were very similar, 30±5 μm, owing to identical cooling rates during solidification. The two groups of cast samples exhibit very similar yield, ultimate tensile strength and hardness values, in spite of a three-fold difference in the grain size evidencing that the DAS, rather than the grain size, controls the mechanical properties in aluminium castings. While similar in the as-cast state, the mechanical properties of the AlSi7Mg0.3 alloy grain refined with Al–3B in T6 temper are markedly superior with higher strength and elongation values. Since the composition and the DAS values of the two groups of samples are nearly identical, the much smaller grain structure is credited for the superior strength and hardness as well as the higher ductility in the B-treated AlSi7Mg0.3 alloy.