Porosity formation in aluminium alloy A356 modified with Ba, Ca, Y and Yb

An increased level of porosity is normally reported to accompany modification of Al-Si alloys. In this study, the effects of additions of barium, calcium, yttrium and ytterbium on porosity formation in an A356.0 (Al-7%Si-Mg) alloy are examined. The permanent mould casting consisted of a vertical plate connected directly to a runner at a hot-spot junction. Casting defects observed include surface shrinkage and internal porosity in the hot spot and also hot tearing of the casting and runner. All additions increased the porosity level compared to the unmodified alloy and it increased with increased addition level. The results show that additions of Ca and Y caused porosity to become increasingly concentrated in the hot spot. Additions of Ba and Yb resulted in small, round, dispersed porosity. When porosity formation is considered based on the feeding mechanisms, particularly interdendritic feeding, it is possible to rationalise the effects of the elements on porosity distribution based on their impact on the eutectic solidification mode. In Ca and Y containing alloys the eutectic evolves from the surface towards the centre of the hot spot, while heterogeneous nucleation of eutectic grains across the hot spot occurs with additions of Ba and Yb.