PNNL

Abstract

Cast aluminum alloys in general show poor fatigue performance due to presence of defects. Friction stir processing (FSP) can be used as a tool for enhancing mechanical properties of cast alloys by eliminating such defects. In the present study FSP led to a five times improvement in fatigue life of an investment cast Al-7Si-0.6Mg hypoeutectic alloy. The reason for such enhancement was linked to closure of casting porosity which acted as crack nucleation sites in the as-cast condition. Porosity acted as notches in the as-cast alloy and led to an order of magnitude higher crack growth rate. As FSP eliminated the porosity and refined Si particles, crack growth rate dropped due to the elimination of the notch effect together with increased crack path tortuosity. Finally, short crack behavior was noted in both cast and FSP specimens. The critical crack length where transition from short crack to long crack behavior took place is related to respective microstructural characteristic dimensions.