Effect of friction stir processing on fatigue behavior of an investment cast Al–7Si–0.6 Mg alloy

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