The effect of microscopic inclusion locations and silicon segregation on fatigue lifetimes of aluminum alloy A356 castings

The microstructural heterogeneity of aluminum alloy A356 permanent mold castings impacts fatigue performance differently depending on location in the casting. Casting conditions, which include casting temperature, gradients in the mold temperature, plunger speed and casting pressure are variables which can affect solidification rates and therefore, the microstructure within the casting. In this paper an experimental investigation of fatigue performance of specimens cut from various locations in a test bar of cast aluminum alloy A356 is reported. The number of cycles to failure for each test was recorded and the fracture surfaces and local microstructure examined. It was observed that local fatigue resistance varied substantially along the solidification path while tensile strength was little affected. The amount of Al Si eutectic and the density of micropores increases along the solidification path. Samples located near the surface and the farthest from the gate end demonstrated the longest lifetimes. Conversely, the samples taken from the centerline of the casting closest to the gate end demonstrated the shortest lifetimes.