Effect of T6 heat treatment on the defect susceptibility of fatigue properties to microporosity variations in a low-pressure die-cast A356 alloy

The present study aimed to optimize the conditions for the T6 heat treatment of low-pressure die-cast A356 alloy in terms of the dependence of fatigue properties on microporosity variation. The fatigue property was evaluated using a high cycle fatigue test, and the microporosity was based on the fractographic porosity measured through SEM imaging of a fractured surface. The number of cycles to failure for the A356 alloy depends on the variations in microporosity through an exponential relationship. Because the fatigue strength coefficient and exponent of the S-N curve is always underestimated in the presence of microvoids, the evaluation of the number of cycles to failure achievable in a defect-free condition has practical significance. Detailed suggestions for improving the number of cycles to failure were accomplished based on the modified Basquin's equation, which fundamentally determines the number of cycles to failure from both the defect susceptibility of the tensile strength and the maximum achievable tensile strength in a defect-free environment. A T6 heat treatment with solutionizing and aging treatment times of 6 h at 540 °C and 16 h at 160 °C, respectively, is proposed as the optimal conditions for improving the number of cycles of A356 alloy at stress amplitudes of less than 200 MPa.