High temperature fatigue deformation behavior of automotive heat resistant aluminum alloys

High temperature high cycle and low cycle fatigue deformation behavior of automotive heat resistant aluminum alloys (A356 and A319 based) were investigated in this study. The microstructures of both alloys were composed of primary Al-Si dendrite and eutectic Si phase. However, the size and distribution for eutectic Si phase varied: a coarse and inhomogeneous distributed was observed in alloy B (A319 based). A brittle intermetallic phase of α-Fe Al₁₂(Fe,Mn)₃Si₂ was detected only in B alloy. Alloy B exhibited high fatigue life only under a high stress amplitude condition in the high cycle fatigue results, whereas alloy A showed high fatigue life when stress was lowered. With regard to the low-cycle fatigue result (250 °C)) showing higher fatigue life as ductility increased, alloy A demonstrated higher fatigue life under all of the strain amplitude conditions. Fractographic observations showed that large porosities and pores near the outside surface could be the main factor in the formation of fatigue cracks. In alloy B, micro-cracks were formed in both the brittle intermetallic and coarse Si phases. These micro-cracks then coalesced together and provided a path for fatigue crack propagation. From the observation of the differences in microstructure and fractography of these two automotive alloys, the authors attempt to explain the high-temperature fatigue deformation behavior of heat resistant aluminum alloys.