Cyclic deformation behavior of a cast aluminum alloy

Due to effective weight reduction and subsequent improvement in fuel economy of vehicles, as well as flexibility in fabrication and high strength-to-weight ratio, cast A356 Al–Si–Mg alloy is being used to replace some relatively heavy components in the automotive industry. However, very limited data on low cycle fatigue (LCF) resistance have been reported on this alloy especially in T5 condition. This study was aimed to evaluate cyclic deformation characteristics, LCF resistance, and fracture mechanisms of A356 alloy in the T5, T6 and modified T6 conditions in relation to their microstructural features. It was observed that the microstructure of this alloy consisted of primary α-Al matrix and eutectic regions containing Si particles, which were acicular in the T5 condition and spherical in the T6 and ModT6 conditions. The ModT6 material had higher yield strength (YS) and ultimate tensile strength (UTS) but lower strain hardening exponent than the T6 material, while the T5 material had lower YS and UTS but higher initial strain hardening exponent than the T6 material. Cyclic stress amplitudes remained basically stable at lower strain amplitudes, and cyclic hardening occurred at higher strain amplitudes. In comparison to the T5 and ModT6 materials, the T6 material exhibited a higher cyclic hardening capacity, giving rise to a longer fatigue life. Crack initiation in both tensile and fatigue tests was observed to occur at the sub-surface pores in all the three material conditions. The T5 material had quasi-cleavage fracture characteristics due to the presence of acicular Si particles, while the T6 and ModT6 exhibited dimple-like fracture features. Fatigue striations, coupled with some secondary cracks and microvoids, were observed to occur in the fatigue crack propagation regime.