Low cycle fatigue behavior of T4-treated Al-Zn-Mg-Cu alloys prepared by squeeze casting and gravity die casting

Gravity die casting (GC) and squeeze casting (SC) T4-treated Al-7.0Zn-2.5Mg-2.1Cu alloys were employed to investigate the microstructures, mechanical properties and low cycle fatigue (LCF) behavior. The results show that mechanical properties of SC specimens are significantly better than those of GC specimens due to less cast defects and smaller secondary dendrite arm spacing (SDAS). Excellent fatigue properties are obtained for the SC alloy compared with the GC alloy. GC and SC alloys both exhibit cyclic stabilization at low total strain amplitudes (less than 0.4%) and cyclic hardening at higher total strain amplitudes. The degree of cyclic hardening of SC samples is greater than that of GC samples. Fatigue cracks of GC samples dominantly initiate from shrinkage porosities and are easy to propagate along them, while the crack initiation sites for SC samples are slip bands, eutectic phases and inclusions at or near the free surface.