Tensile and fatigue properties of gravity casting aluminum alloys for engine cylinder heads

The mechanical properties were evaluated on specimens of AlSi9Cu3–T6 (333-T6) gravity casting (GC) alloy at room temperature. The GC 333-T6 alloy showed higher yield strength (YS), ultimate tensile strength (UTS) and quality Index but lower hardening capacity than GC 333 aluminum alloy without heat treatment. In addition, the GC 333-T6 aluminum alloy offered a five-fold higher hardening capacity in the cyclic deformation than in the monotonic deformation. Cyclic deformation characteristics of GC 333-T6 aluminum alloy were obtained from the LCF test. The alloy exhibited cyclic stabilization at low strain amplitudes (0.2%) and cyclic hardening at higher strain amplitudes (0.25–0.35%). The extent of cyclic hardening increased with increasing strain amplitude. The Basquin' s equation and Coffin–Manson relationships could be used to describe the fatigue lifetime of this alloy. Additionally, micro-cracks initiated at pores would preferentially pass through the elongated Si particles at lower strain amplitudes. The fatigue crack propagation was mainly characterized by the formation of dimples or fatigue striations at different strain amplitudes. Meanwhile, the larger fatigue crack propagation zone and smaller spacing of fatigue striations at the lower total strain amplitude (0.2%) gave rise to a longer fatigue life. Furthermore, final fast fracture tended to preferentially occur from the larger defects in the fast-fracture region, such as large voids, small pits and inclusions.