Effect of microstructure on fatigue crack propagation characteristic of Al-Si-Cu die casting alloys

Fatigue crack propagation characteristics of a eutectic and two hypereutectic Al-Si-Cu die casting alloys were investigated, where the stress ratio was 0.1 and the load frequency was 50 Hz at an ambient temperature. Relationships between fatigue crack propagation characteristic and microstructure have been discussed. The fatigue crack propagation rate decreases with increasing Si content in a low stress intensity factor range, ΔK. In this range, the crack propagation rate is very slow because it is considered to increase effects of crack deflection and branching induced crack closures with increasing Si content. At a high ΔK, because the effects of crack closures are decreased, the higher the Si content, the higher the fatigue crack propagation rate becomes. In this study, the fatigue crack propagation rate for the Al-15%Si-T5 alloy becomes higher than the other two alloys. It is considered to decrease effects of crack deflection and branching induced crack closures with many intermetallic compounds in the Al-15%Si-T5 alloy. The fractographic study on fractured specimens after fatigue crack propagation tests reveals the fracture surfaces consisting of the cleavage fractures of Si and intermetallic compound particles, and the striation pattern in α-aluminum.