Fatigue properties and microstructure of Al–Si–Cu system casting alloys

The effect of microstructure on fatigue crack initiation and propagation behaviour of Al–Si–Cu system casting alloys fabricated by varying the Si or Ca contents and the mould temperature was investigated. The fatigue strength was strongly dependent on the size of casting defects that become crack initiation sites. A comparison of the fatigue characteristics of the different alloys was carried out, in which the data of the S–N_f curves were converted into the ΔK₀–N_f /A^1/2 relationship, where ΔK₀, N_f, and A are, respectively, the initial cyclic stress intensity factor range, number of cycles to fracture, and initial area of casting defects on the fracture surface of the fatigue specimen. The fatigue strength increased with Si content, which was attributed to the small size of casting defects and the reduced crack propagation rate. According to the short crack propagation test results, the crack propagation rate of the alloys with coarse eutectic Si particles increased due to the high stress concentration effect of the particles, although these coarse particles also play a role in decreasing the crack propagation rate, by crack deflection and branching. As a result of these two competitive effects, at least in the present study, the macroscopic crack propagation rate was not influenced by the size of the eutectic Si particles.