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

The effect of microstructure on fatigue crack initiation and propagation be haviour of Al-Si-Cu system casting alloys fabricated by varying the Si or C a contents and the mould temperature was investigated. The fatigue strength was strongly dependent on the size of casting defects that become crack in itiation sites. A comparison of the fatigue characteristics of the differen t alloys was carried our, in which the data of the S-N-f curves were conver ted into the Delta K-0-N-f/A(1/2) relationship, where Delta K-0, 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 co ntent, which was attributed to the small size of casting defects and the re duced 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 part icles, although these coarse particles also play a role in decreasing the c rack propagation rate, by crack deflection and branching. As a result of th ese two competitive effects, at feast in the present study, the macroscopic crack propagation rate was not influenced by the size of the eutectic Si p articles. MST/3934.