Al–Si–Cu系合金鋳物の疲労特性とミクロ組織

抄録

The effect of microstructural features on the fatigue crack initiation and propagation behaviors of Al–Si–Cu system casting alloys with different microstructures by changing Si and Ca contents and mold temperature (cooling rate) was investigated. The fatigue strength is strongly dependent on the sizes of casting defects which become crack initiation site. Data of S−N curves were converted into the ΔK₀–N_f √area relationship to compare the fatigue characteristic where ΔK₀, N_f and area are the range of initial cyclic stress intensity factor, number of cycles to fracture and size of casting defects on the fracture surface of fatigue specimen, respectively, assuming that the crack propagation rate follows the Paris law. The fatigue strength increases in proportional to Si content, which was attributed to small size of casting defect and reduced crack propagation rate. Crack propagation rate of the alloys with coarse eutectic Si particles increases due to the high stress concentration effect of the particles, while these coarse particles also play a role in decreasing the crack propagation rate by crack deflection and branching. Due to the two competitive effects, the macroscopic crack propagation rate is not much dependent on the size of eutectic Si particles.