Effects of surface porosity on the fatigue strength of AE425 and PM390 hypereutectic Al–Si casting alloys at medium and elevated temperatures

The effects of surface porosity on the fatigue life of AE425 and PM390 hypereutectic Al–Si casting alloys were investigated at medium and elevated temperatures (150 °C and 300 °C, respectively) for the purposes of the present study. Surface porosity was found to be the most critical casting defect which influenced the fatigue life of hypereutectic alloys at medium and elevated temperatures. The most frequently observed site for fatigue crack initiation was surface porosity, where 89% of all samples tested tended to fracture because of this particular defect. Thus, surface porosity was shown to have a detrimental effect on the fatigue performance of the alloys in question. It was observed that the fatigue strength decreases as the surface pore size increases, and vice versa. The SEM technique used to identify fatigue crack initiation sites, or surface porosity, may be applied with a high degree of accuracy for measuring single pores responsible for triggering such cracks. When the crack initiation area contains multiple pores or consists of a spongy structure (as in the case of AE425 alloy), the technique used is not in itself sufficient to determine the exact site, and it should, therefore, be re-evaluated to provide greater accuracy of results. It was also observed that, the decohesion and damage of silicon particles in hypereutectic alloys under cyclic loading weakens the overall structure and accelerates the occurrence of fatigue fractures.