Effects of HIPping on high-cycle fatigue properties of investment cast A356 aluminum alloys

This study is concerned with the effects of HIPping on high-cycle fatigue properties of investment cast A356 Al alloys. Tensile and high-cycle fatigue tests were conducted on cast alloys, two of which were HIPped, and then the test data were analyzed in relation with microstructures, tensile and fracture properties, and fatigue fracture mode. Eutectic Si particles were homogeneously dispersed in the matrix of the casting A356 Al alloys, but there were many large pores formed as casting defects. The high-cycle fatigue test results indicated that fatigue strength of the HIPped alloys was higher than that of the non-HIPped alloys because of the significant reduction in volume fraction of pores by HIPping. In the non-HIPped specimens, fatigue cracks initiated at large pores adjacent to the specimen surface and then propagated down to several hundreds micrometers depth while coalescing with other large pores. On the other hand, the HIPped specimens, where pores did not affect the fatigue much, fatigue cracks initiated at eutectic Si particles and propagated along them, thereby leading to improved fatigue strength by 40 to ∼50% over the non-HIPped specimens.