Microstructure effects on fatigue crack initiation in aluminum castings

The effects of microstructures on high-cycle fatigue (HCF) crack initiation have been investigated in a casting aluminum alloy (A356.2) under both axial and bending conditions at various strain/stress ratios. Secondary dendrite arm spacing (SDAS), porosity (maximum size and density distribution) and silicon particles have been quantified in the casting alloy. For specimens removed from directionally solidified ingots, SEM observations show that high cycle fatigue cracks initiate at near surface porosity, oxides or within eutectic area depending on SDAS. When the SDAS is greater than 25similar to28 um, the porosity size exceeds a critical value of similar to75 um. Fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than 25 um, fatigue cracks initiate from the interdendritic eutectics where the silicon particles are segregated. Oxides initiated the fatigue cracks whenever they were near the surface, regardless of the SDAS. The effects of microstructure on both small and long crack propagation have also been observed. In a microstructure with a small SDAS, small cracks propagate around silicon particles (debonding). The silicon particles could prolong the propagation life by deviating the cracks. As the SDAS increases, more silicon particles are cracked in the crack path. HCF crack propagation path is trans-dendrite.