On the role of internal defects in the fatigue damage process of a cast Al-Si-Cu alloy

The effect of different types of defects on the fatigue properties of a secondary Al-Si-Cu-Fe alloy was analyzed by micro-computed tomography and scanning electron microscopy. It was found that extended pores typically occurring at unfavourable cooling conditions possess several regions of similar stress concentration and hence multiple crack initiation is very likely. On the other hand, the Fe-rich inclusions may form impressive clusters and can contribute significantly to deteriorating the tensile properties of the material. However, they do not directly influence the fatigue properties in as cast specimens, as they are virtually absent in the casting skin. However, they may be relevant in pore formation as CT scans show that large pores can be enclosed in the cavities of the iron inclusion clusters.