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

The effect of shrinkage pores and gas pores on the fatigue process of a secondary Al-Si-Cu-Fe alloy was analyzed by micro-computed tomography and scanning electron microscopy. Repeated CT-scans were performed in order to study the damage accumulation process. It was found that there are basically three different failure scenarios. In the first one, lifetime is determined by cracking of the dendritic branches bridging extended pores. This damage process will happen even at moderate load levels as these material bridges are exposed to high local loads. Lifetime in the second case is dominated by crack initiation from complex shaped gas pores followed by a short period of stable crack growth. Crack coalescence then becomes very likely, and failure is imminent. Finally, a two-stage crack growth process was observed for complex shaped shrinkage: fracture of the material bridges is followed by a phase of stable crack extension with the fatigue crack starting from one or several branches of the shrinkage pore. Estimate of the crack growth rate agree with previous results from the literature and confirm that the internal cracks grow much faster than expected long crack vacuum tests.