Defect sensitivity in L-PBF AlSi7Mg0.6 alloy subjected to fatigue load: Effect of load ratio and torsion loading

This study aims to investigate the impact of the defects on the fatigue behavior of the AlSi7Mg0.6 alloy processed by laser powder bed fusion (L-PBF) by considering uniaxial and torsion loadings up to 2 x 106 cycles. Several sets of specimens were produced and characterized using X-ray tomography. Fatigue tests were performed applying axial loading with different load ratios on the one hand, and torsion loading on the other hand. The critical defects responsible for fatigue failure were either isolated pores or clusters of small gas pores. The impact of the loading condition on the sensitivity to the different defect types observed in the material is discussed based on the analysis of the fracture surfaces. In particular, the results obtained for axial loadings showed that the critical defect size required to induce fatigue failure is decreased when a positive mean stress is applied, which impacts the distribution of the fatigue lives compared to pure tension -compression tests. For torsion loading, the clusters of gas pores were the predominant critical defect type, as opposed to axial loading. This increase in the criticality of the clusters when changing the loading type was investigated by means of numerical simulations accounting for real defect geometries.