Influence assessment of artificial defects on the fatigue behavior of additively manufactured stainless steel 316LVM

The laser powder bed fusion of metals (PBF-LB/M) is one of the most promising techniques to realize lightweight optimized parts and structures. Possible design elements are internal cooling channels or topology optimized geometries. However, not only does the process suffer of instabilities causing pores and lack-of-fusion defects but also a low surface quality. A further knowledge about these defects and their influence on the mechanical behavior are needed to use additively manufactured parts in structural relevant applications. In this work, the austenitic stainless steel 316LVM (X2CrNiMo18-15-3) has been processed by PBF-LB/M. In total, four different batches were manufactured with either no intended porosity or specific cubic defects ranging between 0.3 and 1.5 mm edge length. The fatigue behavior was evaluated at stress ratio R = -1 up to 1E7 cycles. The fracture surface was analyzed by scanning electron microscopy and the relationship between artificial defect size and fatigue strength was investigated by Kitagawa-Takahashi (KT) diagram and its modification by El Haddad's intrinsic crack length. The results show that the KT-diagram underestimates the fatigue strength of the investigated steel indicating a high defect tolerance and possible hardening mechanisms during cyclic loading such as possible nano-twinning. An influence of the entrapped process gas could also play a role. As long as this is unclear, the models can only be used conservatively as the full potential of the PBF-LB/M steel cannot be fully exploited.