Non-destructive characterization of process-induced defects and their effect on the fatigue behavior of austenitic steel 316L made by laser-powder bed fusion

Additive manufacturing (AM) offers a high potential for light weight applications due to its possibility to generate complex structures with a high freedom of design compared to conventional techniques. However, the mechanical characterization of additively manufactured materials has become an essential topic in research over the last years to use AM parts for structural components especially in the automotive and aerospace industry. In the current research, specimens for fatigue tests made of the austenitic stainless steel AISI 316L processed by laser-powder bed fusion technique with three different building orientations were investigated. The gauge sections of the fatigue specimens were scanned by microfocus computed tomography (µ-CT) to detect and identify process-induced defects. The 3D information about the defects gained by µ-CT were compared with the crack-initiating defect on the fractured surfaces. The fatigue data were used for a model-based description of the fatigue strength based on the √area-parameter model. The results depict that the fatigue life is significantly influenced by the orientation of the inherent pores caused by the building direction causing a significant scatter in fatigue life. The µ-CT data allow to estimate the fatigue strength using the √area-parameter model based on the identified critical defects and are in good accordance with the results from the fatigue tests and the data obtained by fractography. Additionally, it could be shown that the √area-parameter model is applicable for additively manufactured 316L steel. It can also give an explanation for the anisotropic fatigue behavior, which supports the assumption that mainly the orientation of the pores and their √area-parameter are influencing the fatigue strength.