Self-heating behavior during cyclic loadings of 316L stainless steel specimens manufactured or repaired by Directed Energy Deposition

The purpose of this article is to assess a self-heating testing method for the characterization of fatigue properties of single-track thickness additively manufactured specimens. It also evaluates the impact of the microstructure orientation with respect to the loading direction on the dissipative behavior and the initiation of microcracks. The 316L stainless steel specimens under scrutiny were manufactured by Directed Energy Deposition in two configurations: (i) fully printed specimens (2 orientations) and (ii) repaired specimens. The paper first presents a morphologic and crystallographic texture analysis and second, a series of self-heating tests under cyclic loading. The microstructural analysis revealed elongated grains with their sizes, shapes and preferred orientations controlled by process parameters. The self-heating measurements under cyclic tensile loading proved that the dissipation estimation through infrared measurements can be performed on small scale, thin specimens. The self-heating curves could successfully be represented by the Munier model. Moreover, several links between the printing parameters and self-heating results could be established. For example, a smaller vertical increment between successively deposited layers leads to higher mean endurance limits in all configurations. Repaired specimens had a lower mean endurance limit when compared with fully printed or conventionally manufactured substrate specimens. Finally, anisotropy was highlighted during these cyclic tests: specimens loaded orthogonally to the printing direction (PD) showed higher fatigue limits when compared with the ones tested along the PD. Additionally, post-mortem observations revealed characteristic microcracking patterns initiated during the self-heating experiments. Loading along the printing direction induced a classical dominating crack, whereas orthogonal loading generated a network of microcracks along the printing direction. This suggests that the damage, such as void opening, where concentrated at the interlayers. Additionally these damage patterns can be correlated with patterns of plasticity at the grains scale observed in a previous study.