The Influence of Surface Quality on Fatigue Strength of Wire Arc Additive Manufactured 316L Stainless Steel

This study investigates the influence of surface quality on fatigue strength of WAAM 316L stainless steel. The tests were performed on milled, polished and as-built surfaces. The macrostructure with optical microscope and microstructure with EBSD detector were evaluated. The hardness profile was measured from cross-section of the WAAM 316L part. Tensile strength of WAAM 316L was investigated in built and deposition direction. The bending fatigue tests were conducted to evaluate the influence of surface quality on fatigue strength. No pores or other significant printing defects in the WAAM 316L material were detected in microstructure analysis. The microstructure evaluation indicated that high epitaxial grain growth is present in the WAAM 316L microstructure. Due to this a coarse columnar grain structure was present in the microstructure. Tensile tests revealed the built direction to be the best in terms of static strength and that reflected in the fatigue tests as better fatigue strength compared to the deposition direction. The fatigue limit of the milled WAAM 316L in built direction was 235 MPa. The polished WAAM 316L had lower fatigue strength compared to the milled WAAM 316L, although the surface quality of polished WAAM 316L was better. This is due to the residual stresses generated by the polishing process used in the experiments. The fatigue strength of the WAAM 316L with as-built surface was clearly the lowest. The fatigue limit of as-built WAAM 316L was 125 MPa.