Enhancement in fatigue performance of metastable austenitic stainless steel through directed energy deposition additive manufacturing

The effect of the as-built microstructure and defect distribution on the fatigue performance of a metastable austenitic stainless-steel fabricated by directed energy deposition was analyzed in two fatigue regimes. Strain localization, porosity, microstructural response, and deformation induced phase transformation were studied for samples parallel and perpendicular to the build direction using digital image correlation, optical surface imaging, scanning electron microscopy, x-ray diffraction and electron back scatter diffraction. Pore area fractions between 0.3 and 0.6% played a negligible role in overall fatigue life. Anisotropic fatigue behavior and strain ratcheting were discovered in both fatigue regimes and connected to a high 〈100〉 texture and αMsbcc formation in deformed samples with parallel orientations. Areas of localized strain and slip traces were linked to eventual crack initiation sites, and ultimate fatigue failure. Improved fatigue life by as much as 25% that of standard wrought and DED LENS® material was observed in the transitional fatigue regime (N ≈ 104–106). In the high cycle fatigue regime (N > 106), parallel orientations performed identical to wrought while perpendicular orientations observed a 7% lower fatigue (endurance) limit. In conclusion, metastable 304L produced by directed energy deposition has shown exceptional fatigue life compared to wrought AISI 304L in the as-built condition.