Effects of volumetric energy density on defect structure and fatigue behaviour of powder bed fusion manufactured 316L stainless steel

Three structures of AISI 316L austenitic stainless steel were additively manufactured using the laser beam powder bed fusion (PBF-LB) process with varying volumetric energy density (VED) levels: low (50.8 J/mm³), medium (79.4 J/mm³), and high (84.3 J/mm³). The impact of VED on defects, microstructure, and fatigue behaviour was investigated and compared to wrought 316L steel. Various novel techniques were used to analyse the grain structure and defects. Fatigue behaviour was assessed through low and high cycle fatigue tests and tensile tests at room temperature. Results showed that increasing VED improved material density (from 91.8 % to 99.6 %) and reduced defect size and complexity. Both lack of fusion (LoF) and pore-type defects were identified, with fewer and less complex LoF defects at higher VED. Defects were primarily located at subsurface regions corresponding to the border hatch-fill contour layer. Printed structures exhibited lower high cycle fatigue (HCF) strength than wrought steel, but this difference diminished in low cycle fatigue (LCF) regimes for medium and high VED structures, where twinning and martensitic transformation enhanced fatigue strength. The defect characteristics significantly influenced HCF strength, and achieving a fatigue limit comparable to wrought steel requires very low defect density even at high VED.