Role of printing density in internal defect evolution and hydrogen embrittlement of PBF-LB 316L stainless steel fabricated from recycled powder

Abstract

This study examines the influence of printing energy density and reuse of recycled powder on defect evolution, microstructure, and hydrogen embrittlement (HE) susceptibility in laser powder bed fusion (PBF-LB)-processed 316L stainless steel. Two sets of specimens were fabricated at volumetric energy densities (VED) of 66.0 J/mm³ (low energy density, LED) and 84.3 J/mm³ (high energy density, HED) using a blended batch of recycled powders with different prior histories. X-ray microscope revealed significantly higher porosity in LED specimens (∼1%) compared to HED (∼0.03 %), highlighting the role of volumetric energy input in densification and defect mitigation. Ex-situ hydrogen charging was conducted in NaOH at 80 °C for 24 h, followed by slow strain rate tensile testing. While hydrogen exposure had minimal effect on ultimate tensile strength, it markedly reduced ductility (particularly in LED specimens), indicating a strong correlation between internal porosity and HE susceptibility. Fractography revealed cleavage and intergranular fracture features in H-charged specimens. Electron backscatter diffraction analysis showed increased strain localization in LED specimens, along with a reduction in Σ3 twin boundaries after hydrogen charging. The present study underscores the need for careful control of printing parameters when reusing recycled powder to ensure structural reliability in real-world applications.