Enhancement of Mechanical Properties and Hydrogen Embrittlement Resistance of Laser-Directed Energy Deposition-Fabricated 316L Stainless Steel by Laser Shock Peening

316L stainless steel offers attractive characteristics for hydrogen applications, including low hydrogen diffusivity and high hydrogen solubility. However, its use is limited by relatively low strength and resistance to hydrogen embrittlement (HE) under prolonged hydrogen exposure. Laser-directed energy deposition (L-DED) can not only increase the strength of 316L, but also induce significant tensile residual stresses that promote HE. In this study, 316L stainless steel samples produced by L-DED were post-processed by laser shock peening (LSP) to release the tensile residual stresses and refine the near-surface microstructure. LSP-treated samples showed refined grains, higher hardness, and the introduction of compressive residual stress, which led to improved tensile performance in hydrogen. Notably, after seven passes of LSP, the HE index (reduction in elongation due to hydrogen) was 12.5%, compared with 36.1% for the unpeened material. These results demonstrate that LSP is an effective approach to simultaneously increase strength and significantly improve HE resistance in additively manufactured 316L stainless steel.