Grain morphology transition and synergistic strength-ductility enhancement in laser directed energy deposited nitrogen-alloyed 316L via homologous powder in-situ induction

Controlling the solidification grain architecture is crucial for optimizing the mechanical properties of additively manufactured stainless steel components. In this study a deterministic energy-mass coupling strategy was proposed to trigger in-situ homologous powder nucleation (utilizing the base 316L feedstock without intentional foreign additions). By systematically modulating the powder feed rate under a constant laser power, the non-equilibrium melt pool was precisely tailored to facilitate the entrapment of semi-molten particles and precisely control the nitrogen content via localized melt pool metallurgy. This approach circumvented the need for ex-situ inoculants and induced columnar-to-equiaxed transition. The resulting nitrogen-supersaturated, near-equiaxed grain network —verified as a fully single-phase solid solution devoid of secondary nitride precipitation—endows the LDED 316L with exceptional mechanical responses. At both room temperature and 350°C, the yield strength and elongation comprehensively surpass those of conventional wrought counterparts. Multiscale characterization elucidates that the strengthening mechanisms are highly temperature-dependent: at room temperature, deformation twinning is actively activated, which governs the dynamic Hall-Petch strengthening mechanism; whereas at 350°C, where thermal activation suppresses twinning, the fine grain boundary network acts as robust geometric pinning sites to severely retard dynamic recovery and shear localization. The critical novelty of this work lies in completely decoupling these cross-temperature deformation mechanisms and demonstrating that endogenous powder retention can be synergistically coupled with interstitial solid solution. This provides a transformative processing window for the structural-mechanical design of next-generation nuclear components.