Tailoring solidification and strength enhancement through process parameter optimization in Nb-added 316L stainless steel fabricated by directed energy deposition (DED)

Additive manufacturing (AM), particularly the directed energy deposition (DED) process, enables the fabrication of metallic components with complex geometries and is widely applied across high-value industries. However, the microstructure and mechanical properties of DED-fabricated parts are highly sensitive to processing parameters, making consistent quality control challenging. In this study, 316L stainless steel with 2 wt.% Nb addition was fabricated under systematically varied scan speeds and laser powers to clarify the process–microstructure–property relationships unique to Nb-modified alloys. Results showed that increasing scan speed reduced the melt pool size and promoted a solidification mode transition from dendritic to cellular morphologies. Because dendritic structures provided higher hardness and strength through stronger Nb segregation at dendrite boundaries, the cellular-dominated condition at 1.2 m/min exhibited the lowest mechanical performance. Increasing laser power reduced the cooling rate and led to coarsening of the solidification structure, which further degraded strength. Notably, under rapid cooling conditions, Nb segregation suppressed nucleation and promoted ab-normal growth of the solidification structure, resulting in behavior distinct from conventional 316L stainless steel where refinement is expected. These findings demonstrate that process optimization windows established for conventional 316L cannot be directly ap-plied to Nb-modified alloys. Instead, alloy-specific segregation behavior must be incorporated into parameter optimization strategies to achieve reliable microstructure and mechanical properties in DED-fabricated Nb-added stainless steels.