From negative to positive synergy: Heat treatment-driven microstructural evolution and corrosion behavior of DED 316 L stainless steel

This study elucidates the transition from negative to positive synergy in directed energy deposition (DED) 316 L stainless steel, governed by heat treatment-driven microstructural evolution and its consequent effects on corrosion behavior. Multi-modal characterization reveals that the microstructural evolution is dominated by recovery and precipitation processes, with no recrystallization. The 1150 °C heat treatment (HT1150) fosters an optimal synergistic microstructure, achieving complete elemental homogenization, a stabilized network of recovered low-angle grain boundaries (LAGBs), and minimal porosity (0.28%). This synergy concurrently enhances mechanical properties and corrosion resistance: it delivers substructure strengthening to recover high hardness (170 HV) and facilitates the formation of a dense, Cr₂O₃-rich passive film, yielding the highest corrosion resistance among all conditions. In contrast, the HT900 sample maintains high hardness (190 HV) through σ-phase precipitation strengthening (area fraction: 1.02%), but its corrosion resistance is degraded by σ-phase-induced Cr-depleted zones. The HT1050 sample falls into a “performance trap” due to its incompletely recovered substructure, which forms a continuous network of corrosion pathways that triggers a self-accelerating negative synergy loop, resulting in the poorest corrosion resistance. These findings establish 1150 °C as the optimal parameter for achieving the transition from negative to positive synergy, providing critical guidance for the post-processing of additively manufactured 316 L components.