Microstructure and mechanical properties of high-efficiency laser-directed energy deposited 15-5PH stainless steel

Microstructural and mechanical characteristics of 15-5 precipitation hardening (PH) stainless steel fabricated by laser-directed energy deposition (LDED) were investigated thoroughly for high-efficiency deposition. Different from the typical fully martensitic microstructure in wrought counterparts, high-power LDEDed 15-5PH stainless steel in as-deposited condition mainly consists of lath martensite, skeletal δ-ferrite with small amounts of retained austenite and NbC carbides. Formation mechanism of δ-ferrite was proposed based on the combined effects of Cr element segregation and rapid cooling that suppressed the mesothermal austenitic diffusion transformation during LDED process. Besides, the presence of δ-ferrite gives rise to the precipitation of two-scale Cu-rich precipitates after direct aging treatment. The size of CRPs in δ-ferrite (~28.6 nm) is obviously larger than that in martensite (~11.7 nm). The coarsening CRPs in δ-ferrite could be attributed to the surrounding Cu and Ni-depleted conditions. Due to the low original microhardness of δ-ferrite (5.85 ± 0.77 GPa) and the coarsening CRPs inside, relevant tensile tests indicate the yielding and tensile strength of high-efficiency LDEDed 15-5PH stainless steel drops apparently. The present study reveals the possibility of δ-ferrite formation in LDEDed PH stainless steel, which could provide an experimental basis for the optimization of high-efficiency additive process.