Achieving ultrafine, fully equiaxed grains and heterogeneous structures in laser powder bed fusion 316L stainless steel through in-situ alloying

Although 316L stainless steel (SS316L) exhibits favorable ductility and toughness, its limited strength restricts its applicability. This study addressed this limitation by preparing a series of SS316L-xTC4 alloys (where x indicates Ti6Al4V (TC4) contents of 0.5 wt%, 1 wt%, 2 wt%, and 4 wt%) with equiaxed grains, ultrafine grains, and heterogeneous structures using in-situ alloying via laser powder bed fusion. The Ti, Al, and V in the TC4 alloy were shown to stabilize and promote the formation of the α phase, which is stronger than the γ phase in typical SS316L. Furthermore, these solute elements readily formed nanoparticles with impurities, such as C and O, to increase the nucleation rate and thereby achieve grain refinement. This resulted in the formation of ultrafine grains predominantly within the α phase, where the solute elements were primarily distributed. The formation of the α phase also impeded the growth of the γ phase; coupled with the effects of the nanoparticles, this also significantly reduced the grain size in the γ phase. Notably, the SS316L-2TC4 alloy exhibited fully equiaxed grains, and the coexistence of the α and γ phases as well as ultrafine and coarse grains formed heterogeneous grain and dual-phase structures within. The synergistic effects of equiaxed grains, ultrafine grains, and heterostructures produced an SS316L alloy that exhibited both excellent strength and elongation.