The influence of laser scanning speed on microstructural heterogeneities and phase transformations in additively manufactured medium-manganese steel

Controlling microstructure evolution in metastable steels like medium-manganese steel (MMnS) during additive manufacturing (AM) is complex due to the dependence of the phase transformation behavior on AM conditions. The present study investigates the phase evolution of a X21Mn13 MMnS produced via laser-based powder bed fusion of metals (PBF-LB/M), utilizing both experimental analysis (microscopy and X-ray diffraction) and computer-aided simulations (finite element analysis and phase field method) focusing on the impact of varying laser scanning speeds on microstructure evolution during solidification and subsequent cooling. This comprehensive analysis allowed for a thorough investigation, separating the distinct influences of chemical composition, grain morphology and mechanical factors on the austenite stability. Additionally, a crystallographic examination, comparing primary austenite grains (PAG) and retained austenite grains (γ R ) was conducted to elucidate phase evolution. The findings reveal that increased scanning speed leads to less Mn evaporation, decreased PAG size, lower density of geometrically necessary dislocations (GND) and compressive residual stress in γ R ; influencing the austenite stability. The martensite formation due to austenite-to-martensite transformation was found to occur predominantly within non-<100>-oriented γ R grains, eventually weakening the contribution of this component to the crystallographic texture, whereas <100>-oriented γ R grains were retained and thus, dominate austenite texture.