Systematic study of rapid solidification in laser powder-bed fusion of stainless steels to verify applicability of columnar-equiaxed transition

In recent years, microstructural control in powder-bed fusion (PBF)-type metal additive manufacturing (AM) has attracted considerable attention. The solidification microstructure is generally described by the columnar-equiaxed transition (CET) criterion. However, we previously reported that austenitic stainless steels (SSs) melted by beam irradiation for PBF exhibit an opposite to the CET criteria. It is essential to systematically understand the microstructural formation behavior under the rapid cooling and high-velocity flow conditions unique to the PBF process to obtain guidelines for microstructural control of SSs. In this study, we systematically investigated the solidification behavior in laser-melted regions of austenitic, ferritic, and martensitic SSs to elucidate the dominant factors governing microstructural formation. The liquidus–solidus temperature difference (ΔT₀), which varies among the SSs, was examined as a key parameter characterizing the alloy-dependent solidification behavior. Based on the digital twin analysis, we proposed that the CET criteria serve as a guideline for ferritic and martensitic SSs in the PBF process, whereas they cannot be used for austenitic SSs, which exhibit the opposite trend to the CET criteria. Additionally, the machine-learning analysis, i.e., random forest, indicated that concentrations of Si, Mn, and C particularly affect microstructural formation.