Mechanism of crystallographic lamellar microstructure formation in PBF-LB 316L stainless steel: A numerical simulation study

In this study, finite element thermal simulations were coupled with ExaCA, a cellular automaton model for microstructure evolution, to investigate the solidification behavior of SS316L fabricated by powder bed fusion by laser beam (PBF-LB) under different laser conditions and to elucidate the formation mechanism of crystallographic lamellar microstructure (CLM). A heat transfer model was employed to accurately capture melt pool geometry, and the resulting thermal data were then used as input for ExaCA to simulate microstructure evolution. The framework successfully reproduced both the melt pool geometry and the experimentally observed texture, thereby providing a simulation-validated basis for mechanistic analysis rather than relying solely on inference from experimental observations. The results reveal that the major ⟨110⟩ // Z layer, where Z denotes the build direction, is governed by the thermal gradient direction in the overlap region near the melt pool bottom, which promotes coherent epitaxial growth of mutually perpendicular dendrites across adjacent tracks. In contrast, the minor ⟨100⟩ // Z layer is controlled by the distribution of the thermal gradient components along the transverse (Y) and build (Z) directions near the center of the melt pool bottom, where a sufficiently low lateral gradient enables epitaxial growth along the build direction. These findings demonstrate that CLM formation is governed by the directional distribution of thermal gradients near the melt pool bottom rather than simply by the overall melt pool shape, providing a mechanistic basis for understanding and controlling texture development in materials fabricated by powder bed fusion.