Uncovering microstructure evolution in Fe-6.5Si alloy during powder bed fusion electron beam melting: A multiphysics simulation study

Here, we uncover the microstructure evolution in Fe-6.5Si (wt%) during powder bed fusion electron beam melting using a virtual prototyping framework that integrates the discrete element method for powder-bed generation, computational thermal–fluid dynamics with free-surface melt flow, and a coupled cellular automaton–finite element model incorporating heterogeneous and epitaxial nucleation. Multi-track and multilayer melting were investigated at a constant linear energy of 2.0 J/mm by varying scan speed (200–1000 mm/s) and line offset (50–150 μm). Experiments and simulations consistently showed that reduced line offset enhances heat accumulation, preferentially deepening the melt pool through suppressed lateral heat dissipation and evaporation-driven recoil effects, which broadens the distribution of solidification direction ( SD ) and promotes competitive grain growth. Increasing line offset stabilizes SD toward the build direction but increases the mean undercooling, activating heterogeneous nucleation and reducing columnar aspect ratio. In multilayer melting, microstructure selection could not be explained by SD or mean undercooling alone. Instead, heterogeneous nucleation was governed by the residence time of cells in the supercooled state. Despite the higher undercooling observed at a scan speed of 1000 mm/s, the short and spatially uniform solidification time effectively suppressed new grain formation, enabling sustained epitaxial growth and resulting in the highest texture index and a single-crystal-like microstructure. Based on these findings, processing guidelines for achieving a single-crystal-like microstructure are proposed: (i) suppress heterogeneous nucleation by limiting the effective undercooling, (ii) form a thin melt pool to align the SD with the build direction, and (iii) minimize the solidification time by promoting rapid solid–liquid interface migration.