Columnar to Equiaxed Transition in Electron Beam Powder Bed Fusion (EB-PBF) Characterized Through Microstructure Analytics

Abstract

Electron beam powder bed fusion (EB-PBF) offers the possibility to tailor local thermal histories via electromagnetic beam steering, yet the resulting columnar to equiaxed transition (CET) in Ni-based superalloys remains incompletely quantified. In this work, the CET in IN718 processed by spot-melting EB-PBF is systematically mapped using a microstructure analytics framework built on orientation-corrected pseudo-3D EBSD. Three processing degrees of freedom are explored: beam current, dwell time, and scanning strategy. A global coordinate system is enforced across three orthogonal cross sections, enabling consistent evaluation of grain morphology, crystallographic texture, and lack of fusion (LOF) porosity via percentile-based aspect ratio, grain size, misorientation, and pore-fraction descriptors. At fixed beam energy, comparison of structured (snake, single-direction) and nonstructured (single-direction-shifted, stochastic) patterns demonstrates that increasing spatial and temporal randomness disrupts thermal field coherence, suppresses epitaxial grain extension, and promotes fine equiaxed morphologies. Extending the analysis to a two-dimensional beam-current-dwell-time matrix under stochastic spot melting, experimentally determined process maps reveal a narrow processing window in which defect-lean, equiaxed, and nearly isotropic microstructures are obtained. This window is bounded on one side by insufficient energy input causing LOF defects, and on the other by excessive energy input driving strong morphological and crystallographic texture and abnormal grain growth. The results demonstrate that dwell time primarily governs crystallographic texture, whereas beam current and dwell time jointly control grain morphology and defect formation.