Instability in single-crystal epitaxy during laser powder bed fusion driven by solidification interface morphology

The direct fabrication of single-crystal nickel-based superalloys via laser powder bed fusion ( L -PBF) remains hindered by epitaxial instability, primarily caused by dynamic solidification conditions associated with the solidification interface morphology of the melt pool. This study reveals that single-crystal epitaxial instability manifests as stray grain formation, their cross-layer evolution, and cumulative crystallographic orientation deviation. The inclination angle of the solidification interface relative to the epitaxial growth direction governs these behaviors. The angle controls the transition of the dominant factor in stray grain formation between directionality and magnitude of solidification parameters, making melt pool regions with large deviation angle or low thermal gradient prone to nucleation. It also determines the competitive growth relationship between stray grains and epitaxial dendrites, resulting in cross-layer growth or truncation of stray grains. Furthermore, it gives rise to horizontal orientation deviations during intertrack overlap, which accumulate across layers and form vertical grain boundaries along the building direction. These insights overcome the limitations of understanding local microstructural evolution based solely on the magnitude of solidification parameters, highlight the critical role of interface morphology during high-dynamics solidification in additive manufacturing, and establish a unified morphology–microstructure causality framework, providing strategic guidelines for stabilizing single-crystal epitaxy in L -PBF via melt pool geometry control, alloy design, and texture optimization.