Epitaxial Growth in Additive Manufacturing of High-$\gamma'$ Nickel-based Superalloys: Solidification Dynamics, Defect Mitigation, and Hybrid Synergy

Additive manufacturing (AM) is redefining the limits of directional solidification (DS) and single-crystal (SX) fabrication for nickel-based superalloys. By reconciling classical Bridgman theory with the extreme thermal gradients (G) and solidification velocities (V) inherent to AM, this review establishes a unified framework for controlled epitaxy. It dissects the kinetics of grain competition and the columnar-to-equiaxed transition (CET), highlighting how scan strategies and thermal management dictate melt pool geometry and the G/V ratio. A comparative assessment of laser powder bed fusion (L-PBF), electron beam powder bed fusion (EB-PBF), and directed energy deposition (DED) delineates distinct process windows for direct fabrication versus epitaxial repair. Addressing the conflict between printability and performance in high-$\gamma'$ alloys, the text links defect mechanisms, specifically stray grains (SGs) driven by constitutional supercooling, complex cracking modes, and surface anomalies, to targeted mitigation strategies ranging from virtual grain selection to alloy tailoring and synergistic post-processing. Ultimately, a multi-process hybrid architecture is proposed that positions EB-PBF as the foundational platform for bulk SX formation, augmented by L-PBF or DED, to close the "manufacture–service–repair–remanufacture" engineering loop.