Powder bed fusion of hard-to-weld Ni-based superalloys: A review of thermal history, in situ monitoring, and crack suppression

Powder bed fusion offers geometric freedom for hard-to-weld Ni-based superalloys, but processing-induced cracks and their evolution during post-processing limit reliable use. This review examines the evidence connecting powder characteristics, thermal history, microstructure, residual stress, and cracking in laser and electron beam powder bed fusion. Representative results are compared with attention to alloy chemistry, measurement method, sampling location, and post-processing condition. The comparison shows why increased density, reduced final residual stress, and absence of observed cracks are not interchangeable measures of process success. In situ monitoring and machine learning are assessed in terms of calibration, label reliability, transferability, and validation against physical defects. Thermo-mechanical models and alloy-design strategies are evaluated alongside their computational and service-performance limitations. An integrated framework and research roadmap identify the measurements and independent validation needed to translate laboratory crack suppression into repeatable component manufacture. Particular priorities are spatially registered thermal and defect data, uncertainty-aware control, and qualification that connects fabrication with fatigue, creep, and environmental durability.