Ex-Situ and In-Situ Hybrid Processing Opportunities in Metal Additive Manufacturing.

Metal additive manufacturing (AM) enables near-net-shape fabrication of complex components, but process-induced defects and heterogeneities continue to limit performance and broader adoption. This review examines ex-situ and in-situ hybrid processing across multiple AM processes and alloy systems, including heat treatment, deformation-based processing, hot isostatic pressing, machining, friction stir processing, ultrasonic and magnetic-field assistance, alloying, stirring, and surface preparation. Ex-situ routes modify the completed build through bulk densification, homogenization, phase control, and surface improvement, whereas in-situ routes modify defects, microstructure, residual stress, and phase evolution during fabrication by interacting with the liquid melt pool, viscoplastic material, or solidified layer(s). The in-situ hybrid processing literature is dominated by wire-based directed energy deposition (DED) because its open architecture facilitates secondary-tool integration. Deformation-based methods were the most common and frequently promoted pore closure, recrystallization, texture modification, residual-stress redistribution, and improved mechanical performance. Hybrid processing was most effective when integrated to target the dominant material-specific limitation. Aluminum alloys benefited primarily from porosity control, stainless steels from grain and texture modification, titanium alloys from prior-β and α/β morphology control, nickel-based superalloys from segregation and Laves-phase mitigation, and steels from control of transformation microstructures. This material-state-aware framework may also extend to polymers and composites.