Metal Additive Manufacturing of Engineering Alloys: Process–Material Interactions, Defect Formation, Property Optimization and a Decision-Making Framework

Metal additive manufacturing (MAM) has evolved into a strategic manufacturing route for producing, repairing, and remanufacturing high-value components; however, process selection and optimization remain challenging because manufacturing conditions, alloy behaviour, defect formation, microstructural evolution, and final properties are strongly interdependent. This review provides a critical and comparative assessment of four representative MAM technologies—Laser Powder Bed Fusion (L-PBF), Laser Metal Deposition (LMD), a laser-based variant of Directed Energy Deposition (DED), Wire Arc Additive Manufacturing (WAAM), and Metal Binder Jetting (MBJ)—with particular emphasis on their interactions with titanium alloys, nickel-based superalloys, steels, and aluminium alloys. Rather than comparing these technologies solely in terms of productivity, cost, or geometrical capability, the review examines the physical and metallurgical mechanisms governing thermal history, solidification or densification, phase evolution, defect formation, microstructure, and resulting component performance. Particular attention is given to porosity, lack of fusion, cracking, residual stresses, distortion, anisotropy, and surface integrity, together with process-specific mitigation and post-processing strategies. Based on the relationships identified across the literature, an integrated Process–Material–Defect–Property (PMDP) framework is proposed to connect manufacturing conditions and alloy-specific responses with defect and microstructural evolution, resulting properties, and in-service performance. A complementary PMDP-based decision pathway is introduced to support performance-driven alloy–process selection, risk identification, thermal and parameter optimization, post-processing definition, and final performance validation. The analysis demonstrates that no universally optimal MAM technology exists; instead, process suitability depends on the compatibility between material behaviour, manufacturing physics, component requirements, and the targeted material state. Future progress is expected to rely on transferable process–material maps, additive manufacturing (AM)-specific alloy design, mechanism-informed in-situ monitoring, physics-informed data-driven modelling, and adaptive closed-loop control. The proposed framework therefore provides a structured pathway towards material- and performance-driven, state-controlled, and ultimately performance-certified metal additive manufacturing.