Joining techniques for metal additive manufacturing parts: A literature review

Additive Manufacturing (AM) is a revolutionary technology emerging as power tool of manufacturing for different geometry components at reduced costs and better functionality. Various metal additive manufacturing technologies such as Laser Powder Bed Fusion (LPBF), Wire Arc Additive Manufacturing (WAAM), and Directed Energy Deposition (DED) enable the fabrication of complex and customized metallic components. However, the integration of AM parts into functional assemblies often requires reliable joining to other AM components or conventionally manufactured materials, which introduces unique metallurgical and process challenges due to AM-specific microstructures, inherent defects, residual stresses, and surface conditions. This review provides a comprehensive assessment of both similar and dissimilar joining of base metals (AM–AM and AM–wrought) using a broad range of joining techniques applicable to metal AM parts, including fusion welding processes, solid-state welding methods, brazing, and hybrid joining methods. This review also critically elaborates the influence of AM process characteristics on joint formation, defect evolution, mechanical performance, and strategies for improving joint quality, such as optimization of parameters, tailoring filler materials, interlayers, and pre/post-processing treatments. Emerging developments, including machine-learning-assisted process optimization, in-situ monitoring, closed-loop control, functionally graded interfaces, multi-material AM, and digital-twin-based process planning, are highlighted as promising pathways toward robust integration of AM components, which identify critical knowledge gaps and outline future research directions for achieving reliable and scalable joining solutions for metal AM systems.