Welding of thermoplastics printed by fused filament fabrication: A critical review of joining mechanisms and relationships between processing, structure, and properties

Fused filament fabrication (FFF) technology, also known as fused deposition modeling (FDM), enables the production of thermoplastic components with complex geometries, but remains limited by the printer’s build volume. Welding can join separately printed components. However, layered structures, infill configurations, porosity, and interlayer bonding affect joint formation and strength. This article critically reviews six welding techniques for FFF-printed thermoplastic components: friction stir welding, friction stir spot welding, rotary friction welding, ultrasonic welding, laser joining, and microwave-assisted welding. The review focuses on joining mechanisms, process parameters, material characteristics, mechanical performance, and failure mechanisms. The results indicate that joint quality depends on the interaction among welding parameters, polymer type, infill structure, tool or interface design, and joint configuration. Increasing process energy does not always improve joint strength because excessive heating can cause thermal degradation and deformation, while insufficient material consolidation can limit joint integrity. Direct comparisons between techniques are still limited by differences in materials, specimen geometries, joint configurations, and testing methods. Evidence regarding microwave-assisted welding for joining separately printed components is also still limited. Future research needs to integrate printing and welding parameters, correlate structural characteristics and defects with mechanical performance, and improve the consistency of testing and reporting. Such an approach is necessary to clarify the relationships between process, structure, and properties and to support the development of reliable joints for engineering applications.