AddJoining is a novel additive manufacturing approach to produce metal-polymer hybrid structures. The technique was developed as an alternative to produce layered metal-polymer hybrid structures in substitution to the ones obtained by adhesive bonding and lamination process, such as co-curing and co-bonding, with higher manufacturing flexibility and shorter cycles, out of autoclave and without mechanical fasteners. The present work is aimed at presenting an overview of the AddJoining technique with respect to its principles, state of the art, material combinations, mechanical properties and possible application scenarios. In specific, two examples of material combinations are focused: the AA2024/CF-PA6 (continuous fibers) and the Ti-6Al-4V/PA-CF (short fibers).

On the aluminum-based hybrid joints, an ultimate lap-shear strength (ULSS) of 22.0 ± 1.2 MPa was achieved, which was 19% higher than adhesively-bonded joints using the same adherends. Under fatigue, those hybrid joints surpassed 30% of the ultimate lap-shear force at 105 cycles, which is a conventional aircraft certification benchmark. As for the titanium-based hybrid joints, the ULSS reached 23.9 ± 2.0 MPa, despite resorting to considerably lower substrate temperatures. Moreover, by applying a modified three-point bending test (ISO 14679:1997), a strong correlation between substrate roughness and joint strength was observed; this is a result of the polymer being able to flow around and penetrate into intricate surface irregularities, thereby enhancing the micro-mechanical interlocking. Finally, among the possible application scenarios, the examples of a hybrid skin-stringer and a topology-optimized hybrid floor beam were demonstrated, whereby a decrease in weight was achieved either via materials selection or material removal.