Metal-polymer friction stir spot welding enhanced by meso-mechanical interlocking

Abstract The main goal of this study is to present an alternative method of pretreating metal surfaces, entitled mechanical grooving, for metal-polymer joining by friction stir spot welding (FSSW). This pretreatment consists of the successive passage of a rotating engraving tip on the metal surface, creating a multilinear textured pattern, being an innovative pretreatment for the production of hybrid structures by FSSW. Mechanical grooving results in the creation of a well-defined serrated profile that, when interacting with the polymer during welding, promotes the mutual anchoring of the dissimilar materials. Mechanically grooved and grinded aluminium were joined to polyamide 6 by FSSW, and the welds were characterised based on cross-section morphology, fractography, tool axial force, and tensile-shear testing. FTIR-ATR characterization demonstrated that the chemical degradation is not significant during welding and that at the metal/polymer interface, there is adhesion between the materials. The axial force during the plunging stage of welds produced with the mechanically grooved aluminium is around 900 N, while for the welds produced with grinded aluminium is close to 1900 N. Furthermore, the resulting meso-mechanical interlocking mechanism provides superior mechanical performance (4.00 kN ± 82 N) compared to welds executed using grinded aluminium (3.02 kN ± 420 N). Finally, different fracture modes were observed depending on the surface pretreatment used. For instance, while adhesive failure was observed in the grinded aluminium welds, a mixed adhesive-cohesive failure was observed in the mechanically grooved aluminium welds. The cohesive failure is mainly attributed to the good performance of the interlocking mechanism.