Enhancing bonding synergy and mechanical response of metal/composite hybrid joints through physicochemical surface pretreatment

Light metal/carbon fiber reinforced polymer (CFRP) hybrid structures have been considered a key component for lightweight and advanced multi-material structures in modern automotive and aerospace applications. In this study, directional physicochemical surface pretreatment routes based on the conceptual process of geometric configuration and chemical modification were designed for the reliable joining of A6061 aluminum alloy (Al alloy) and carbon fiber reinforced polyamide 66 (CFRP) via friction stir lap welding (FSLW). The effects of surface pretreatment parameters were clarified and an effective coupled treatment strategy was proposed. Micro-textures with uniform geometric grooves with average width w=13.17±3.27μm and average depth d=122.45±12.83μm processed by laser ablation (LA) and flower-like AlOOH nanostructures synthesized by hot water treatment (HWT) promoted mechanical interlocking and chemical bonding at the macro/micro/nanoscale. For this reason, the high interfacial bonding strength of 30.2 MPa and joint efficiency of 67.2% were achieved, and continuous superficial resin of the CFRP matrix adhered to the fracture surface of Al alloy during the failure process. More effectively, the interfacial structures and chemical compositions were analyzed by transmission electron microscopy (TEM), and the formation of C-O-Al and hydrogen bonds was attributed to the γ-Al2O3·yH2O layer and AlOx·yH2O atom clusters, which was verified to enhance the interfacial bonding. This first exploratory study on Al alloy/CFRP interfacial bonding at the nanoscale based on physicochemical coupling surface pretreatment provides an in-depth understanding of the joining of light metals and polymer-matrix composites.