A novel chemical compound interlayer design strategy for improving metal/CFRTP friction stir lap welding joint strength based on melting characteristic and solubility parameters of polymer

Aluminum/carbon fiber reinforced thermoplastic (Al/CFRTP) composite structures exhibit significant potential for aerospace and rail transportation applications due to their low density and high specific strength. Enhancing the interfacial bonding strength of welded joints through surface pretreatment is a critical focus of current research. Traditional interfacial chemical modification strategies often require separate treatments of both aluminum and CFRTP surfaces, based on the characteristics of their functional groups. However, these methods are hindered by complex reaction pathways and low processing efficiency. This study proposes a novel chemical interlayer design strategy for Al/CFRTP interfaces, employing reversible addition-fragmentation chain transfer (RAFT) polymerization and leveraging the high-temperature melting properties of CFRTP. The developed chemical interlayer integrates aluminum alloy-affinitive monomers with polymer chains derived from CFRTP. Moreover, by replacing nonreactive polyamide 66 (PA66) with polyvinyl pyrrolidone (PVP) based on solubility parameter theory, this approach effectively overcomes the challenges of solvent selection and modification typically encountered with original PA66 molecular chains. The synthesized interlayer features a straightforward synthesis pathway, with design and synthesis strategies that are broadly applicable to common metals and CFRTP. Interfacial morphology analysis reveals that this chemical interlayer significantly reduces interface defects in friction stir lap welded Al/CFRTP joints, thereby enhancing interfacial integrity. The interfacial shear strength improved by 74.6 % compared to untreated joints, with the fracture mode shifting from interfacial failure to cohesive fracture of the resin. Preliminary Fourier transform infrared spectroscopy (FTIR) analysis indicates that enhanced hydrogen bonding interactions are the primary mechanism strengthening Al/CFRTP interface.