In situ covalent bond engineering at CFRTP/aluminum interfaces via zinc-assisted friction lap soldering Welding: Mechanisms and performance enhancement

The weak interfacial reliability of carbon fiber-reinforced thermoplastic (CFRTP)/aluminum alloy hybrid joints remains a critical bottleneck in lightweight transportation applications. Current work processes an in situ covalent bond engineering within the hybrid structures and develops a novel friction lap soldering welding (FLSW) technique. Integrating a Zn solder interlayer reconstructs bonding behavior by synchronously removing oxide barriers, enhancing atomic diffusion, and promoting covalent interactions. The CFRTP/AA5052 aluminum (5052) FLSW hybrid structures achieve a tensile shear strength of 126.75 N/mm, surpassing the spontaneous fracture joints manufactured by traditional friction lap welding. Importantly, the Zn interlayer enhances interfacial compatibility, increasing the work of adhesion by 40.23 % compared to direct CFRTP/5052 bonding. Microstructural analysis confirms a defect-free Zn–Al eutectic layer formed via Al–Zn interdiffusion. Systematic experimental and density functional theory (DFT) analyses indicate the dual covalent bonding pathways of Zn–O and Al–O covalent bonds formed within the FLSW hybrid structures. Higher Al doping content in α-Zn enhances mechanical properties, lower anisotropy, and stronger covalent bonds. The Zn–Al eutectic layer exposes both Zn and Al atoms for covalent bonding with oxygenated groups of CFRTP, whereas traditional hybrid joints relied solely on sparse Al–O bonds from surface oxides. Current work establishes FLSW as a scalable, oxide-removal strategy for high-performance CFRTP/aluminum joints, advancing lightweight hybrid structure fabrication through interfacial covalent bond engineering.