Underwater dissimilar friction stir lap welding of aluminum alloy and CFRTP

With the rapid development of the transportation and electronic equipment industries, the demand for robust joining technology for metal/polymer has become imperative, especially in the pursuit of lightweight solutions. Underwater welding is one of the methods to reduce the formation of holes and other welding defects, which is conducive to improve the quality and strength of joints. In this study, the underwater friction stir lap welding (UFSLW) of 6061-T6 Al alloy and carbon fiber reinforced thermoplastic (CFRTP) joints is investigated. The welds are evaluated by optical microscope (OM) image, scanning electron microscope (SEM), and energy dispersive X-ray spectrometer (EDS) of the fracture, tensile, and hardness analysis. Research reveals that the presence of cracks and tunnel defects similar to those in air FSLW (AFSLW) is reduced in the underwater welding process. From the microscopic result analysis, the rapid dissipation of frictional heat and substrate viscosity in the cooling medium of circulating water in the stirring zone reduces the interaction between Al alloy and molten CFRTP, which leads to the rapid transformation of the plasticized material into a solid state and reduces the formation of thick stacked layer. In addition, finer Al alloy fragments are obtained in the welding zone due to the fast cooling rate. Ultimately, the maximum tensile strength of 32.12 MPa is achieved, which is 10.76% higher than that of the same welding parameters in air. Based on UFSLW technology, the joint strength of this study is 69% higher than that of Al alloy and polymer joints, which demonstrates enormous application potential and opens up new prospects for the joining technology of dissimilar materials.