Study on thermal-assisted underwater friction stir welding for carbon fiber-reinforced polyetherimide

Friction stir welding (FSW) of carbon fiber-reinforced thermoplastic has shown attractive applications in industrial fields. However, the heat input during the FSW is hard to control, which leads to significant material loss and carbon fiber damage. In this paper, a carbon fiber-reinforced polyetherimide (CF-PEI) plate is joined via thermal-assisted underwater friction stir welding (UFSW). The evaluation of heat-assisted UFSW joints is conducted by studying the surface morphology, microstructure of the weld interface, the state and distribution of carbon fibers, and tensile analysis. The findings indicated that water can reduce heat input, and the water temperature has a significant impact on the weld properties. Specifically, at a water temperature of 60°C, there is a considerable reduction in material loss and carbon fiber damage. The study also reveals that this temperature facilitates the interweaving of carbon fibers into a stable structure at the interface and their integration with the PEI substrate. This reduces the inhibition of the material on carbon fiber elongation, ultimately enhancing the bonding interface and improving tensile shear properties. The maximum tensile strength value of the welds reached 33.6 MPa, equivalent to 79.6% of the CF-PEI sheet. Compared to FSW, joint tensile strength is increased by 45.5%.