Fiber-reinforced thermoplastics (FRTPs) have become a new generation of lightweight materials due to their superior mechanical properties, good weldability and environmental resistance, potential for recycling, etc. The market for FRTPs is expected to grow at a compound annual growth rate (CAGR) of 7.8% from 2022 to 2030. Many researchers have been trying to solve the problems in their processing and joining process, and gradually expand their application. Resistance welding is one of the most suitable techniques to join FRTPs. This paper summarizes the research progress of FRTP resistance welding in terms of the basic process of FRTP resistance welding, factors affecting joint performance, joint failure behavior, numerical simulation, weld quality control, and resistance welding of thermoplastic/thermoset composites. The objective of this paper is to provide a deeper insight into the knowledge of FRTP resistance welding and provide reference for the further development and application of FRTP resistance welding.

This study employs 304 stainless steel perforated mesh (SS mesh) as the heating element for the resistance welding of continuous carbon fiber-reinforced polycarbonate (CCF/PC) sheets. An electro-thermal coupled finite element model is developed to investigate the effect of SS mesh structural parameters (aperture shape, aperture area, mesh thickness) and clamping distance on the welding temperature field. The model accurately predicts peak temperatures, with errors of 1–4% compared with experiments. Under identical aperture area, the SS mesh with longer effective current path length and smaller effective cross-sectional area has higher resistance. In addition, the resistance increases significantly with decreasing mesh thickness and increasing aperture size. Reducing the clamping distance effectively improves temperature uniformity across the weld zone and mitigates edge overheating. A novel mesh structure—featuring larger aperture in the welding region and smaller aperture in non-welding region, is designed to improve the temperature uniformity and joint quality. Under optimized welding parameters (14 A, 40 s welding/holding, 0.3 MPa), the joint achieves a maximum tensile shear force of 9.851 kN, a 13.1% improvement over conventional uniform-aperture mesh (8.713 kN).

Thermoplastic composite resistance welding boasts stable process, low cost and reliable quality, making it a dependable joining technique for such materials. This process employs a heating element (HE) as the sole heat source and therefore, it is critical in controlling the welding process. This study proposed a perforated stainless-steel mesh (SSM) as the HE and investigated the effect of welding parameters and insulation film thickness on the joint performance of resistance welded carbon-fiber-reinforced polyamide 6 (CF/PA6). The results showed that the joint lap shear strength (LSS) increased first then decreased as the welding pressure, welding time and welding current increased. The maximum LSS reached 24.4 MPa when 0.2-mm-thick films were used. The joint failure mode was identified as blocky fiber peeling with compromised fiber continuity for the joints welded with 0.1-mm-thick and 0.3 mm-thick PA6 films. For the joints made with 0.2-mm-thick PA6 films, the joint failure mode was characterized by resin peeling from the fiber surface.

During resistance welding of fiber-reinforced thermoplastic composites (FRTP), lower temperatures are observed at the edge of the welding area that is vertical to the direction of the current (longitudinal edge, L-edge). This paper proposes a perforated heating element (HE) with reduced edge mesh size to increase the temperature in the L-edge, thereby mitigating the edge effect during the resistance welding of FRTPs. By reducing the mesh size in the L-edges of the welding area, a temperature gradient can be created during the heating process, thereby increasing heat generation at the L-edges of the HE. Through experimental and numerical investigations, the influence of this HE on temperature evolution and joint performance during FRTP resistance welding was investigated. A resistance model was proposed to analyze the resistance and heat generation of regions with different mesh sizes. The results show that the tailored HE increased the temperature in the L-edge area. Furthermore, the reduced mesh size decreased the resistance of the HE in the welding area, mitigating the excessive temperature in the transverse edge (T-edge) (parallel to the current direction) of the welding area. The joints welded with the tailored HE achieved a lap-shear strength (LSS) of 26.6 ± 0.8 MPa, which is 31% higher than the LSS of joints welded with unmodified HE. The proposed resistance model could predict the changing trends of resistance and heat generation in regions with different mesh sizes at the qualitative level. Further work needs to be carried out to improve its quantitative prediction ability.