Carbon fiber-reinforced thermoplastic (CFRTP) composites are now widely used in many fields. Ultrasonic welding (UW) is a key technology for joining these materials. The control mode of UW has a great effect on the quality of the welded joints. However, there is still not enough research comparing the different welding control modes. This paper investigates the effects of the time control, energy control, and displacement control modes on the ultrasonic welding quality of carbon fiber-reinforced polycarbonate (CF/PC). A flat PC film is used as the energy director (ED). The evaluation focuses on the lap-shear strength (LSS), macro- and micro-morphology, fracture surface characteristics and power–displacement curves of the welding process. Furthermore, significant differences are observed in the temperature field evolution and joint failure modes across the different control modes and process parameters. Results indicate that the displacement control mode achieves the highest joint quality and process stability, yielding a maximum LSS of 30.6 MPa. A correlation analysis reveals that the displacement–energy relationship exhibits the strongest coupling, and the Pearson correlation coefficient r is 0.896.

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.