Eliminating the adverse effects of substrate misalignment in ultrasonic welding of CF/epoxy composites using structured energy directors

Ultrasonic welding of carbon fiber reinforced thermoset composites (CF/TSCs) has been successfully achieved in recent advancements through using a coupling layer applied to the TSC surface. However, substrate misalignment during the ultrasonic welding of TSCs can lead to significant temperature inhomogeneities at the welding interface, resulting in localized thermal decomposition of epoxy and ultimately compromising the weld strength. To address this issue, this study introduces a PEI hollow mesh as the energy director (ED) for ultrasonically welding PEI-topped TSCs fabricated via a co-curing process. The mesh features uniformly distributed circular holes with a diameter of 0.8 mm and a center-to-center spacing of 1.4 mm. Compared to film EDs, the usage of mesh EDs significantly improved the temperature uniformity at the welding interface, leading to reduced maximum temperature difference across the welding area from about 94.5 °C to 29.1 °C. This improved thermal consistency led to a more uniform welding line, as confirmed by C-scan analysis, with the percentage of defective regions reduced from approximately 14.2 %–1.4 %. Consequently, the weld strength, assessed through single-lap shear tests, was significantly improved, reaching a maximum lap-shear strength of 28.0 MPa. Moreover, the fatigue life of the joints was over doubled compared to that using film EDs.