Investigation of the room- and low-temperature performance of resistance-welded CF/PEKK joints: effects of welding time and double-welding

This study examined the combined influence of welding time, double-welding (DW), and service temperature on the integrity and mechanical performance of resistance-welded CF/PEKK joints. A DW methodology was introduced and evaluated as an additional processing route for the purpose of joint re-activation, rework, and reconsolidation. A resistance-welding processing window was established through thermal characterisation to enable controlled melting and crystallisation. Single- and double-welded joints produced at different heating-stage durations were assessed by optical microscopy, micro-CT, post-weld comparative crystallinity analysis, single-lap shear testing at room temperature and −50 °C, and dynamic mechanical analysis. Increasing welding time improved consolidation, reduced voids, and increased room-temperature lap-shear strength in single-welded joints. DW improved joint quality at shorter welding times; however, at longer welding times, it reduced room-temperature strength despite lower void content, indicating welding-cycle-dependent changes in the post-weld state. At −50 °C, both joint types exhibited higher strength and a more brittle response, with behaviour governed primarily by matrix stiffening rather than porosity differences. The results demonstrate that the performance of resistance-welded CF/PEKK joints is controlled by the combined effects of welding time, the second welding cycle, and service temperature. Double-welding, therefore, introduces an additional processing degree of freedom to improve robustness in resistance-welded CF/PEKK joints.