Investigating the reparability of fusion bonded metal-plastic composites for improved circularity

Abstract

Multi-material structures have long been established for various components in the automotive industry in the context of lightweight construction, with metal and fiber-reinforced plastic in particular being advantageously joined together in the form of so-called hybrid designs. The joining technology plays a decisive role here with regard to the hybrid component's performance and aging resistance. However, the complexity of component manufacture and the feasibility of subsequent repair, reprocessing and recycling concepts are largely determined by the choice of joining technology. The aim of the present work is to investigate the reparability of joints produced fusion bonding on a fundamental level. Lap shear specimens consisting of a laser-structured, metallic joining partner and a fiber-reinforced thermoplastic (FRTP) sheet are used as a basis for evaluation. The test sequence comprises an initial joining and three consecutive repair processes of the joint by means of fusion bonding, with a destructive lap shear test taking place after each joining process. Two repair approaches (with and without an additional polyamide-6 film) are investigated. Over three repair processes, a slightly decreasing trend in lap shear strength is found if the repair is carried out without additional polyamide-6 film. Using a polyamide-6 film during the repair leads to higher strength in every case tested. Thermal analysis and microindentation on the FRTP substrate takes place in order to quantify the influence of the heating required for repair. A negative effect on the degree of crystallinity and the strength of the polyamide-6 matrix close to the steel-FRTP interface is found. Nevertheless, based on the criteria derived from DIN EN 45554, the conclusion is drawn that the reparability of metal-FRTP joints produced by fusion bonding is possible on a sample scale. For future work, a more in-depth study of the joining surfaces, especially the metal part, as well as the inclusion of an aging step before each lap shear test should be considered.