Resistance insert spot welding: a new joining method for thermoplastic FRP–steel component

Recently, multi-material designs consisting of metal and FRP components are being increasingly used in vehicle body structures to reduce weight and thus energy consumption and emission. Because the most commonly used resistance spot welding (RSW) technology for body assembly cannot be applied to join sheet metals and FRPs, the usage of FRPs is strongly limited. Therefore, a new resistance insert spot welding (RISW) method was developed. During the manufacturing process of FRPs, here compression molding of PA6 GF40 materials, specially designed steel inserts are directly integrated into the FRP part. Later, via the steel inserts, the FRP part can be spot welded to other steel parts in the structure. In this work, the design and development process of RISW is presented. Starting with the design concept of inserts, welding tests were conducted, and the parameters for the welding simulation were calibrated. After that, the detailed insert design was performed using welding simulation and tests with several restrictions, such as insert dimensions, the temperature in FRPs, the weldability range, and joining strength. In the end, an appropriate insert geometry was found, which met all requirements. The mechanical properties of RISW joining are better than or equal to those of SPRs. The new RISW technology was also demonstrated on a real vehicle component. A seat cross member was designed using FRP with 42% weight reduction and produced by compression molding with 30 simultaneously integrated inserts. The cross member was finally successfully welded to a steel closing plate using RISW.