Multimaterial design with fibre reinforced plastic (FRP) and metals offers a high potential for resource- and cost-efficient lightweight constructions. However, the availability of suitable joining processes has so far often been an obstacle to the use of FRP in series production. Joining technologies developed specifically for FRP are mostly not adapted to the process chains of series production.

In industry in particular, there is a demand to reduce the number of different joining processes despite the increasing diversity of materials. In this research project, therefore, a new precompetitive technology for low-damage, force-flow-compatible FRP/metal joints was developed using established spot joining processes such as clinching and resistance spot welding.

In this process, metallic multifunctional inserts are embedded into a thermoplastic FRP structure during component manufacture using a pin tool. As a result, the reinforcing fibres are not severed but reoriented in the molten matrix. In the same process step, the insert is embedded and a counter punch located on the die side presses the material from the resulting draft back into the laminate plane, creating a form fit between the insert and the FRP.
Subsequently, the FRP component can be reliably joined to metal structures by means of established spot joining processes with conventional joining tools using the insert as an interface.

Within the scope of the research project, both specific interfaces adapted to the respective joining constraints and multifunctional interfaces that can be used for both clinching and resistance element welding (WES) were developed using simulation. Suitable parameter sets for the process-reliable production of high-quality joints were identified for both the embedding of the interfaces and the subsequent joining processes.

The structural and damage analyses carried out by means of microscopic images and CT examinations enabled the quality-relevant parameters of the joints to be verified and the material structure in the forming zone of the FRP to be described. The joint properties were investigated comprehensively by means of tensile tests under quasistatic and cyclic loading.

Finally, the obtained results were used for the manufacturing of a three-dimensional de-monstrator structure. On the basis of this, application-relevant requirements such as small flange widths, different accessibility of the joints and the simultaneous molding of several interfaces into an FRP structure could be accomplished.