The trend of economical lightweight construction leads to multi-material design. Since resistance spot welding represents a reliable and cost-effective joining method in steel construction and OEMs want to reuse the production lines as far as possible over several generations of vehicles, the use of resistance spot welding is also of great importance in a multi-material design. By means of that, the investment costs and the project risk can be reduced.
In order to enable the direct welding of aluminum components to the steel structure with the existing conventional spot welding systems, a novel joining method "rivet resistance spot welding" (RRSW) was developed at the Institute of Automotive Light Weight Design.
The method is based on the use of a rivet-like steel element as welding adapter in aluminum sheet metal component. During the forming process, this steel element is punched via a riveting tool, which is integrated in the forming tools. Subsequently, the steel rivets are joined to the steel component by means of spot welding.
In this presented study, the appropriate rivet geometry, as well as the joining process parameters, are systematically analyzed and designed by FEM forming simulations. The welding parameters are based on experimental results. The welding processes can be applied with traditional welding machines. For the quantitative evaluation of the connection carrying capacity, quasi-static tensile tests and high-speed tensile tests were carried out. The performance of this new joining technique was observed to be partly better and partly comparable to those of the Self-Pierced Riveting (SPR). By testing on tensile and shear specimens as well as KS-2 specimens an FE-model for the connection was established. The model was then validated by T-bar profiles both quasi-statically and dynamically (with drop tower). Finally, a section of a passenger car roof made of aluminum sheet was produced for validation of the process. For this purpose, a forming tool was developed with the rivet punching devices integrated. The aluminum partial roof was subsequently welded to the adjacent steel components by WNS using robot handling similar to the series process. More importantly, the contact corrosion between steel and aluminum is avoided due to the special geometry of the rivet element.