Hybrid joining of a modular multi-material body-in-white structure

The production of pure steel and steel-aluminum (Al) bi-material body-in-white (BIW) in the same assembly line requires the use of joining techniques with high flexibility, and this is not currently possible with state-of-the-art mechanical joining techniques. This work therefore developed a hybrid joining technique, rivet resistance spot welding (RRSW). In this process, a special FEM-optimized rivet clamps the Al sheet on both sides, with one flat and one nearly flat surface. It functions as an adapter to weld the Al part with the other steel parts and separates the steel and Al parts to avoid contact corrosion. Experiments demonstrated that, with optimized rivet geometry and proper welding parameters, RRSW created steel-Al combinations in BIW that were of at least equal strength to, and usually stronger, than self-piercing riveting (SPR). RRSW's corrosion resistance was also superior to SPR's. To verify the sample level development, a steel roof was redesigned to use Al, and this was then welded to the adjacent steel parts using fully-automated spot welding unit. Using the same welding conditions as for the steel roof the RRSW quality was good. The same welding system can thus be used both for pure steel and for steel-Al multi-material BIW assembly. RRSW is therefore a flexible and economical joining method and a viable alternative to SPR.