There are several ways for improving passenger vehicle fuel efficiency; among which reduction in the automobile weight emerges at the top of the list. Of the many weigh reduction schemes, a multi-material automotive body-in-white (BIW) design presents a cost-efficient way to reduce vehicular weight. Based on recent research, the components which have low package restriction and functions as shear web to enhance the BIW stiffness are appropriate for Aluminum (Al) usage. Mechanical joining methods are appropriate for joining dissimilar materials in BIW structures. There exist a variety of mechanical joining methods that use different processes. Overall, an in-depth probe of the most industrially feasible mechanical joining methods (i.e. self-piercing riveting (SPR), Clinching, flow-drill screwing, and highspeed joining)) reveals that they are suitable for joining dissimilar materials; however, as a result of their various technical limitations, they are deemed insufficiently flexible. As such, they cannot be used to easily produce BIWs made both of mono- and multi-materials. Additionally, the use of these approaches would also require significant changes to the existing resistance spot welding (RSW)-based BIW assembly lines, which results in retooling and investments in new manufacturing facilities.
Therefore, with the ongoing campaigns against fossil fuels gaining pace by the day, it is imperative that a joining technique that could overcome the aforementioned shortfalls of current techniques be developed. In this view, researchers from the Institute of Automotive Lightweight Design at the University of Siegen in Germany: Professor Xiangfan Fang and Dr. Fan Zhang proposed a novel hybrid joining method called resistance rivet spot welding (RRSW) for aluminum and steel. The researchers aspired that their novel approach would somewhat overcome the technical limitations of the existing joining methods and be easily assimilated within the existing spot-welding units in BIW assembly lines. Their work is currently published in the Journal of Materials Processing Technology.
Generally, the proposed RRSW technique also made use of a steel element as a medium to make two different materials (Al and steel) weldable. The RRSW consisted of two main procedure: riveting and welding. For the researchers to realize the process concept for RRSW, they first determined the proper rivet material and geometry, as well as the form of the die. Overall, a special FEM-optimized rivet was used to clamp the aluminum sheet on both sides, with one flat and one nearly flat surface. The rivet had two functions: as an adapter to weld the aluminum part with the other steel parts, and to separate the steel and aluminum parts to avoid contact corrosion.
The authors reported that with the optimized rivet geometry and proper welding parameters, RRSW created steel-aluminum combinations in BIW that were of at least equal strength to, and usually stronger, than self-piercing riveting. In addition, the researchers observed that the RRSW’s corrosion resistance was superior to SPR’s. Moreover, based on their sample verification study, the researchers noted that by using the same welding conditions as for the steel roof the RRSW quality was good.
In summary, the study presented the development of a novel joining method termed “resistance rivet spot welding (RRSW),” coupled with a detailed description of the welding process and the resulting weld joint. Generally, the study demonstrated that with the right dimensions of the rivet and welding parameters, very good welding results could be achieved. In a statement to Advances in Engineering, Professor Xiangfan Fang highlighted that their novel technique; i.e. RRSW, was flexible, economically justifiable and therefore presented a viable alternative to SPR.
Xiangfan Fang, Fan Zhang. Hybrid joining of a modular multi-material body-in-white structure. Journal of Materials Processing Technology, volume 275 (2020) page 116351.