Dissimilar metal joining of aluminum to steel by hybrid process of adhesive bonding and projection welding using a novel insert element

Multi-material design offers cost-efficient lightweight solutions for automotive body-in-white production. It can be realized by joining ultra-high strength steels with aluminum alloys, using adhesive bonding, combined with another commonly used joining process. For these applications, processes like riveting, clinching, resistance spot welding have been pushed to their technological limits and a further technological development is needed. The present research describes a new joining technology based on resistance welding process for joining aluminum-silicon coated hot stamped ultra-high strength steel with aluminum. The technology consists of a two-stage projection welding process with an additional insert element. Its implementation allows joining aluminum and steel sheets using extremely short welding time with high energy concentration in combination with adhesive bonding. Due to low heat input in the welding process, the adhesive around the insert element is damaged significantly less, compared to classic resistance spot welding. Depending on insert element size and orientation, short flanges can be joined, offering potential for further weight reduction of the car body. The elements are easily applicable under different production constraints. One of possible applications of the investigated joining process is welding a thin outer aluminum sheet on a B-pillar. Welds were made using insert elements - cylinders of Cu- and Fe-based wires with diameter of 1.6mm and length of 10mm employing a medium frequency direct current resistance welding machine. Both tested materials can be successfully used and the weld current ranges are sufficient for industrial application with and without the use of adhesive. Mechanical properties of joints, evaluated using tensile-shear tests, are comparable with those of the existing mechanical joining methods used in automobile manufacture.