Resistance Element Welding of Ultra High Strength Steel to Aluminium

Sammanfattning: The automotive industry strives towards decreased vehicle weight in order to decrease green house gas emissions. To create lighter vehicles, new lighter materials need to be implemented. One issue with new materials is that the traditional joining methods in many cases meet their limitations. New methods must be included in the production line to ensure good joints between the new materials. Historically, cars have been built with steel and the joints have consisted of steel-to-steel combinations. The new materials are for example ultra-high-strength steels and aluminium. These are lighter but are difficult to weld to each other, as steel and aluminium have very different properties. It is also possible to avoid welding and instead join materials mechanically, with rivets and screws for example. Then the steel and aluminium do not need to be welded together. At the same time, it is possible to combine mechanical joining with welding. The method investigated in this study deals with just such a method, Resistance Element Welding (REW). It is based on mechanical joining by punching a steel element, into the aluminium component. Then resistance spot welding is used which is well established in the industry today. Electric current is sent through the two metals to be welded. With the current, the sheets are resistively heated, and fused together. The idea of resistance element welding is that the element punched in the aluminium sheet is welded to the steel. At the same time, the element forms a mechanical joint with the aluminium. In this way the steel and the aluminium are joined together. Various ultra-high strength steels and aluminium alloys were tested to be welded to each other by resistance element welding. The result was presented in 1D-lobes that show which current interval gives an approved joint. The minimum weld size was set to 4,5 mm for all experiments performed in the study. The combinations gave different results, some were more easily welded and had larger approved welding intervals. The reasons for the variations in weldability maybe caused by the material properties and thickness of the steels and the type of coating. The aluminium was not assumed to have a major impact on the welding as it did not participate in the melting process. Sheartensile and cross-tension tests were also carried out to obtain a maximum force. From this it emerged that the most combinations fulfilled the requirements on weldability. But more investigations should be done, for example how it works to weld with adhesive and how it affects the weld if there is a gap between the sheets or if the electrode does not hit the element in the right position.