Steel-aluminium transition joints for car parts of aluminium weldable to the steel body

A process for joining steel and aluminium sheets to cold-formable hybrid blanks was further developed and a pilot line was built, on the basis of experience gained, with an existing laboratory rolling mill. In the process, the aluminium and the steel sheets converge with an overlap of 5-10 mm and are guided between the joining rolls. One of the joining rolls is grooved, so that the roll forces act mainly on the overlapping area. This limits the necessary roll pressure and simultaneously restricts the material flow in a longitudinal direction. The joining zone is activated and heated by means of a laser, immediately before undergoing a cold-rolling pass. The development work was concentrated on hot-dip galvanised steel in the combination with 6xxx aluminium alloys. Tensile tests have shown that fractures typically occur near to the joining line with a tensile strength near to that of the base aluminium material. The hybrid blanks have been characterised with particular reference to formability, impact resistance, fatigue behaviour and with numerical simulations to describe the effect of thermal cycling during the production of the body-in-white. In collaboration with an industrial partner, an appropriate sealing system was developed with excellent behaviour in typical OEM corrosion tests. The optimisation of the profile of the grooved roll by means of FEM modelling resulted in a better process stability and a more constant width of the overlap area.