Joining by upset bulging – tooling design and new concepts for online process control using servo presses and local heating

Conventional fusion-welding techniques pose limitations in modern multi-material assemblies due to the heat input. The negative influence of the heat-affected zone on the material properties must be avoided particularly in high-strength steels. During mechanical joining processes, which may also be used for joining of different materials e.g. steel and aluminium, high-performance joints can be produced without degradation of the material properties. Joining by upset bulging is an innovative joining method based on plastic buckling of tubes under axial compression. In previous investigations of joining by upset bulging the process parameters were determined and joints of tubes with sheet metal were tested under static and dynamic loads. The results of these studies have shown that the process of upset bulging has a large potential for the production of joints of tubes with sheets and plates. The present paper focuses on the tooling concepts for an efficient use of the technology and describes methods to control the forming and joining process. Two aspects are covered: (i) the application of servo-presses for joining by upset bulging and their possibilities for on-line process control, (ii) a moderate, process-integrated local heating of the forming area to avoid the formation of cracks and to further increase the strength of the joints.