Effect of the forming behavior on the impact flash during magnetic pulse welding of tubes

Magnetic pulse welding (MPW) utilizes the oblique high-speed collision of an electromagnetically accelerated metallic tube or sheet with a joining partner. The arising interface pressure leads to a cold weld without critical intermetallic phases at dissimilar material combinations. Realizing the MPW process with as low pressures as possible helps to increase the tool coil's life time, since mechanical and thermal loads are reduced compared to welding trials with higher energy input. Therefore, a guideline for choosing suitable combinations of acceleration gap, working length (i.e. the relative axial position between coil and part), and charging energy was developed earlier. The working length was found to be a key parameter in the weld formation process, although the appearance of the impact flash was almost identical for different values. In this paper, in order to explain this phenomenon, numerical simulations with LS-DYNA are applied to study the forming and collision behavior of the outer tube at varying working lengths. It is shown that the collision angle decreases with increasing working length. Additionally, experiments in vacuum atmosphere are carried out to avoid the interaction with the ambient air. The appearance of the impact flash in vacuum is also influenced by the working length. This leads to the assumption that the collision angle determines the thermodynamic parameters in the joining gap and, thus, the weld formation. These findings underline the reasonability of complementary numerical and experimental methods during the adjustment of MPW at the lower energy level.