Effects of reactive interlayers in magnetic pulse welding

Surface coatings affect the joint formation in magnetic pulse welding processes (MPW). Two types of coatings were identified in former studies. Anodized layers, for instance, are detrimental for the weld formation if they are not removed before or during welding. Contrastingly, a nickel layer on a steel parent part was found to be advantageous since it increased the weld seam length when it was impacted by an aluminum flyer. This paper gives insights into the welding mechanism with nickel coatings during MPW and explains one reason for the improved weld formation. Metallographic analyses showed that the coating is evidently not fractured, but an interlayer between aluminum and nickel is formed. Scanning electron microscopy and energy dispersive X-ray spectroscopy revealed that nickel and aluminum have interacted. The energy release rate of the exothermic reaction is higher than the reaction of aluminum with steel in direct contact. Since all other parameters were kept constant, it is assumed that the additional heat of the nickel-aluminum reaction promotes the welding effect, especially at positions with lower impact pressures. This effect, for instance, enables a significant reduction of the required impact energy for MPW. The formation of the interaction zone was studied for different well defined collision conditions. A newly developed process measurement system was utilized, which records the characteristic process light emission and enables insights into the prevalent collision conditions.