| The main objective of this research is to investigate the magnetic pulse welding (MPW) process, the resulting joint strength and the interface welding metallurgy. It is known that traditional fusion welding is not able to weld dissimilar materials with different thermal properties, but MPW has been applied to both similar and dissimilar materials. MPW is a solid state impact welding technology that may provide metallurgical bonding without melting and solidification, and there is no significant heat affected zone (HAZ). It shares the same physical principle with explosive welding (EXW) and is good for workpiece with length scale in the order of millimeter to centimeter. This research first compares three impact welding technologies, EXW, MPW and laser impact welding (LIW), for different length scale with different driving system. The joining is likely the result of an instability associated with jetting, which scours the surface clean during impact. It was found that the metallurgical bonding by oblique impact was only realized at the combination of certain impact velocity and impact angle. Taking 0.254mm thick copper alloy 110 plate to plate joint as an example, the suitable impact velocity was larger than 250m/s and the suitable impact angle was within 2º&sim7º. As a result, the joint interface undergoes high strain rate deformation and the strain rate was established in the order of 106&sim10 7s-1. In this research, aluminum alloy, copper alloy, and low carbon steel were welded by MPW using electromagnetic force to accelerate the metal. The strengths for both similar and dissimilar material joints were examined by lap shearing test, peeling test and microhardness test, and the similar materials joint was also tested by nanoindentation. The joint strength was greater than that of the base metals and the welded region was hardened by high velocity impact. The tensile failure and peeling failure were outside of the welded region. The welded interface microstructure was characterized by optical microscopy, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and field ion atom probe microscopy. The welded interface indicated wave interface morphology. Due to adiabatic heating and high strain rate deformation, the welded interface microstructure exhibited various features, including grain refinement with large grain boundary misorientation angle, formation of band structure, deformation twins, lamellar grains, and high dislocation density (&simestimated to be 1011m -2). Similar materials welding suggested pure solid state bonding, while dissimilar materials joints sometimes contained small amount of intermetallic phase distributed discontinuously in the vertex region of certain waviness along the welded interface. In order to identify the coupled thermal, electromagnetic, and mechanical properties during the impact welding process, a finite element analysis was attempted via LS-DNYARTM. Some of the simulation results were compared to the instrumented experiments and the validated model can be used to investigate the successful welding criterion with regard to the impact angle and impact velocity for other materials. |
