Five aluminum plates with 0.3mm-thick were overlapped on a one-turn coil of the discharge circuit and seam welded by magnetic pressure. Deformation and collision behavior of each plate was detected using a high-speed video camera to reveal the characteristic instantaneous welding process. Traveling velocity of each plate was also calculated based on the detected discharge and collision signals. Morphological change of the joint plates and microstructure of the welded interface were examined and the effect of welding condition was discussed. The first collision took place between the first and the second plate over the coil. The two plates coalesced and then hit the third plate. This event occurred repeatedly and multi-layered joining was achieved. Two parallel seam-welded areas were formed along the side edges of the coil, but the area between them was left un-welded. The width of the un-welded area gradually changed through the interface. The maximum wavelength and amplitude decreased with the distance from the coil. Such differences in macro- and microscopic features of interfacial morphology can be explained by the required collision velocity and collision angle for the high-speed solid-state welding by oblique collision of two metal plates

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