Metal jet emission and weld interface formation in impact welding were investigated for similar- and dissimilar-metal lap joints. Numerical simulation of oblique collision between metal plates was performed using smoothed particle hydrodynamics (SPH) method for various plate thicknesses, collision velocities, and collision angles. Metal jet emission and formation of the characteristic wavy weld interface in impact welding were reproduced successfully. The composition of the metal jet was governed by the degree of relative density difference between two metals. When the density difference was large, such as Al/Cu and Al/Ni lap joints, the metal jet was mainly composed of the metal component with lower density, Al. On the other hand, when the density difference was small or zero, such as for Cu/Ni and Al/Al lap joints, the metal jet was composed of both metal components. Several types of lap joints were fabricated by magnetic pulse welding (MPW). Metal jets emitted from Al/Cu and Cu/Al lap joints were collected, and their components were analyzed by X-ray diffraction. The microstructure of the weld interface was also examined. The experimental results were in good agreement with the simulation results.