The cause of wavy interface morphology during impact welding is debated in the literature. In this paper, the effect of internal stress waves is investigated by varying the target (Cu 110) thickness during vaporizing foil actuator welding with constant thickness CP-Ti flyers. Experiments and smooth particle hydrodynamics simulations show that increasing target thickness causes the interfacial wavelength to increase, until it reaches about two times the flyer thickness. Also, to demonstrate the effects of consistency and transients of interfacial parameters, i.e., flyer velocity and wavy interface morphology, multi-probe photon Doppler velocimetry, electron microscopy and X-ray computed micro-tomography results are presented.