High-velocity impact welding has usually been presumed as a solid-state manufacturing process. In this study, by integrating meshfree numerical simulations, advanced interfacial characterizations, and diffusion calculations, the relationships between the formation of microstructural defects within the bonding area and melting phenomena were determined. It was also inferred that the emergence of significant grain refinement in the vicinity of the weld interface could be associated to melting and subsequent recrystallization due to the ultra-fast heating and cooling rate of the high pressure impact process. The new results challenge the prevailing belief regarding the solid-state nature of the impact-based welding processes .