The bonding interface of pure Al – galvanized steel spot magnetic pulse welds was characterized at the atomic scale. The Al-Zn interface microstructure is worthy of attention as it contains a few micrometers thick biphased layer. This singular layer consists of an aggregate of grains with a 200 to 400 nm size containing islands of either Al or Zn solid solutions oversaturated in Zn or Al, respectively. Inside a grain, some mottled contrasts with a size in-between 5 and 10 nm correspond to fluctuant solute contents. Besides, some native oxides have been dissolved at the interface suggesting the incorporation of oxygen atoms in the biphased layer. Large and elongated (Al,Fe,Si) particles primitively contained in the base Al were also dissolved which gives rise to some round shaped Si particles in the biphased layer. All these microstructural features and changes are consistent with a ballistic origin. Due to both the absence of solidification defects and the rather planar morphology of the bonding interface, the process is expected to occur at the solid state.