In the process of magnetic pulse welding the joining between two plates is achieved by the collision of joining partners after acceleration of the dynamic partner (flyer) towards the other, static partner (target). The typical acceleration velocity reaches values of about 10³ m/s. During impact, a characteristic welding zone is formed. In this study we locally characterize the interfacial regions of similar (aluminum/aluminum) and dissimilar (aluminum/steel) joints, with a special focus on the cohesive bonding between plates. The bonding process of two surfaces is associated with strong microstructural changes in the adjacent regions. The microstructures are analyzed by optical, scanning, transmission electron microscopy and selected area electron diffraction. We show that the cohesive bonding of both joint types is achieved through the formation of an intermediate layer between the joint partners. The microstructure of this layer differs from the bulk material and in both types of joints consists of new, recrystallized grains of aluminum. Additional nanoindentation measurements reveal increased hardness values in this intermediate layer. These mechanisms lead to the conclusion that the interfacial layer can be described as a thin region that is formed by rapid melting and subsequent crystallization during the impact process. These microstructural investigations of magnetic pulse welded joints provide relevant information on the key mechanisms that determine the quality of cohesive bonding in impact welding methods.