Gradient of nanostructures at the interface of Al/Cu welded joints produced by the high strain-rate collision during magnetic pulse impact welding

This paper focuses on the nanoscale characterization of a welded Al/Cu interface produced by magnetic pulse welding. Welding tests were performed using three field shapers with similar dimension but having strongly different electrical conductivity. Other welding parameters were strictly the same. The three tests produce similar welded joints that consist of a similar morphology of wavy interface at the macroscopic scale. Thus, the Al to Cu transition zone across the location of an interfacial wave is identified as a suitable repetitive site that enables for repetitive fine-scale characterization across a distance less than 1 µm. Transmission Electron Microscopy (TEM) observations of the welded Al/Cu interface reveal a gradient of nanostructures which consists of an amorphous Al_xCu_y nanolayer (∼30 nm) and then, a nanocrystalline layer with a thin thickness of a few tens of nanometres at the Cu side and at the Al side as well. This hierarchical nano-featured structure is confined within a very short total distance of about 200 nm. Outside these confined nanostructures, the interface exhibits a crystalline structure. These nanofeatures were observed at the interface of the Al/Cu welded joint produced by three different field shapers, that shows a reproducibility of the interface structure at the nanoscale level. A thermomechanical analysis of the high strain process at a collision point allows for depicting the mechanism of microstructure formation confined at the Al/Cu interface. The shear strain rate across the interface shows peaks where the temperature distribution also reaches a peak beyond the melting point of Al. The thermal kinetics within the molten zone is characterized by a high cooling rate up to 10³ °C/ns during the melting/solidification stage that explains the observation of the amorphous nanolayer revealed by the TEM observation. The formation of nanocrystalline structure confined at both sides of the amorphous layer can be explained by a nucleation of crystals at a very early stage governed by the thermal kinetics at the boundaries (Al side and Cu side) of the melted zone, and by a dynamic recrystallization governed by the gradient of shearing at high strain rate confined at the collision point. Together, these processes of highly transient thermomechanical responses confined at the Al/Cu interface explain the formation mechanism of interface microstructure that results in the hierarchical nanostructure as identified by TEM characterizations.