Multiscale investigation of gradient growth of intermetallic compounds and joint weakening induced by severe lattice distortion at the interface of Al7075/TC4 friction stir welds

In this work, the microstructural mechanisms responsible for performance degradation induced by intermetallic compounds in Al7075/TC4 dissimilar alloy friction stir lap welded joints were systematically investigated. A multiscale approach was employed to elucidate the heterogeneous distribution of interfacial intermetallic compounds and their structure–property relationships with the macroscopic mechanical behavior of the joint. The results indicate that, during welding, the retreating side on both the Al7075 and TC4 sides experienced higher thermal exposure and pronounced material accumulation. This condition promoted the preferential formation of a high density of TiAl 3 phases with a compositional gradient near the Al7075 side, whereas TiAl phases were mainly generated within the central interfacial region and in the vicinity of the TC4 side. A pronounced interfacial distortion was observed at the TiAl/TC4 interface. Combined numerical simulations and experimental observations confirmed the strong thermodynamic driving force for TiAl 3 formation, facilitated by the rapid diffusion of Al, followed by the subsequent formation of TiAl. Mechanical testing coupled with in situ digital image correlation revealed that joint fracture initiated at the retreating-side interface, and the fracture path was directly associated with the severe deformation localized at the TiAl/TC4 interface.