Deformation behavior of dissimilar ultrasonic spot-welded joints of a clad 7075 aluminum alloy to galvanized high-strength low-alloy steel

The microstructural and mechanical properties of ultrasonic spot-welded (USWed) joints of AA7072-clad AA7075-T6 alloy to galvanized HSLA steel are investigated. The tensile lap shear strength increased with increasing welding energy and reached ∼145 MPa at a welding energy of 2500 J. The dissimilar joints also exhibited a longer fatigue life than other USWed Al-steel joints reported in the literature. In the present dissimilar ultrasonic spot welding, direct Al–Fe interactions to form intermetallic compounds were prevented and accelerated inter-diffusion between Al and Zn occurred along with localized melting at higher welding energy levels. The subsequent fast cooling led to the formation of an Al–Zn eutectic layer (∼40 μm thick) with fine structures at the interface of the dissimilar joints, which displayed a robust sticking capacity as demonstrated by the extensive shear dimples on both tensile and fatigue fracture surfaces. While interfacial failure occurred in the majority of tensile lap shear and fatigue tests, both modes of interfacial failure and TTT (transverse through-thickness) failure were present at lower cyclic stress levels during fatigue, with the interfacial failure being slightly more dominant in the two competitive failure modes.