Tailoring nanotwinned Cu interlayers for localizing anisotropic plastic deformation during low energy input ultrasonic welding of robust Cu-Cu joints

Ultrasonic welding, known for its severe plastic deformation, faces the challenge of balancing sufficient deformation at the welding interface with minimizing damage to the substrate. This study utilizes the anisotropic deformation mechanisms and mechanical properties of nanotwinned Cu (nt-Cu). Specifically, Cu coatings featuring nanotwin layers aligned parallel to the ultrasonic vibration direction were employed as interlayers in ultrasonic welding of Cu-Cu joints. The effects of the nt-Cu interlayer on the welding quality and the deformation mechanisms under the various welding pressures are investigated. Experimental and molecular dynamics simulations demonstrate that at low welding pressures, the nt-Cu interlayer undergoes deformation and detwinning primarily through twin boundary migration. This mechanism effectively mitigates work hardening during the welding process, localizes deformation at the welding interface, and significantly enhances the strengths of the Cu-Cu joints. The maximum enhancement proportion occurs at a welding pressure of 8 psi, up to 26.75% compared to conventional coarse-grained copper. As the welding pressure increases, the strengthening effect gradually weakens. The deformation mechanism of nt-Cu transitions to dislocation transverse and threading. The interaction between dislocations and twin boundaries forms incoherent twin boundaries and 9R phases, resulting in work hardening of the interfacial regions and reduction of the strengthening effect.