Improving weld formation in ultrasonic Cu–Cu joining via tailoring the twin structure of interlayers

In the ultrasonic welding (UW) process, achieving reliable bonding under low ultrasonic energy input is crucial for minimising surface damage and improving long-term service stability of joints. In this research, three Cu interlayers with horizontal (H–TBs), inclined (I–TBs) and vertical twin boundaries (V–TBs) were designed to accelerate weld formation during UW. At a welding time of 0.3 s, lap shear tests indicated that the I–TBs joint exhibited 38.6% and 54.9% higher strength than the H–TBs and V–TBs joints, respectively. The moderate microhardness, elevated coefficient of friction and activation of soft-mode dislocation motion in the I–TBs joint generated localised plastic instability, accelerated the flattening process and improved metallurgical bonding under the low energy inputs.