In-situ measurement of relative motion during ultrasonic spot welding of aluminum alloy using Photonic Doppler Velocimetry

Ultrasonic Spot Welding (USW) is a solid-state welding process widely used for light metal joining. Improved understanding of the bonding mechanism for USW aluminum alloys is essential for achieving sound joints. In the present study, the relative motion of the sonotrode, aluminum specimens, and anvil in USW is investigated using an in-situ velocity measurement technique, Photonic Doppler Velocimetry (PDV). The relative motion analysis is correlated to destructive testing results, including lap-shear tensile testing and weld microstructure characterization, to understand and quantify bond formation during USW. The results indicate that four bonding stages are present: a slip stage, slip-stick transition stage, stick stage, and over-welding stage. In particular, the end of the stick stage, marked by the sonotrode tip and foils vibrating at the same velocity, is essential to achieve peak bond strength as well as a fracture mode change from “interfacial failure” to “nugget pull-out”.