Characterization of multilayer ultrasonic welding based on the online monitoring of sonotrode displacement

Ultrasonic spot welding (USW) of multilayer metal sheets is a critical process of lithium battery manufacturing. However, inconsistent weld quality is still an issue in multilayer USW which makes the online quality monitoring very necessary to provide rapid and reliable feedbacks of each workpiece's weld quality. In this work, the displacement of the sonotrode in the clamping direction was monitored by a high-frequency displacement sensor in order to establish the relationship with the weld quality. Features of displacement signals were analyzed combined with changes in the plastic deformation and welding qualities which were characterized by the laser scanning confocal microscopy, the optical microscopy and the electron backscattered diffraction (EBSD). Besides, effects of welding parameters on the displacement signal were studied. Finally, the relationship between the displacement signal, the plastic deformation and the weld quality was established which can be used for the online quality monitoring of the multilayer USW.