Acoustic monitoring of weld strength in ultrasonic metal welding by tracking welding stages

Ultrasonic metal welding (USMW) is a welding technique often used in the electronic and automobile industries. However, the strength of the welds it produces is influenced by a large number of variables, making USMW difficult to monitor. There is therefore a need for monitoring parameters which can be used across a large number of influencing variables. Such parameters should be closely related to the welding process and the evolution of the weld strength during welding. The goal of this paper is to investigate the usability of the horn and anvil vibrations at the welding frequency as such monitoring parameters. In this study, the horn and anvil vibrations were tracked during welding of copper sheets using laser vibrometers, compared to results in the literature to identify welding stages, and compared to the evolution of the weld strength. To avoid any low sample size bias, 120 welds were used. Their envelopes and their rate of change were extracted and used to identify welding stages. The results showed that the strength of a weld plateaus when it reaches its stage of maximum strength, and that this stage can be identified by both the horn and anvil vibrations using features derived from the envelopes and their rate of change.