11 Dynamics and Vibrations of Battery Tabs under Ultrasonic Welding
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The effect of structural vibration of the battery tab on the required sonotrode force during ultrasonic welding is studied by applying a longitudinal vibration model for the battery tab. It is found that the sonotrode force is greatly influenced by the kinetic properties, quantified by the equivalent mass, equivalent stiffness, and equivalent viscous damping, of the battery tab and cell pouch interface. This study provides a fundamental understanding of battery tab dynamics during ultrasonic welding and its effect on weld quality, and thus provides a guideline for design and welding of battery tabs.
The effects of longitudinal and flexural vibrations of the battery tab during ultrasonic welding on the development of axial normal stresses that occasionally cause cracks near the weld area are studied by applying a continuous vibration model. Analysis results show that fracture could occur near the weld area, due to low cycle fatigue as a result of large dynamic stresses induced by resonant flexural vibration of the battery tab during welding. The axial normal stresses due to longitudinal waves traveling along the battery tab are shown to be insignificant compared to those due to flexural waves as the longitudinal wavelength at a typical ultrasonic welding frequency (e.g., 20 kHz) is much larger than the battery tab length, whereas the flexural wavelength is much shorter.
It has been observed that sufficient energy is required to produce proper bonding of battery tabs, whereas excessive energy can cause quality issues such as weld fracture and perforation. Therefore, it is important to have a product/process design in ultrasonic welding to ensure efficient energy conversion from ultrasonics to welding energy, minimizing energy loss in the process. Vibrational energy loss due to material damping of the Cu coupon during ultrasonic welding is discussed, where the material damping is modeled as Kevin-Voigt damping and determined experimentally. It is shown that substantial energy loss can occur during welding due to the flexural vibration of the Cu coupon, especially when the overhang (the upper part of the Cu coupon extended from the anvil) of the Cu coupon resonates at or close to the welding frequency, degrading the weld quality of battery tabs.