Chapter 11 Kang et al. 1 CHAPTER 11 DYNAMICS AND VIBRATIONS OF BATTERY TABS UNDER ULTRASONIC WELDING Bongsu Kang Civil & Mechanical Engineering Department Purdue University Fort Wayne Wayne Cai Manufacturing Systems Lab General Motors Global R&D Center Chin-An Tan Mechanical Engineering Department Wayne State University Abstract 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 while the flexural wavelength is much shorter. It has been observed that sufficient energy is required to produce proper bonding of battery tabs, while 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. Keywords: Ultrasonic welding, Battery tabs, Dynamics of battery tabs, Vibration of battery tabs, Vibrational energy loss 11.1 Introduction Ultrasonic metal welding for battery tabs must be performed with 100% reliability in battery pack manufacturing as the failure of one weld essentially results in a battery that is inoperative or cannot deliver the required power due to the electrical short caused by the failed weld. Moreover, this stringent weld quality control is of great concern for battery pack manufacturers as automotive batteries are exposed to harsh driving environment such as vibration, severe temperature, and possibly crash, all of which can affect battery performance and safety. Therefore, one of the main issues arising in ultrasonic welding of battery tabs is to ensure consistent weld quality that meets design specifications such as electrical conductivity and shear strength of the weld. The quality of ultrasonic metal welds depend on a number of factors such as