Improving process robustness in ultrasonic metal welding of lithium-ion batteries

Ultrasonic metal welding is a solid-state joining method popularly adopted in the assembly of lithium-ion battery cells, modules, and packs for electrical vehicles due to its numerous advantages over traditional fusion welding techniques. Ultrasonic metal welding process yields quality welds under optimal conditions, but can result in poor welds when there are disturbances, such as the presence of oil contamination. State-of-the-art controllers cannot detect those disturbances or control the welding process accordingly. In this research, two methods are proposed to improve the process robustness, namely, a real-time controller and a new tool geometry design for the sonotrode. The developed controller monitors the on-line power signal and adjusts the weld clamping pressure through a calibrated step function accordingly. Experimental results show that with oil contaminated workpieces, the new controller yielded an average improvement of 14.5-440% in the T-peel strength over the current controller, depending on the level of oil contamination. Additionally, the process robustness was shown to be improved by the use of a spherical tool in place of a flat tool. Improvements are achieved for all tested clamping pressures, especially when the pressure is outside the optimal range for the flat tool.