A Novel Numerical Model of Ultrasonic Assisted Resistance Spot Welding Process to Unravel the Weld Formation Mechanism in Multilayer Foil Welding

Abstract Ultrasonic-assisted resistance spot welding (URW) has emerged as a superior technique compared to conventional resistance spot welding (RSW) for multiple thin aluminum foils-to-tab welding applied in the manufacturing of pouch cell batteries, producing larger and higher quality welds. However, in situ experimental observation of the nugget formation during URW is challenging due to the enclosed weld region and the transient nature of the process. In this study, a numerical modeling framework is implemented, leveraging a baseline finite element model (FEM) of RSW to systematically evaluate individual and coupled impacts of various ultrasonic effects on the thermal, mechanical, electrical, and flow fields of the weld stack. A coupled FEM-computational fluid dynamics model and a cavitation energy coupled FEM have been utilized for the first time to study the melt flow under ultrasonic pressure variation and the effects of cavitation energy on temperature distribution, respectively. To experimentally verify the occurrence of acoustic cavitation during URW, in situ acoustic signals have been monitored with a microphone. Coupling all ultrasonic effects, including reduced contact resistance, acoustic softening, and increased electrical conductivity, along with cavitation energy, underpredicts the nugget size, contrasting experimental observations. The reduction in contact resistance proves to be a dominating factor that results in a smaller-sized URW nugget in joining thin foils to the tab. These findings highlight the need to modify the contact resistance model and to incorporate additional ultrasonic mechanisms to enable predictive modeling of weld nugget evolution for URW.