Transient Temperature and Heat Flux Measurement in Ultrasonic Joining of Battery Tabs Using Thin-Film Microsensors

Abstract

Process physics understanding, real time monitoring, and control of various manufacturing processes, such as battery manufacturing, are crucial for product quality assurance. While ultrasonic welding has been used for joining batteries in electric vehicles (EVs), the welding physics, and process attributes, such as the heat generation and heat flow during the joining process, is still not well understood leading to time-consuming trial-and-error based process optimization. This study is to investigate thermal phenomena (i.e., transient temperature and heat flux) by using micro thin-film thermocouples (TETC) and thin-film thermopile (TETP) arrays (referred to as microsensors in this paper) at the very vicinity of the ultrasonic welding spot during joining of three-layered battery tabs and Cu buss bars (i.e., battery interconnect) as in General Motors's (GM) Chevy Volt. Microsensors were first fabricated on the buss bars. A series of experiments were then conducted to investigate the dynamic heat generation during the welding process. Experimental results showed that TETCs enabled the sensing of transient temperatures with much higher spatial and temporal resolutions than conventional thermocouples. It was further found that the TETPs were more sensitive to the transient heat generation process during welding than TETCs. More significantly, the heat flux change rate was found to be able to provide better insight for the process. It provided evidence indicating that the ultrasonic welding process involves three distinct stages, i.e., friction heating, plastic work, and diffusion bonding stages. The heat flux change rate thus has significant potential to identify the insitu welding quality, in the context of welding process monitoring, and control of ultrasonic welding process. The weld samples were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to study the material interactions at the bonding interface as a function of weld time and have successfully validated the proposed three-stage welding theory.

Microstructure, welding mechanism, and failure of Al/Cu ultrasonic welds

“…(3) Ni Ni bonding was established by the friction-induced physical bonding, in which ultrasonic frictional sliding initiated plastic deformation in the very top surface skins (while the tab layers still remained flat with no interlayer mixing), bringing fresh Ni atoms into contact and adhesion. The frictional heat (as measured in [37]) is expected to provide assistance for plastic deformation, but the diffusional flow is limited by the short time and high heat conduction. The bonding strength was strongly affected by the surface topology (locally) and the applied energy (globally) that determine the local contact stress and deformation.…”

“…Such measured temperature rise during ultrasonic welding of Ni-coated Cu was as high as 650 • C for Cu/Cu welding. With this technique, Li et al [37] studied the transient thermal process during ultrasonic welding of battery tabs that helps better understand the welding mechanism and resultant microstructure and properties. The high temperature, along with the earlier mentioned high strain rate deformation studied by Panteli et al [27] on Al/Mg USW, results in a special microstructure evolution, including dynamic recrystallization in Cu/Cu ultrasonic welding reported by Lee et al [44].…”

Welding techniques for battery cells and resulting electrical contact resistances

“…As explained in the previous sections, with resistance spot welding melting temperatures are reached and with laser keyhole welding boiling temperatures are reached. For ultrasonic welding, 30-60% of the melting temperature of the material is reached, but the exact values strongly depend on the welding parameters and properties of the material [13,34]. The highest temperatures occurring in a hot spot at the external conductor will not harm a battery cell but the temperatures inside the casing, which arise at the electrochemically active materials, are of special interest.…”

Simulating microstructure evolution of battery tabs during ultrasonic welding

“…Thus, a constant surface heat flux was applied in this model at the interface between the top and bottom copper sheets. The applied surface heat flux was 18.15 W/mm 2 in this model, which was calibrated by comparing the simulated temperature histories at different locations to the measurement [11]. The friction coefficients between the knurling tools and workpieces were considered to be temperature-dependent.…”

“…The thermal contact conductance between thin metal sheets was determined as a function of contact pressure [9]. The real-time temperature and heat flux change have been measured and monitored near the weld zone using thin-film thermocouples [10] and thin-film micro-sensors [11]. Different weld formations were studied for various welding time durations from 0.6 s to 1.5 s. Based on the history of heat flux change rate, Li et al [11] proposed a bonding mechanism for ultrasonic welding consisting of three continuous stages within one operation, which were firstly friction heating, then bonding by plastic work, and finally diffusion bonding.…”