Optimization Of Ultrasonic Welding Of Metal Tabs During Layer-Built Lithimm Battery Manufacturing

Layer-built lithium battery, as the power source of new-energyvehicles, research of its key materials and manufacturing process isbecoming a hot issue nowadays. The manufacturing process of lithiumbattery mainly includes slurrying, coating, welding, assembly and testingprocesses, among which welding is a key process for joining Al/Cu foil,tab and bus. Any joining defects will significantly affects the consistencyof lithium batteries. Therefore, how to guarantee the joining quality of themetal tabs is one of the key issues to be solved.Ultrasonic metal welding, as a high-efficiency, low-consumption andsolid-state joining technology, is attracting increasing attentions ofautomobile manufacturers and battery manufacturers because of itssuperiority for joining materials with high electrical conductivity andthermal conductivity such as Al/Cu. However, due to changes of thematerial properties and thickness (10μm-1mm), mechanism of heatgeneration and plastic deformation are not clear, which results in narrowwelding lobe and fluctuant joint quality.Ultrasonic metal welding experimental system for battery tabs werebuilt to study the effect of process parameters on joint strenth, includinghorn area and morphology, welding force, welding time and amplitude.Secondly, infrared thermography was empoyed to investigate thetemperature at the horn-workpiece interface and to reveal the influence ofthe frictional heating behavior during the welding process on the formation of the joint. Then, the plastic deformation occurred at theworkpiece interface has been investigated to reveal the mechanism ofjoint formation. Finaly, response surface methodology (RSM) wasadopted to optimize the selection of welding parameters and improve theweld quality. The main contents and conclusions are as follows.(1) Temperature measurement at horn/workpiece interfaceThe variation of temperature at horn/workpiece interface wasmeasured to investigate the frictional heat generation during the weldingprocess. Correlationship between the temperature at horn/workpiece andjoint strength were built and utilized to achieve on-line detection ofwelding quality. The results showed that the temperature didn’t reach themelting point of workpiece, which confirms that UMW is a solid-statejoining process. When the temperature on workpiece surface is equal tothe critical temperature, the highest joint strength will be achieved;temperature below or above the critical temperature will result in adecrease in the joint strength. Critical temperatures of0.2mm Cu/Cu,Al/Al, Al/Cu are179.5℃,77.4℃,79.1℃, respectively.(2) Study of plastic deformation at workpiece interfaceThe effect of weld parameters on plastic deformation at the joininginterface was studied by metallographic examination, scanning electronmicroscopy(SEM) and Energy dispersive spectroscopy(EDS). Plasticdeformation at the interface would lead to a mechanical interlockingbetween materials. Stable intermetallic compound layer wasn’t found atthe bonding areas when joining dissimilar metals of Al/Cu, howeverelement interdiffusion was observed at the joining interface. Two newconceptions,“effective thickness” and “effective connection length”,were proposed to represent plastic deformation. Increase of weldingpressure, welding amplitude and welding time can all improve theeffective connection length. Finally, a relationship model between the plastic deformation and weld strength was established to determine thecritical effective connection size.(3) Optimization of welding process parameters via response surfacemeothodologyRSM was then used to develop mathematical models to predict therelationship between the processing parameters (e.g. horn area andmorphology, welding force, amplitude and time) and the temperature atthe horn/workinterface. The results turned out that horn morphology andwelding time were the two most influential factors. According to thecritical temperature, the optimum parameters were determined as hornarea33.65mm~2, depth of horn texture0.31mm, pressure1,52kN,amplitude24.4μm and welding time0.067s.