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Pulsed Arc Welding of Battery Tabs for Vehicle Electrification

Team of Automotive Engineers Working on Electric Car Chassis Platform, Taking Measures, working with 3D CAD Software, Analysing Efficiency. Vehicle Frame with Wheels, Engine and Battery.

Evaluating this process for several tab battery material combinations

BY TIM FRECH, JOLENE TRAN, AND KATE NAMOLA

TIM FRECH (tfrech@ewi.org) is senior engineer, JOLENE TRAN is
a former intern , and KATE NAMOLA is a former project manager,
EWI, Columbus, Ohio. A presentation about this paper was given at
the Sheet Metal Welding Conference XIX, held Nov. 2–4, 2021, in
Livonia, Mich.

Reprinted with permission: The AWS Welding Journal

Batteries used in electro-mobility applications are generally prismatic with a combination of copper, aluminum, or nickel tabs, or cylindrical with either steel or aluminum shells. Interconnection of these batteries to produce packs can be very easy; for example, nickel tabs to steel shells is a mature resistance welding process. More difficult material combinations involve dissimilar metal welds of copper-to-aluminum, aluminum-to-steel, and others. Though welding processes have been developed for many of these combinations, all have some limitations. EWI addressed these limitations by evaluating pulsed arc welding for several tab battery material combinations. The welding pulse shapes were investigated to accurately control weld penetration and weld solidification behavior. In this article, the pulsed arc welding process will be described, the various joint combinations and weld data will be discussed, and recommendations will be provided for real-world battery pack applications.

Lithium-ion Cells

Lithium-ion cells are commonly used in consumer electronics and electric vehicle applications, and their adoption within the aerospace industry is growing rapidly. With the increased demand in recent years, EWI has pursued different paths for optimization in battery packs manufacturing with lithium-ion cells. The most recent area of study has been the joining of tabs and busbars to the cylindrical cells and tabs from prismatic batteries. Historically, lithium-ion battery casings have been made up of a variety of materials. Nickel-coated steel and aluminum have been the go-to materials for their chemical resistance and corrosion protection. The structural integrity of a lithium-ion casing is critical for public safety.

Battery Manufacturing Joining Applications

Typically, connecting each individual lithium-ion cell involves joining a tab or busbar to the cell casing — Fig. 1. Sometimes the busbar is connected directly to the cell; other designs use a tab that is thinner than the busbar, and span between the busbar and the cell. Many pack designs are assembled using resistance welding. Ultrasonic wire bonding is also common for cell-to busbar interconnections in large-format batteries. Laser welding, a third process, is often chosen for high-speed, noncontact welding of certain material combinations. Resistance welding provides the opportunity for in-process quality assurance via weld process monitoring. One negative aspect of resistance welding can be the lower production throughput, as compared to laser welding. However, multihead resistance welding systems, combined with a welding power supply shared across multiples of weld heads, can be less costly and weld at rates competitive with laser welding. Over the years, EWI has investigated several joining processes for many material combinations of tabs and busbars to cells and tabs.

Pulsed Arc Welding

Pulsed arc welding is a relatively new process that creates a high-energy density arc between a tungsten electrode and the workpiece. This results in high local temperatures to melt the metals to be welded, with minimal heat-affected zones (HAZs). Pulsed arc welding is typically used in small-scale welding processes. Controlling current and welding duration allows for a stable welding process. Since the system is a closed loop of power, there is the possibility of providing in-process quality assurance.

Experimental

EWI recently conducted a project to evaluate pulsed arc welding and determine its effectiveness in common battery welding material combinations. A Sunstone Engineering Orion 250i-EV pulsed arc welding unit with a tungsten electrode was used for joining copper-to-nickel plated steel and nickel-to-nickel plated steel. Further investigation was completed for copper to 304L
stainless steel. The thickness of the tabs for the research ranged from 0.005 to 0.010 in.

Results

Most material combinations provided excellent results. The initial tests yielded excellent weldability for two of the most common combinations, copper-to-nickel plated steel and nickel-to-nickel plated steel. Copper-to-nickel plated steel saw a large welding lobe curve that showed superior-looking welds with consistent strengths (see Fig. 6). Metallographic analysis of the welded samples demonstrated that longer welding times resulted in nearly complete joint penetration fusion. Shorter welding times led to porosity in the weld. The energy had less effect on penetration and visual appearance than the welding time. In nickel-to-nickel plated steel welding, there were no porosity issues. Peel testing showed that the bonds had a peel strength of 70 to 120 Newtons (N) when complete joint penetration was achieved. Subsequent work on copper to 304L stainless steel welding resulted in partial penetration welds. Peel testing of the samples showed that the strength is an average of 20 to 40 N, less than half that of copper-to-nickel plated steel and nickel-to-nickel plated steel. In this case, the joint strength could be increased by making multiple welds.

Conclusion

These studies of the pulsed arc welding process show developing potential for implementation into the manufacturing of lithium-ion cells, especially when copper or nickel tabs are utilized. This welding technique minimized the HAZ and the risk for damage to the cell. Material combinations of copper-to-nickel plated steel and nickel-to-nickel plated steel showed acceptable peel strength. Pulsed arc welding has the potential to be a replacement for resistance welding and laser welding in the manufacturing of packs using cylindrical cells or prismatic batteries. Plans for future work include investigation of in-process monitoring to help understand the relationship of the weld current and voltage profiles to the weld quality. Other material combinations, such as plated and laminated busbars, will be investigated for pulsed arc welding to steel and aluminum cell cases.

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