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Full text release has been delayed at the author's request until August 04, 2027

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Ultrasonic-assisted Resistance Spot Welding of Multilayered Thin Films for Battery Cell Manufacturing

Kwon, Ho
http://orcid.org/0000-0002-6757-8195
http://rave.ohiolink.edu/etdc/view?acc_num=osu1721355309645998

Abstract Details

2024, Doctor of Philosophy, Ohio State University, Welding Engineering.
Li-ion batteries (LIB) have been spotlighted as a promising power source to replace traditional fuel gases owing to their high energy density, lightweight, and greenhouse gas emission free characteristics. Nevertheless, the catastrophic failure of the LIB is usually connected to safety issues, and the solutions must be addressed from the perspective of materials and designs. The representative materials for the current collectors in LIB are the commercial pure-grade aluminum (Al) and copper (Cu) foils because of their high electrical conductivities, electrochemical stabilities, and low density. However, they degrade during the multiple charge/discharge cycles when the applied voltage exceeds their corrosion potentials. The current flows generate the resistance heating during the charge/discharge cycles at the joint between the foil stacks and the lead tab, increasing the cell temperature. This accelerates the degradation of the foil and reduces the life cycle of the LIB. To reduce the electrical resistance, the increase in conductive area is desirable. Therefore, securing the large joint area with minimal weld discontinuities not only improves the mechanical properties helps but also impedes the resistance heating in LIB. The conventional resistance spot welding (RSW) process has not been widely used to produce the joint between the current collectors and the lead tab so far because of the high thermal, electrical conductivity and thinness of the Al and Cu weld stacks. These aspects generally lead to the smaller weld nugget size, and the sticking of the weld stacks to the electrodes. A recently developed hybrid joining process, known as ultrasonic-assisted resistance spot welding (URW), shows the great effects on increasing the weld nugget size as well as mechanical properties in various pairs of similar and dissimilar metal sheets by the microstructure modification. In the present study, multiple thin pure aluminum (Al) and copper (Cu) foils and tab stacks are joined by a recently developed ultrasonic-assisted resistance spot welding (URW) process. The ultrasonic vibration, which is perpendicular to the weld interface, is applied from the onset of the preheating current and maintained until the end of the main weld current. A graphite interlayer is placed between electrode and weld stack to effectively avoid the sticking issue. Various number of foils ranges from 25 to 96 layers were welded under the optimized process parameters, including weld force and current. The comparison between conventional RSW and URW is discussed in the view of mechanical properties and microstructure. Firstly, solidification cracking and gas porosities were substantially reduced in the multilayered Al foil stacks of URW compared to that of conventional RSW. The nugget center consisted of mixed microstructures of equiaxed dendrites and non-dendritic structures with minimal boundary segregation. Unique wavy structures were observed at the foil side fusion boundary. As the number of foil layers increased from 25 to 75, the amount of equiaxed dendrites increased while that of wavy structures decreased. The sizes of all URW nuggets exceeded the critical diameter, which led to button pullout failure mode. The hardness of weld nuggets and heat affected zone (HAZ) were higher in URW than in RSW. The ultrasonic vibration also reduced HAZ width at the tab of weld stacks. Secondly, mechanisms of the microstructure evolution and the strengthening effects in multilayered Cu foil stacks under the presence of ultrasonic vibration are proposed through EBSD analysis. The effects of vibration amplitudes and times on the microstructure and following mechanical properties are examined in more detail. The weld nugget shows distinct microstructural differences at the center and the fusion boundary. The increase of vibration amplitude refines the microstructures near the fusion boundary, while coarsens at the center of the nugget. On the other hand, the increase in welding time continuously reduces the overall grain size in the nugget. The peak loads of the lap shear tensile specimens increase with vibration amplitudes and time. Especially when the number of foil layers is 96, double fractures are observed, the first one at the nugget center and the second one at the tab side HAZ. The more refined grains along the fusion boundary suppress the first crack propagation and lead to the second crack initiation. Lastly, the fundamental experimental analysis is carried on with a simple Al stack configuration to examine the process mechanism of URW. The correlation of the process signals to the high-speed images proposes the possible hypothesis of the modified microstructure and enlarged diameter of the URW nugget. Generally, the dynamic resistance in URW tends to be lower compared to that in RSW. Specifically, the application of ultrasonic vibration during the preheating notably decreases the contact resistance at the beginning of the welding stage. The ultrasonic vibration helps initiate melting faster and maintain the melting for a longer time, even though at the lower heat generation. In other words, the ultrasonic vibration accelerates melting potentially through decreasing the melting temperature and producing extra heat sources in the melt. The strong melt convection is observed during the melting, which is potentially due to the acoustic streaming and cavitation. It is expected that the synergetic effects of these can significantly contribute to the increase in weld nugget diameter.
Xun Liu (Advisor)
Glenn Daehn (Committee Member)
Avraham Benatar (Committee Member)
144 p.
Engineering; Materials Science
Resistance spot welding; Power ultrasound; Li-ion battery; Multilayer foil joining; Microstructure; Mechanical properties

Recommended Citations

Citations

  • Kwon, H. (2024). Ultrasonic-assisted Resistance Spot Welding of Multilayered Thin Films for Battery Cell Manufacturing [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1721355309645998

    APA Style (7th edition)

  • Kwon, Ho. Ultrasonic-assisted Resistance Spot Welding of Multilayered Thin Films for Battery Cell Manufacturing. 2024. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1721355309645998.

    MLA Style (8th edition)

  • Kwon, Ho. "Ultrasonic-assisted Resistance Spot Welding of Multilayered Thin Films for Battery Cell Manufacturing." Doctoral dissertation, Ohio State University, 2024. http://rave.ohiolink.edu/etdc/view?acc_num=osu1721355309645998

    Chicago Manual of Style (17th edition)

osu1721355309645998
© 2024, some rights reserved.Creative Commons License Ultrasonic-assisted Resistance Spot Welding of Multilayered Thin Films for Battery Cell Manufacturing by Ho Kwon is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by The Ohio State University and OhioLINK.