The ultrasonic spot welding (USW) is widely used in the joining of multilayer Cu or Al tabs in the lithium-ion battery. Due to the high-frequency vibration of the sonotrode and various deformation in each interface, the friction behavior is complex which makes it difficult to simulate the welding process of multilayer specimens. In this paper, an efficient and accurate finite element model (FEM) was proposed via introducing the interface heat flux to equivalent the heat generation by the friction. The total heat flux was determined by the heat transfer analysis and the proportion of each interface was determined based on the analysis of the friction behavior. Then, the FEM was validated by comparing the simulated temperature and deformation with experimental results. Finally, the FEM was applied to simulate the USW process of 4-, 5- and 10-layers of copper and aluminum foils in order to characterize the gradient of the ultrasonic energy. It was found that the heat generation concentrated in middle interfaces which was 65% of total in the welding of 4-layers copper foils. The heat generation was mainly related to the welding parameters and the proportion was related to the size of tips and the structure of specimens. The plastic strain was various in specimens because of the gradient of the input energy. It was most obvious in the welding of 10-layers aluminum foils that the maximum equivalent plastic strain (PEEQ) in the fifth interface was 92.9% smaller than the top interface.
Lithium-ion battery for electric vehicles contains a large number of battery tabs with multiple and thin sheets. Dissimilar metals are often used for different electrodes, for example, the copper and nickel are often used for negative electrode and the aluminum is often used for positive electrode. Meanwhile the thickness of metal sheets are various at different locations. Ultrasonic spot welding is very capable of welding similar and dissimilar combinations. However, there are less heat generation and plastic deformation in the bottom interfaces where ultrasonic energy is difficult to reach. This will result in the unbonded region occurrence caused by ultrasonic energy attenuation.
In this work, experimental investigation was conducted to identify the effect of ultrasonic energy on joint characterization in ultrasonic welding multiple tabs by comparing the plastic deformation in different layers and various joint interfaces. Two sonotrodes with different knurl size were used to produce the welding energy for the tabs joining. Samples were cross-sectioned along vibration direction to obtain hardness profiles and metallographic maps. The hardness profile can be used to identify the changes of grain morphology. Hardness and grain size changes in different layers were also studied to reflect how ultrasonic energy decrease from top to bottom in battery tabs. Thereby, the relationship between material attributes and ultrasonic energy loss was established based on the experimental results.
Lithium battery pack, made of aluminum alloys, consisted of hundreds of welding seams. Due to the complicate distribution of welding seam and low stiffness of aluminum alloys, large welding deformation was found in the lithium battery pack. This paper analyzed the effect of welding parameters and the welding sequences on the deformation of lithium battery pack, then proposed a method to reduce the welding deformation of lithium battery pack maintaining the welding quality of single weld seam of aluminum alloys. By the coupling optimization of welding sequences and welding parameters, the welding deformation of lithium battery pack decreased from 1.69 to 1.29 mm with the reducing rate of 23.7% and hundreds of welding seams contours met the requirements of manufacturing quality. These findings could pave the way to improve the manufacturing quality of lithium battery pack made of aluminum alloys.
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