Joining multiple thin foils of current collectors to a conductive tab is one of the critical processes to ensure battery life. At the cell-level, the wide joint area is generally preferred for reliability, strong mechanical and electrical properties. Resistance Spot Welding (RSW) has not been widely used to cell-level joining of the pouch type due to the following reasons. The high conductivity of Al and the thinness of foils and tabs make it challenging to form large nuggets in RSW. The electrode-sheet sticking is another issue for RSW on Al alloys. The direct contact between the molten aluminum and Cu electrode, mainly due to the thinness of the stacks, shortens electrode life.
In this study, multilayered thin pure aluminum (AA1060) foils and tabs were joined by a recently developed ultrasonic-assisted resistance spot welding (URW) process. The URW equipment was developed by attaching an ultrasonic transducer to a conventional RSW machine. The 20 kHz ultrasonic vibration along the axial direction is triggered by preheating current, continued during the weld current, and stopped when the current is off. The number of foils (t) varies from 25 to 75.
Compared with RSW, solidification cracking and gas porosities were significantly reduced in URW. Mixed microstructures of equiaxed dendrites and non-dendritic structures were observed at the nugget center. Unique wavy structures were observed at the foil side fusion boundary. Segregation of impurity elements was highly reduced at interdendritic regions. These microstructural changes are attributed to the strong fluid flow in the liquid nugget, which reduces the thermal gradient and activates dendrite fragmentation. As the t increased from 25 to 75, the number of equiaxed dendrites increased while that of wavy structures decreased. Since oxides, which originate from foil surfaces, serve as heterogeneous nucleation sites in the melt, the equiaxed dendritic region is enlarged when they are dispersed homogeneously. Additionally, total volume fractions of oxides increase along with t, which can further activate heterogeneous nucleation and enlarge the equiaxed dendritic region. The wavy structure is affected by peak temperature at the fusion boundary. Since the propagation distance of the ultrasound increases as a function of stack thickness, the flow intensity reduces. Additionally, the higher heat input for joining higher t increases the peak temperature as well. These increase the peak temperature at the fusion boundary and contribute to the reduction of wavy structure.
The sizes of all URW nuggets exceeded the critical nugget diameter to ensure button pullout failure. The hardness of weld nuggets and heat affected zone (HAZ) were higher in URW than in RSW, which is attributed to the better dispersion of finer oxides. The ultrasonic vibration also reduced HAZ width at the tab of weld stacks. It represents that the peak temperature in the liquid nugget of URW can be reduced in the presence of fluid flow.
The URW successfully joined multilayered Al foil stacks with up to 75t. Compared to the RSW, weld discontinuities were significantly reduced. The ultrasonic vibration modified the solidification microstructure by reducing the thermal gradient and activating heterogeneous nucleation. All of the lap shear tension specimens showed pullout mode failure, exceeding the critical nugget diameter. Despite the higher total energy input in URW, the hardness drop and the width of the HAZ reduced. These findings not only show the applicability of URW on cell-level welding but also show that ultrasonic vibration improves microstructural and mechanical properties.
Keywords: Power ultrasound; Li-ion battery; Multilayer foil joining; Resistance spot welding; Microstructure; Mechanical properties