Characteristics of Solidification Cracking for Ni-P Coated Cu-Al5052 Single-Mode Fiber Laser Welds
Article information
Abstract
Abstract
This study fundamentally examines the characteristics of solidification cracking in Cu-Al5052 single-mode fiber laser welding by utilizing uncoated and Ni-P coated Cu plates for the high durability busbar welding of pouch-type lithium-ion battery packs. Weld solidification cracking was confirmed regardless of Ni-P coating for both weld materials. However, there were differences in the regions of crack occurrence. Unlike the uncoated Cu-Al5052 welds where cracks occurred randomly within the fusion zone, for the Ni-P coated Cu-Al5052 welds, solidification cracks were concentrated near the faying surface of the upper and lower materials, particularly near the fusion line. The effect of the Ni and P components on the solidification cracking susceptibility, i.e., the welding solidification temperature range, was considered. Diffusion-controlled Scheil’s solidification calculations revealed that the mushy zone range enlarged with the use of a Ni coating layer. It was confirmed that an extremely wide mushy zone range of 1060 K was calculated at the solidification cracking region. Furthermore, P was also identified as an element that expands the solidification temperature range based on the Cu-P binary system. Consequently, during the laser welding of Cu-Al5052 dissimilar materials, the behavior of solidification cracking in the welds is influenced by the composition of the coating material and the incorporated composition within the weld fusion zone. Hence, it is necessary to select the optimal coating material and layer thickness considering weld solidification cracking susceptibility.
1. Introduction
The global market for eco-friendly electric vehicles (EVs) is rapidly growing due to increasingly stringent regulations on greenhouse gas emissions. According to the data published by the International Energy Agency (IEA) in 20241), the global market share of EVs increased from 5% in 2020 to approximately 9% in 2021 and 18% in 2023. Accordingly, research on battery packaging technology has been actively conducted in the automobile industry to secure mileage and safety. Battery cells are classified into three types: cylindrical, pouch, and prismatic cells. Among them, pouch cells are preferred as battery cells for EVs by several companies, including BMW and Chevrolet, due to their high energy density and space utilization. Battery packaging consists of cell-module-pack stages. In the case of Chevrolet Bolt EV 2nd generation, a representative vehicle that applies pouch cells, 24 to 32 cells are welded in one module and a total of 192 cells are finally installed in one vehicle2,3). Since each cell is connected by bonding busbars, which are conductors, to the terminals referred to as tabs, EVs have a considerable number of battery cell busbar welds. In general, Cu materials with high electrical conductivity are used for busbars. Depending on the battery type, Cu-Fe dissimilar welds are performed for cylindrical cells while Al and Cu materials are used for anode and cathode tabs, respectively, for pouch cells. Therefore, single-material welds and Cu-Al dissimilar welds are required depending on the busbar type and electrical connection characteristics. For Cu-Al dissimilar welds, various studies have been conducted to secure weldability and mechanical properties considering the differences in physical and chemical properties between the two materials.
Previous studies on Cu-Al dissimilar welds, which are required for battery packaging, were mostly conducted in terms of the intermetallic compounds formed in the weld zone and pore behavior4-11). In particular, mechanical properties are secured by controlling brittle intermetallic compounds, such as Al9Cu4 and AlCu2, by inserting Ni and Ag foil and applying coating materials to the weld zone12-17). Meanwhile, for Cu-Al dissimilar welds, an important weldability issue referred to as hot cracking has been reported in many cases in addition to the control of mechanical properties due to the formation of intermetallic compounds18,19). Hot cracking in welds is particularly concerning because it can negatively affect the overall performance of EVs by causing local resistance heating during electrical conduction in the weld zone. According to a study by Michaud that examined the solidification cracking behavior of Cu-Al welds by applying different laser pulses20), the solidification rate and dendrite arm spacing in Cu-Al welds are directly related to solidification cracking sensitivity. In addition, according to a study by J. Zhang that performed spot welding by applying the Nd:YAG pulse laser21), the solidification rate and the mushy zone of the weld zone significantly affect the solidification cracking behavior of the weld zone. Most previous studies on weld solidification cracking, however, are focused on uncoated materials, and there are few studies on the effects of coating materials that are applied for the aforementioned control of intermetallic compounds on weld solidification cracking.
In this study, the possibility of improving weld integrity through the use of coating materials was basically examined by evaluating the solidification cracking behavior of Cu-Al dissimilar welds according to the presence or absence of coating layer application. In particular, considering that low-speed high-heat-input welding conditions with a scan speed of 10 to 350 mm/s and a heat input of 4 to 96 J/mm were applied in previous studies22-27), the high-speed and low-heat-input single-mode fiber laser, which was relatively less reviewed, was applied.
2. Materials Used and Experimental Method
2.1 Materials used
Table 1 shows the chemical compositions of the materials used. High-purity C1100P Cu (100 × 30 × 5 mm) and Al5052 (100 × 30 × 5 mm) alloys were used. As for Cu plates, uncoated and Ni-P coated materials were applied. The coating layer contained approximately 5 mass % of P. The coating layer thickness was 25 ㎛ for the single side and 50 ㎛ for both sides at the top and bottom.
Chemical composition of materials used (mass%)
2.2 Experimental method
2.2.1 Single-mode laser welding
Fig. 1 shows the schematic description of laser welding and specimen arrangement. The detailed conditions of laser welding are summarized in Table 2. A single-mode fiber laser with a maximum power of 2 kW was used for welding, emitting a laser beam with a diameter of 38 ㎛ onto the surface of the material. The laser beam scan speed was set to 750 mm/s and the welding heat input to 2.67 J/mm so as to apply the single-mode laser with a 374% higher scan speed compared to the average scan speed of 158 mm/s in previous studies and an 89% lower heat input compared to the average heat input of 25 J/mm in the studies. Considering the laser beam reflection rate of uncoated Cu, Al 5052 was arranged as the top material and Cu as the bottom material for all welds. Welding was performed while there was no gap between the top and bottom materials using a pressurized jig.
Schematic description of (a) single-mode fiber laser welding facilities and (b) weld specimen arrangement
Specific conditions for single-mode fiber laser welding
2.2.2 Microstructure analysis
For the microstructure analysis of the weld zone, a Scanning Electron Microscope (SEM) and an Electron Probe Micro Analyzer (EPMA; JXA-8530F, JEOL) were used.
3. Experiment Results and Discussion
3.1 Microstructure and solidification cracking behavior in Cu-Al5052 single-mode fiber laser welds
Fig. 2 shows the EPMA analysis results for Cu- Al5052 dissimilar welds. Fig. 2(a) and 2(b) are the results for uncoated Cu while Fig. 2(c) and 2(d) are the results for Ni-P coated Cu materials. In the case of the Ni-P coated Cu weld compared to uncoated Cu (Fig. 2(c) and 2(d)), the coating layer elements, i.e., Ni and P, are clearly identified on the top and bottom surfaces of Cu, and the inflow of Ni and P into the fusion zone (FZ) can also be seen. A narrow bead width of approximately 0.28 mm (±0.02 mm) was obtained due to the nature of single-mode laser welding, and penetration weld beads could be obtained using a heat input of 2.67 J/mm. In addition, the element distribution in which the top and bottom materials are not completely mixed can be seen due to the high-speed welding speed. This can be said to be a general behavior that occurs in dissimilar welds under the application of high-speed single-mode laser scanning. Regardless of Ni-P coating, pore generation was significantly inhibited for all dissimilar welds under the two conditions.
Element (Al, Cu, Ni, P) distribution for uncoated and Ni-P coated Cu-Al5052 dissimilar welds, (a) and (b) uncoated Cu, (c) and (d) Ni-P coated Cu
Fig. 3 shows the magnified SEM observation results (backscattered electron (BSE) images) for the Cu- Al5052 laser weld cross-section. It can be seen that FZ solidification cracking occurred in both the uncoated Cu (Fig. 3(a)) and Ni-P coated Cu (Fig. 3(b)) dissimilar welds. The crack positions, however, were different in the two welds. While cracks were scattered in FZ for the uncoated Cu dissimilar welds, a considerable amount of solidification cracking tended to be concentrated on the fusion line (FL), especially at the faying surface between the top and bottom materials, for the Ni-P coated Cu welds. The analysis results of Fig. 2 and 3 show that solidification cracking that occurs during the welding process of lithium-ion battery production has a length of 50 to 100 ㎛. This suggests that such micro-scale weld solidification cracking should be identified, inhibited, and controlled to produce high-quality battery busbar welds.
BSE image of Cu and Al5052 dissimilar welds, (a) uncoated Cu (b) Ni-P coated Cu
Fig. 4 shows the EPMA element analysis results at the same positions as Fig. 3 for the uncoated Cu welds (Fig. 4(a) and 4(b)) and Ni-P coated Cu welds (Fig. 4(c) and 4(d)). While Al and Cu were scattered across the entire area of diluted FZ at specific ratios in the case of uncoated Cu (Fig. 4(a) and 4(b)), solidification cracking was concentrated in Ni- and P-rich areas where the coating layer elements (Ni and P) were introduced into FZ at the faying surface between the top and bottom materials in the case of the Ni-P coated Cu welds (Fig. 4(c) and 4(d)). In other words, it was judged that failure to optimize the thickness and elements of Ni-P coating, which can be applied to Cu-Al5052 dissimilar welds, is highly likely to degrade weldability, such as an increase in solidification cracking sensitivity at local positions due to the inflow of coating layer elements.
Element distribution for uncoated and Ni-P coated Cu-Al5052 dissimilar welds, (a) and (b) uncoated Cu, (c) and (d) Ni-P coated Cu
3.2 Investigation of the effect of Ni on solidification cracking behavior by calculating the weld solidification temperature range
Since it was judged that Ni and P from the coating layer directly affected the occurrence of solidification cracking through the solidification cracking area microstructure and element analysis, the solidification cracking behavior caused by the inflow of coating layer elements was investigated in this section by calculating (Scheil’s model, Thermo-Calc) the mushy zone range during non-equilibrium weld solidification. Considering that it is impossible to calculate the impact of P on the Thermo-Calc (version 2021a) software, only the effect of Ni was considered in the calculation of the weld solidification temperature range and the effect of P was examined from the Cu-P binary phase diagram. The detailed calculation conditions of the weld solidification temperature range and Cu-Al5052 dissimilar weld FZ elements were derived in the same way as a previous study by the authors of the present study on cylindrical battery busbar welding (Cu-Steel)28). Table 3 shows the input compositions of each dissimilar weld FZ applied for the calculation while Table 4 lists the cooling rate, solidification completion ratio, and secondary dendrite arm spacing reflected in the weld solidification calculation.
Calculated chemical composition of Cu-Al5052 dissimilar weld FZ by rule of mixture (i.e. input parameters of Thermo-Calc mushy zone calculation) in mass%
Input parameters of Thermo-Calc calculation
Fig. 5 shows the results of calculating the weld solidification temperature ranges of uncoated Cu and Ni coated Cu-Al5052 dissimilar welds. In FZ, the solid-liquid coexistence temperature range, i.e., the mushy zone, increased by approximately 60 K for Ni coating compared to the uncoated Cu weld. Since the mushy zone range of the weld solidification process is well known as a key factor that intensifies weld solidification cracking sensitivity, the calculation results confirmed that Ni coating increases the solidification cracking sensitivity of Cu-Al5052 dissimilar welds.
Calculated mushy zone temperature range of Cu-Al5052 fusion zone, (a) uncoated Cu (b) Ni coated Cu
The calculation results of Fig. 5 could be verified through the quantitative element analysis of the weld solidification cracking area. Fig. 6 shows the results of recalculating the mushy zone temperature range from the measured compositions in the solidification cracking area based on the quantitative element analysis results at the positions where solidification cracking occurred in the Ni-P coated Cu weld. The quantitative element analysis results of the solidification cracking area used in the calculation are shown in Table 5. In an area with a high Ni content, an extremely wide mushy zone range of up to 1,060 K was calculated. This indicates that solidification cracking sensitivity varies in the actual bead depending on the Ni content, and it significantly increases especially in areas with locally high Ni contents.
Calculated mushy zone temperature range from the measured compositions at solidification cracking area of Ni-P coated Cu-Al5052 dissimilar weld
Measured chemical composition of Ni-P coated Cu-Al5052 dissimilar weld solidification cracking area (in atom%)
In addition, as can be confirmed from the Cu-P phase diagram29), adding a trace amount of P extends the solidification temperature range of Cu by more than 300 K. Therefore, it is judged that the coating layer elements used in this study, i.e., Ni and P, are factors that increase solidification cracking sensitivity.
4. Conclusions
This study investigated the characteristics of solidification cracking under the application of Ni-P coating during Cu-Al5052 laser welding for high-integrity busbar welding of pouch lithium-ion battery packs from a metallurgical perspective. The conclusions of this study are summarized as follows.
1) The weld zone with inhibited pores was secured under all conditions through Cu-Al laser penetration welding. The occurrence of solidification cracking was confirmed regardless of Ni-P coating application, but the crack position was slightly different. While Al and Cu were scattered across the entire area of diluted fusion zone (FZ) at specific ratios in the case of uncoated Cu welds, solidification cracking was concentrated in Ni- and P-rich areas where the coating layer elements (Ni and P) were introduced into FZ in the case of Ni-P coated Cu welds.
2) Since the inflow of a considerable amount of coating layer elements was confirmed at the solidification cracking positions in Ni-P coated Cu welds, the effects of the coating layer elements on solidification cracking sensitivity, i.e., the weld solidification temperature range, were investigated. In the diffusion-control-type Scheil model calculation results using Thermo-Calc, the mushy zone increased for Ni coating compared to the uncoated Cu weld. In the actual quantitative analysis results, a high Ni content and a wide mushy zone of up to 1,060 K were also confirmed from compositions near the solidification cracking area.
3) P, one of the coating elements, was also identified as an element that extends the solidification temperature range during solidification with the Cu element. Therefore, it was judged that the solidification cracking sensitivity of Cu-Al5052 dissimilar welds increases during Ni-P coating, and solidification cracking is concentrated at positions where the direct inflow of the coating layer occurs.
4) In conclusion, the solidification cracking behavior of the weld zone can be exacerbated by the specific elements of the coating material and the resulting mixed composition in the bead of Cu-Al5052 dissimilar welds. Therefore, it is necessary to select the optimal coating material and coating layer thickness.