1. Introduction
As the power output of electric vehicles and hybrid electric vehicles continues to increase, enhancing the current-carrying capacity of busbars for stable transmission of high currents within power modules has emerged as an important technical requirement1-3). In particular, with the expanding adoption of next-generation silicon carbide (SiC)-based power semiconductors and the accompanying increase in system power density, busbars are required to provide greater current-carrying capability and thermal stability. To meet these demands, the use of thicker Cu busbars is being gradually expanded compared with conventional designs4). Such increases in busbar thickness are important not only for improving current-carrying capacity but also for ensuring joint reliability under high-power operating conditions. Consequently, a more detailed evaluation of weldability is required5-7).
In Part 1 of this study, the authors investigated the heat-affected zone (HAZ) formation behavior and solidification cracking characteristics of large-area multi-pass welding using a single-mode fiber laser and a rectangular spiral (ㄹ-shaped) scanning pattern for Cu-Cu busbars with a thickness combination of 0.8-0.8 mm. Under ultra-low heat input conditions below 5 J/mm and optimized overlap ratio conditions, sound welds free from solidification cracking were successfully obtained, and HAZ-free characteristics associated with rapid cooling behavior on the order of 5 × 104 K/s were confirmed. Furthermore, the large-area multi-pass welds exhibited a maximum shear tensile load approximately 920% higher than that of single-pass linear welds, demonstrating the applicability of single-mode fiber laser welding for Cu busbar joining in power module applications. However, as discussed above, practical power module busbars are expected to employ a variety of thickness combinations depending on the required current capacity and system output. As busbar thickness increases, higher heat input is likely to be required to achieve equivalent penetration depth and joint area. Such increases in heat input may influence weldability characteristics that are the focus of this study, including HAZ formation, solidification cracking behavior, and mechanical performance8-10).
Accordingly, this study investigates thicker Cu busbar combinations of 0.8-1.2 mm and 0.8-1.5 mm based on the optimized large-area multi-pass welding conditions established in Part 1 for the 0.8-0.8 mm Cu busbar combination. Particular attention is given to the effects of increased heat input associated with increased busbar thickness on HAZ formation behavior, solidification cracking characteristics, and maximum shear tensile load. In addition, the applicability of HAZ-free single-mode fiber laser welding to various Cu busbar thickness combinations for high-power semiconductor module applications is discussed through correlation with thermodynamics-based thermal history analyses of the rapid heating and cooling behavior occurring during welding.
2. Materials and Experimental Procedures
2.1 Materials
Table 1 presents the chemical composition of the high-purity C1100P Cu used in this study. For the welding experiments, the C1100P Cu sheets were machined into dimensions of 100 × 30 × 0.8 mm, 100 × 30 × 1.2 mm, and 100 × 30 × 1.5 mm.
2.2 Experimental Procedures
2.2.1 Single-Mode Fiber Laser Welding
Fig. 1 illustrates the schematic configuration of the laser welding system and specimen arrangement. The detailed welding conditions are summarized in Table 2. A single-mode fiber laser was employed for all welding experiments. During welding, the laser power, laser beam diameter on the specimen surface, and scanning speed were controlled, resulting in welding conditions corresponding to heat inputs ranging from 5.7 to 13.3 J/mm and energy densities ranging from 191.5 to 446.7 J/mm2. A multi-pass welding pattern with a weld area of 8 × 6.3 mm2 was applied using a rectangular spiral scanning path. As in Part 1, the overlap ratio along the scanning path was selected as a primary process variable. Four overlap ratio levels were investigated for each welding condition and are designated as Condition A (Low), Condition B (Medium), Condition C (Medium High), and Condition D (High). For all welding experiments, a clamping jig was used to ensure intimate contact between the upper and lower materials without any gap.
Fig. 1
Schematic illustrations of (a) the single-mode fiber laser welding system, (b) the weld specimen configuration, and (c) the laser beam scanning patterns used for single-pass and multi-pass welding, respectively
Table 2
Specific conditions for single-mode fiber laser welding
2.2.2 Microstructural Analysis and Mechanical Property Evaluation
After sectioning, the specimens were sequentially ground using SiC abrasive papers (#400-#2000), followed by final polishing witath 3 μm and 1 μm diamond paste. For microstructural observation, the polished specimens were etched at room temperature for 30 s using a solution consisting of 50 mL distilled water and 50 mL nitric acid (HNO3, 68-70%). Microstrutcural characterization of the welds was performed using an optical microscope (OM; DR IRM, Leica).
The mechanical properties of the laser welds were evaluated through shear tensile testing. The test specimens were prepared according to the configuration shown in Fig. 1(b), and the welding conditions were applied as specified in Table 2. A crosshead speed of 3 mm/min was used during testing. The maximum tensile load obtained from the load-displacement curve was used as the indicator of weld mechanical performance. For each welding condition, the maximum tensile load was measured five times and the average values were compared.
3. Experimental Results and Discussion
3.1 Weldability of Large-Area Multi-Pass Welds with Different Busbar Thickness Combinations of 0.8-1.2 mm and 0.8-1.5 mm
This section focuses on representative welding conditions capable of achieving approximately 25% penetration into the lower material, consistent with the criteria established in Part 1. The selected conditions were a heat input of 5.7 J/mm for the 0.8-1.2 mm busbar combination and 13.3 J/mm for the 0.8-1.5 mm busbar combination. The welding results were compared with those obtained for the 0.8-0.8 mm busbar combination in Part 1, where heat inputs of 3.5-4.5 J/mm were employed. Furthermore, the effects of increased busbar thickness were systematically evaluated from three perspectives: solidification cracking behavior as a function of overlap ratio during large-area welding using the rectangular spiral scanning pattern, macrostructural characteristics and HAZ formation under optimized welding conditions, and the maximum shear tensile load of the welded joints.
3.1.1 Solidification Cracking Behavior as a Function of Overlap Ratio for the 0.8-1.5 mm Busbar Thickness Combination
Fig. 2 compares the solidification cracking behavior observed on the weld bead surface under four overlap ratio conditions at a heat input of 13.3 J/mm. Similar to the results obtained for the 0.8-0.8 mm busbar combination, solidification cracking was clearly observed under the non-optimized high-overlap conditions shown in Figs. 2(b), 2(c), and 2(d), whereas a crack-free weld bead was obtained only under the optimized overlap condition shown in Fig. 2(a). The same tendency was also confirmed for the 0.8-1.2 mm busbar combination. Notably, the heat input applied to the 0.8-1.5 mm busbar combination was approximately 270% higher than that used for the 0.8-0.8 mm combination. Under these conditions, an increased tendency toward solidification cracking was observed during multi-pass welding. These results indicate that more precise control of the overlap ratio becomes increasingly important for suppressing solidification cracking as busbar thickness increases.
3.1.2 Macrostructures of Welds with 0.8-1.2 mm and 0.8-1.5 mm Busbar Thickness Combinations
Fig. 3 presents the results of large-area multi-pass welding performed under overlap ratio Condition A for busbar thickness combinations of (a) 0.8-1.2 mm and (b) 0.8-1.5 mm. Heat inputs of 5.7 J/mm and 13.3 J/mm were applied for the 0.8-1.2 mm and 0.8-1.5 mm combinations, respectively. As described in Table 2, the weld area was 8 × 6.3 mm2. Cross-sectional observations revealed penetration depths of approximately 1.43 mm for the 0.8-1.2 mm combination and 1.74 mm for the 0.8-1.5 mm combination. Under overlap ratio Condition A, defect-free multi-pass welds without solidification cracking were successfully obtained on both the bead surface and cross-section despite the increased heat input required for the thicker busbars. Compared with the welding results obtained for the 0.8-0.8 mm busbar combination in Part 1, the heat input increased moderately. Nevertheless, clear boundaries between individual passes remained visible, no grain coarsening was observed adjacent to the pass boundaries, and continuity with the base metal microstructure was maintained. Based on these macrostructural observations, no evidence of HAZ formation was detected. Although the welding heat input increased compared with the 0.8-0.8 mm busbar combination, the heat inputs of 5.7 and 13.3 J/mm remain considerably lower than those typically employed in multimode laser welding processes. Therefore, even with increased busbar thickness, multi-pass welds produced using a single-mode laser can maintain HAZ-free behavior.
3.2 Mechanical Properties of Large-Area Multi-Pass Welds with Different Busbar Thickness Combinations
Fig. 4 presents the shear tensile test results for the welds shown in Fig. 3. Maximum loads of 3.46 kN and 3.56 kN were obtained for the 0.8-1.2 mm and 0.8-1.5 mm busbar combinations, respectively, with both conditions exhibiting joint strengths exceeding 3.40 kN. As shown in Fig. 4, these values are comparable to those obtained for the 0.8-0.8 mm busbar welds. Overall, the welding conditions established for each busbar thickness combination indicate that equivalent mechanical performance can be achieved despite moderate variations in heat input, provided that similar weld dimensions, including bead surface area and cross-sectional area, are maintained.
Fig. 4
Maximum load obtained by shear tensile test for multi-pass Cu-Cu welds at an overlap ratio of condition A
In the secondary battery industry, numerous studies have reported laser welding of tab-to-busbar and cell terminal-to-busbar joints using various laser sources, including single-mode lasers. Depending on the material combination, tab and busbar thicknesses, and welding configuration, maximum shear loads ranging from several hundred newtons to approximately 1300 N have been reported. Kumar et al. reported maximum loads of approximately 285-510 N for Al tab-Cu busbar joints and approximately 298-1320 N for other tab-Cu busbar combinations produced by laser lap welding between a 0.3 mm tab and a 1.5 mm busbar11). In addition, laser wobble welding of 21700 cylindrical cell terminals to thick busbars has also been reported to produce joint strengths on the order of several hundred newtons12). Therefore, the mechanical performance achieved for the power module busbar welds in the present study (Fig. 4) is superior to the maximum shear loads typically reported for laser-welded joints in battery manufacturing applications.
3.3 Thermal Analysis of Rapid Heating and Cooling Behavior during Large-Area Multi-Pass Welding of 0.8-1.5 mm Busbars
To investigate the HAZ-free behavior observed in Fig. 3, thermal histories associated with the heating and cooling behavior of large-area multi-pass welds produced using a single-mode laser were calculated using the Thermo-Calc Additive Manufacturing Module. The cooling rate was determined from a single thermal cycle, defined as the period from the peak temperature reached after passage of the heat source to subsequent cooling to room temperature. Figs. 5-7 present the calculated thermal histories and cooling rates corresponding to selected locations on the bead surface at the 6th, 13th, 20th, and 26th passes (Fig. 5), overlapped regions within the U-shaped segments of the rectangular spiral scanning pattern (Fig. 6), and HAZ locations along the bead depth direction (Fig. 7), respectively. Among the two busbar thickness combinations investigated, the 0.8-1.5 mm busbar weld was selected as the representative case for analysis because it represents the more severe condition from the perspective of maintaining HAZ-free behavior.
Fig. 5
Calculated thermal histories and corresponding cooling rates at selected points on the bead surface during single-mode fiber laser multi-pass welding at a heat input of 13.3 J/mm, (a) extracted surface locations (6th, 13th, 20th, and 26th passes) and (b) corresponding cooling rate profiles
Fig. 6
Calculated thermal histories and corresponding cooling rates at overlapped regions of the multi-pass scanning path during single-mode fiber laser welding at a heat input of 13.3 J/mm, (a) extracted overlap locations and (b) corresponding cooling rate profiles
Fig. 7
Calculated thermal histories and corresponding cooling rates along the bead depth direction during single-mode fiber laser multi-pass welding at a heat input of 13.3 J/mm, (a) extracted subsurface locations and (b) corresponding cooling rate profiles
The total welding time required to complete the 8 × 6.3 mm2 weld area was calculated to be approximately 0.79 s. An average cooling rate of approximately 4 × 104 K/s was obtained at the bead surface, as shown in Fig. 5. Furthermore, no evidence of thermal accumulation was observed in the overlapped regions shown in Fig. 6, and cooling rates comparable to those observed at the bead surface were obtained. Compared with the cooling rate of approximately 7 × 104 K/s reported for the 0.8-0.8 mm busbar combination in Part 1, a reduction in cooling rate was observed. Nevertheless, a cooling rate of approximately 4 × 104 K/s still represents extremely rapid cooling compared with those encountered in conventional welding processes. Therefore, the HAZ-free behavior observed in the macrostructures of the 0.8-1.5 mm busbar welds shown in Fig. 3(b) is believed to be attributable to this rapid cooling behavior. In addition, the calculated HAZ location immediately beneath the weld boundary along the bead depth direction exhibited a maximum temperature of 491.7 K, as shown in Fig. 7. Maintaining a peak temperature below 500 K during the ultra-high-speed large-area welding process completed within approximately 0.8 s suggests that thermal damage can be effectively suppressed not only within the HAZ beneath the weld bead but also in surrounding components adjacent to the welded region.
4. Conclusions
In this study, the effects of busbar thickness on the optimal welding conditions, solidification cracking behavior, HAZ formation, and mechanical properties of single-mode laser welds for power semiconductor module busbars were systematically investigated to establish weld integrity and process guidelines. The main conclusions are summarized as follows.
1) For the 0.8-1.2 mm busbar thickness combination, the optimal welding condition was identified as a heat input of 5.7 J/mm, whereas a heat input of 13.3 J/mm was required for the 0.8-1.5 mm combination. Compared with the optimal welding conditions established in Part 1 for the 0.8-0.8 mm busbar combination (3.5-4.5 J/mm), these heat inputs represent increases of approximately 60% and 270%, respectively.
2) For both the 0.8-1.2 mm and 0.8-1.5 mm busbar combinations, solidification cracking was observed under non-optimized overlap ratio conditions. Sound welds free from solidification cracking were obtained only under overlap ratio Condition A.
3) Despite variations in busbar thickness and welding heat input, maximum shear tensile loads exceeding 3.40 kN were consistently achieved. These results indicate that uniform mechanical performance can be maintained across various busbar thickness combinations.
4) Thermal analysis of the 0.8-1.5 mm busbar welding condition using the Thermo-Calc Additive Manufacturing Module revealed that the HAZ-free behavior observed in the macrostructures was attributable to the rapid cooling behavior of approximately 4 × 104 K/s.
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