In the pursuit of advancing manufacturing processes for energy storage systems, our research focuses on enhancing the laser welding quality of aluminum alloy components used in energy storage cell modules. The transition to renewable energy sources has amplified the demand for efficient and reliable energy storage solutions, where battery modules serve as fundamental units. The welding integrity of battery terminals and connecting busbars directly impacts electrical conductivity, vibration resistance, and overall safety. Traditional welding methods often introduce defects such as cracks and porosity in aluminum alloys, prompting the adoption of laser welding for its precision and minimal distortion. However, conventional fiber laser welding faces challenges like spatter and depth instability. To address these issues, we explore the application of annular dual-beam laser welding, a novel technique that combines central deep penetration with peripheral thermal conduction to stabilize keyhole formation and improve weld quality. This study systematically investigates the effects of key parameters—laser energy ratio, total power, welding speed, and defocus distance—on weld morphology and mechanical properties, aiming to establish optimal conditions for high-yield production of energy storage cells.
The experimental setup utilized a continuous-wave laser system capable of delivering up to 4,000 W through a 50 μm core fiber for the central beam and 2,000 W through a 150 μm core fiber for the outer ring. The beams were focused using a 400 mm focal length lens, resulting in spot diameters of 0.11 mm and 0.33 mm for the center and ring, respectively. Specimens consisted of 2 mm thick 1060 aluminum busbars and battery covers from a commercial 280 A·h energy storage cell, with chemical compositions detailed in Table 1. Prior to welding, surfaces were treated with a 200 W laser cleaner to remove oxides and cleaned with alcohol to minimize contamination. Welding was performed under a nitrogen shield gas flow of 10 ± 2 L/min, with a spiral trajectory having an inner diameter of 9.4 mm and outer diameter of 12.6 mm. Post-weld analysis included sectioning, polishing, etching with NaOH solution, and examination under an optical microscope to measure fusion zone dimensions. Tensile strength was evaluated using a universal testing machine to correlate process parameters with mechanical performance.
The laser energy ratio, defined as the power distribution between the central and outer beams, significantly influences keyhole stability and defect formation. In energy storage cell welding, maintaining a stable keyhole is crucial for minimizing porosity and achieving consistent penetration. We varied the energy ratio from 50/50 to 90/20 while keeping total power at 4,000 W, speed at 80 mm/s, and defocus at 0 mm. The results, summarized in Table 2, indicate that lower energy ratios (e.g., 50/50) promote a wider keyhole opening, facilitating vapor escape and reducing porosity, but result in shallower penetration. Conversely, higher ratios (e.g., 90/10) increase penetration depth but elevate the risk of thermal cracks due to excessive energy concentration. The optimal energy ratio of 80/20 yielded a balanced profile with maximum tensile strength, as it minimized defects while ensuring adequate fusion. The relationship between penetration depth (D) and tensile strength (σ) can be expressed empirically for shallow penetrations as: $$ \sigma = k_1 \cdot D $$ where \( k_1 \) is a proportionality constant. However, beyond a critical depth, cracking leads to strength reduction, modeled as: $$ \sigma = k_2 \cdot \frac{1}{D} $$ for deeper welds, highlighting the non-linear dependence on energy distribution.
| Element | Al | Si | Fe | Cu | Mn | Mg | Cr | Zn | Ti |
|---|---|---|---|---|---|---|---|---|---|
| Busbar | 99.58 | 0.054 | 0.243 | 0.0047 | <0.002 | <0.001 | <0.002 | 0.0084 | 0.023 |
| Cover | 99.68 | 0.077 | 0.141 | 0.0044 | 0.0032 | 0.0021 | <0.002 | 0.0044 | 0.023 |
| Energy Ratio (Center/Ring) | Penetration Depth (mm) | Fusion Width (mm) | Tensile Strength (MPa) | Defects Observed |
|---|---|---|---|---|
| 50/50 | 0.45 | 1.20 | 85 | Minor Porosity |
| 60/40 | 0.58 | 1.35 | 92 | None |
| 70/30 | 0.72 | 1.40 | 105 | None |
| 80/20 | 0.88 | 1.42 | 118 | None |
| 90/10 | 1.05 | 1.38 | 95 | Thermal Cracks |
Total laser power directly affects the heat input and penetration depth in energy storage cell welding. We tested powers from 3,500 W to 5,000 W with an 80/20 energy ratio, 80 mm/s speed, and 0 mm defocus. As shown in Table 3, penetration depth increases linearly with power, described by: $$ D = \alpha \cdot P $$ where \( \alpha \) is a material constant, and \( P \) is the total power. However, tensile strength peaks at 4,000 W, beyond which defects like porosity and cracks emerge due to excessive thermal energy. This aligns with the general heat input equation: $$ E = \frac{P}{v} $$ where \( E \) is energy per unit length and \( v \) is welding speed. At lower powers, insufficient melting leads to weak joints, while higher powers cause overheating, reducing strength. The optimal power of 4,000 W ensures adequate penetration without compromising integrity, crucial for the longevity of energy storage cells.
| Total Power (W) | Penetration Depth (mm) | Fusion Width (mm) | Tensile Strength (MPa) | Defects Observed |
|---|---|---|---|---|
| 3,500 | 0.65 | 1.30 | 98 | Incomplete Fusion |
| 4,000 | 0.88 | 1.42 | 118 | None |
| 4,500 | 1.10 | 1.45 | 102 | Porosity |
| 5,000 | 1.32 | 1.48 | 88 | Cracks |
Welding speed governs the linear energy density and cooling rates, impacting the microstructure of welds in energy storage cells. Speeds from 40 mm/s to 120 mm/s were evaluated at 4,000 W total power, 80/20 energy ratio, and 0 mm defocus. Table 4 summarizes the inverse relationship between speed and penetration depth, approximated by: $$ D = \frac{\beta}{v} $$ where \( \beta \) is a constant. At lower speeds, prolonged heat exposure increases penetration but raises crack susceptibility due to high thermal gradients. The optimal speed of 80 mm/s achieves a balance, providing sufficient fusion time without defect formation. This parameter is critical for mass production of energy storage cells, as it influences both quality and throughput.
| Welding Speed (mm/s) | Penetration Depth (mm) | Fusion Width (mm) | Tensile Strength (MPa) | Defects Observed |
|---|---|---|---|---|
| 40 | 1.25 | 1.60 | 105 | Cracks |
| 60 | 1.02 | 1.50 | 112 | None |
| 80 | 0.88 | 1.42 | 118 | None |
| 100 | 0.70 | 1.35 | 100 | Minor Porosity |
| 120 | 0.55 | 1.28 | 90 | Incomplete Fusion |
Defocus distance, representing the displacement from the focal plane, alters the power density distribution and penetration characteristics. Tests ranged from -6 mm to +4 mm at 4,000 W power, 80/20 energy ratio, and 80 mm/s speed. Negative defocus (focus below surface) concentrates energy internally, enhancing penetration, as described by the power density formula: $$ P_d = \frac{P}{\pi r^2} $$ where \( r \) is the effective beam radius, which increases with defocus magnitude. As shown in Table 5, tensile strength and penetration peak at -2 mm, beyond which excessive defocus causes beam divergence, reducing efficiency. This parameter optimization is vital for adapting to variations in energy storage cell component thicknesses and geometries.
| Defocus Distance (mm) | Penetration Depth (mm) | Fusion Width (mm) | Tensile Strength (MPa) | Defects Observed |
|---|---|---|---|---|
| -6 | 0.95 | 1.38 | 108 | Undercut |
| -4 | 1.05 | 1.40 | 115 | None |
| -2 | 1.12 | 1.43 | 120 | None |
| 0 | 0.88 | 1.42 | 118 | None |
| +2 | 0.75 | 1.39 | 105 | None |
| +4 | 0.60 | 1.35 | 92 | Incomplete Fusion |
In conclusion, our investigation demonstrates that annular dual-beam laser welding is a highly effective method for improving the manufacturing yield of energy storage cells. The optimal parameters—80/20 energy ratio, 4,000 W total power, 80 mm/s welding speed, and 0 mm defocus—produce welds with stable morphology and superior tensile strength. We established that penetration depth and strength correlate positively up to a critical point, after which thermal cracks degrade performance. The interplay of parameters underscores the importance of balanced heat input to mitigate defects like porosity and undercut. Implementing these findings in industrial settings has enabled the production of over 10,000 energy storage cell modules with a first-pass yield exceeding 99.95%, highlighting the practical viability of this approach. Future work could explore real-time monitoring and adaptive control to further enhance welding consistency for next-generation energy storage systems.