Welding test of domestic industrial grade 2kW fiber laser cell with fine core diameter

Laser welding is widely used in the manufacture of new energy automotive batteries because of its non-contact, high energy density, fast welding speed, accurate control of heat input and easy automation.

With the intensification of energy crisis and environmental pollution caused by automobiles worldwide, the automobile industry chain is accelerating its transformation to low-carbon, electrification and other directions, and new energy vehicles are gradually replacing traditional fuel vehicles. Battery system is the key component to determine the performance of new energy vehicles. The battery system is a complex system composed of battery cell, battery module and battery pack. The system contains thousands of welding spots/welds. Each welding will directly affect the function and efficiency of the entire battery system, so the welding process is crucial. Therefore, the high reliability of battery welding technology and process has become an urgent problem for the development of automobile manufacturing industry. Laser welding is widely used in the manufacture of new energy automotive batteries because of its non-contact, high energy density, fast welding speed, accurate control of heat input and easy automation.

The materials to be welded during the production and assembly of new energy batteries are mainly 3003 series aluminum alloy (battery shell), aluminum and copper (electrode, busbar, busbar). The difficulties in laser welding process are mainly reflected in the high reflectivity of aluminum alloy to laser, which is easy to produce pores and hot cracks; Al/Cu dissimilar metal materials are mixed to form brittle, high resistance intermetallic compounds, etc. At present, a variety of methods and process studies have been reported, such as lifting beam oscillation welding, pulsed laser welding, laser beam quality and reducing focus spot, which can improve the problems to some extent. In June 2021, Hunan Dake Laser Co., Ltd. developed an industrial 2kW thin core diameter resonant cavity single-mode continuous fiber laser using 976 nm wavelength semiconductor laser pumping technology, and achieved the μ 2.25kW pure single mode (M 2 ≤ 1.1) laser is output in m core diameter QBH. The laser can not only maintain the near diffraction limit laser output, but also obtain smaller focus spot under the same welding joint, which is particularly suitable for high-quality welding of copper and aluminum materials.

fourteen μ The m core diameter 2kW single-mode fiber laser adopts double end pumping mode, and its structure is shown in Figure 1, including 976nm wavelength semiconductor laser (LD), forward pump combiner (FC), high reflection grating (HR), ytterbium doped fiber (YDF), low reflection grating (OC), backward pump/signal combiner (BC), cladding optical filter (CPS), energy transmission fiber (GDF) and output end cap (QBH). The center wavelength of the fiber grating pair used is 1080 nm, the gain fiber is a commercial double clad ytterbium doped fiber, and the core mode field area is 150 μ M 2, the absorption coefficient of the gain optical fiber at 976nm is about 2.5dB/m, and the energy transmission optical fiber (core diameter is 14 μ m. 0.07 for NA) up to 7 m. While ensuring the full absorption of the pump light and the laser light conversion efficiency, the laser shortens the length of the gain fiber and improves the stimulated Raman scattering threshold of the laser. When the total pump power is 2720W, the output power reaches 2250W, and the Raman suppression ratio is - 38dB, as shown in Figure 2. The winding diameter of the gain fiber is about 9cm, which can suppress the mode instability while ensuring that the conversion efficiency does not decrease, filter out high-order modes, and improve the output beam quality. The output beam quality M2=1.06, as shown in Figure 3.

Figure 1 2kW 14 μ Structure diagram of m laser

Fig. 2 Core diameter 14 μ m. The Raman suppression ratio is - 38dB when the QBH optical cable is 7m long and the output power is 2250W

Fig. 3 Beam quality M at 2kW output ² 1.06 (LQM test)

To test 14 μ The performance of the m core diameter single-mode 2kW laser in the welding of high reflective materials, we first carried out the red copper laser welding experiment. The material used is red copper sheet with a thickness of 2mm. The welding method is swing oscillating welding. The welding speed (moving speed of focus spot) is 300mm/s, the swing diameter is 1mm, and the focus is on the surface of red copper (without defocusing). Figure 4 shows the curve of weld penetration width and penetration depth changing with output power. It can be seen that when the output power is increased from 1kW to 2kW, the penetration depth is increased from 0.91mm to 1.53mm, while the penetration width is basically maintained at 1.76mm. This indicates that 14 μ The m core diameter single-mode laser can gradually increase the penetration depth while keeping the fusion width basically unchanged, so that the penetration depth can be adjusted, and it has a wider process window.

Fig. 4 Core diameter 14 μ Variation of Weld Width and Penetration with Laser Power in the Process of Welding Red Copper with m2kW Single mode Laser

In order to further verify the advantages of thin core diameter single-mode laser in high anti material welding, we based on 14 μ M core diameter single-mode laser, 20 μ M core diameter single-mode laser, 50 μ The contrast experiment of copper sheet overlay welding was carried out for a m core diameter multimode laser. The output power of the three lasers is set to 2kW, and the welding mode is swing oscillating welding, with a swing diameter of 1mm. The welding material used is two red copper sheets with a size of 30mm × 50mm, the thickness of the upper plate is 1mm, and the thickness of the lower plate is 2mm. When the penetration reaches 1.5mm μ The maximum welding speed of m core diameter multimode laser is 100mm/s, 20 μ The maximum welding speed of m core diameter single-mode laser is 300mm/s. And 14 μ Because the focal spot of the m-core single-mode laser is smaller and the energy density is higher, the maximum welding rate corresponding to the penetration is 450mm/s, which is 20 μ 1.5 times of m single mode laser, 50 μ M multimode laser. The above results show that compared with the ordinary 20 μ M core diameter single-mode laser and 50 μ M core diameter multimode laser, using 14 μ M core diameter single-mode laser can greatly improve the welding rate.

In addition to significantly increasing the welding rate, 14 μ M core diameter single-mode laser is also conducive to reducing thermal effects and defects. Figure 5 shows the red copper overlay welding effect of the three lasers. The output power is 2kW, the welding speed is set at 450mm/s, and the swing diameter is 1mm. fifty μ The width of the heat affected zone corresponding to the m core diameter multimode laser is 5.820mm, and the penetration depth is 0.878mm. Because the penetration depth is less than 1mm, 1+2mm copper plate overlap welding cannot be achieved at this speed, and there are obvious holes and a large number of spatters on the weld surface, as shown in Figure 5a. Figure 5b shows 20 μ The weld effect picture of the m-core single-mode laser shows that the width of the corresponding heat affected zone is 4.241mm, and the penetration depth is 1.085mm. Because the penetration just penetrates the upper plate, the high-strength combination of the upper and lower copper plates cannot be achieved. In addition, there are also holes on the weld surface. fourteen μ The weld effect diagram of the m-core single-mode laser is shown in Figure 5c. The width of the heat affected zone is 2.455mm, and the penetration depth is 1.310mm. Compared with the first two lasers, the width of the heat affected zone is significantly reduced, and no obvious holes and splashes are found on the surface. Therefore, 14 μ M core diameter single-mode laser can also obtain better process stability.

The generation of weld defects is usually closely related to the instability of the weld pool. Due to the fluctuation of the workpiece surface, the defocusing amount changes, which leads to uneven and unstable weld fusion width, and finally produces holes and other defects. For further analysis 14 μ The root cause of the obvious defect suppression effect in the welding of the m-core single-mode laser is that the weld width changes with the defocusing amount. The adopted welding method is swing oscillating welding. The swing diameter is 1mm, the welding speed is 450mm/s, the output power is 2kW, and the base is 2mm red copper sheet. The results are shown in Figure 6. It can be seen that the change of melting width is only ± 0.25mm within the defocusing range of - 3~2.5mm. This is because the strict single-mode laser output (M 2=1.06) makes 14 μ M core diameter single-mode laser has larger rayleigh length, and the spot shape near the focus changes slowly. Because of its extremely high beam quality, 14 μ The weld width produced by the m-core single-mode laser is uniform and stable, and has obvious defect suppression effect.

Fig. 6 Core diameter 14 μ Variation of melting width with defocusing amount during welding of red copper with m2kW single-mode laser

In conclusion, due to the smaller focus spot and the extremely high beam quality μ The single mode laser with m core diameter of 2kW can obviously reduce the thermal effect in welding, thus reducing the spatter of molten pool and the rate of weld defects; The penetration depth can be controlled without changing the weld width, compared with the common 20 μ M single mode laser and 50 μ The m multimode laser greatly improves the welding speed, expands the welding window, and reduces the generation of welding defects. It is suitable for welding between aluminum alloy, copper and other high inversion materials and copper aluminum dissimilar materials. At present, the laser has been applied to the welding of the lug of the flexible battery, the welding of the bus bar, the sealing of the square battery, the explosion-proof valve, the pole welding, etc. In the future, the process parameters of the laser will continue to be optimized to further improve its efficiency and stability in processing.

Source: Progress in Laser and Optoelectronics