Technical Column | The Application Of Red And Blue Composite Laser In Welding.

In the manufacturing process of the batteries used in electric vehicles, the copper material needs to be welded at high speed and without splashing. Infrared lasers with wavelengths close to 1000 nm are commonly used, however, this presents two major challenges for welding copper materials: low energy absorption and process instability. The absorption rate of copper to infrared laser increases with the increase of temperature. When the high power infrared laser irradiated the copper surface, the energy absorption rate of the copper surface suddenly increased after the formation of small holes;Small holes are unstable and easily form splashes. At the same time, because the power of the infrared laser will be very large, it will cause damage to the laser. The absorption rate of copper materials for blue laser is about 60%, which is much higher than the absorption efficiency of IR laser. The feasibility of blue diode lasers for processing copper has been reported in some literature. The blue laser can weld copper foil or copper plate efficiently and with high quality. However, the cost of blue lasers is much higher than that of near-infrared lasers, and the maximum output power is limited to 2000 W. Combining the shortcomings of low absorption rate of infrared laser energy, unstable process and low output power of blue laser, we can propose a blue-infrared composite laser welding process. In this welding process, we can first melt the surface of the base material with a blue laser with high absorptivity, and then increase the depth of the weld pool with an infrared laser. Yang et al. studied the near-blue-infrared composite laser welding of 3 mm thick copper plates based on experiments and numerical simulations;A low-power blue laser is first used to heat the copper plate, and then a high-power infrared laser illuminates the high-temperature surface of the copper plate to form deep holes. Fujio et al. developed a blue-infrared laser composite welding system and found that the welding efficiency of the hybrid laser is 1.45 times higher than that of the infrared laser. Kaneko et al. used a coaxial composite blue-infrared laser to enlarge the molten pool and the small hole, stabilizing the internal heat convection. In blue-infrared laser composite welding, the absorption of laser energy will not only affect the stability of the welding process, but also affect the service life of the equipment. If the temperature of the copper surface is low after the blue laser irradiation, the IR laser energy reflected by the copper surface is high, which may damage the laser head.

Fujio, S et al. developed a composite laser system with a blue semiconductor laser as the preheating source and a single mode fiber laser as the welding source. The welding test of 2.5×3.0×50 mm copper wire was carried out by using the composite laser system. Figure 1 shows the melting and solidification kinetics of pure copper captured with a high-speed camera at 0.1, 0.2, and 0.3 seconds under (a) composite laser and (b) single-mode fiber laser. In the case of a single-mode fiber laser with an output power of 1 kW, the melting of copper starts from about 0.3 seconds. On the other hand, for a hybrid laser with a single-mode fiber laser with an output power of 1 kW and a blue diode laser with an output power of 200 W, the melting of copper starts from 0.2 seconds. Therefore, as shown in Figure 2, the melting volume of copper in the composite laser becomes larger than that of the single-mode fiber laser.

Because the blue diode laser is used for preheating, the temperature of the copper is raised to about 800 ° C. The increase of temperature will lead to a local increase in the absorption rate of copper to the fiber laser. At the same time, the copper melting volume of the composite laser is larger than that of the single mode fiber laser. Therefore, it is considered that the light absorption rate and welding efficiency of the single-mode fiber laser can be improved by preheating the blue diode laser.

Figure 1. Melting and solidification kinetics of copper samples at 0, 0.1, 0.2, and 0.3 seconds (a)1 kw single-mode fiber laser +200 w blue laser, (b)1 kw single-mode fiber laser

Figure 2. Copper sample after radiation

Wu et al. established a new blue-infrared laser heat source model for copper materials with a thickness of 0.5 mm by using a coaxial composite blue-infrared laser welding process, and combined with a virtual mesh thinning method, conducted numerical simulations of the dynamic behavior of the molten pool and laser energy absorption. Compared with the blue laser welding, the maximum melting temperature and speed of the coaxial composite blue-infrared laser welding fluctuate greatly, and the total energy efficiency of the laser is low, but it can still obtain a good weld. Compared with the infrared laser welding, the energy efficiency of the infrared laser is improved and stabilized by the blue laser in the coaxial composite blue-infrared laser welding.

At t= 0.1s, a new simulation with 0 W blue laser power, 1400 W infrared laser power and 1.2 m/min welding speed is restarted from the coaxial composite blue-infrared laser welding situation. As shown in Figure 3(a), only small pools are formed. The maximum melting temperature is 1798 K and the maximum melting speed is 0.11 m/s. As shown in Figure 3(b), after t= 0.232s, the absorbed infrared laser power and efficiency are 190.4W and 13.60%, respectively. Compared with infrared laser welding, the infrared laser energy efficiency of coaxial composite blue-infrared laser welding is increased by 16.99%, and the total laser energy efficiency is increased by 165.22%. As shown in Figure 3(c), the standard deviation of infrared laser efficiency in coaxial composite blue-infrared laser welding and infrared laser welding is 0.014% and 0.215%, respectively. It can be concluded that in composite blue-infrared laser welding, blue laser improves and stabilizes the energy efficiency of infrared laser.

Figure 3. Numerical results of laser welding