During the manufacturing process of batteries used in electric vehicles, copper materials need to be welded at high speeds and without spatter. Infrared lasers with wavelengths close to 1000 nm are typically used, however this presents two main challenges for welding copper materials: low energy absorption and process instability. The absorption of infrared laser light by copper materials increases with temperature. When a high-power IR laser irradiates a copper surface, the energy absorption rate of the copper surface suddenly increases after the formation of small holes; the holes are unstable and spatter is easily formed. At the same time, because the power of the infrared laser will be large, it will make the laser damaged. The absorption of the blue laser by the copper material is about 60% , which is much more efficient than that of the IR laser. The feasibility of blue diode lasers for processing copper has been reported in some literature. Blue lasers can weld copper foils or sheets with high efficiency and quality. However, the cost of blue lasers is much higher than that of NIR lasers and the maximum output power is limited to 2000 W. Combining the drawbacks of low IR laser energy absorption, unstable process and low output power of blue laser, we can propose a blue-IR composite laser welding process. In this welding process, we can first melt the surface of the base material with a blue laser with high absorption, and then increase the depth of the molten pool with an infrared laser.Yang et al. investigated the near-blue-infrared composite laser welding of a 3 mm-thick copper plate based on experiments and numerical simulations; first, the copper plate was heated with a low-power blue laser, and then a high-power infrared laser irradiated the high-temperature surface of the plate to form a deep small hole.Fujio et al. developed a blue-infrared laser composite welding system and found that the welding efficiency of the hybrid laser was 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 small holes and stabilize the internal thermal convection. In composite blue-infrared laser welding, the absorption of laser energy affects not only the stability of the welding process, but also the service life of the equipment. If the temperature of the copper surface is low after exposure to the blue laser, the IR laser energy reflected from the copper surface is high, which can damage the laser head.
Fujio, S et al. developed a composite laser system using a blue semiconductor laser as the preheating source and a single mode fiber laser as the welding source. Welding tests were performed on 2.5 × 3.0 × 50 mm copper wires using this composite laser system. Fig. 1 shows the melting and solidification kinetics of pure copper captured with a high-speed camera at 0.1, 0.2, and 0.3 s under (a) the composite laser and (b) the single-mode fiber laser. For a single-mode fiber laser with an output power of 1 kW, the melting of copper starts from about 0.3 s. The melting kinetics of the single-mode fiber laser are shown in Fig. 2.1.2. 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 Fig. 2, the melting volume of copper becomes larger in the hybrid laser than in the single-mode fiber laser.
Because of the preheating with the blue diode laser, the temperature of the copper rises to about 800 °C. The temperature of the copper increases to about 1.5 °F (0.5 °F). The increase in temperature leads to a local increase in the optical absorption of copper into the fiber laser. At the same time the composite laser obtains a larger copper melting volume than the single mode fiber laser. Therefore, it is concluded that by preheating of the blue diode laser, the light absorption of copper to the single-mode fiber laser is increased and the welding efficiency is improved.
Wu et al. developed a new blue light-infrared laser heat source model for copper materials with a thickness of 0.5 mm by using a coaxial composite blue light-infrared laser welding process, and numerically simulated the dynamic behavior of the molten pool and laser energy absorption by combining with the virtual mesh refinement method. Compared with blue laser welding, the maximum melting temperature and velocity of coaxial composite blue-IR laser welding fluctuate more, and the total laser energy efficiency is lower, but good welds can still be obtained. Compared with infrared laser welding, in coaxial composite blue-IR laser welding, the blue laser improved and stabilized the energy efficiency of the infrared laser.
A new simulation with 0 W of blue laser power, 1400 W of IR laser power, and a welding speed of 1.2 m/min was restarted from the coaxial composite blue-IR laser welding case at t = 0.1 s. The new simulation is shown in Fig. 3(a). As shown in Fig. 3(a), only a small molten pool is formed. The maximum melting temperature is 1798 K and the maximum melting speed is 0.11 m/s. As shown in Fig. 3(b), the absorbed IR laser power and efficiency are 190.4 W and 13.60%, respectively, after t = 0.232 s. The IR laser power and efficiency of the welded material are also shown in Fig. 3(c). Compared with the IR laser welding, the IR laser energy efficiency of the coaxial composite blue-IR laser welding was increased by 16.99%, and the total laser energy efficiency was increased by 165.22%. As shown in Fig. 3(c), the standard deviations of the IR laser efficiencies in coaxial composite blue light-IR laser welding and IR laser welding were 0.014% and 0.215%, respectively. It can be concluded that the blue laser improves and stabilizes the energy efficiency of the infrared laser in composite blue-IR laser welding.
In view of the cost as well as the maximum power limitation of the blue light and the drawbacks of the infrared laser's low energy absorption and process instability, a composite blue-red laser welding process was proposed. By preheating the material with the high absorption rate of blue light, thus achieving a rise in the absorption rate of red light, and at the same time, due to the small power density of blue light compared to fiber laser, it is possible to realize the combination of stable heat conduction welding and deep melting welding to achieve efficient welding of highly anticorrosive alloys (Aluminum, Copper).
