1. Material vs. laser wavelength
Since the birth of the first laser in 1960, after more than 60 years of development, laser, as the sharpest and most precise knife, has been gradually used in our lives. Laser is combined with biology, medical treatment and diagnosis, and pharmaceutical science, and has gradually penetrated into daily life in aspects such as laser treatment, laser surgery, and laser diagnosis. In the field of equipment manufacturing, high-power laser equipment plays an increasingly important role in cutting, welding, measuring, marking and other aspects of high-end equipment manufacturing fields such as aviation, aerospace, automobiles, high-speed rail, and ships. In terms of fine micromachining, ultrashort pulse lasers play an irreplaceable role in drilling, engraving, grooving, surface texturing, surface modification, trimming, cleaning and other aspects of photovoltaics, liquid crystal displays, semiconductors, LEDs, OLEDs and other fields. role. With the rapid development of semiconductor pump technology, 1um wavelength near-infrared lasers, after years of development, have formed a complete industrial chain and occupy a pivotal position in industrial processing applications. In particular, 1um near-infrared fiber laser has become a widely used laser type due to its wide power coverage, excellent beam quality, stability and reliability. Copper is the third most used metal in the world after iron and aluminum. Copper material is one of the most widely used metal materials in modern industrial processing. The terminal demand structure of the copper industry chain covers more than 30 sub-sectors, such as aerospace, high-speed trains, intelligent terminal products, electronic communications, and automobiles, and is the main benchmark for high-end industrial applications. The 1 micron band infrared fiber laser currently used on a large scale has shortcomings such as large spatter and uncontrollable penetration depth in the processing of copper materials due to its weak absorption of copper. Figure 1 shows the absorption curves of commonly used metal materials for lasers of different wavelengths. It can be seen that the absorption rates of lasers by different metals vary greatly at different wavelengths. Figure 2 shows the absorption rate comparison curves at different wavelengths for metallic copper alone. At room temperature, copper's absorption rate of near-infrared wavelengths (about 1 micron) is less than 5%, so using infrared light to process copper materials is extremely inefficient. 95% of the laser will be reflected and will also cause damage to the laser itself; The absorption rate of copper at green light wavelengths (515nm and 532nm) is as high as more than 40%. The selectivity of the laser wavelength of the material itself determines that the most ideal wavelength for precision processing of highly reflective materials is a short wavelength (≤700nm).
Compared with the short wavelength of ultraviolet laser, the current bottleneck limitation of materials science cannot support the realization of stable high-power ultraviolet laser output. Ultraviolet lasers exceeding one hundred watts are extremely rare. On the contrary, through the efforts of scientists from various countries, commercial green lasers have made great progress in recent years. Germany's TRUMPF and the United States' IPG have obtained ultra-high power green light output of more than 3 kilowatts and 1 kilowatts, respectively, through disc laser technology and fiber laser technology. High-power continuous green light lasers play an extremely important role in two important problems in current industrial applications: 3D printing and precision welding of copper materials.
2. Application prospects and advantages of high-power green light
At the 14th China International Battery Technology Exhibition in 2021, Germany's TRUMPF debuted its 3-kilowatt high-power continuous green light disc laser. The average output power of this product is as high as 3 kilowatts, which represents the strongest power in the current green laser series and is very suitable for welding of highly reflective materials such as copper and aluminum. Especially in the lithium battery industry represented by new energy vehicle power batteries, TRUMPF green lasers (1000–3000W) can weld up to 120 layers of copper foil with almost no spatter and precise and controllable penetration. In addition, high-power green light also has outstanding advantages in 3D printing applications of pure copper materials. At present, there is still a gap in high-power green lasers in China.
2.1 High-reflection metal welding
Due to the outstanding electrical conductivity of copper materials, copper materials are widely used in the lithium battery industry, especially in new energy vehicle power batteries. At present, the mainstream still uses high-power infrared fiber lasers for copper welding. Compared with the infrared band, copper welding using green light is more efficient and has almost no spatter. Splash is fatal to battery processing, and splashes will affect the production safety, performance and life of the battery.
Figure 3 shows the absorption of 1064nm infrared laser by copper. As can be seen from Figure 3, as the melting temperature increases from 0 to 1400K, copper's absorption of infrared light slowly rises from 5% to about 10%; when copper reaches the melting point (1400K), copper The absorption rate of infrared band laser will rise stepwise from 10% to about 17%, and then as the temperature continues to rise, the absorption rate will slowly increase. This sudden change in absorptivity around the melting point can cause some of the molten material to be discharged in the form of splatter, and can also cause small holes to collapse, forcing the entire process to have to be restarted. Especially for the back-end process welding of lithium batteries, the welding yield has a direct impact on the cost of the battery.
Figure 4 shows the absorption curves of copper for different wavelengths (infrared, green and blue light) at different temperatures. The green lines in the figure represent the absorption rate of green light by copper at 20°C (solid state) and 1600°C (molten state). At room temperature of 20°C, when copper is in a solid state, its absorption rate in the green light band is about 40%. However, when the temperature rises to 1600°C and copper is in a molten state, the absorption rate drops by about 5%. That is, the absorption of green light decreases slightly after copper melts. This feature helps achieve stable holes and virtually zero spatter when machining copper. This is the obvious advantage of green-light laser welding over infrared laser welding.
2.2 3D printing of pure copper materials
Copper material is widely used in high-end manufacturing due to its excellent thermal conductivity, electrical conductivity and other excellent properties. For example, in aerospace, high-speed trains, automobile industry and other fields, there is a direct application demand for pure copper material 3D printing technology.
The laser light source for 3D printing of metal materials currently mainly uses 1um near-infrared single-mode fiber laser. The 1um near-infrared single-mode fiber laser has the disadvantage of low absorption coefficient due to the absorption coefficient of copper material, and a large influence with temperature, resulting in low density of printed samples and poor process robustness. Green laser, as the best light source for 3D printing of highly reflective metal materials, can effectively solve related problems and achieve a density of greater than 99.95% for 3D printing of pure copper materials.
3. High-power continuous single-mode green light from OUHK Laser
Shenzhen Gongda Laser Co., Ltd. is mainly engaged in the research and development, production and sales of "advanced short-wavelength fiber lasers" and "laser precision processing solutions". It is a company focusing on the research and development, production of medium and high power short-wavelength (green and ultraviolet) fiber lasers. and Application Solutions Laser Inc. The current main products are 50-500W high-power single-mode green lasers and 100-1000W MOPA single-mode pulsed fiber lasers. OUHK Laser has been focusing on the research and development of high-power short-wavelength fiber lasers, and has taken the lead in launching a 500W single-mode green laser: GCL-500 that can be used for highly reflective metal 3D printing and precision welding. The GCL-500 green light laser adopts an all-fiber fundamental frequency plus extracavity frequency doubling solution, achieving a single-mode continuous green light output of up to over 500W, filling the domestic gap in this type of product.
4. Committed to the advanced application of short-wavelength and high-power lasers
The GCL-500 continuous single-mode green laser has good output power stability, excellent beam quality, and high absorption rate for highly reflective materials, especially copper, making it promising for 3D printing of pure copper materials. By further adding a spatial modulator, pulsed green light with high-speed modulation frequency can also be obtained, which makes it also have broad application prospects in precision cutting and welding of highly reflective materials. GCL-500 continuous single-mode green laser uses free-space output, which helps ensure excellent beam quality. The laser can also provide a flexible transmission method coupled to optical fiber, which can more easily match automated control and be used in high-reflective material welding processes. After long-term exploration of the laser welding process, it has been shown that better welding results can be achieved by using output spots (beam shaping) with different energy distributions. In addition, based on OULD Laser's GCL-500 single-module single-mode green laser, it can also perform multi-module space or fiber beam combining. On the one hand, green light output with flexible beam energy distribution can be obtained; on the other hand, continuous fiber-optic green light output of several kilowatts or even tens of thousands of watts can be obtained, providing core high-end laser welding capabilities for high-quality, high-efficiency, and high-yield laser welding. Powerful short wavelength light source. Continuous high-power green laser can provide an effective solution for the processing and application of copper materials, and is expected to shine in pure copper 3D printing and high-reflective metal precision welding.