1. Background

At room temperature, copper's absorption rate for near-infrared wavelengths (about 1 um) is less than 5%, which is equivalent to 95% of the laser being reflected. The absorption rate of copper for green and blue wavelengths (about 0.43-0.53 um) exceeds 40%, which is nearly an order of magnitude higher than the near-infrared band. Compared with the common 1 um band near-infrared lasers, the short wavelengths of green and blue lasers naturally have lower beam divergence angles and smaller focused spots, so they have more advantages in processing. In particular, green fiber lasers based on the currently booming high-power fiber laser technology will usher in broad application prospects due to their many advantages such as high average power, good beam quality, and strong stability. With the continuous development of the new energy vehicle industry, copper has been widely used in new energy vehicle busbars, fuses and other parts due to its excellent physical properties such as conductivity and corrosion resistance, and its short-wavelength lasers with high absorption rates have attracted much attention.

Table 1 Summary of cutting-edge short-wavelength laser output indicators

Compared with the parameters in Table 1, the energy density of the semiconductor-coupled blue laser is relatively low, with a BPP of 15-80 mm·mrad. At present, high-power blue lasers are mainly used for auxiliary applications, and are rarely directly used in precision processing such as precision welding and 3D printing. The 1000-3000W high-power disk green laser launched by TRUMPF in recent years uses fiber-coupled output, and the output beam quality is slightly better than that of blue light, which is 2-8 mm·mrad. Shenzhen Gongda Laser, a domestic laser manufacturer focusing on high-power fiber green light, has successively launched similar high-power fiber green lasers in recent years, with a maximum power of 2000W single-mode fiber green light, and an output beam quality (BPP) of 0.27mm·mrad, which is about an order of magnitude smaller than TRUMPF's disk green light. In theory, the better the beam quality of the light source, the stronger the beam focusing ability, the smaller the input heat impact, and the higher the controllability of the processing format and processing distance, which is conducive to the laser processing equipment to obtain a higher process window and help improve the production application yield.

This article mainly compares the performance parameters of the kilowatt single-mode and kilowatt multi-mode launched by Gongda Laser, analyzes the two application directions of welding and 3D printing, and further compares the actual application differences of the two single-mode and multi-mode green lasers, providing a reference for customers' subsequent application selection.

The lasers compared are 1000W single-mode fiber green laser (see Figure-1) and fiber-coupled output multi-mode kilowatt fiber green laser (see Figure-2).

Figure 1 1000W single-mode fiber green laser

Figure 2 Fiber-coupled output - multi-mode kilowatt fiber green laser

2. Comparison of key data of single-mode and multi-mode continuous green light

1. The optical parameters of the 1000W single-mode fiber green laser and multi-mode green laser developed by Gongda Laser are compared as follows:  

*: Test conditions, 25℃ room temperature environment Table-2 Comparison of key data of single-mode and multi-mode continuous green light

2. Comparison of output spot distribution: the energy of single-mode laser is distributed in Gaussian shape, with concentrated energy distribution in the center; the energy distribution of multi-mode laser is distributed in a flat-top shape, with relatively dispersed energy distribution. The energy distribution diagram is shown in Figure 3:

3. Using the same input spot diameter and the same fiber configuration, compare the differences in optical characteristics between single-mode and multi-mode. The configuration table is as follows:

Table 3 Comparison of single-mode and multi-mode continuous green light experiment configurations

3. Single-mode and multi-mode kilowatt continuous green light--copper welding ability test

A single-pass welding test was conducted by using a conventional 1mm copper sheet to draw 5 lines. The power of the single-mode and multi-mode lasers was set to 70% (the actual power was about 700W), and the scanning speed was 150mm/s. The penetration values of each group of welding samples at different defocus values were tested (the average value of the five-line test). An example of the penetration test is shown in Figure 5:

Figure 5 (Top: Single Mode; Bottom: Multi Mode) Example of penetration test

Table 4 below compares welding under the same conditions using single-mode lasers and multi-mode lasers. It can be clearly seen that single-mode lasers have greater depth of focus and penetration than multi-mode lasers, and can achieve a larger process window when welding products. Figure 5 uses a histogram pattern to provide detailed statistics on the distribution of penetration depth under different defocus amounts, and Figure 6 is a photo of the penetration width and penetration depth under the same parameters for single-mode and multi-mode lasers. Since single-mode lasers have a large aspect ratio, their welding accuracy is much higher than that of multi-mode lasers. Precision welding can be achieved without a large laser power, and the heat impact and deformation after welding are small. The spatter generated by single-mode and multi-mode lasers during welding is comparable when detected by a high-speed camera.

Table 4 Melting depth values of single-mode and multi-mode laser welding at different defocus values -- 150mm/s scanning speed

Figure 6 Melting depth values at different defocus values for single and multi-mode

Figure 7 Melt width and depth under the same parameters for single and multi-mode

4. Application Case Display

The material's penetration depth is inversely proportional to the laser scanning speed. The faster the scanning speed, the shallower the welding penetration depth. The above single-mode and multi-mode kW continuous green laser output optical parameter characteristics and welding penetration test data are combined. Comparing the above welding data, we can get:

(1) Multi-mode welding can obtain a wider welding track and a shallower molten pool; it is conducive to thin-sheet welding and obtains a smaller contact resistance and a higher welding tension.

(2) Single-mode continuous green light welding has a smaller welding track area, a deeper molten pool, a small heat input, and a small heat effect, which is conducive to precision welding or thicker material welding. By adjusting the defocus, it can also be used for precision thin-sheet welding. The working window is larger than that of multi-mode continuous green light, and a higher welding strength can be obtained to meet different welding requirements. In terms of welding capability, single-mode continuous green light is superior.

Here are two application cases of single-mode continuous green light:

1. Fuse welding

A single-mode continuous green laser is used to weld fuses and terminals of different materials and thicknesses.

(1) Optical configuration: galvanometer: 30mm; field mirror: F254; beam expander: 2-10 times.

(2) The power gradient of the three materials is 70-90%, and the speed gradient is 120-150mm/s. All of them can be welded. The welding fracture position is checked again. The fuses are firmly welded to the substrate terminals.

The welding effect is shown in the figure below:

2. Precision 3D printing

Compared with the corresponding single-mode infrared laser, since the wavelength of green light (532nm) is half shorter than that of infrared (1060-1080nm), the focusing spot is more than twice as small under the same configuration. Therefore, in green light precision 3D printing, single-mode continuous green light has obvious advantages in material absorption rate, energy density, printing quality, etc.

From the optical parameters, it can be seen that the focusing ability of single-mode continuous green light is much higher than that of multi-mode continuous green light. Under the conditions that single-mode continuous green light uses F420 focusing field lens and multi-mode continuous green light uses F255 focusing field lens, the focusing spot size of single-mode continuous green light is about 27.4um, while the focusing spot size of multi-mode continuous green light is about 141um. It can be clearly seen that there are obvious differences in the processing accuracy and working format of the two.

If the same processing format and height are considered, and the same F255 field lens is used for focusing, single-mode continuous green light has a more obvious advantage. Pure copper powder is a good conductor. The beam quality, the size of the focused spot, the thermal conductivity of the material, the powder particle size, and the thickness of the powder all have a direct impact on the printing accuracy. Currently, using single-mode continuous green light for precision 3D printing, Xihe Additive can achieve a maximum printing accuracy of about 50um. The printed sample is shown in Figure 9 below:

5. Conclusion

Due to the stable absorption rate, the spatter generated during green light welding can be better controlled by properly controlling the defocus amount. Comparing the application tests of kilowatt single-mode green light and kilowatt multi-mode green light, the main results are as follows:

1. Advantages of single-mode continuous green light application:

(1) Compared with multi-mode laser, the focal depth and melting depth of single-mode laser are larger. Under the same parameters, the depth-to-width ratio of single-mode laser is 7.6, while the depth-to-width ratio of multi-mode laser is 1.05. Therefore, single-mode laser has stronger penetration than multi-mode laser;

(2) The energy density is higher, and a larger process window can be achieved when welding products with the same power;

(3) Since the single-mode has a large depth-to-width ratio, its welding precision is much higher than that of multi-mode. It does not require a large laser power to achieve precision welding, and the heat effect and deformation after welding are small;

(4) Since the focused spot is smaller, more delicate welding can be obtained, achieving higher processing accuracy;

(5) The beam quality is better, and a longer focal length field lens can be used in actual use, resulting in a larger processing area. This advantage can greatly increase the size of printed samples in 3D printing and reduce the impact of material splashing on the field lens.

(6) Due to the high energy density, the average power required for precision welding is smaller, so the power used by single-mode continuous green light is generally lower, and the energy efficiency ratio is higher than that of multi-mode continuous green light.

2. Advantages of multi-mode continuous green light:

(1) Multi-mode kilowatt continuous green light uses optical fiber transmission, and the optical path is relatively flexible;

(2) When welding thin sheets, a single pen can obtain a wider weld, which helps to improve efficiency;

(3) Flat-top spot, better uniformity, more precise back mark control when welding on ultra-thin sheets.

Kilowatt single-mode continuous green light and kilowatt multi-mode continuous green light each have their own advantages in precision welding applications; in precision 3D printing applications, single-mode continuous green light has obvious precision advantages.

It is recommended that you choose single-mode or multi-mode continuous fiber green light lasers according to your needs for the precision processing of high-reflective materials such as copper.