Laser cleaning with continuous-wave vs. pulsed fiber lasers

For emerging laser cleaning applications, there are questions about whether the CW fiber laser or pulsed fiber laser should be used.

FIGURE 1. Cleaning performance of aluminum alloy with white paint.
FIGURE 1. Cleaning performance of aluminum alloy with white paint.

LULU SONG, JIANFENG SHANGGUAN, CAIJIE LI, and DAISHU QIAN

Laser cleaning uses a focused laser beam to rapidly vaporize or strip the contaminants on a material’s surface. Compared with other traditional physical or chemical cleaning methods, laser cleaning is non-contact, with no consumables or pollution, high precision, and small residual damage. This process is an ideal choice for the new generation of industrial cleaning technology.

Furthermore, fiber lasers with high reliability, stability, and flexibility have become the best choice as a laser cleaning beam source. Two types of fiber lasers, continuous-wave (CW) fiber lasers and pulsed fiber lasers, occupy the market for macromaterial processing and precise material processing. For emerging laser cleaning applications, there are questions about whether the CW fiber laser or pulsed fiber laser should be used. Here, we compare the laser cleaning applications of CW fiber lasers and pulsed fiber lasers, and analyze their respective characteristics and applicable application scenarios. This article provides useful reference when choosing the corresponding laser cleaning technology.FIGURE 2. Cleaning performance of carbon steel with white paint.FIGURE 2. Cleaning performance of carbon steel with white paint.

We use two models of lasers to clean different kinds of materials: one is aluminum alloy with white paint (around 20 µm) and another is carbon steel with white paint (around 40 µm). The cleaning performance is achieved by adjusting the pulse width (100 ns, 200 ns, and 500 ns), frequency (2060 kHz), and scanning speed (15009600 mm/s). FIGURES 1 and 2 show the cleaning performance by the test lasers detailed in the TABLE.Table

For pulsed fiber lasers, a laser with lower frequency is more likely to hurt the substrate during the cleaning process, and a laser with a narrow pulse width (around 100 ns) could clean the paint more easily. It’s essential to balance the heat between cleaning the paint and melting the substrate (heat effect). A pulsed fiber laser with a master oscillator power amplifier (MOPA) structure offers the advantage of precise heat control, which is a critical point during the cleaning process.

With a CW fiber laser, the slower the scanning speed is, the greater the damage to the substrate. However, when the speed is higher than the threshold, a faster speed will cause insufficient cleaning. Thus when using the CW fiber laser to perform laser cleaning, it’s critical to choose the right scanning speed.

The following details show the difference between CW fiber lasers and pulsed fiber lasers from three main aspects: cleaning performance, cleaning efficiency, and roughness after the cleaning process.FIGURE 3. Cleaning performance comparison on aluminum by CW (left) and MOPA pulsed fiber laser (right).FIGURE 3. Cleaning performance comparison on aluminum by CW (left) and MOPA pulsed fiber laser (right).

Intuitively, the materials are darker after cleaning by a CW fiber laser when compared with a MOPA pulsed fiber laser (FIGURES 3 and 4). Improper heating will result in the substrate metal melting during the cleaning process, which is not acceptable especially in the module cleaning industry.FIGURE 4. Cleaning performance comparison on carbon steel by CW (left) and MOPA pulsed fiber laser (right).FIGURE 4. Cleaning performance comparison on carbon steel by CW (left) and MOPA pulsed fiber laser (right).

The different microscopic surface of the substrate after cleaning can be easily shown under the microscope (FIGURES 5 and 6). The metal on the substrate melts during the CW fiber laser cleaning process, even though the paint is removed. However, when cleaning by the MOPA pulsed fiber laser, the damage to the substrate is small and the surface is smoother.FIGURE 5. Microscopic surface after cleaning on aluminum by CW (left) and MOPA pulsed fiber laser (right).FIGURE 5. Microscopic surface after cleaning on aluminum by CW (left) and MOPA pulsed fiber laser (right).

FIGURE 6. Microscopic surface after cleaning on carbon steel by CW (left) and MOPA pulsed fiber laser (right).FIGURE 6. Microscopic surface after cleaning on carbon steel by CW (left) and MOPA pulsed fiber laser (right).FIGURE 7 shows the roughness (Ra) for the surface from CW and MOPA pulsed fiber lasers. The damage brought by MOPA fiber laser cleaning is very small, and the roughness value is close or even lower than the original surface (the laser cleaned some dust on the original surface, too). While cleaned by CW lasers, the roughness value will be 1.5X more than the original surface.FIGURE 7. Comparison for the roughness of the surface after CW and MOPA pulsed fiber laser cleaning.FIGURE 7. Comparison for the roughness of the surface after CW and MOPA pulsed fiber laser cleaning.

Another advantage brought by the MOPA fiber laser is high cleaning efficiency. When cleaning the dust on the aluminum alloy, the cleaning efficiency of the MOPA pulsed fiber laser is 2.77 m2/h, which is 7.7X the cleaning efficiency by a CW fiber laser (0.36 m2/h). When cleaning the dust on the carbon steel, the cleaning efficiency of the MOPA pulsed fiber laser is 1.06 m2/h, which is 3.5X the cleaning efficiency of the CW fiber laser (0.3 m2/h).

In conclusion, dust could be removed by both the MOPA pulsed fiber laser and CW fiber laser. Using the same average output power, the cleaning efficiency of the MOPA pulsed fiber laser is quicker than the efficiency of the CW fiber laser. In the meantime, precise heat control between cleaning and melting produces good cleaning performance, without damaging the substrate.

However, the cost of a CW fiber laser is lower, which compensates for the disadvantage of the cleaning efficiency by increasing the average output power. However, it will cause a heat effect, which will hurt the substrate.

Thus, different cleaning applications will require different laser models. For precise cleaning such as mold cleaning, it is better to choose the MOPA pulsed fiber laser. For some large steel structures, pipes, etc., due to their large volume, fast heat dissipation, and low requirement for substrate damage, the CW fiber laser will be a good choice. JPT has a specialized application lab with highly trained engineers and various types of equipment used for different applications (marking, cutting, welding, etc.), which helps customers to evaluate the processing characteristics before choosing the appropriate laser.

LULU SONG, JIANFENG SHANGGUAN, CAIJIE LI, and DAISHU QIAN are all with JPT Opto-electronics Co., Ltd., Shenzhen, China; en.jptoe.com.

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