Why laser cleaning will become the general trend?

Why laser cleaning will become the general trend?

Traditional industrial cleaning methods mainly include high-pressure water, chemical reagents, ultrasonics, and mechanical polishing. However, these cleaning methods have problems such as damage to the substrate, poor working environment, pollution, incomplete cleaning of some locations, and high cleaning costs. With the intensification of environmental pollution, scholars from all over the world are actively developing new energy-saving, environmentally-friendly and efficient cleaning technologies. Because laser cleaning technology has multiple advantages such as low damage to the matrix material, high cleaning accuracy, zero emission and no pollution, it is gradually being valued and favored by academia and industry. There is no doubt that the application of laser cleaning technology to the cleaning of dirt on metal surfaces has a very broad prospect.

The development history and status quo of laser cleaning technology

In the 1960s, the famous physicist Schawlow first proposed the concept of laser cleaning, and later applied the technology to the restoration and maintenance of ancient books. The range of decontamination by laser cleaning abroad is very wide, from thick rust layer to fine particles on the surface of the object can be removed, including the cleaning of cultural relics and art, the removal of rubber dirt on the surface of tire molds, the removal of silicon oil contaminants on the surface of gold films, and microelectronics. Industry's high-precision cleaning. In China, laser cleaning technology really started in 2004. China began to invest a lot of manpower and material resources to strengthen the research on laser cleaning technology. In the past ten years, with the development of advanced lasers, from low-efficiency and bulky carbon dioxide lasers to lightweight and compact fiber lasers; from continuous output lasers to short-pulse lasers of nanoseconds and even picoseconds and femtoseconds; from visible light output To long-wave infrared light and short-wave ultraviolet light output...lasers have developed by leaps and bounds in terms of energy output, wavelength range, laser quality, and energy transfer efficiency. The development of lasers has naturally promoted the rapid development of laser cleaning technology. Laser cleaning technology has achieved fruitful results both in theory and application.

Principles of laser cleaning technology

The process of pulsed laser cleaning relies on the characteristics of the light pulse generated by the laser, based on the photophysical reaction caused by the interaction between the high-intensity beam, the short-pulse laser, and the contamination layer. The physical principle can be summarized as follows (Figure 1):

A) The light beam emitted by the laser is absorbed by the contamination layer on the surface to be treated;

B) Absorption of large energy forms a rapidly expanding plasma (a highly ionized unstable gas), which generates shock waves;

C) The shock wave turns pollutants into fragments and is removed;

D) The light pulse width must be short enough to avoid heat accumulation that damages the surface to be processed;

E) Experiments show that when there are oxides on the metal surface, plasma is generated on the metal surface.

Figure 1. Schematic diagram of laser cleaning

Plasma is only generated when the energy density is higher than the threshold, which depends on the contamination layer or oxide layer being removed. This threshold effect is very important for effective cleaning while ensuring the safety of the base material. There is a second threshold for the appearance of plasma. If the energy density exceeds this threshold, the base material will be destroyed. In order to perform effective cleaning under the premise of ensuring the safety of the base material, the laser parameters must be adjusted according to the situation so that the energy density of the light pulse is strictly between the two thresholds.

Advantages of laser cleaning

Compared with traditional cleaning methods such as mechanical friction cleaning, chemical corrosion cleaning, liquid and solid strong impact cleaning, and high-frequency ultrasonic cleaning, laser cleaning has five obvious advantages:

Environmental protection advantages: Laser cleaning is a "green" cleaning method. It does not need to use any chemicals and cleaning fluids. The cleaned waste is basically solid powder, small in size, easy to store, recyclable, no photochemical reaction, no Will produce pollution.

Effect advantage: The traditional cleaning method is often contact cleaning, which has mechanical force on the surface of the cleaning object, damages the surface of the object or the cleaning medium adheres to the surface of the object to be cleaned, and cannot be removed, resulting in secondary pollution. Grinding and non-contact, no thermal effect will not damage the substrate, so that these problems can be easily solved.

Control advantages: The laser can be transmitted through optical fibers, cooperate with robots and robots to easily realize remote operation, and can clean parts that are not easily reached by traditional methods. This can ensure the safety of personnel when used in some dangerous places.

Convenient advantage: Laser cleaning can remove various types of contaminants on the surface of various materials to achieve a cleanliness that cannot be achieved by conventional cleaning. It can also selectively clean the contaminants on the surface of the material without damaging the surface of the material.

Cost advantages: laser cleaning is fast, efficient, and time-saving; although the initial one-time investment is relatively high when purchasing a laser cleaning system at this stage, the cleaning system can be used stably for a long time, with low operating costs, and more importantly, it can be easily automated. . It is foreseeable that the cost of laser cleaning system will drop significantly in the future, thereby further reducing the cost of using laser cleaning technology.

Classification of laser cleaning technology

Laser cleaning methods can be divided into the following three categories:

1. Laser dry cleaning

Laser radiation is used for direct decontamination. After the laser is absorbed by the object or dirt particles, it will vibrate to separate the substrate from the contaminants. There are two main ways to remove dirt particles in laser dry cleaning: one is the instantaneous thermal expansion of the surface of the substrate, which generates vibration, so that the particles adsorbed on the surface are removed. The other is the thermal expansion of the particles themselves, which causes the particles to leave the surface of the substrate.

2. Laser wet cleaning

Laser wet cleaning is to first uniformly cover a layer of liquid medium film on the surface of the substrate to be cleaned, and then use laser radiation to remove stains. According to the absorption of the laser by the dielectric film and the substrate, wet cleaning can be divided into strong substrate absorption, strong dielectric film absorption, and dielectric film substrate co-absorption. When the strong matrix absorbs, the matrix absorbs the laser energy and transfers heat to the liquid medium film. The liquid layer at the interface between the matrix and the liquid is overheated and boils, and the liquid layer and stains are removed together.

3. Laser + inert gas cleaning

While the laser is irradiated, the inert gas is blown to the surface of the workpiece. When the contaminants are peeled from the surface, the gas is blown away from the surface to avoid contamination and oxidation of the clean surface.

Part 2 Application field and application process of laser cleaning technology

Application areas of laser cleaning

With the development of laser cleaning technology, laser cleaning technology has been popularized in many industries in industrial applications, as follows:

Microelectronics field: semiconductor components, microelectronics devices, memory templates, etc.;

Cultural relics protection: stone carvings, bronzes, glass, oil paintings and murals, etc.;

Abrasive tool cleaning: rubber molds, composite molds, metal molds, etc.;

Surface treatment: hydrophilic treatment, surface roughening, treatment of the weld before and after welding, etc.;

Paint and rust removal: aircraft, ships, weaponry, bridges, metal pressure vessels, metal pipes, etc.; aircraft parts, electrical product parts, etc.;

Others: urban graffiti, printing rollers, building exterior walls, nuclear industry, etc.

Research on the Application of Laser Cleaning Technology on Metal Surface

1. Laser cleaning not only removes dirt but also improves corrosion resistance

Laser cleaning technology can overcome the shortcomings of traditional cleaning technology, such as time-consuming, laborious and environmental pollution, and plays an important role in removing dirt on metal surfaces. In addition, the laser cleaning parameters can be further controlled to cause chemical reactions on the surface of the metal to be cleaned and form a protective layer several microns thick to prevent further corrosion of the metal. The use of laser cleaning technology for decontamination can improve the corrosion resistance of metal devices. Increase 3 to 4 times.

2. The choice of laser type and wavelength has an important influence on the cleaning effect

Figure 2 shows the variation trend of the absorption coefficient of various metals with wavelength. When λ=916nm-1200 nm, most metals have higher absorption coefficients in this waveband, and organics have relatively strong laser absorption in this waveband. Because of this, in terms of absorptance, the fiber laser has its unique advantages in all aspects, combining the comparative advantages in all aspects. The organic pollution layer absorbs the laser strongly, and the temperature of the organic pollution layer rises rapidly to the evaporation point to vaporize, so as to achieve the purpose of removing the pollution layer without damaging the substrate. Then determine the energy threshold of laser cleaning. The energy threshold of laser cleaning will determine the effect of laser cleaning. Choosing an appropriate laser cleaning energy threshold requires comprehensive consideration of the material's performance, microstructure, morphological defects, and the influence of factors such as the wavelength and pulse width of the laser.

Figure 2. Variation curve of absorption coefficient of various metals with wavelength

Figure 3. Absorption of light of different wavelengths by 921A steel and epoxy zinc-rich paint

3. The appropriate laser incident angle makes the cleaning effect twice the result with half the effort

When the laser is incident at a certain oblique angle, the laser is directly radiated under the adhered particles, resulting in higher thermoelastic stress. Compared with normal incident, the contaminants are easier to be removed. In addition, the study found that as the tilt angle increases, the laser radiation area becomes wider (see Figure 4). When the tilt angle is 20 degrees, the area to be cleaned is about 10 times that of normal incidence, which effectively increases the laser The efficiency of cleaning.

Figure 4. The relationship between the laser incident angle and the radiation area

4. The correct defocusing amount improves the laser cleaning effect

The cleaning mechanism will be different for different defocusing amounts. When the defocus is positive, cleaning is a mechanism of surface material bursting and cracking, and when the defocusing amount becomes larger, the removal of the paint layer changes from cracking to vaporization. In the red box in Figure 6 (2), in the defocus experiment, there will be dust around the cleaning area, which is obviously the remaining ashes from the burning of the paint layer. In 6(1), the fragmentation principle is used, and there is no ashes.

Figure 5. Fragmentation process and vaporization process comparison

Figure 6. Comparison of fragmentation processing results and vaporization processing results

In order to optimize the effect of laser cleaning on metal surfaces, it is necessary to comprehensively consider process parameters such as laser cleaning method, cleaning model, laser type, laser wavelength, energy density, power, pulse frequency, pulse time, and laser incident angle. As shown in Table 1, the pulsed laser can effectively clean the corrosion on the surface of carbon steel. When the wavelength is 1064nm, the laser power is 500W, the pulse frequency is 10kHz, the pulse width is 120ns, the cleaning speed is 60mm/s, and the overlap rate is 5%. The laser cleaning and rusting effect is the best under the parameters, and there is no oxygen element found on the rusted surface, micro areas, lines and points of laser cleaning. Only by systematically studying process parameters can an efficient laser cleaning system be formed.

Table 1 Laser rust removal effect under the main cleaning parameters

Laser cleaning will become a major trend in the development of the cleaning market

Although laser cleaning cannot completely replace traditional cleaning technology at present, with the advancement of science and technology and the improvement of environmental protection requirements, laser cleaning will become a major trend in the development of the global cleaning market. By developing new laser cleaning systems and equipment and optimizing the cleaning process, improving cleaning efficiency, reducing cleaning costs, and reducing damage to the surface to be cleaned, achieving green, high-efficiency and automation of the metal surface cleaning process will inevitably be the future market demand.

As a key high-tech enterprise of the National Torch Program specializing in the research and development, production and sales of fiber lasers and their key components and materials, Wuhan Raycus Fiber Laser Technology Co., Ltd. has a complete industrial chain from materials, core components to complete machines. Integration. At present, Raycus Laser has launched 100W-2000W pulsed special cleaning laser for laser cleaning, which has been widely used in surface treatment, mold cleaning, paint and rust removal and other hardware manufacturing, automobiles and rail transit fields. Raycus Laser can also provide customized cleaning solutions for different application needs.