Introduction
Laser cleaning technology can be traced back to the 1960s [1], and has gradually become one of the research and development hotspots in the field of industrial manufacturing in the world in recent years. It is known as "the green cleaning technology with the most development potential in the 21st century [2- 6]". Compared with traditional cleaning methods such as mechanical friction cleaning, chemical agent cleaning, high-frequency vibration cleaning, etc., laser cleaning technology has many remarkable features as follows:
(1) Green and environmental protection.
Laser cleaning technology does not need to use chemicals, and will not produce waste liquid to pollute water bodies;
(2) The damage to the base material is small.
Does not require contact with the substrate, nor does it require any mechanical force to be applied to the substrate;
(3) It has a high degree of flexibility.
Easy to automate;
(4) The cleaning effect is excellent.
Choosing lasers with different wavelengths and different processing parameters can easily clean various pollutants attached to the surface of the substrate;
(5) High cleaning efficiency.
Laser cleaning equipment using scanning technology can convert point light sources into line or surface light sources, which results in a cleaning speed much higher than traditional cleaning technologies.
While laser cleaning technology is making great strides towards practical application, relevant scholars have also done in-depth research on its cleaning mechanism, evaluation of cleaning effect, improvement of cleaning effect, and expansion of cleanable materials. So far, laser cleaning technology has been able to stably and effectively clean the surface of various regular substrates. The materials that can be cleaned include but are not limited to metals, alloys, glass and various composite materials. The scope of application of this technology is gradually expanding to various fields such as industry, military industry, shipbuilding, aviation, and aerospace.
Principle and Application of Laser Cleaning
Laser cleaning technology refers to that when the high energy density and high repetition frequency laser beam is irradiated on the surface of the substrate, after the stains on the surface of the substrate absorb the energy carried by the laser beam, the binding force between the two (covalent bond, double even Pole, capillary effect, Van der Waals force) are destroyed, and finally the stain is detached from the surface of the substrate.
The schematic diagram of laser cleaning is shown in Fig. 1.
Laser cleaning is a very complex process, which involves physical processes such as ablation, melting, gasification, and expansion, as well as chemical processes such as decomposition, ionization, degradation, and combustion. simultaneously exist . Generally speaking, laser cleaning technology can be divided into three typical methods: laser ablation cleaning method, laser liquid film assisted cleaning method and laser shock wave cleaning method.
The main mechanism of laser ablation cleaning method includes thermal expansion, gasification, ablation and phase explosion [7], as shown in Figure 2. When the laser beam comes into contact with the substrate after passing through air, rare gas or vacuum, the heated substrate expands simultaneously with the stain. With prolonged exposure, depending on the stain and substrate thresholds, two situations may occur:
(1) When the ablation threshold of the stain is lower than the substrate, by adjusting the energy density of the laser beam, the stain can be effectively cleaned without damaging the substrate;
(2) When the ablation threshold of the stain is greater than the substrate, the stain cannot be completely removed if the substrate is not to be damaged. At this time, if the purpose of cleaning must be achieved, it is necessary to carefully adjust various laser parameters to minimize the damage to the substrate.
Regardless of the above cases, various complex physical and chemical changes will occur during the cleaning process, such as molecular bond breaking, decomposition, degradation, phase explosion, gasification, ionization, and plasma generation. In addition, due to the difference in the thermal expansion coefficients of the substrate and the stain, the shape of the interface between the two changes simultaneously, causing the stain jets to fly off the surface of the substrate.
In industrial production and manufacturing, when using laser technology to clean corresponding workpieces, not only can quickly remove the pollutant particles on the surface of the workpiece, but also form a protective film on the surface of most metal materials, which can prevent the substrate from rusting and greatly improve Corrosion resistance of metal materials, as shown in Figure 3 and Figure 4.
In addition, most production equipment will gradually accumulate rust, stains, waste, etc. on the surface of the equipment after long-term use [8]. Using conventional sandblasting, chemical solvents and other cleaning processes to clean the surface of the equipment will inevitably lead to the loss of the target surface and reduce the life of the equipment. The use of laser cleaning technology can greatly improve the cleaning efficiency, reduce the company's expenditure on equipment maintenance, and prolong the service life of the equipment.
The main mechanism of the laser liquid film assisted cleaning method includes liquid film boiling, evaporation, vibration, etc., as shown in Figure 5. In this method, the surface of the cleaning object is pre-covered with liquids such as water and ethanol to form a liquid film. When the laser beam is irradiated on the substrate, the liquid film will absorb the energy carried by the laser, the liquid will explode violently, and the boiled liquid will hit the stain on the surface of the substrate at high speed. This instantaneous explosive force can effectively remove the stains on the substrate, so as to achieve the effect of laser cleaning.
The laser shock wave cleaning method does not directly focus the laser beam on the surface of the object to be cleaned, but uses plasma shock waves for cleaning. The cleaning objects are mainly fine particles, as shown in Figure 6. First, adjust the height of the laser head so that the focus of the laser beam is as close as possible to the dirt particles to be cleaned, and the focus position of the laser is kept at a small distance from the surface of the substrate. At this time, the air at the laser focus point will be ionized, resulting in the generation of spherical shock waves and spreading to the surroundings. The shock wave will soon act on the surface of the substrate. When the lateral force applied by the shock wave parallel to the surface of the substrate is greater than the adhesion force of the stain particles in the vertical direction, the stain will move along the direction of the shock wave force, thereby detaching from the substrate surface.
Among the above three methods, when using the laser ablation cleaning method, it is only necessary to select the appropriate laser, and the stains can be effectively cleaned by selecting the correct process parameters. This method does not require prior treatment of the object to be cleaned, nor does it require the use of expensive special lasers, but this method produces less impact and is not ideal for removing some firmly attached stains. When using the laser liquid film assisted cleaning method, it is generally pre-covered with a liquid with a high light absorption rate, which effectively compensates for the lack of impact of the laser ablation cleaning method. But, the weak point of this method is: the technological process is comparatively complicated, and needs to carry out accurate test to the composition distribution ratio of liquid film just can obtain better cleaning effect; Materials and stains undergo chemical reactions, which may produce new substances and cause secondary pollution. When using the laser shock wave cleaning method, the relevant cleaning process requirements are very strict. It is necessary to ensure that the laser beam is not in contact with the substrate, and the distance between the two must be adjusted at any time to ensure sufficient impact on the stain particles. , to achieve the best cleaning effect.
In the manufacture of aircraft, automobiles and ships, laser cleaning technology has already occupied a place. Audi has long used fiber lasers to clean the surface of car welds, which can improve the strength of welds while removing stains. Laser cleaning technology can also be used for automobile engine maintenance, quickly remove engine carbon deposits and oil stains. In addition, in the tire mold manufacturing industry, the application of laser cleaning technology has also been well received by industry personnel [9], as shown in Figure 7.
Laser cleaning technology also shines in the field of military equipment maintenance. Initially, the United States pioneered the use of laser cleaning technology to clean the surface coatings of military aircraft. However, due to the limitation of laser performance at that time, coupled with its large size, this technology can only stay in the experimental stage.
With the development of laser technology, the power of lasers continues to increase, and is developing towards miniaturization and flexibility, which provides a solid material foundation for the large-scale application of laser cleaning technology. So far, when the Edison Research Institute of the United States uses a 1kW laser to clean the F-16 fighter jet, the cleaning rate has reached 2.36cm3/min, as shown in Figure 8. In addition, the exterior paint cleaning of the H-53 and H-56 helicopters of the US military has adopted laser cleaning technology [12-13].
In the fields of cultural relics protection and building cleaning, laser cleaning technology is also widely used, as shown in Figure 9. Scientists have used krypton fluoride excimer lasers to effectively clean stone carvings, handicrafts, and bone fossils. Although laser cleaning technology has many advantages, for some precious cultural relics with various colors, such as enamel glass and other items, different colors have different absorption effects on laser light, which may cause excessive stress gradients on parts with high laser absorption. large, causing damage [14-16]. Therefore, not all cultural relics are suitable for laser cleaning.
Laser Cleaning and Application Development Trends
Laser cleaning technology has developed rapidly along with the continuous progress of laser technology itself. In 2015, the TAKAHASHI[18] research team successfully completed the modification of the shape of the nanoscale 3D workpiece when using ultraviolet laser to process the tiny workpiece. This method can not only modify the shape of the tiny workpiece, but also can be used to remove the surface of the workpiece of particles. IVANOVA et al. [19] also proposed a method for cleaning nanoscale microscopic particles in 2016. Compared with the method of directly irradiating and cleaning nanoparticles with pulsed or continuous laser, this method uses laser thermal capillary effect The cleaning method produces less thermal effect and less damage to the substrate. Photos of cultural relics cleaned by laser are shown in Figure 11.
In the same year, the PALOMAR[20] research team conducted a series of cleaning experiments on the silver surface with a 532nm nanosecond pulsed laser. The results show that compared with the traditional laser cleaning method, there is no color change or quality decline on the surface of silver cultural relics, and the best conditions for cleaning silver objects are finally determined. In 2017, RADOJKOVIC et al. [21] used similar technology to use Nd:YAG laser to laser clean silver-coated copper wires, effectively removing the stains covered on silver cultural relics without damaging the cultural relics themselves. This strongly proves the feasibility of the above method. Since then, the cleaning of silver products has been officially included in the scope of laser cleaning, and the application range of laser cleaning technology has been expanded again.
In 2018, GU et al. [22] conducted research on the effect of laser plasma shock wave cleaning of nano-scale particles, successfully cleaned aluminum particles with a diameter of about 100 nm, and sorted out a set of effective particle cleaning methods. Although the mechanism of laser cleaning particles is not very clear, the cleaning method of particles can only be explored step by step through experiments, but with the deepening of research, laser cleaning technology will eventually occupy a pivotal position in the field of particle cleaning[23 ].
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In addition, laser cleaning technology has also achieved fruitful research results in the maintenance of automobiles, ships and other equipment in recent years. In 2018, LIU et al. [24] conducted a nanosecond pulse laser cleaning test on Q235 and 45# steel materials frequently used in ocean-going equipment, and summarized a set of laser derusting experience for Q235 and 45# steel materials. The laser maintenance of equipment has very important practical value. The oil stain on the workpiece equipment is removed as shown in Figure 12.
CHANG et al. [25] used picosecond lasers with high power density and low power density to clean Q235 steel alternately. It was found that compared with the traditional laser direct cleaning method, this alternate cleaning method greatly improves the effect of removing stains attached to Q235 steel. This technology is very suitable for cleaning stains on ocean-going equipment.
After that, CHEN [26] conducted a series of experiments on the reliability of laser cleaning and traditional cleaning methods on the adhesion of paint on automobile surfaces, and the results once again strongly proved the value of laser cleaning technology in the automobile manufacturing and maintenance industries.
In 2019, QI et al. [27] carried out laser cleaning on the train collector rings that have been corroded for many years. After many tests, it was found that the cleaned collector rings can not only remove the residual stains on the surface, but also effectively improve the current collection efficiency. The service life of the ring greatly improves the safety of the train at high speed.
With the continuous advancement of laser technology and the continuous decline of laser prices, the popularization of laser cleaning technology has set off a wave of upgrading the traditional cleaning industry around the world. As of 2018, the market value of laser cleaning technology in various industries has reached 589 million U.S. dollars [28], and it is estimated that this figure may be close to 724 million U.S. dollars in 2023, and the five-year compound annual growth rate may reach 4.22. %. The above results all show that this new type of clean technology has great potential for market development.
New Laser Hybrid Cleaning Technology and Future Prospects
Although laser cleaning technology has many advantages that cannot be achieved by traditional cleaning methods, many problems and deficiencies have also been exposed during its development. In order to ensure the cleaning effect, lasers with different wavelengths can be selected according to the difference in laser absorption rate of the object to be cleaned at different wavelengths. However, for the cleaning of composite materials, if the absorptivity of different components in the material to the same laser wavelength is greatly different, there may be a phenomenon that a certain component in the material is successfully cleaned while other components cannot be removed. . At the same time, the price of laser cleaning equipment is also relatively expensive, which has caused a major obstacle to the promotion of laser cleaning technology. In order to solve related problems, various new laser composite cleaning technologies have emerged as the times require.
As early as 1977, KIBLER et al. started research on laser cleaning of composite materials [32]. They used continuous CO2 lasers to emit laser beams of different sizes, shapes and power densities, and conducted comparative experiments on different thicknesses and types of carbon fiber epoxy resin materials, and studied the effects of different cleaning parameters on the laser cleaning effect.
In order to analyze the effect of different laser power and scanning speed on the cleaning morphology of resin stains on the surface of carbon fiber composite materials, the domestic ZHAN research team used high repetition frequency infrared lasers to carry out cleaning experiments on such stains, and obtained the best results for cleaning such stains. Optimum process parameters [33].
In addition, laser cleaning of carbon fiber reinforced matrix composites is also one of the current research hotspots [34-38]. In 2017, the ZHANG team used a 22.93kW/cm2 laser with a repetition frequency of 100kHz to quickly clean carbon fiber reinforced matrix composites at a rate of 6cm/s [39]. It was found that the cleaning process not only effectively removed the stains on the surface of the substrate, but also saw a small increase in strength due to the infiltration of the adhesive into the carbon fibers.
The above-mentioned researchers have found that the laser composite cleaning method is generally better than the conventional traditional cleaning method in the cleaning of composite materials [40]. Different from the traditional cleaning method that only uses a single wavelength laser, the practical laser composite cleaning technology generally uses a pulsed semiconductor laser and a pulsed fiber laser at the same time. These two laser beams can act on the object to be cleaned at the same time. At this time, when the semiconductor laser scans the surface of the object to be cleaned at high speed, the irradiated area heats up rapidly and carbonizes, and the resulting thermal stress can loosen the stain layer to be cleaned; while the high-power pulsed fiber laser acts on the object to be cleaned The shock wave generated during the process can break the target and quickly break away from the surface of the workpiece.
Compared with the traditional laser cleaning method, the laser composite cleaning method is faster, more efficient, and the cleaning quality is more uniform, and the sparks and smoke generated are smaller. For different materials, lasers with different wavelengths can also be used for cleaning at the same time to achieve the purpose of removing stains on composite materials. Therefore, a composite cleaning laser system composed of many different types of lasers can meet the cleaning needs of many different materials. At the same time, the spatial power combination and extended output technology of multi-unit pulsed laser, the adaptive control and active optimization control algorithm based on the fiber end cap to realize the optical axis stability control of multiple beams and the spatial power combination technology, the far-field synthesis based Energy time-series measurement and active optical path control to realize time-domain synchronization of multi-unit pulsed lasers have been developed rapidly in recent years, which has strongly promoted the development of laser composite cleaning technology.
Although after years of development, the relevant technologies of laser cleaning have been relatively mature, but because the laser cleaning equipment is too complicated, there are still some problems that need to be solved urgently.
(1) Harm to the human body. In general, most of the lasers used for laser cleaning have high power, and this technology will definitely develop in the direction of higher power in the future, and most of the lasers are in the invisible frequency band, which contains great hidden dangers. As we all know, high-energy lasers can cause skin burns, more serious damage to eyes, and are very likely to cause permanent visual damage to users, and even blindness in severe cases. This puts forward higher safety requirements for laser cleaning technology. The sealing of the optical path should be completed in the design of the laser, and strict usage specifications need to be formulated. When using it, it is mandatory to wear protective clothing, goggles and other equipment to ensure personal safety.
(2) The cost of equipment is high. Although laser cleaning technology has many advantages, compared with traditional cleaning technology, the equipment price of hundreds of thousands makes most enterprises discouraged. The core of laser cleaning technology is laser, and reducing the production cost of laser is the key to fundamentally popularize laser cleaning technology. It is necessary to continuously increase the investment in the research and development of laser core technology to mobilize the enthusiasm and creativity of scientific researchers, vigorously promote the formation of the laser cleaning industry chain, and finally realize the goal of laser cleaning technology entering the small and medium-sized enterprises.
(3) The mechanism of laser cleaning has not yet been fully clarified. Generally, the parameters used for laser cleaning include laser wavelength, power, repetition rate and scanning rate, etc. If the parameters are not selected properly, the stain may not be completely removed, or the substrate may be irreversibly damaged. Therefore, in the laser cleaning process, it is necessary to select appropriate cleaning parameters according to the threshold of different cleaning materials. Researchers generally conduct multiple cleaning experiments in order to obtain the best parameters required for cleaning different materials. These time-consuming and lengthy experiments undoubtedly greatly hinder the further application of laser cleaning technology. In addition, a set of systematic standards for evaluating the effect of laser cleaning has not yet been formulated, which has brought great obstacles to the development of laser cleaning technology.
In recent years, with the continuous deepening of related research, researchers have gradually begun to get involved in solving these problems. For example, by establishing corresponding finite element models, the cleaning process can be effectively simulated, which provides a reference for the selection of cleaning parameters. In addition, machine learning technology is used to fit and predict laser cleaning parameters, which greatly reduces the complexity of the test and provides a new way of thinking for the development of laser cleaning technology.
(4) The cleaning efficiency of complex workpieces is low. In the actual cleaning process, some workpieces are irregular in shape, and there are many complex structures and gaps that are not easy to clean. However, at present, the main equipment for laser cleaning is mostly hand-held or 2D mobile platforms, and it is difficult to effectively clean such complex workpieces. To this end, it is necessary to improve the existing cleaning equipment, and use robotic arms, 3D platforms, and testing equipment to achieve efficient, automated, and dead-angle-free laser cleaning. However, due to the complex structure of this type of equipment, its price usually fluctuates in the millions, and there is still a long way to go before it can be widely used.
Conclusion
Compared with traditional cleaning technology, laser cleaning technology is undoubtedly a more efficient and green new cleaning technology. After the efforts of scientific researchers in recent years, laser cleaning technology has also developed three typical cleaning methods: laser ablation cleaning method, laser liquid film assisted cleaning method and laser shock wave cleaning method. The application range of laser cleaning technology is not limited to traditional The rust removal of workpieces has been extended to the maintenance fields of cultural relics cleaning, ocean-going equipment, high-speed trains, etc. At the same time, the scale of laser cleaning is constantly making new breakthroughs, and the development of new laser composite cleaning technology has effectively solved the problem that traditional laser cleaning technology is difficult to clean composite materials.
Secondly, laser cleaning technology still has weaknesses such as expensive equipment and lengthy tests, and will inevitably encounter various resistances in the process of popularization. However, people in the industry still clearly realize that laser cleaning technology contains huge development potential and commercial value in future industrial development, and the market value of laser cleaning equipment is still showing a trend of rapid growth year by year. In the foreseeable future, laser cleaning technology will develop rapidly, injecting new impetus into related industries.
