1. Research Background
With the application of aluminum and magnesium alloy materials and composite materials in the aircraft manufacturing industry, the performance of the aircraft, especially the flying speed, has been greatly improved, and higher requirements have been put on the performance of the aircraft skin coating, such as the plane facing the wind and aircraft.
Wings require higher temperature resistance, UV resistance, resistance to freeze-thaw cycles, abrasion resistance, and resistance to rain, etc., and therefore have a coating on the surface of the aircraft skin coating process and coating performance very high demand.
When the aircraft skin is painted, it must fully consider the matching of primer, topcoat and substrate. Before the painting, the surface of the aircraft skin needs to be pretreated. The surface treatment method cannot be used to enhance the adhesion by sandblasting. Generally, the chromic acid anodization, the arodine oxidation treatment and the phosphating primer are generally selected. Treatment methods. After surface treatment, it is necessary to coat the surface of the aircraft with a primer. For the aircraft skin, the primer should have good adhesion to the aircraft skin, excellent corrosion resistance, heat resistance, impact resistance, elasticity and liquid oil resistance to the machine, and it should also be with the aircraft skin finish. Should have a good matching. At present, a wide range of skin primers used in aircraft are epoxy primers, acrylic primers, vinyl chloride primers and polyurethane primers. After a layer of primer is applied, a high-performance special protective topcoat is applied to the surface of the aircraft skin, mainly including phenolic coatings, alkyd coatings, epoxy coatings, and fluorocarbon coatings.
Damage to the surface of the aircraft or normal maintenance every few years (normally 4 to 6 years) requires the old coating to be removed and new coating applied again. In recent years, with the rapid development of the aviation industry, the number of passenger aircraft, transport aircraft, and fighter jets is increasing. The removal of paint on the surface of the skin is often one of the problems in repainting. Various paint removal technologies have emerged.
2. Overview of traditional paint removal techniques
At present, there are mainly three kinds of traditional cleaning methods:
(1) Mechanical cleaning method, which uses scraping, wiping, brushing, or sandblasting to achieve the purpose of removing surface dirt;
(2) Wet chemical cleaning method, which uses organic Cleaning agent, by spray, shower or high-frequency vibration to remove oil and other surface attachments;
(3) ultrasonic cleaning method, the method is to put parts into water or organic solvents, the use of ultrasonic vibration effect cleaning dirt.
For the removal of coatings on aircraft skins, suitable traditional cleaning methods are mechanical cleaning and wet chemical cleaning methods. The mechanical method is simple and the operation is flexible, but the labor intensity is large, the noise pollution is serious, and the dirt that is separated from the surface is easily re-adsorbed on the clean surface to form secondary pollution. Moreover, if the operation process is not properly selected, it will be difficult to maintain the accuracy of the decontamination surface, and it will also cause permanent damage to the surface of the components. This must be done with special care when cleaning the aircraft skin. In order to improve accuracy, the use of advanced equipment is very expensive. Taking an imported sand-sanding machine as an example, the cost of such a device is as much as US$300,000. The cost is extremely high. For different materials and hardness surfaces, the shape of the abrasive grains and the operating technology (such as injection pressure, compressed air volume, etc.) are all special. The requirements and regulations limit the scope of their use, but it is also difficult to guarantee that the cleaning surface will not be damaged. Due to the special shape of the aircraft, using mechanical cleaning is obviously not the best choice.
Wet chemical cleaning methods are more extensive than mechanical cleaning methods. It generally uses acid, alkaline solution and special cleaning agent to remove the coating. Although the corrosion inhibitor is added in the cleaning solution, it is difficult to control effectively because of the time, and the acid and alkaline solution still cause different degrees of corrosion to the substrate. For example, when the metal surface is cleaned with an acid solution, the corrosion of the substrate with the acid solution begins with the dissolution of the surface scale skin; since the metal is an electrochemical corrosion process characterized by the precipitation of atomic hydrogen during the acid etching process, When the hydrogen atoms produced by the metal during the corrosion process do not rapidly combine with the constituent hydrogen atoms, some of the hydrogen atoms will diffuse through the metal surface into the metal and cause hydrogen embrittlement. Although fighter skins and their main structural components are still dominated by aluminum alloys, composite materials are increasingly used on passenger aircrafts, transport aircrafts, and modern fighter aircraft. Some new composite materials are very resistant to chemicals. Poor, not suitable for chemical cleaning. At the same time, waste discharged after chemical cleaning will cause serious pollution to the environment.
It can be seen that the traditional method of paint removal has been greatly limited in the cleaning of the surface of the aircraft skin, and it is still a last resort to use it. Obviously, it is necessary to find an efficient and quick “green” cleaning method to meet the high-precision, high-cleanliness and non-damaging cleaning requirements on the substrate surface.
3. Laser cleaning technology applied to paint removal is the focus of future development
After nearly four decades of development, laser technology has become increasingly sophisticated, and has been widely used in industrial production, medical care, military, and entertainment. Laser cleaning technology is a new technology that has only emerged in recent decades. The related research started in the mid-1980s, but it was not until the early 1990s that researchers gradually paid attention to it and developed rapidly. Its emergence opened up laser technology in the industry. New areas of application and become a new member of the laser processing technology family. As a new cleaning technology, laser cleaning technology has now become a supplement and extension of traditional cleaning methods, and it has begun to be applied in microelectronics, construction, nuclear power plants, aerospace, automotive manufacturing, medical care, cultural heritage protection and other fields.
3.1 Laser Cleaning Technology and Its Features
Laser cleaning technology refers to the use of high-energy laser beam irradiation on the surface of the workpiece, so that the surface of the dirt, rust, particles or coatings instantly evaporated or peeled, so as to achieve a clean process. Compared with the traditional cleaning process, laser cleaning technology has the following characteristics:
(1) It is a "dry" cleaning, does not require cleaning fluid or other chemical solutions, will not lead to secondary pollution, and cleanliness is much higher In the chemical cleaning process;
(2) The laser cleaning technology is targeted at a wide range of cleaning objects, such as various oxide particles, rust, coating and surface organic matter, while not limited by the matrix material;
(3) Traditional cleaning The method is often a short-distance operation, which has a mechanical force on the substrate surface and is easy to damage the substrate; and the laser cleaning is a non-contact processing, and can perform a long-distance operation, and the laser processing parameters can be adjusted without damaging the surface of the substrate. Effectively remove contaminants and make surfaces renewed as new;
(4) Laser cleaning can be easily automated with modern methods;
(5) Laser decontamination equipment can be used for a long period of time with low operating costs;
(6) Laser cleaning Technology is a kind of "green" cleaning process, eliminating the waste is a solid powder, small size, easy to store, basically the environment create pollution.
3.2 Selection of laser when laser removing paint
When using laser for cleaning, the choice of laser is related to the subsequent cleaning work, so the choice of laser is extremely important. At present, there are many types of lasers, such as CO2 lasers, Nd:YAG solid-state lasers, and excimer lasers. These kinds of lasers can meet the requirements of laser cleaning. Which kind of laser should be used depends on the specific situation. Pulsed lasers are commonly used for cleaning, and continuous lasers are sometimes used. For the removal of the coating (lacquer layer), CO2 lasers work best, especially the pulsed TEA CO2 lasers are promising in this area. However, due to the fact that CO2 lasers cannot be transported by optical fibers, they are limited in remote cleaning applications. However, the use of YAG lasers or even diode lasers for laser cleaning provides a vast space.