The traditional industrial cleaning methods mainly include high-pressure water, chemical reagents, ultrasonic waves, and mechanical polishing. However, these cleaning methods have problems such as damage to the substrate, poor working environment, pollution, partial cleaning, and high cleaning costs. With the intensification of environmental pollution, scholars from various countries are actively developing energy-saving, environmentally friendly and efficient new cleaning technologies. Because laser cleaning technology has multiple advantages such as low damage to substrate materials, high cleaning accuracy, zero emissions 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 very broad prospects.
Development history and current status of laser cleaning technology:
In the 1960s, the famous physicist Schawlow first proposed the concept of laser cleaning, and then applied the technology to the repair and maintenance of ancient books. The decontamination range of laser cleaning abroad is very wide, from the thick rust layer to the fine particles on the surface of the object, including the cleaning of cultural relics, the removal of rubber dirt on the tire mold surface, the removal of silicone oil contaminants on the surface of the gold film, and microelectronics. High-precision cleaning in the industry. In China, laser cleaning technology really began in 2004, and China began to invest a lot of manpower and material resources to strengthen the research on laser cleaning technology. In the past decade, with the development of advanced lasers, from inefficient and bulky carbon dioxide lasers to light and compact fiber lasers; from continuous output lasers to short pulse lasers with nanoseconds or even picoseconds and femtoseconds; from visible light output To the output of long-wave infrared light and short-wave ultraviolet light ... lasers have developed by leaps and bounds in terms of energy output, wavelength range, or laser quality and energy conversion efficiency. The development of lasers has naturally promoted the rapid development of laser cleaning technology. Laser cleaning technology has achieved fruitful results in theory and application.
The principle of laser cleaning technology:
The process of pulsed laser cleaning depends on the characteristics of the light pulses generated by the laser and is based on the photophysical reaction caused by the interaction between the high-intensity light beam, the short-pulse laser and the contamination layer. The physical principle can be summarized as follows (Figure 1):
A) The beam emitted by the laser is absorbed by the pollution layer on the surface to be treated;
B) Absorption of large energy forms a rapidly expanding plasma (highly ionized unstable gas), generating shock waves;
C) Shock waves make pollutants debris and removed;
D) The light pulse width must be short enough to avoid heat accumulation that would damage the treated surface;
E) Experiments show that when there are oxides on the metal surface, plasma is generated on the metal surface.
Plasma is only generated when the energy density is above the threshold, which depends on the contaminated layer or oxide layer being removed. This threshold effect is very important for effective cleaning while ensuring the safety of the substrate 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 carry out effective cleaning under the premise of ensuring the safety of the substrate material, the laser parameters must be adjusted according to the situation, so that the energy density of the light pulse is strictly between two thresholds.
Advantages of laser cleaning:
Compared with traditional cleaning methods such as mechanical friction cleaning, chemical corrosion cleaning, liquid solid strong impact cleaning, and high-frequency ultrasonic cleaning, laser cleaning has obvious five advantages:
Environmental protection advantages: Laser cleaning is a "green" cleaning method, without the use of any chemicals or cleaning fluids. The cleaned waste is basically solid powder, small in size, easy to store, recyclable, no photochemical reaction, no Will cause pollution.
Effect advantage: The traditional cleaning method is often contact cleaning, which has mechanical force on the surface of the cleaning object, the surface of the damaged object or the cleaning medium is attached to the surface of the object to be cleaned, and cannot be removed, resulting in secondary pollution Grinding and non-contact, no thermal effects will not damage the substrate, making these problems solved.
Control advantages: The laser can be transmitted through the optical fiber, cooperate with the robot hand and the robot, and realize the long-distance operation conveniently.
Convenient advantage: laser cleaning can remove various types of contaminants on the surface of various materials, reaching a degree of cleanliness that cannot be achieved by conventional cleaning. It can also selectively clean contaminants on the surface of the material without damaging the surface of the material.
Cost advantage: laser cleaning speed is fast, high efficiency, save time; although the initial investment of laser cleaning system is high at the current stage, the cleaning system can be used stably for a long time, the operating cost is low, and more importantly, it can be easily automated . It is foreseeable that the cost of the laser cleaning system will be greatly reduced in the future, thereby further reducing the cost of using laser cleaning technology.
Classification of laser cleaning technology:
The methods of laser cleaning can be divided into the following three categories:
1. Laser Dry Cleaning
Using laser radiation to directly decontaminate, after the laser is absorbed by objects or dirt particles, it generates vibration, which separates the substrate and the pollutants. In the laser dry cleaning, there are two main ways to remove dirt particles: One is the instantaneous thermal expansion of the substrate surface, which generates vibrations to remove the particles adsorbed on the surface. The other is the thermal expansion of the particles themselves, causing the particles to leave the surface of the substrate.
2. Laser wet cleaning
Laser wet cleaning is to uniformly cover the surface of the substrate to be cleaned with a layer of liquid dielectric film, and then use laser radiation to remove stains. According to the absorption of laser light by dielectric film and substrate, wet cleaning can be divided into strong substrate absorption, strong dielectric film absorption and dielectric film substrate absorption. When the strong substrate absorbs, after the substrate absorbs the laser energy, the heat is transferred to the liquid dielectric film, the liquid layer at the interface between the substrate and the liquid is overheated and the liquid layer and the stain are removed together.
3. Laser + inert gas cleaning
At the same time of laser radiation, the surface of the workpiece is blown with inert gas. When the contaminants are peeled from the surface, they are blown away from the surface by the gas, avoiding the contamination and oxidation of the clean surface.
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