Table of Contents
The development of laser cleaning and the principle
Traditional industrial cleaning methods mainly include high-pressure water, chemical reagents, ultrasonic and mechanical grinding methods, but these cleaning methods exist damage to the substrate, poor working environment, pollution, incomplete cleaning of some locations and cleaning costs and other problems. With the intensification of environmental pollution, scholars in various countries are actively developing energy-saving, environmentally friendly, efficient new cleaning technology. As the laser cleaning technology has the multiple advantages of low damage to the substrate material, high cleaning accuracy, zero emissions and no pollution, is gradually by the academic and industrial sector attention and favor.
Laser cleaning technology development history and the current situation
In the 1960s, the famous physicist Schawlow first proposed the concept of laser cleaning, and then the technology was applied to the restoration and maintenance of ancient literature. Foreign laser cleaning decontamination range is very wide, from the thick rust layer to the object surface microscopic particles can be removed, including the cleaning of cultural relics and works of art, tire mold surface rubber dirt removal, gold film surface silicone oil contaminants removal and microelectronics industry, high precision cleaning. In China, laser cleaning technology really began in 2004, China began to invest a lot of human and material resources to strengthen the research of laser cleaning technology. In the past decade or so, along with the development of advanced lasers, from inefficient, bulky carbon dioxide lasers to lightweight, compact fiber lasers; from continuous output lasers to nanosecond or even picosecond, femtosecond short pulse lasers; from visible light output to long-wave infrared and short-wave ultraviolet output …… lasers both in the energy output, the Wavelength range or in the laser quality, energy efficiency have been leaps and bounds. The development of lasers naturally also promotes the rapid development of laser cleaning technology, laser cleaning technology in the theory and application have achieved fruitful results.
Principle of laser cleaning technology
The process of pulsed laser cleaning depends on the characteristics of the light pulses generated by the laser, based on the photophysical reaction caused by the interaction between the high-intensity beam, short pulses of laser light and the contaminated layer.
a) The laser emits a beam of light that is absorbed by the contamination layer on the surface to be treated.
b) The absorption of large energy forms a rapidly expanding plasma (highly ionized unstable gas), which generates a shock wave.
c) The shock wave turns the contaminant into fragments and is rejected.
d) the width of the light pulse must be short enough to avoid heat accumulation that would cause damage to the treated surface.
e) Experiments have shown that plasma is produced on metal surfaces when oxides are present on the surface.
Plasma is only generated at energy densities above a threshold value that depends on the contamination or oxide layer being removed. This threshold effect is important for effective cleaning while keeping the substrate material safe. There is also a second threshold value for the appearance of plasma. If the energy density exceeds this threshold, the substrate material will be destroyed. In order to clean effectively while ensuring the safety of the substrate material, the laser parameters must be adjusted to the situation so that the energy density of the light pulse is strictly between the two thresholds.
Advantages of laser cleaning
Compared with mechanical friction cleaning, chemical corrosion cleaning, liquid solid strong impact cleaning, high-frequency ultrasonic cleaning and other traditional cleaning methods, laser cleaning has five obvious advantages.
laser cleaning is a “green” cleaning method, without the use of any chemicals and cleaning solutions, cleaning down the waste is basically a solid powder, small size, easy to store, recyclable, no photochemical reaction, will not produce pollution.
traditional cleaning methods are often contact cleaning, cleaning the surface of the object has a mechanical force, damage to the surface of the object or cleaning media attached to the surface of the object to be cleaned, can not be removed, resulting in secondary pollution, laser cleaning of non-abrasive and non-contact, non-thermal effect will not damage the substrate, so that these problems are solved.
the laser can be transmitted through optical fiber, with robotic hands and robots, easy to achieve long-distance operation, can clean the traditional method is not easy to reach parts, which can be used in some dangerous places to ensure the safety of personnel.
laser cleaning can remove various types of contaminants from the surface of various materials, to achieve a cleanliness that can not be achieved by conventional cleaning. And also can selectively clean the surface of the material without damaging the material surface contaminants.
laser cleaning speed, high efficiency, saving time; at this stage to buy laser cleaning system, although the initial one-time investment is high, but the cleaning system can be used for long-term stability, low operating costs, more importantly, can be easily automated operation. It can be expected that the future cost of laser cleaning system will also be a significant decline, thus further reducing the cost of using laser cleaning technology.
Classification of laser cleaning technology
Laser cleaning methods can be divided into the following three main categories.
Laser dry cleaning
The use of laser radiation direct decontamination, the laser is absorbed by the object or dirt particles, resulting in vibration, so that the substrate and the contaminant separation. Laser dry cleaning in the dirt particles are removed in two main ways: one is the instantaneous thermal expansion of the substrate surface, resulting in vibration, so that its surface adsorption of particles are removed. Another is the thermal expansion of the particles themselves and make the particles leave the surface of the substrate.
Laser wet cleaning
Laser wet cleaning is first to be cleaned on the substrate surface is uniformly covered with a layer of liquid media film, and then use laser radiation to stain. According to the media film and substrate absorption of laser light, wet cleaning can be divided into strong substrate absorption, strong media film absorption and media film substrate co-absorption. Strong substrate absorption, the substrate absorbs laser energy, the heat will be transferred to the liquid media film, the substrate and the liquid interface at the liquid layer superheated boiling, the liquid layer and stains are removed together.
Laser + inert gas cleaning
Laser radiation at the same time, with inert gas blowing to the surface of the workpiece, when the contaminants from the surface stripped, it is blown far away from the surface by the gas to avoid the clean surface is contaminated and oxidation.
Laser cleaning machine in the industrial industry applications
Laser cleaning applications, mainly from the market need for a non-toxic non-abrasive cleaning methods, this method can be used as an alternative to the previous use of chemical cleaning, manual cleaning and abrasive jet cleaning methods laser cleaning applications: 1, steel manufacturing …
Laser cleaning applications, mainly from the market needs a non-toxic non-abrasive cleaning methods, this method can be used as an alternative to the previous use of chemical cleaning, manual cleaning and abrasive spray cleaning methods
Laser cleaning applications
Surface polishing and rust removal in steel manufacturing
1) When metals are exposed to moisture, they react with water to form ferrous oxide, which rusts. This rust reduces the quality of the metal and makes it unsuitable for use in a wide range of applications.
2) On the other hand, due to the heat treatment process, an oxide layer is formed on the surface of the metal. The oxide layer discolors the surface of the metal and thus hinders subsequent finishing operations.
Cleaning the anode assembly
1)The presence of dirt and other contaminants will increase the resistance of the anode, which leads to more power consumption of the cell.
2) The presence of contaminants also shortens the life of the anode by increasing its consumption rate during the melting process.
Prepare for metal bonding
1)In order to improve process stability, surface adhesion and better welding quality, the surface of the metal material to be joined must be prepared before applying welding and other joining techniques.
2) If the surface of the metal material is not treated with the necessary cleaning, joints and seams are prone to degradation, increased wear and even catastrophic failure.
Pre-treatment for brazing and welding
1)It helps to remove ferrous and non-ferrous metals, lubricants and other contaminants from metal and aluminum surfaces in preparation for high-quality welding.
2)It also ensures smooth and non-porous brazing welds.
Local removal of coatings
1)It can be used on almost all surface types, whether chemically anodized, oxide or organic coatings.
2)Laser cleaning can be used in the automotive and other industries to remove coatings and paints while maintaining the integrity of the substrate material.
Selective paint removal
The use of lasers for polishing, surface cleaning and removal of coating applications, is rapidly expanding. Depending on the application, the pulse frequency, energy and wavelength of the laser must be precisely selected to clean, polish and ablate the target material. At the same time, any kind of damage to the substrate material must be prevented. Currently, laser cleaning technology is mainly used for cleaning small parts. Of course, it is highly likely that this technology is also applicable to cleaning large surfaces and large equipment/structures. With the current progress in this field, these visions are expected to become a reality in the near future.
Factors affecting the effectiveness of laser cleaning
The right angle that laser cleaning machine work
When the laser is incident at a certain tilt angle, the laser radiation is directly underneath the adhering particles, generating higher thermoelastic stress, and the contaminants are more easily removed compared to vertical incidence. In addition, it was found that with the increase in the angle of tilt, the laser radiation area is wider (see Figure 4), when the angle of tilt is 20 degrees, the area to be cleaned is about 10 times the area of vertical incidence, effectively improving the efficiency of laser cleaning.
The correct amount of defocusing to enhance the laser cleaning effect
Different amount of defocusing, the cleaning mechanism will be different. In the positive defocusing time cleaning is the surface material blasting fragmentation mechanism, while in the defocusing amount becomes larger, the removal of the paint layer is from fragmentation to vaporization, there will be dust around the cleaning place, which is obviously the remaining ash of the burning paint layer. In contrast, in the fragmentation process, there is no ash.
In order to optimize the effect of laser cleaning of metal surfaces, a comprehensive consideration of laser cleaning methods, cleaning models, the type of laser, laser wavelength, energy density, power, pulse frequency, pulse time and the laser incidence angle and other process parameters. As in Table 1, pulsed laser can effectively clean the carbon steel surface rust, when the wavelength of 1064nm, laser power of 500W, pulse frequency of 10kHz, pulse width of 120ns, cleaning speed of 60mm / s, lap rate of 5% of the cleaning parameters of laser cleaning rust effect is best, laser cleaning rust surface, micro-zone, line and point at the presence of oxygen elements are not found. Only the systematic study of process parameters to form a set of efficient laser cleaning system.
Test Data for laser cleaning in different power
|Object surface iron content
|Cleaning surface effect
|The rust layer has been removed, the surface is bright white metallic color, but a little gray-black spots remain on the surface.
|The rust layer is removed and the surface is bright white metallic
|The rust layer is removed and the surface is bright white metallic
|The rust layer has been removed, the surface is bright white metallic color, but the surface remains a little gray-black spots
|The rust layer has been removed and the surface is bright white metallic