1. The principle of supersonic arc spraying

 

The principle of arc spraying is to use two continuously fed metal wires as consumable electrodes, generate an electric arc at the ends of them as a heat source, atomize the molten wire with compressed air, and spray it to the work piece at supersonic speed to form a A thermal spraying method for coatings with high bonding strength, low porosity and low surface roughness. Its working principle is the same as ordinary arc spraying (subsonic atomization), supersonic arc spraying is a continuous melting-atomization-deposition process. But in terms of atomization method, supersonic arc spraying is fundamentally different from ordinary arc spraying, that is, supersonic arc spraying uses supersonic atomization. The advantages are: good atomization effect, fine and uniform particles after atomization, and high speed, which is conducive to obtaining high-quality coatings. Supersonic arc spraying uses Laval nozzles to increase the speed of the airflow from subsonic to supersonic speed, which strengthens the airflow to the particles, thereby increasing the particle speed. The particle speed has a great influence on the performance of the coating. When the particle speed is high, the impact effect on the substrate when the particles are deposited is strong, and the particle deformation is sufficient. It is conducive to the combination of particles and substrates, particles and particles, thereby improving the bonding strength and cohesive strength of the coating; the particle speed is high, and the flying time of the particles in the air before deposition is short, and the oxides produced during the flight are less. It is conducive to the combination of particles, thereby improving the cohesive strength of the coating and reducing the porosity of the coating. The higher the particle velocity, the more favorable it is to obtain a high-quality coating. With the upgrading of thermal spraying equipment, the particle speed is continuously increasing, and the quality of the coating is also continuously improved.

 

Supersonic atomization reduces the particle size and the roughness of the coating. Roughness is an important performance index of the coating, and it depends on the particle size after atomization. The supersonic atomization strengthens the effect of the air flow on the melting metal at the end of the wire, and the atomized particles are fine and uniform, which greatly reduces the roughness of the coating. At the same time, the reduction of the particle size also reduces the splashing during the flattening of the particles, which is beneficial to reduce the porosity of the coating.

 

Supersonic atomization is the starting point of supersonic arc spraying, and is the fundamental difference between it and ordinary arc spraying.

 

Supersonic arc spraying equipment includes power supply, spray gun, wire feeding mechanism and its accessories. The key equipment is the supersonic arc spray gun. Our company uses imported nozzles, and the spraying current is stable, and the spraying area can reach 20 square meters under the premise of ensuring the wire atomization effect and coating quality.

 

During arc spraying, the temperature of the arc area is as high as 5000-6000℃, and the nozzle is cooled by air cooling.

 

2. Technical advantages of supersonic arc spraying

 

Compared with ordinary flame spraying, supersonic arc spraying has the following technical advantages:

 

1. High thermal efficiency. Most of the heat generated by flame spraying is lost to the atmosphere and cooling system, and the utilization rate of heat energy is only 8-15%. Arc spraying directly uses electric energy to convert heat energy to melt the wire, and the heat energy utilization rate is as high as 70-80%.

 

2. High production efficiency. The production efficiency of arc spraying is high, which is reflected in the amount of metal wire sprayed per unit time. Under normal circumstances, its production efficiency is more than 8 times that of flame spraying.

 

3. Simple operation, safe and reliable. Arc spraying equipment has no complicated operating mechanism. As long as the working current and voltage are selected within the specified range according to the different spraying materials, the spraying quality can be guaranteed. Only electric power and compressed air are used on site, and flammable gases such as oxygen and acetylene are not used, which is safe and reliable.

 

4. The coating has high bonding strength. Arc spraying can obtain higher bonding strength without increasing the working temperature and using precious metals, generally up to 20MPa, which is 2.5 times the strength of flame sprayed coatings. The bonding strength of the supersonic arc spray coating can reach 60MPa, which is close to the quality of plasma.

 

5. The porosity of the coating is low. The porosity of the coating surface is generally less than 1%, which improves the wear and corrosion resistance of the coating.

 

Due to the unique advantages of supersonic arc spraying, it has been applied in the fields of anti-corrosion and wear resistance of materials, repairing old and recycling waste, and power production, and has produced great economic and social benefits.

 

The development and success of supersonic arc spraying shows that the technology is highly advanced and scientific. The specific performance is as follows:

 

1. Using the principles of aerodynamics, computer design, and Laval nozzles, the supersonic arc spray gun was successfully developed, which greatly increased the particle velocity, and the measured particle velocity exceeded the current domestic and international arc spray particle velocity. Among them, aluminum ion The speed is 385.7m/s.

 

2. The designed supersonic arc spraying power supply has reliable arc starting, and the acceleration of the Laval nozzle on the airflow, which elongates the arc and keeps the arc stable, and effectively improves the atomization effect of the wire particles and the coating performance. Density, bonding strength.

 

3. The spray gun is air-cooled, which solves the problem of bonding between metal particles and the inner wall of the nozzle, improves the quality of the air flow, and improves the speed of the particles and the quality of the coating.

 

4. The technology has high practical value and promotion value. The supersonic arc spraying technology is at the international leading level in the current arc spraying field.

 

3. Application of supersonic arc spraying in thermal power plants

 

In our country, the abrasion and corrosion of the "four boiler tubes" has always been a bottleneck problem that plagues CFB boiler power plants. The economic loss and social impact caused by boiler tube burst, leakage, corrosion, and boiler shutdown for maintenance are very huge.

 

The four boiler tubes have been exposed to sulfide corrosion, high temperature oxidation and particle erosion for a long time, and the working environment is very harsh. In order to solve the problem of wear and corrosion failure, arc spraying technology is used in foreign countries, such as the United States, Germany, the former Soviet Union, Japan and other countries, to spray different alloy materials on the four boiler tubes, which can slow down the wear and corrosion of the boiler tubes and extend the service life. , Is very effective. Supersonic arc spraying technology has also been widely used in China. For example, our company has provided four-tube boilers, external heat exchangers, furnace buried heating surfaces, heat exchangers in slag coolers, and water for dozens of domestic power plants. The membrane wall of the cold cyclone separator, the combustion belt of the water-cooled wall of the furnace and the wear-prone parts around the outlet of the furnace roof were sprayed, all of which have achieved obvious results.