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       Supersonic flame spraying, also known as high-speed oxy-fuel spraying (HVOF, HighVelocityOxy-fuel), is a new type of thermal spraying technology developed on the basis of ordinary flame spraying and explosive spraying in the early 1980s. Flame spraying is a high-energy and high-speed spraying method developed because of its potential advantages in providing kinetic energy.



The heat source of supersonic flame spraying is to use combustible gas (such as H2, C3H3 or C3H6, etc.) and high-pressure O2, or use liquid fuels such as kerosene and alcohol and high-pressure O2 to mix and ignite in a special combustion chamber, or in a special nozzle. The high-temperature and high-speed flame flow produced by the combustion, and the violently expanded gas is constrained by the water-cooled nozzle to form a supersonic high-temperature flame flow, and the combustion flame flow speed can reach more than 1500--2000m/s. In a typical supersonic flame, the presence of a Mach cone can be observed. The powder is sent to the powder channel in the center of the combustion chamber by nitrogen gas through the powder inlet, and the powder is fed axially or laterally into the flame flow, which can reach supersonic speed through the flame flow acceleration, that is, the heating and acceleration of powder particles and coating can be realized. deposition. Due to the extremely high velocity of the flame flow, the sprayed particles can be accelerated up to 300--650m/s while being heated to a molten or semi-molten state, resulting in a high-bond, dense, high-quality coating.



In HVOF technology, the spray gun is a necessary guarantee to generate a stable high-speed flame, and is the key to ensure the quality of the coating. The HVOF spray gun is mainly composed of three parts: the combustion chamber for mixing air and fuel, the Laval nozzle for accelerating the flame to supersonic speed, and the constant-section long nozzle for fully heating and accelerating the spray material particles. Due to the limitation of the temperature of the combustion flame, the supersonic flame has a lower temperature (about 3000°C) and higher speed than the plasma heat source. For WC-Co cemented carbide, it can effectively suppress the decomposition of WC during the spraying process. The coating not only has high bonding strength and compactness, but also can retain the hard and wear-resistant WC phase in the powder to the greatest extent. Therefore, the coating has superior wear resistance, which is comparable to the explosive spray coating, and greatly exceeds the plasma spray coating. It has been widely used in electroplating hard chromium layer and spray welding layer.

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