Supersonic spraying

       Supersonic spraying

In order to fight against the explosive spraying of the American carbide company, in the early 1960s, the American J. Browning invented the supersonic flame spraying technology, called "Jet-Kote", and obtained a US patent in 1983. In recent years, foreign supersonic flame spraying technology has developed rapidly, and many new devices have appeared, which are replacing traditional plasma spraying in many fields. In China, Wuhan Institute of Materials Protection, Beijing Iron and Steel Research Institute, Beijing Tide New Technology Materials Co., Ltd. are also conducting research in this area, and have produced supersonic spraying devices with their own characteristics.

The fuel aviation kerosene and the combustion accelerant (O2) are introduced into the combustion chamber in a certain proportion to mix and explode explosively. At the same time, the powder feeding gas (Ar or N2) is fed into the high-temperature gas along the tungsten carbide central sleeve in the combustion head, and sprayed together to form a coating on the workpiece.

The velocity of the flame flow generated at the nozzle outlet of the sprayer is generally 4 times the speed of sound, that is, about 1520m/s, up to 2400m/s (specifically related to the type of combustion gas, mixing ratio, flow rate, powder quality and powder flow rate, etc.) . The velocity of the powder hitting the workpiece surface is estimated to be 550-760 m/s, which is comparable to explosive spraying. The reason why the Jet-Kote method can have such a high speed lies in the rational design and manufacture of a nozzle according to the principle of fluid mechanics, which is called the expansion tube of the Laval tube.

As long as the design of the tube is reasonable, the fluid can reach the speed of sound at a certain section (such as AB) of the tube when the velocity is low, as long as it undergoes sufficient compression. After passing this section, the supersonic speed will be obtained.

The supersonic spraying method has the following characteristics:

① The temperature of the powder particles is lower and the oxygen is relatively light (this is mainly due to the short residence time of the powder particles in the high temperature and the short exposure time in the air, so the oxide content in the coating is low, and the chemical composition and phase composition have Strong stability), but only suitable for spraying metal powder, Co-Wc powder and low melting point TiO2 ceramic powder;

② The particle movement speed is high.

③ The particle size is small (10～53>; μm) and the distribution range is narrow, otherwise it cannot be melted.

④ Coating bonding strength, high density, no delamination phenomenon.

⑤ The surface roughness of the coating is low.

⑥The spraying distance can be changed in a wide range without affecting the spraying quality.

⑦ Thicker coating can be obtained than explosive spraying, and residual stress is also improved.

⑧The spraying efficiency is high and the operation is convenient.

⑨ The noise is large (greater than 120dB), and sound insulation and protective devices are required.