What is the difference between high pressure stationary and low pressure portable cold spray systems

       

Cold spraying technology has become a research hotspot in the field of spraying, and many scientific researchers have developed and assembled cold spraying equipment.

 According to the system structure and function, it is divided into high-pressure cold spraying equipment and low-pressure cold spraying equipment. In the high-pressure cold spraying equipment, the powder is introduced from the shrinking section near the front end of the nozzle throat. The powder feeding gas needs to have a high pressure to generate a pressure difference to overcome the pressure of the working gas, so that the powder is sent to the open section through the nozzle throat for acceleration. Spray on the workpiece surface. High pressure will undoubtedly make the powder feeder, gas heating device, gas parameter control device, and gas source relatively complex and bulky, so the equipment is mostly arranged in a fixed way. High-pressure cold spraying equipment is also called fixed cold spraying equipment. In the low-pressure cold spraying equipment, the powder is directly fed into the open section downstream of the nozzle throat to be accelerated and sprayed to the surface of the workpiece. Since the gas flow rate in the open section is increased while the pressure and temperature have been significantly reduced, the lower pressure powder feeding gas can be used. fulfil requirements. Parts of the powder feeder, gas heating device, gas parameter control device, and gas source are relatively simple in structure, so it is also called a portable cold spray system. Obviously, the two devices have different characteristics. The former powder introduction method makes the gas-solid two-phase flow easy to concentrate and evenly distribute with extremely high gas pressure, the powder is easy to be uniformly and densely deposited, and the coating quality is excellent and stable; however, the gas consumption is large, the equipment structure is complex, and it is not easy to handle. . The latter's powder introduction method and lower gas pressure reduce gas consumption, and the equipment is relatively simple and flexible, which is conducive to on-site construction; the concentration and uniformity of powder flow are not ideal, which affects the quality of the coating to a certain extent.

The comparison of powder introduction methods in cold spray equipment is shown in Figure 2-26, mainly because the powder injection methods are different. In the high-pressure fixed structure, the powder is injected axially along the constricted section at the front end of the nozzle throat, which is the booster section of the Laval nozzle, and the working gas pressure is still maintained in the range of 1.5~3.0MPa. It should be slightly above this pressure range and always maintain a stable pressure differential. In the low-pressure portable structure, the powder is injected radially along the expansion section at the rear end of the nozzle throat, which is the decompression section of the Laval nozzle. The pressure of the working gas has been reduced from 2.5 to 3.0 MPa to 0.5 to 1 MPa, which is very small at this time. The powder can be introduced into the working air flow by the high pressure of the powder feeding gas, and the solid-gas volume ratio in the two-phase flow is increased from 10^-3 ~10^-6 in the high-pressure structure to 10^-4~10^-5. an order of magnitude. However, in this structure, the injection method of the powder along the extended pressure relief section at the rear end of the nozzle throat determines that it is a radial introduction method, that is, the powder delivery pipe is perpendicular to the Laval nozzle or at an angle, and it is impossible to be like a high-pressure structure. In this way, the powder can be introduced along the axial direction of the Laval nozzle. In addition, the residence time of the powder in the nozzle is extremely short, and it cannot be fully mixed with the working gas. Therefore, it is difficult to control the mass distribution and velocity distribution of the powder along the radial direction. Such non-uniformity will adversely affect the quality of the coating.