Process principle and technical characteristics of cold spraying

Cold spray is a metal spray process, but it is different from traditional thermal spray (overspeed flame spray, plasma spray, explosion spray and other traditional thermal spray), it does not need to melt the sprayed metal particles, so the temperature generated on the surface of the sprayed substrate is not Will exceed 150 degrees Celsius. Cold spray technology is an effective method for forming thick aluminum coatings on the surface of magnesium alloys. This method does not require high surface preparation and does not require any consideration for the mechanical or thermal properties of the plated parts. The aluminum coating exhibits the ability to prevent various and electrical corrosion of the magnesium component. In many cases, battery corrosion protection is only required around steel fasteners, and cold spray is an innovative technique for local protection of exposed magnesium surfaces.

Cold spray anti-corrosion is a revolutionary technology that allows the direct and in-situ formation of thick aluminum coatings on magnesium alloys to reduce or eliminate the hazards of common or electrical corrosion. This technology is expected to overcome the shortcomings of the original magnesium alloy anti-corrosion technology, which helps to use magnesium for external components of automobiles.

Cold spray is an up-and-coming solid state process. The method can convert the kinetic energy of the ultrasonic particles accelerated by ultrasonic into thermal energy when striking the surface of the plating member, thereby completing metallurgical welding. The principle of the process is that each metal has its own specific temperature-dependent critical particle velocity that is soldered to the plated part when the particle motion exceeds this speed.

In the traditional thermal spraying process, due to the higher temperature, both the plating and the plating material are oxidized, metallurgical deformation and residual tensile stress. Conversely, the coating produced by the cold spray process has a low porosity, anti-oxidation and anti-phase change, and can reduce the tensile stress for various metals, cermets or other materials.

In high pressure cold spray technology, high pressure helium or nitrogen (350 to 450 psi) is used as a carrier gas to accelerate the spray material to ultrasonic speed. The gas is heated and forced through a focus-diverging nozzle (deLaval) where it is accelerated to ultrasonic velocity (greater than 1000 m/sec). The spray particles are injected axially upstream of the spray head.

In the low pressure cold spray technique, nitrogen or air is pressurized to 70-15 psi, while the spray powder is injected radially downstream of the diverging portion of the spray head. The low pressure cold spray system is portable and economical, with particle speeds of up to 800 m/s. Portable cold sprayers can be used for the spraying of aluminum, copper, zinc and other metal combinations. The portability features make the low pressure cold spray machine more suitable for field maintenance and repair.

Cold spray technology is an effective method for forming thick aluminum coatings on the surface of magnesium alloys. This method does not require high surface preparation and does not require any consideration for the mechanical or thermal properties of the plated parts. The aluminum coating exhibits the ability to prevent various and electrical corrosion of the magnesium component. In many cases, battery corrosion protection is only required around steel fasteners, and cold spray is an innovative technique for local protection of exposed magnesium surfaces.

The cold spray technology is relative to the thermal spray technology. When spraying, the sprayed particles hit the surface of the substrate at a high speed (500-1000 m/s). During the whole process, the particles do not melt, maintain the solid state, and the particles undergo pure plastic deformation polymerization. coating. Cold spray technology has been developed rapidly in Russia, the United States, Germany, etc. in recent years.

In the cold spraying process, since the spraying temperature is low, the driving force for the phase change is small, the solid particle grains are not easily grown, and the oxidation phenomenon is difficult to occur. It is therefore suitable for spraying temperature sensitive materials such as nanophase materials, amorphous materials, oxygen sensitive materials (such as copper, titanium, etc.), phase change sensitive materials (such as carbides, etc.). At present, the research of nano-powders is more and more extensive, the particles themselves are small, completely different in performance from solids, and exhibit many new unique properties superior to the bulk structure. In recent years, nano-coating preparation has attracted interest. Studies have shown that nano-coatings have better performance than conventional coatings due to grain size effects and the presence of a large number of grain boundaries. Surface nanocrystals can improve the mechanical and chemical properties of the surface (and overall) of the material to varying degrees. Spraying onto the surface of the substrate by conventional spraying methods will cause changes in composition, properties and structure; while cold spraying will retain its basic structure and properties, enabling the coating of the nano-coating to be achieved.