[China Aluminum Industry Net] Introduction of Cold Spraying Cold spray technology is an effective method for generating thick aluminum coatings on the surface of magnesium alloys. This method does not require high surface preparation and does not require mechanical or thermal properties of the coated parts. . The aluminum coating exhibits the ability to prevent various and electrolytic corrosion of magnesium components. In many cases, battery corrosion protection is required only around steel fasteners, and cold spraying is precisely an innovative technique for the partial protection of exposed magnesium surfaces.
Technical Features of Cold Spraying Cold spray anti-corrosion is a revolutionary technology that enables the direct, on-site generation of thick aluminum coatings on magnesium alloys to reduce or eliminate common or electrocorrosive damage. This technology is expected to overcome the shortcomings of the original magnesium alloy anti-corrosion technology, which will help to use magnesium for the automotive exterior components.
The technological principle of cold spraying technology Cold spraying is a budding solid-state process. This method can convert the kinetic energy of the ultrasound-accelerated solid particles into heat energy when they hit the surface of the plated part, thereby completing the metallurgical welding. The principle of the process is that each metal has its own specific, temperature-dependent critical particle velocity, which will be welded to the plating when the particle moves beyond this velocity.
In the traditional thermal spraying process, due to the high temperature, both the coating and the plating material are oxidized, resulting in metallurgical deformation and residual tensile stress. On the contrary, the coating made by cold spray process has a very low porosity (<0.5%), and it is also resistant to oxidation and phase change. It can reduce the tensile stress for various metals, cermets or other material combinations.
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 velocity. The gas is heated and forced through a focus-spray nozzle (deLaval) where it is accelerated to ultrasonic velocity (greater than 1000 m/sec). The sprayed particles are injected axially upstream of the spray head.
In the low pressure cold spray technique, nitrogen or air is pressurized to 70 to 15 pounds per square inch, and the spray powder is injected radially in the downstream of the divergent portion of the spray head. The low-pressure cold spray system is portable and economical to operate, with particle speeds of up to 800 m/s. The portable cold sprayer can be used to spray aluminum, copper, zinc and other metal combinations. The portability makes the low pressure cold sprayer more suitable for field maintenance and repair.
Cold spraying technology is an effective method to generate a thick aluminum coating on the surface of magnesium alloys. This method does not require high surface preparation and does not need to take into account the mechanical or thermal properties of the coating. The aluminum coating exhibits the ability to prevent various and electrolytic corrosion of magnesium components. In many cases, battery corrosion protection is required only around steel fasteners, and cold spraying is precisely an innovative technique for the partial protection of exposed magnesium surfaces.
However, in order to understand and improve the cold spray process, it is necessary to carry out more thorough research, especially for the continuous development and innovation of various material combinations and the cold spray process itself, as well as the better use of materials for future technologies. jobs.
The difference between thermal spraying and cold spraying techniques Thermal spraying technology is to heat a certain solid material to a molten or semi-molten state and spray it onto the surface of the substrate at a high speed to form a film layer with desired properties, thereby achieving the surface modification of the substrate surface. Processing technology. Because the thermal spray coating has a special layered structure and a number of tiny pores, the combination of the coating and the substrate is generally mechanical, and its bonding strength is low. In many cases, thermal spraying can cause phase transitions, decomposition of some elements and volatilization [1,2] and oxidation of some elements [3,4].
Cold spraying technology is relative to thermal spraying technology. When spraying, 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 and remain in a solid state, and the particles undergo pure plastic deformation polymerization. coating. Cold spray technology has been rapidly developed in Russia, the United States, and Germany in recent years. The applicable materials for cold spray technology are in the cold spraying process. Due to the lower spraying temperature, the driving force for phase change is smaller and the solid particle crystals are smaller. The particles are not easy to grow and oxidation is difficult to occur. It is suitable for spraying temperature-sensitive materials such as nano-phase materials, amorphous materials, oxygen-sensitive materials (such as copper, titanium, etc.), phase-change sensitive materials (such as carbides, etc.). At present, the research of nanopowders is more and more extensive. The particles themselves are small, completely different from the solid in performance, and exhibit many new unique properties superior to the bulk structure. In recent years, the preparation of nano-coatings has attracted people's interest. Studies have shown that due to grain size effects and the presence of a large number of grain boundaries, nano-coatings have superior properties to conventional coatings.] Surface nanocrystals can improve the mechanical and chemical properties of the material surface (and overall) to varying degrees. Spraying onto the surface of the substrate with a conventional spray method will cause changes in its composition, properties, and structure; while the use of cold spray will retain its basic structure and properties, so that the nano coating can be sprayed to achieve.
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