Current situation and future application of cold spraying technology
cold spraying is the name of a solid-state spraying process that uses a high-speed gas nozzle to accelerate powder particles to the substrate. In this process, metal particles produce plastic deformation and solidify under impact. The term "cold spraying" refers to a process that requires a relatively low processing temperature, which is generally much lower than the melting point of the spraying material
although the concept of cold spraying metal materials onto substrates can be traced back to the early 20th century, the applicability of this technology, including new energy materials, nano materials, advanced composite materials, eco-environmental materials, high-performance structural materials, new functional materials, new chemical materials, etc., was not shown in the Institute of theoretical and applied machinery of the Russian Academy of Sciences in novosibirisk until the 1980s. Due to the adhesion of metal powder reaching the substrate and precipitation materials, it forms a solid state. Therefore, the characteristics of cold spray precipitation are very unique, so it is very suitable for precipitation of a wide range of traditional and advanced materials on various types of substrate materials, especially for non-traditional applications that are sensitive to process temperature
Figure 1: low pressure cold spraying is used to repair the corroded surface on site
some characteristics of cold spraying include the formation of dense sediment with extremely low oxygen content, no residual tensile stress, particle increase, recrystallization area and phase change. Some materials even have nano grain refinement. Because of these properties, cold spraying is particularly suitable for precipitation of various advanced and temperature sensitive materials. Today, cold spraying is increasingly used in various industrial fields, including aerospace, energy and military industries (Figure 1 and Figure 2)
Figure 2: manufacturing conductive bus duct by cold spraying on tin oxide coated glass for heating glass how does cold spraying work
in the 1980s, in the application and development of cold spraying technology, two methods of spraying materials into the nozzle were patented. Today, we call them high pressure (before using high-pressure supply gas to spray powder into the nozzle throat) and low pressure (using low-pressure supply gas to spray powder into the nozzle diversion area)
in high-pressure cold spraying (Figure 3), preheat high-pressure helium or nitrogen (up to 1000 psi) (up to 1000 ℃), and then force it through the DeLaval expansion nozzle. Above the nozzle, gas expansion converts enthalpy into kinetic energy, accelerates the gas flow to the ultrasonic range (1000 m/s), and reduces its temperature. Before the throat of the nozzle, the powder feed is introduced into the air stream axially. The accelerated solid particles impact the substrate with enough kinetic energy to produce mechanical or metallurgical bonding effect
Figure 3: working principle of high-pressure cold spraying
in low-pressure cold spraying (Figure 4), air or nitrogen (PSI) with relatively low pressure is also preheated (up to 550 ℃), and then forced through DeLaval nozzle. The heated gas on the enlarged side of the nozzle is accelerated to about 600 m/s. The powder feed is directed downward into the enlarged part and accelerated to the substrate
Figure 4: working principle of low-pressure cold spraying
since the application of cold spraying technology has been extended to new and unique application fields, more and more commercial cold spraying systems have been introduced to the market (Figure 5)
Figure 5: commercial low-pressure cold spraying system for field operation portable system
(picture provided by ceterlie widsor, Ltd.)
cold spraying technology now has new development, including spraying induced by shock wave, in which the control valve downstream of high-pressure gas source quickly opens/closes to generate pulse (Hz) heating ultrasonic flow (Figure 6). Pulsed flow is used to simultaneously accelerate and heat the powder introduced into the cylindrical nozzle. Compared with cold spraying, it does not require the use of DeLaval nozzle, and the powder can obtain additional energy during acceleration. This can effectively bond various engineering materials, including steel, titanium and cement. In addition, intermittent airflow can also reduce gas consumption and improve energy efficiency
Figure 6: shock wave induced spray (SISP)
cold spraying technology belongs to the larger thermal spraying process of the "family", and it is not used to replace any mature thermal spraying method here. On the contrary, cold spraying technology is expected to supplement and expand the application range of thermal spraying
at present, cold spraying is increasingly used in various industries to slow down the corrosion of sensitive materials, such as magnesium aluminum alloy, surface repair, spray target manufacturing, bus duct manufacturing on heating glass, WC Co (tungsten carbide cobalt) deposition of hard chromium replacement coating, conductivity and thermal conductivity coating of transition surface, copper welding preparation, and deposition of thermal barrier and NiCrAlY bonding coating. In many such applications, cold spraying is more economical because it can really eliminate or reduce manufacturing steps
for other applications, cold spraying is only the only feasible solution, especially in non-traditional applications. As environmental and health and safety regulations become more stringent, interest in cold spraying has grown as a potential greener alternative
with the progress of technology, it is expected that the application of cold spraying will continue to expand to more non-traditional applications, such as photoelectric, wind energy, medical and construction fields. In optoelectronic applications, cold spraying can be used for the manufacture of complex conductive solar cells (Figure 7). Wind power generation can use cold spraying to strengthen the surface performance of components manufactured by advanced polymer matrix and even standardized waterproof material enterprises
Figure 7: the thin-film photoelectric panel series provides power for the entire building of the Toledo university campus in Toledo, Ohio
in the medical field, cold spraying can effectively spray hydroxyapatite (HAP), a famous biocompatible material, onto a large number of substrates, but it will not affect the integrity of HAP at the same time. Architects can use cold spraying to create infinite beautiful metal patterns on any metal or ceramic substrate
nanotechnology and intelligent structures
some other non-traditional applications include the use of advanced materials, such as nanostructures and amorphous materials. In nanostructured materials, the particle size is very small, which has extreme fracture toughness while maintaining high strength and other mechanical advantages. Nanocrystalline is extremely sensitive to process temperature, and cold spraying can be used effectively without affecting the beneficial microstructure (Fig. 8)
figure 8: nanostructured 2618 aluminum alloy deposited by cold spraying process
due to the low deposition temperature, cold spraying can embed micro sensors and their functional coatings on the surface to form intelligent structures. These structures can provide real-time information related to material properties or environmental conditions. Many emerging enterprises are focusing on providing sensing, database management and pre analysis solutions for bridges, electricity, wind turbines, aircraft, automobiles, ships, pipelines and construction equipment. The future of cold spraying depends on depositing advanced materials on various substrates while minimizing heat loss and cost. This is the cornerstone to define the future direction and opportunities of this technology
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introduction to the author
Dr. Julio villafuerte is the R & D director of ceterlie widsor Ltd., one of the design and manufacturers of high-quality metal forming and welding equipment in North America. The company provides products to many world leading automobile manufacturers. At present, Dr. villafuerte is also an adjunct professor of Waterloo University. He has actively participated in lectures and research, and has won the doctor's degree and master's degree in mechanical and material engineering of Waterloo University. He has more than 15 years of rich experience in welding and material processing technology, and has been committed to the research and dissemination of new cooling aerodynamic spraying technology in recent yearsDr. villafuerte is very active in the industrial field, including the international thermal spraying Association and ASM thermal spraying Association. He has written many technical publications and often serves as a technical/strategic consultant for professional and academic committees. Dr. villafuerte is the technical consultant of ASM Journal of material engineering and properties, Journal of photochemistry and Photobiology and chemistry, and surface and coating technology. (end)