The process of gas-dynamic spraying is the fixing of metal particles on metal, glass, ceramic or concrete products at the moment of collision of the gas-powder mixture with the outer surface-substrate. This happens due to the preliminary acceleration of these particles in the nozzle for supersonic particle acceleration, while the temperature of the accelerated metal particles does not exceed their melting temperature. The metal layer deposited on the product by the method of cold gas-dynamic spraying is distinguished by high-quality adhesion to the base surface and is resistant to mechanical damage.

The history of the discovery of the phenomenon and the facts discovered empirically

The fact that for the formation of a metal coating on the substrate surface there is no need to bring metal particles to a state of melting or close to it, as is usually the case when using standard deposition techniques, was discovered at the end of the 20th century by Russian scientists. The results of a number of experiments carried out by researchers of the Russian Academy of Sciences have shown that surface deposition can also be obtained by heating solid metal particles to a temperature that is much lower than their melting point.

In addition, the following important facts were recorded during the experiments:

• The main parameter in the technology of cold gas-dynamic spraying, on which the quality of adhesion depends, is the acceleration rate of the gas-powder mixture. It is this parameter that affects the degree of adhesion of the coating to the surface on which it is applied, as well as such characteristics of the deposited layer as porosity and mechanical strength. At a speed of solid particles above 500-600 m/s, erosive processes are transformed into a durable spray layer;
• Empirically, the critical limit of particle consumption was found, at which the metal layer is not formed for any duration of exposure of the gas-powder flow to the substrate;
• if the powder consumption exceeds the critical value, then there is a strong and reliable adhesion of the particles on the sprayed surface, and a dense sprayed layer is formed;
• of the total volume of solid particles dispersed by the supersonic flow, only a small amount forms a surface spray layer. The main mass of particles is sprayed and does not have the ability to gain a foothold on the treated surface. Accordingly, the amount of metal particles deposited and fixed on the product directly depends on the amount of powder material consumed;
• the surface of the substrate during the formation of the deposition layer heats up insignificantly.

Types of cold gas-dynamic spraying and their advantages

Cold gas-dynamic spraying has 2 types:

1. Spraying high pressure, which uses helium, nitrogen, or a mixture of gases. Powder material consumption is 4.5-13.5 kg/h.
2. Cold gas-dynamic spraying of low pressure, carried out with the help of compressed air. The volume of the consumed powder ranges from 0.3-3 kg/h.

Both types of spraying have their advantages and disadvantages:

• when used in a high pressure process, the coating is of better quality, despite the fact that the size of the solid particles of the metal powder can vary from 5 to 50 microns, and not within 5-30 microns, as in compressed air technology;
• The low-pressure spraying process uses smaller equipment, the cost of which is much lower than that used for high-pressure spraying.

Technological process of high and low pressure spraying

In the high pressure cold spray process, the gas is heated and combined with the solid particles of the powder material. This gas-powder mixture enters a supersonic nozzle, accelerates there to supersonic speed and, under a pressure of 7-40 bar, is directed to the surface of the product on which it is necessary to form a metal coating.

Cold spraying, which uses compressed air, is technologically different from the high-pressure spraying method in that the main processes take place immediately in the nozzle to accelerate particles to supersonic speed: the gas is heated directly in it, and the powder enters the nozzle perpendicular to the gas flow. In addition, when using the low-pressure spraying method, powders are used in which ceramic particles are present in addition to metal particles. Such additives improve the surface condition of the product, which is sprayed, and improve the quality of adhesion of materials. In addition, during the passage of the mixture flow through the equipment, the ceramic particles additionally clean the walls and the outlet of the nozzle.

Scope of cold gas-dynamic spraying

Cold gas-dynamic coating is used to solve the following problems:

• restoration of metal parts that were prone to chips, cracks, abrasion and other mechanical damage;
• coating of metal products with spraying in order to increase their anti-corrosion and heat-conducting properties;
• protection of contact surfaces of ferrules of metal cables.

Candidates of Physical and Mathematical Sciences O. KLYUEV and A. KASHIRIN.

When the first metal tools of labor first appeared, it turned out that, being solid and durable, they often deteriorated under the influence of moisture. Time passed, people created mechanisms and machines, and the more perfect they became, the more difficult conditions their metal parts had to work. Vibrations and alternating loads, huge temperatures, radiation exposure, aggressive chemical environments - this is not a complete list of "tests" to which they are subjected. Over time, people have learned to protect the metal from corrosion, wear and other phenomena that reduce the life of parts. In fact, there are two approaches to providing such protection: either alloying elements are added to the base metal, which give the alloy the desired properties, or a protective coating is applied to the surface. The operating conditions of machine parts dictate the properties that coatings must have. The technologies for their application are diverse: there are common and relatively simple, there are very thin ones that allow you to create coatings with unique properties. And restless engineers continue to invent new coatings and come up with ways to get them. The fate of these inventions can become happy if the coating is much superior to its predecessors in terms of useful properties or if the technology has a significant economic impact. In the development of physicists from Obninsk, both of these conditions were combined.

Metal particles flying at great speed upon impact with the substrate are welded to it, and ceramic particles compact the coating (a); stuck ceramic particles are visible on the microsection of the metal layer (b).

Diagram (top) and general form(bottom) apparatus for spraying metal coatings.

By means of the device it is possible to apply coverings in any rooms and even in field conditions.

A zone of negative pressure arises behind the critical section of the nozzle, and the powder is sucked in here. Thanks to this phenomenon, it was possible to simplify the design of the feeder.

Defects in body parts (left) and the result of spraying (right): a - a crack in an automatic transmission; b - a cavity in the cylinder head.

Coated with a layer of copper or aluminum, tools can be used in fire hazardous areas: when they hit metal objects, they do not spark.

TEMPERATURE PLUS SPEED

Of the methods of metallization of surfaces in modern technology most often used galvanic deposition and immersion in the melt. Vacuum deposition, vapor deposition, etc. are used less frequently. The closest thing to the development of Obninsk physicists is gas-thermal metallization, when the deposited metal is melted, sprayed into tiny drops and transferred to the substrate by a gas jet.

Metal is melted with gas burners, electric arc, low-temperature plasma, inductors and even explosives. Accordingly, metallization methods are called flame spraying, electric arc and high-frequency metallization, plasma and detonation-gas spraying.

In the process of flame spraying, a metal rod, wire or powder is melted and sprayed in the flame of a burner operating on a mixture of oxygen and combustible gas. In electric arc metallization, the material is melted by an electric arc. In both cases, metal droplets are moved to the sprayed substrate by air flow. In plasma spraying, a plasma jet is used to heat and spray the material, which is formed by plasmatrons of various designs. Detonation-gas spraying occurs as a result of an explosion that accelerates metal particles to tremendous speeds.

In all cases, the particles of the sprayed material receive two types of energy: thermal - from the heating source and kinetic - from the gas flow. Both of these types of energy are involved in the formation of the coating and determine its properties and structure. The kinetic energy of particles (with the exception of the detonation-gas method) is low compared to the thermal one, and the nature of their connection with the substrate and between themselves is determined by thermal processes: melting, crystallization, diffusion, phase transformations, etc. Coatings are usually characterized by good adhesion to the substrate (adhesion) and, unfortunately, low uniformity, since the spread of parameters over the cross section of the gas flow is large.

Coatings, which are created by gas-thermal methods, have a number of disadvantages. These include, first of all, high porosity, unless, of course, the goal is to specifically make the coating porous, as in some parts of radio tubes. In addition, due to the rapid cooling of the metal on the surface of the substrate, high internal stresses arise in the coating. The workpiece inevitably heats up, and if it has a complex shape, then it can be "led". Finally, the use of combustible gases and high temperatures in the work area make it difficult to ensure the safety of personnel.

The detonation-gas method stands somewhat apart. During the explosion, the speed of the particles reaches 1000-2000 m/s. Therefore, the main factor determining the quality of the coating is their kinetic energy. Coatings are characterized by high adhesion and low porosity, but explosive processes are extremely difficult to control, and it is practically impossible to guarantee the stability of the result.

SPEED PLUS TEMPERATURE

The desire to create a more advanced technology arose a long time ago. The engineers had a goal - to preserve the advantages of traditional technologies and get rid of their shortcomings. The direction of the search was more or less obvious: firstly, the coatings should be formed mainly due to the kinetic energy of the metal particles (the particles should not be allowed to melt: this will prevent the part from heating and oxidation of the substrate and coating particles), and, secondly, the particles should acquire high speed not due to the energy of the explosion, as in the detonation-gas method, but in a jet of compressed gas. This method is called gas dynamic.

The first calculations and experiments showed that it is possible to create coatings with quite satisfactory characteristics in this way if helium is used as the working gas. This choice was explained by the fact that the speed of the gas flow in the supersonic nozzle is proportional to the speed of sound in the corresponding gas. In light gases (hydrogen was not considered because of its explosiveness), the speed of sound is much higher than in nitrogen or air. It is helium that would accelerate metal particles to high speeds, giving them enough kinetic energy to attach to the target. It was believed that the use of heavier gases, including air, was doomed to failure.

The operation of experimental sputtering installations gave a good result: particles from most industrially used metals accelerated in a helium jet adhered well to the substrate, forming dense coatings.

But the engineers were not completely satisfied. It was clear that equipment running on light gases would inevitably be expensive and could only be used at enterprises producing high-tech products (only there are lines with compressed helium). And lines with compressed air are available in almost every workshop, at every car service enterprise, in repair shops.

Numerous experiments with compressed air seemed to confirm the worst expectations of the developers. However, an intensive search nevertheless led to a solution. Coatings of satisfactory quality were obtained when the compressed air in the chamber in front of the nozzle was heated, and fine ceramics or hard metal powder were added to the metal powder.

The fact is that when heated, the air pressure in the chamber increases in accordance with Charles's law, and, consequently, the speed of the outflow from the nozzle also increases. The metal particles, which have gained tremendous speed in the gas jet, soften when they hit the substrate and weld to it. Ceramic particles play the role of microscopic sledgehammers - they transfer their kinetic energy to the underlying layers, compact them, reducing the porosity of the coating.

Some ceramic particles get stuck in the coating, others bounce off it. True, coatings are obtained in this way only from relatively ductile metals - copper, aluminum, zinc, nickel, etc. Subsequently, the part can be subjected to all known methods of machining: drilling, milling, sharpening, grinding, polishing.

THE MAIN CONDITION - SIMPLICITY AND RELIABILITY

The efforts of technologists will be in vain if the designers cannot create simple, reliable and economical equipment in which the process invented by technologists would be implemented. The basis of the apparatus for spraying metal powders was a supersonic nozzle and a small-sized electric compressed air heater capable of bringing the flow temperature to 500-600 o C.

The use of ordinary air as a working gas made it possible to simultaneously solve another problem that faced the developers of systems using light gases. We are talking about the introduction of the sprayed powder into the gas jet. To maintain tightness, the feeders had to be installed up to the critical section of the nozzle, that is, the powder had to be fed into the high pressure area. Purely technical difficulties were exacerbated by the fact that, passing through the critical section, metal particles caused wear of the nozzle, worsened its aerodynamic characteristics, and did not allow stabilization of the coating deposition modes. In the design of the apparatus with an air jet, the engineers applied the principle of a spray gun, known to everyone from school experiments in physics. When a gas passes through a channel of variable cross section, then in a narrow place its velocity increases, and the static pressure drops and may even be lower than atmospheric pressure. The channel through which the powder came from the feeder was located just in such a place, and the powder moved into the nozzle due to air suction.

As a result, a portable apparatus for applying metal coatings was born. It has a number of advantages that make it very useful in various industries:

for the operation of the apparatus, only an electrical network and an air line or a compressor are needed, providing a compressed air pressure of 5-6 atm and a supply of 0.5 m 3 / min;

when applying coatings, the temperature of the substrate does not exceed 150 ° C;

coatings have high adhesion (40-100 N/mm2) and low porosity (1-3%);

the equipment does not emit harmful substances and radiation;

the dimensions of the device allow it to be used not only in the workshop, but also in the field;

coatings of almost any thickness can be sprayed.

The installation includes the actual sprayer weighing 1.3 kg, which the operator holds in his hand or fixes in the manipulator, an air heater, powder feeders, a unit for monitoring and controlling the operation of the sprayer and feeder. All this is mounted on a rack.

I had to work hard on the creation of consumables. Commercially available powders have too large particle sizes (of the order of 100 microns). A technology has been developed that makes it possible to obtain powders with grains of 20-50 microns.

FROM SPACE VEHICLES TO SEEDERS

The new method of spraying metal coatings can be used in a wide variety of industries. It is especially effective in repair work, when it is necessary to restore parts of products, for example, to repair a crack or a sink. Due to the low temperatures of the process, it is easy to restore thin-walled products, which cannot be repaired in another way, for example, by surfacing.

Since the spraying zone has clear boundaries, the sprayed metal does not fall on defect-free areas, and this is very important when repairing complex-shaped parts, such as gearbox housings, engine cylinder blocks, etc.

Spraying devices are already used in the aerospace and electrical industries, at nuclear power facilities and in agriculture, at auto repair enterprises and in foundry production.

The method can be very useful in many cases. Here are just a few of them.

Restoration of worn or damaged areas of surfaces. With the help of spraying, parts of gearboxes, pumps, compressors, molds for investment casting, molds for the manufacture of plastic packaging that are damaged during operation are restored. New method has become a great help for employees of auto repair enterprises. Now, literally "on their knees", they close up cracks in cylinder blocks, mufflers, etc. Without any problems, they eliminate defects (cavities, fistulas) in aluminum castings.

Elimination of leaks. The low gas permeability of the coatings makes it possible to eliminate leaks in pipelines and vessels when sealing compounds cannot be used. The technology is suitable for repairing tanks operating under pressure or at high and low temperatures: heat exchangers, car radiators, air conditioners.

Application of electrically conductive coatings. By sputtering it is possible to apply copper and aluminum films on a metal or ceramic surface. In particular, the method is more cost-effective than traditional methods when copper-plating current-carrying busbars, galvanizing pads on grounding elements, etc.

Anti-corrosion protection. Aluminum and zinc films protect surfaces from corrosion better than paint and many other metal coatings. The low productivity of the installation does not allow processing large surfaces, but it is very convenient to protect such vulnerable elements as welds. With the help of spraying zinc or aluminum, it is possible to stop corrosion in places where "bugs" appear on the painted surfaces of car bodies.

Restoration of plain bearings. Plain bearings usually use babbitt liners. Over time, they wear out, the gap between the shaft and the sleeve increases and the lubrication layer is broken. Traditional repair technology requires either replacement of the liner or welding of defects. And spraying allows you to restore the liners. In this case, ceramics cannot be used to seal the layer of the sprayed metal. Hard inclusions in a matter of minutes after the start of work will disable the bearing, and the surfaces of both the bushings and the shaft will be damaged. I had to use a nozzle of a special design. It allows the coating of pure babbitt in the so-called thermokinetic mode. Powder particles immediately behind the critical section of the nozzle are accelerated by a supersonic air flow, then the flow velocity sharply decreases to transonic. As a result, the temperature rises sharply, and the particles are heated almost to the melting point. When they hit the surface, they deform, partially melt and stick well to the underlying layer.

TO A SPECIALIST - A NOTE

Literature

Kashirin A. I., Klyuev O. F., Buzdygar T. V. Device for gas-dynamic coating of powder materials. RF patent for invention No. 2100474. 1996, MKI6 C 23 C 4/00, publ. 12/27/97. Bull. No. 36.

Kashirin A. I., Klyuev O. F., Shkodkin A. V. The method of obtaining coatings. RF patent for invention No. 2183695. 2000, MKI7 C 23 C 24/04, publ. 06/20/02. Bull. No. 17.

Contacts of developers and conditions for purchasing their technologies or products can be found in the editorial office.

In fact, it is a more advanced version of the gas-thermal method of restoring various metal parts and surfaces that has long been proven. Cold Spray or simply CGN significantly expands the possibilities of the "hot" method of processing products.

Today, it is undoubtedly the most advanced technology for the recovery and protection of materials, which has become widespread in both the industrial sector and the civil sector.

The principle of action, the pros and cons of CGN

It has two main differences from the gas-thermal method of restoration. Firstly, the deposition of a protective or restorative coating occurs at a low temperature not exceeding 150 °C, which in turn does not cause stress in the workpieces and their deformation. Secondly, the "cold" technology allows you to create a layer of adjustable thickness and within precisely defined boundaries. We will talk about other pros and cons a little later, but for now, about the authors of the method and how it works.

Its developer is "Obninsk powder coating center"(Russia). The equipment they produce is called DIMET ®. It is certified according to the GOST R system and is protected by patents in Russia, the USA, Canada and other countries. The technology is based on the principle of supersonic impact of the smallest particles of low-melting and other materials on the treated surface. Basically, these are polymers or alloys of carbides with metals with a particle size of 0.01-0.5 microns. Mixing with gas, they are fed to the product at a speed of 500-1000 m/s.

Depending on composition consumable(powder) and changing the modes of its application, you can get a homogeneous or composite coating with a solid or porous structure and its own functional task. This can be: restoration of the geometry of the product, hardening and protection of the metal from corrosion, increase in the thermal and electrical conductivity of the material, as well as the formation of a wear-resistant coating that can withstand the effects of chemically active environments, high thermal loads, etc.

By the way, Obninsk engineers have already developed several modifications of DIMET ® units. Given the wide demand for this equipment, both manual and automated cold gas-dynamic spraying machines are now mass-produced, which allows them to be used in industry, the oil and gas industry, as well as in small businesses for processing small parts. Moreover, there is nothing particularly complicated in the technology itself. For the operation of the complex (in addition to the material for spraying), only compressed air is needed (supplied at a pressure of 0.6-1.0 MPa and a flow rate of 0.3-0.4 m3/min.) And a 220 V power supply.

Now more about the advantages and disadvantages of the method. Firstly, in contrast to the gas-thermal method, CGN can be effectively used at normal pressure, in any temperature range and humidity level. Secondly, it is environmentally absolutely safe. Thirdly, due to the high speed, it can also be used for abrasive surface cleaning. Well, the only drawback of the technology is the possibility of applying coatings only from relatively ductile metals, such as copper, aluminum, zinc, nickel, etc.

Scope of CGN

I would like to dwell in more detail on the areas of application of the technology of cold gas-dynamic spraying with powder materials in order to clearly show how much it is in demand today.

Elimination of defects, restoration of surfaces and sealing

All this is a job that even small businesses can do. For example, in small workshops, it is possible to repair parts made of light alloys (parts of an automotive structure, for example), primarily aluminum and aluminum-magnesium. Moreover, defects that have arisen both in the production process and during operation are easily eliminated. And the absence of strong heating and the low energy of the method make it possible to repair even thin-walled products.

CGN is also excellent for restoring worn surfaces. For example, such a labor-intensive process as “building up” metal in bearing seats can now be carried out even by small enterprises, not to mention the restoration of sealing (when the use of liquid sealants is impossible) in pipelines, heat exchangers or vessels for working gases, liquids.

It is very effective in the repair of complex products, where accurate restoration of geometric parameters is required, elimination of hidden defects, but at the same time with the preservation of all operational characteristics, as well as presentation. That's why this method actively used in the military-industrial complex, railway and aviation industries, agriculture, gas transmission, etc.

You can not do without this technology in the creation of contact pads. Due to the possibility of easy coating on any metal, ceramic and glass surfaces, CGN is also used in the production of electrical products. For example, in the processes of copper plating, the creation of power current-carrying networks, the application of current leads, the manufacture of sublayers for soldering, etc.

Anti-corrosion treatment and elimination of deep defects

Spraying the so-called anti-friction coating is a highly effective way to get rid of local damage (deep chips, scuffs, scratches). This avoids the procedure of complete refilling or even replacement of the product, which, of course, is not economically viable.

And in anti-corrosion treatment and protection against high-temperature corrosion of various communications, this method has no equal at all. By the way, various modifications of equipment DIMET ® provide high-quality processing of the inner surface of pipes with a diameter of 100 mm and a length of up to 12 m.

Kawasaki robots are used in spraying complexes using DIMET technology. This technology allows you to apply a metal layer on various surfaces: metal, glass, ceramics, stone. A feature of the technology is the possibility of applying metal powder to metals that are incompatible for welding and soldering. For example, it is possible to effectively deposit copper on aluminum, which is of great value for electrical engineering.

About technology

The technology of gas-dynamic spraying of powder metal and its transformation into a monolithic coating is implemented on DIMET equipment manufactured by the Obninsk Center for Powder Spraying. Coatings are formed on any solid surface such as metal, glass, ceramics, stone. The coating material is chosen when solving a specific production or creative problem, since the solution can be obtained using different types of powder materials.

Compressed air (5-8 atm) is heated (300-600°C) and fed into the nozzle, where a supersonic flow is formed:

• powders containing metal and ceramic particles are introduced into this flow
• particles are accelerated by the gas flow to a speed of several hundred meters per second and are directed to the substrate in the unmelted state
• upon impact with the substrate, the kinetic energy of the particles is converted into heat, and then into the binding energy of the particles with the substrate
• as a result of such high-speed impacts, the particles are fixed on the substrate and form a dense coating.

The main processes that determine the adhesion of particles to the substrate and to each other are:

1. Close contact between the crystal lattices of particles and the substrate (or different particles) until the formation of metallic bonds, at least in certain areas of the contact spot. In this case, the particle or substrate does not melt anywhere. This clutch mechanism is similar to the clutch mechanism in explosion welding.
2. On individual protrusions and irregularities of the falling particles, their melting can occur and spot microwelding can be carried out.
3. In close contact of juvenile surfaces of dissimilar materials, intermolecular interactions of these materials can manifest themselves. A typical example of such a mechanism is the deposition of a mirror aluminum coating on glass.
4. Mechanical cohesion can play a certain role under the condition of deep penetration of particles into the substrate. The specific ratio of the relative role of various linkage mechanisms in different cases can differ significantly from each other and is the subject of a separate study.

Areas of use

Industry	Application	Coatings
Foundry	Repair of molded parts defects Under pressure In a chill mold Lost Wax Models	Coatings for restoring the shape and dimensions of parts. Sealing coatings (low gas permeability)
Metallurgical production	Reducing the electrical resistance of the contacts of electrolyzers High temperature corrosion protection	Electrically conductive coatings Heat resistant coatings
Automotive	Cast parts repair	Sealing coatings Anti-corrosion coatings
	Coatings for the repair of mechanical damage to the cylinder head, BC, units Sealing cracks in cylinder head, BC, radiators, pipelines, air conditioners Local corrosion protection Restoring the shape of body parts made of aluminum without putty	Sealing coatings Anti-corrosion coatings
Aircraft building, aircraft repair	Repair of casting and manufacturing defects of aluminum parts	Coatings for restoring the shape and dimensions of parts. Sealing coatings
Rocket and space technology	Special	Coatings for sealing products made of heat-strengthened aluminum Heat-radiating coatings
Shipbuilding, ship repair	Protective protection of welded seams Restoration of bearing seats	Coatings for restoring the shape and dimensions of parts Anti-corrosion coatings Sealing coatings
Oil and gas industry	Restoration of the geometry of parts of gas compressor units Seizure prevention of highly loaded threaded connections Restoration of plain bearings	Coatings for restoring the shape and dimensions of parts Anti-seize coatings Antifriction
Electrical production	Metallization of electrical contact pads Application of electrically conductive galvanically compatible coatings Metallization for heat transfer Sublayers for aluminum and glass for soldering	Electrically conductive coatings
Tool production	Restoration of molds for plastic and glass packaging Restoration of molds for pressing rubber products Restoration of equipment for pressing parts from press materials (AG4, DSV, carbolite) Making a spark-proof tool	Coatings for restoring the shape and dimensions of parts Intrinsically Safe Coatings
Restoration of monuments and sculptures	Restoration of the lost elements of monuments. Corrosion protection	Coatings for restoring the shape and dimensions of parts Anti-corrosion coatings

Completed project

Robotic complex for coating contact surfaces of current-carrying busbars, which are used in the tokamak reactor of the ITER project. The developer of the complex is Acton LLC (partner and system integrator of Robowizard).

Complex scheme:

Solved problem:

Sputtering of a two-layer copper coating on flat electrical contact surfaces of aluminum busbars. The spraying area is up to 0.5 m 2, the tires themselves reach a length of 12 meters and a mass of 4 tons.

The composition of the complex:

1. PLC Aries;
2. Robot Kawasaki RS006L;
3. Spray chamber;
4. Controller E01;

The implemented complex makes it possible to perform the following tasks:

• execution of the technological process with the function of program control and parameter management;
• movement of the sprayer along a given trajectory, synchronized with the work technological equipment, through the transmission of information messages;
• visualization of process parameters on the operator's touch screen, as well as means for changing operating modes, organized on the basis of dialog box elements.

If you have a need for such a solution, leave your contact details in the application form. Our experts will advise you and discuss the details of cooperation.

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Cooperation scheme

Supersonic cold gas-dynamic spraying (SCD).

The essence of the method lies in the formation of coatings due to the high kinetic energy of unmelted metal particles. This method is currently known as cold spray- cold spray.

It should be noted that in the most common gas-thermal coating methods for their formation from a stream of particles, it is necessary that the particles falling on the substrate have a high temperature, usually above the melting point of the material. With gas-dynamic spraying, this condition is not mandatory, which determines its uniqueness. In this case, particles that are in an unmelted state, but with a very high speed, interact with a solid base.

In contrast to the hot plasma spraying method, a gas-dynamic method of cold coating deposition was developed, the essence of which was that a certain threshold speed was established at which cold plastic particles formed a dense coating. With different granulation (large and small particles in a single flow), smaller particles with a higher speed settled on the substrate, while larger particles with a lower speed bounced off the surface and did not participate in the formation of the coating.

This particle behavior allowed larger abrasive particles to be introduced into the coating material flow. Simultaneous sandblasting and coating took place. From the point of view of surface preparation, when the juvenile surface of the substrate loses its activity due to the adsorption of gases on the surface during deposition delay, such a coating scheme is optimal. At the same time, an installation was developed in which gas (air, nitrogen) at a pressure of 2.5-3.5 MPa is heated to 350-600 ° C in a metal coil passing through it electric shock from a welding transformer. The atomizer is equipped with a Laval nozzle, which provides a supersonic outflow of a two-phase jet.

On fig. 2.48 is a diagram of the process. Gas-dynamic cold spraying makes it possible to apply coatings of ductile metals with the addition of other materials.

On fig. 2.49 shows the dependences of the speed and temperature of gas and particles along the Laval nozzle for a two-phase jet (nitrogen + solid particles of copper with a size of 5 and 25 microns) at pressure R= 2.5 MPa and temperature T 0= 950°С. In this case, the ratio of the output diameter /) in to the critical /) k is /) in / Г> k \u003d 9.

Rice. 2.48.

Rice. 2.49. Air temperature T d, air velocity and temperature and speed of copper particles with a diameter of 5 and 25 microns in a profiled supersonic nozzle

The domestic installation "DIMET" is produced by the Obninsk powder spraying center in two versions - manual with a power of 2 kW and stationary with a power of 7 kW. Recommendations for the use of powder materials are presented in Table. 2.10.

The main application of HDN is the application of anti-corrosion coatings of a tread type based on aluminum and zinc. Wear-resistant coatings are applied based on plastic materials - babbitt, copper, nickel, etc. Compared to the GN and EDM methods, when the metal is melted and saturated with gases, including hydrogen, which worsens the protective properties of the coating, HDN does not have these disadvantages. Hydrogen does not dissolve in solid phase particles. The coating effectively protects the steel from corrosion. The method has found wide application for protecting car bodies from corrosion in the area of ​​welds.

Main Components coatings	working
Aluminium, zinc		Sealing leaks in metal pipes, radiators, condensers, heat exchangers, etc., including sealing leaks in welds, repair of corrosion and mechanical damage. Sealing cracks, gullies and other defects in aluminum, steel and cast iron parts
Aluminium, zinc		Restoring the shape of metal parts. Filling cavities, pores, cracks and other defects in products made of aluminum and its alloys (including engine parts, molds etc.). Restoration of bearing seats in aluminum, steel and cast iron parts
Aluminium, silicon carbide		Filling cavities, cracks and other defects in aluminum, steel and cast iron engine body parts
Aluminium oxide		Cleaning and jet-abrasive preparation of the surface of steel and cast iron for the application of metal coatings
		Electrically conductive coating (on steel, aluminium, ceramics). Undercoat for tin soldering on aluminium, steel and cast iron parts
Copper, zinc		Filling of cavities, cracks and other defects in steel and cast iron body parts of the engine

Main Components coatings	working	Appointment of coatings, objects of repair and restoration
		Anti-corrosion protection. Sealing of defects, microcracks, threaded connections
		Heat resistant coating to protect against high temperature corrosion. Electrically conductive coating for contact pads of electrical equipment
Nickel, zinc		Filling cavities, burnouts and other defects in steel products. For high temperature products
		Electrically conductive coating for contact pads of electrical equipment
		Anti-corrosion protection of steel parts and welds on steel structures

On fig. 2.50 shows the installation diagram of the company Linde(USA). Recent advances in the implementation of the method - the manufacture of hand sprayers, the characteristics of which are given in table. 2.11.

Table 2.11

Characteristics of HDN sprayers

Characteristics	Model 412	Model 403
Productivity according to A1, g/min
Number of temperature modes
Dimensions (mm) and weight (kg):
spraying block	450 x 64 x 85 mm; 1.3 kg	450 x 64 x 85 mm; 1.3 kg
	340 x 260 x 320 mm; 8 kg	560 x 260 x 490 mm; 16 kg
Characteristics coatings:
adhesion strength, MPa
porosity, %
surface roughness, µm	R, = 20-40

Rice. 2.50. Scheme of the company's cold spraying plant Linde:

1 - tanker with liquefied gas (AG); 2 - evaporator; 3 - compressor; 4 - air heater; 5 - powder feeder; 6 - atomizer

Low requirements for the accelerating gas and low power consumption make it possible to create portable units using the DIMET technology.