Gas-dynamic spraying of metal: purpose, purpose, types of technology. Advantages and disadvantages of the method. Application area. Equipment and features of the use of cold spraying.

Gas-dynamic spraying of metal is carried out in order to give surfaces metallic and non-metallic metal products required properties. This can be an increase in electrical and thermal conductivity, strength, protection from exposure corrosion processes, restoration of geometric dimensions, etc. At the same time, depending on specific task, depending on the metal of the product, is selected necessary equipment, consumables and spraying technology. Most often, surfaces are subject to metallization, while the applied coating has high adhesion with the material on which it is applied, and the product is mechanically strong. Pure metal powders or mixtures can be deposited, in which, in addition to the metal component, ceramic powder is introduced into certain quantities. This significantly reduces the cost of powder coating technology and does not affect its properties.

The essence of the method of cold gas-dynamic spraying is the application and fixing on the surface of a product or part of solid particles of metal or a mixture of materials ranging in size from 0.01 to 50 microns, accelerated to the required speed in air, nitrogen or helium. Such material is called powder. These are particles of aluminum, tin, nickel, babbits of different brands, a mixture of aluminum powder with zinc. The medium with which the material is moved can be cold or heated to a temperature not exceeding 700 °C.

Upon contact with the surface of the product, a transformation of the plastic type occurs, and the energy of the kinematic type passes into adhesive and thermal, which contributes to the production of a durable surface layer of the metal. The powder can be applied not only to metal surfaces, but also to those made of concrete, glass, ceramics, stone, which greatly expands the scope of the method for creating surfaces with special properties.

Depending on the pressure, the following types of cold gas-dynamic spraying are distinguished:

• high;
• low.

In the first case, helium and nitrogen are used as a working medium that moves powder material ranging in size from 5 to 50 microns. Metal particles, if they move, have a pressure of more than 15 atm. In the second case, it is used compressed air, which is supplied under pressure not exceeding 10 atm. These types also differ in such indicators as heating power and consumption of the working medium.

The spraying steps are as follows:

• preparation of the surface of the product for spraying by mechanical or abrasive means;
• heating of the working medium (air, nitrogen, helium) up to the technological process temperature;
• supply of heated gas to the nozzle of the equipment together with the powder under the required pressure.

As a result, the powder is accelerated in the flow to supersonic speeds and collides with the surface of the part or product. There is a deposition of a metal layer with a thickness, the value of which depends on the heating temperature of the supplied gas and pressure.

The preparation of the surface of the product by an abrasive method is performed using the equipment itself for applying gas-dynamic spraying by simply changing the mode parameters.

The scope of this type of spraying is quite extensive. The method is used to seal leaks in tanks and pipelines, repair parts and castings made of light alloys, apply electrically conductive, anti-corrosion and anti-friction coatings, eliminate mechanical damage, restore seats in bearings.

The main advantages of the method

The advantages of the technology include:

• performance of work under any climatic conditions (pressure, temperature, humidity);
• the possibility of using equipment of a stationary and portable type, which in the latter case allows you to carry out work at the place of their implementation;
• the possibility of applying a coating to local areas (defective places);
• the ability to create layers with different properties;
• the possibility of creating a layer of the required thickness or different in thickness in multilayer coatings;
• the process does not affect the structure of the product, which is sprayed, which is an important advantage;
• safety;
• environmental friendliness.

The disadvantage of this type of spraying is only one fact. Layers can be applied to ductile metals such as copper, zinc, aluminium, nickel and their alloys.

Manufacturers different countries produce stationary and portable equipment for manual and automated coating of different capacities on different metals.

Applied equipment

The apparatus for gas-dynamic metal spraying consists of the following main parts:

• powder containers;
• systems for supplying the working medium, including a cylinder for compressed gas and all the necessary accessories for it;
• nozzles (as a rule, there are several of them, they are of different configurations and are used for different spraying modes);
• control panel.

In the Russian Federation, high-quality equipment for spraying by the gas-dynamic method is produced by the powder spraying center in Obninsk under the trademark "DIMET". It meets the requirements of domestic GOSTs, is certified and protected by patents in many countries, including Russia.

The process of repairing a part by gas-dynamic spraying is shown in the video:

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By the way, Obninsk engineers have already developed several modifications of DIMET installations. Given the widespread demand this equipment, now both manual and automated devices of cold gas-dynamic spraying are mass-produced, which allows them to be used in industry, oil and gas industry, as well as in small business 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 about the advantages and disadvantages of the method. Equipment for metal spraying from China? 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.

High-precision restoration of parts of various mechanisms, current conduction

CGN very effective in the repair of complex products, where precise restoration of geometric parameters is required, elimination of hidden defects, but at the same time with the preservation of all performance characteristics, as well as trade dress. That is 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. Prices for equipment for spraying metals? 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.

Additional Information:

Heat-resistant coatings are applied by the gas-dynamic method, which provide protection up to 1000-1100 degrees Celsius. The electrical conductivity averages 80-90% of the electrical conductivity of the bulk material. Corrosion resistance depends on the characteristics of the aggressive environment.

The operation of the DIMET equipment, developed and mass-produced by the “Obninsk Center for Powder Spraying” (OOO “OCPN”), is based on the effect of fixing metal particles, if they move at supersonic speed, on the surface when they collide with it, gas dynamic spraying metals DIMET. The technology makes it possible to apply metal coatings not only on metals, but also on glass, ceramics, stone, and concrete. By now, the DIMET technology makes it possible to apply coatings of aluminum, zinc, copper, tin, lead, babbits, nickel and apply them not only to metals, but also to glass, ceramics, stone, and concrete.

Plakart specialists produce gas-dynamic coatings for industrial equipment (for example, in the photo - anti-corrosion coating of a heat exchanger without dismantling). In addition, we supply turnkey cold gas-dynamic spraying installations (adjustment, service, training).

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: restoring the geometry of the product, strengthening and protecting the metal from corrosion, increasing 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.

In Browning's description of the invention, these problems are discussed but not resolved. exit from this provision opens up a spraying method in which the powder is not heated to a molten state. The idea of ​​the possibility of "cold welding" of small metal particles during their high-speed collision with a solid surface was expressed in the invention of Shestakov back in 1967. The proposal for cold welding of particles in a dynamic mode was not developed at that time.

Equipment for cold gas-dynamic spraying of metals? Because to implement the cold spraying regime, new proposals were needed for the design of the nozzle assembly.

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 the composition of the consumable material (powder) and changing the modes of its application, it is possible to obtain a homogeneous or composite coating with a solid or porous structure and its own functional task. This can be: restoring the geometry of the product, strengthening and protecting the metal from corrosion, increasing 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 the presentation. That is why this method is actively used in the military-industrial complex, the 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.

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. High pressure spraying, which uses helium, nitrogen or a mixture of gases. Powder material consumption is 4.5-13.5 kg/h.
2. Cold gas-dynamic spraying low pressure carried out with 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 spraying under high pressure.

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 to be 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.

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 and so on.). 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.