Do-it-yourself cold gas-dynamic spraying of metals equipment. The essence and objectives of gas-dynamic spraying. Principle of action, pros and cons of CGN

Metal spraying is a technology for changing the surface structure of a product in order to acquire certain qualities that increase performance characteristics. Processing is performed by applying a homogeneous metal layer to a part or mechanism. Special powder compositions are used as consumables, which are subjected to heat treatment and give them significant acceleration. Upon impact contact with the surface, particles are deposited on the plane.

The technology appeared at the beginning of the 20th century as an alternative to traditional methods of surface modification of metals. As methods of spraying metal products were studied and developed, a separate industry was formed - powder metallurgy. This is a technology for producing powders for making various products from them.

In modern industry, metal spraying is considered one of the most economical processing methods. Compared to bulk alloying, the technology makes it possible to obtain the necessary performance properties of the surface at lower costs.

Ultrasonic pressure treatment and ultrasonic hardening

The main technological parameters of ultrasonic hardening (USH) are the duration of exposure (t), ball diameter (dsh) or rounding radius of the working part of the tool (r), vibration amplitude (Ak), effective mass of the tool (Gin), longitudinal feed (s), number passes (i), the speed of movement of the part being hardened (v), the initial surface roughness (Ra) and the quality of the surface layer.
To improve the physical and mechanical properties of parts, finishing and strengthening treatment by surface plastic deformation (with a spherical or cylindrical tip) is used. In this case, the metal of the asperity protrusions moves in both directions from the point of contact with the deforming element. The height of the irregularities decreases, forming a new microrelief. To obtain the required surface roughness, it is necessary to apply the minimum necessary force to the deforming element, sufficient for plastic deformation to occur.

When rolling and rolling with roller and ball heads, mandrating, and pulling with smoothing broaches, the shape of non-rigid parts and parts of variable rigidity may be distorted. The application of ultrasonic vibrations (UV) to the deforming tool reduces the magnitude of the static load during plastic deformation of metals.

The installation diagram of the ultrasonic device (Fig. 1) includes an ultrasonic generator, a magnetostrictive transducer 5, a waveguide 3 and a deforming tip 2. The acoustic system is mounted in a movable housing 4, which can move along the axis of the fixed housing. The installation and regulation of the required radial force is carried out using a calibrated spring 7 and a screw 8. The tip 2 performs ultrasonic testing and with a small force P is pressed against the workpiece 1.

Rice. 1. Scheme of ultrasonic hardening with a spring-loaded ball or diamond tip

In practice, steel or carbide balls can be used as a tool, loosely or rigidly connected to the waveguide of the transducer, and in diamond burnishing, polished diamond crystals are used, sealed in steel holders. The radius of curvature of the working part of the diamond tip is 1. . . 4 mm and depends on the processing conditions, the material of the processed surface and the rigidity of the technological system. It has been established that under the influence of ultrasonic sound with an amplitude Ak = 10 μm, the rate of deformation of the surface layers increases 100 times and is accompanied by hardening. Serial equipment is used as equipment for performing ultrasonic hardening. The ultrasonic emitter is fixed in the tool holder (Fig. 2).

The main factors influencing the formation of a particle coating during thermal spraying

To begin with, we will simply list all the main factors that, according to the results of practical experience, influence the formation of coatings. We will divide these factors into five large independent groups (within individual groups, parameters may depend on each other):

1. Spraying parameters
2. Powder parameters
3. Substrate parameters
4. Parameters of movement of the spraying device relative to the sprayed part
5. Cooling parameters

Spraying parameters:

• Spraying atmosphere: air, vacuum or water (in the special case of underwater plasma spraying)
• Flame size and shape
• Flame temperature distribution
• Flame luminosity
• Thermal power of flame
• Composition of flame gases
• Flame gas speed
• Flame gas consumption
• Powder injection method (axial or radial)
• Powder gas speed and pressure
• Powder gas composition
• Powder gas consumption
• Powder mass flow

Powder parameters:

• Powder particle size distribution
• Chemical composition of simple particles (the parameter includes all physical properties of the material of simple particles, such as thermal conductivity, thermal conductivity, heat capacity, melting point, strength, brittleness, hardness and others)
• Phase composition, geometric distribution and phase size in agglomerate particles
• Chemical composition of individual phases in agglomerate particles
• Form of simple particles or agglomerates
• Porosity of simple particles or agglomerates
• Specific gravity of simple particles or agglomerates

Substrate parameters:

• Sprayed surface shape
• Type of surface roughness
• Surface roughness depth
• Presence of surface defects
• Presence and type of surface contamination
• Degree of surface contamination by foreign substances (for example, oil)
• Surface hardness
• Chemical and phase composition of the substrate (the parameter includes all physical properties of the material, such as thermal conductivity, thermal conductivity and heat capacity)
• Chemical composition of oxides on the surface
• Surface oxide thickness
• Size, weight and shape of the part
• Part temperature before spraying

Parameters of movement of the spraying device relative to the sprayed part:

• Relative speed of movement of the spraying device relative to the sprayed surface of the part
• Spray distance
• Spray angle
• Spray spot size
• Layer thickness per pass

Cooling parameters:

• Type of refrigerant: compressed air, water, solid carbon dioxide or liquid nitrogen
• Cooling method: general cooling of the entire part; local cooling combined with a spraying device; combined cooling (local plus general)
• Velocity and pressure of the refrigerant gas relative to the surface to be cooled (includes heat transfer coefficient)
• Presence or absence of crossing of spraying and cooling flows
• General cooling capacity
• Local cooling power
• Distance of the “cooling spot” from the “spray spot” for local cooling
• Relative location of sputtering spots and cooling spots as they move: the cooling spot “catches up” or “runs ahead” of the sputtering spots

Alternatives to galvanic chrome plating.

Excellent wear resistance drives the use of electroplated chrome plating in the aerospace and automotive industries. Hard chrome is used to protect surfaces from abrasive wear and to restore worn parts to their original size and shape.

Hexavalent chromium electrolyte plating technology has been used for the past 120 years.

However, the toxicity of chromium and chromium-containing waste has led to the development of new technologies for its application.

The most acceptable solutions to the problem are:

• thermal spraying;
• chemical vapor deposition (CVD);
• physical vapor deposition (PVD). Let's consider these options in more detail.

Metallization

During metallization, all three stages of coating formation are realized. The transfer of filler material particles into the liquid phase ensures, upon contact with the surface of the part, the formation of contact zones of significant area (Fig. 3.2).

The high heating temperature of the molten particles promotes the activation of the surface layer of the part and causes the development of chemical interaction. Such interaction over the contact area of ​​the liquid drop with the surface of the part leads to welding of filler material particles, which occurs through the formation of centers of setting. The more foci of setting, the higher the adhesion strength of the particles to the base. Welding of a particle of filler material occurs only when the critical temperature of chemical interaction (Tcr) is reached in the contact zone with the base. The absence of the required degree of heating leads to the appearance of areas of non-fusion and pores in the metallized layer.

Impact of Particle Velocity on Coating Quality and Efficiency

1. The coating particle has reached the minimum impact speed, which is necessary to excite the mechanism of interaction with the surface of the substrate (processed sample). This so-called “critical speed” affects the properties of the coating material.
2. As the impact velocity is higher than the critical velocity, the deformation and quality of particle adhesion increase.
3. If the impact velocity is too high (the "erosion rate"), more material is destroyed than added. No coating is formed.
4. To form a dense and well-formed coating, the particle impact velocity must be between the critical velocity and the erosion rate.

Rolling and rolling out surfaces

Similar to ultrasonic hardening treatment, the same equipment is used to perform finishing and hardening treatment of the external surfaces of parts by rolling, and the internal surfaces by rolling.

Rice. 2. Ultrasonic hardening of the shaft surface on a 16K20 screw-cutting lathe

Depending on the material of the part, the pressure on the roller is 5. . . 20 MN/m2 with a number of passes up to 4. Rolling provides a roughness of the treated surface Ra = 0.4. . . 0.05 µm. The rolling tool shown in Fig. 3, installed in the tool holder with shank 7.

Rice. 3. Hardening treatment of the outer surfaces of parts by rolling

Rolling of the surface to be treated is carried out by ball 2, which rests against the outer race of bearing 10, mounted on axis 9, and is kept from falling out by cap 8. Under the influence of rolling force, ball 2 is pressed out and moves quill 3 in the bore of housing 4, which compresses spring 5. With the help of Screw 6 adjusts the compression force of the spring. For rolling processing, the tool holder of a lathe with a rolling tool is brought until the ball comes into contact with the surface of the pre-processed part. Then, using the caliper transverse feed screw along the dial, a preload of 0.5 is created. . . 0.8 mm. Set the spindle speed to 1200...1500 min-1 and longitudinal feed 5 = 0.3. . .1.5 mm/rev. , turn on the machine and make 2-3 longitudinal passes to the right and left°. Spindle oil is used as a cutting fluid.

Balls and rollers for rolling (rolling) are made of hardened steel or hard alloy

Powder spraying

The sprayed powder enters the burner from above from the hopper through an opening, is accelerated by a flow of transport gas (a mixture of “oxygen - combustible gas”) and, at the exit from the nozzle, enters the flame, where it is heated. Entrained by a stream of hot gas, powder particles fall onto the sprayed surface. In powder burners, as in wire burners, the sprayed material is supplied to the flame and the resulting molten particles are dispersed using a jet of compressed air.

In most cases, acetylene is used as the flammable gas. You can also use propane and hydrogen. Propane is often used for spraying plastics.

Units in which the sprayed material is supplied in powder form include a gas-flame burner of the Rototec-80 type from the Swiss company Castolin-Eutectic (Fig. 2).

Rice. 2. Rototec-80 gas flame burner

The sprayed material with a particle size of up to 100 microns is poured into a special cone-shaped container. Structurally, the gas-flame burner is designed in such a way that during its operation the container with the powder is located in the upper part of the gas channel. Therefore, in addition to injection, the force of gravity plays a significant role in the uniform supply of powder to the heating area. The burner is made in a portable version. Case dimensions 50030080 mm. When spraying powder materials with different thermophysical properties, the burners provide regulation of the working gas mixture, which makes it possible to obtain high-quality coatings from both refractory (Al2O3 and TiO2) and low-melting (bronze, babbitt) materials.

Using this burner, it is possible to restore the geometric dimensions of the seats of large shafts for rolling and sliding bearings, main and connecting rod journals of internal combustion engine crankshafts, diesel vehicles and compressor units.

Spraying methods, equipment used

At the dawn of the development of technology, the processing of products was carried out using a burner nozzle and a conventional compressor, which provided heating of the consumable material and high-speed transfer to the deposited surface.
As technology developed, new methods for obtaining protective coatings were developed. The next stage of development was the use of electric arc equipment. The design of such a wire-type metallizer was developed in 1918. There are two types of spraying process:

1. Gas-dynamic. Processing is carried out with the smallest particles, the size of which does not exceed 150 microns.
2. Vacuum. The procedure takes place under conditions of reduced pressure. The formation of a protective layer occurs during the process of condensation of the sprayed material on the base surface.

Let's consider the main processing methods, as well as the features of the spraying equipment used.

Sputtering in magnetron installations

Magnetron vacuum metallization technology is based on the action of a diode gas discharge in crossed fields. During operation of the installation, gas ions are formed in the plasma of the glow charge, which affect the sprayed substance. The main elements of the magnetron system are:

• anode;
• cathode;
• magnetic node.

The function of the last element is to localize the plasma at the base of the sputtered substance - the cathode.
Any magnetic system consists of central and peripheral permanent magnets. The cathode is supplied with a constant voltage from a power source. Under the influence of current, the target is sputtered, provided that the charge remains consistently high throughout the entire procedure. Advantages of the magnetron method:

• high performance;
• accuracy of the chemical composition of the deposited substance;
• uniformity of coverage;
• no thermal effect on the workpiece being processed;
• possibility of using any metals and semiconductor materials.

Using installations, thin protective films are produced in a special gas environment. The sprayed material can be metals, semiconductors or dielectrics. The rate of layer formation depends on the current strength and working gas pressure.

Ion plasma sputtering

One of the types of vacuum deposition of metal onto a surface.
The method is the next stage in the development of thermal deposition technology, which is based on heating the starting materials to the boiling point with their further condensation on the workpieces. The schematic diagram of equipment for ion plasma planting includes the following elements:

• anode;
• target cathode;
• hot cathode;
• camera;
• workpiece

Installation algorithm:

1. A reduced pressure is created in the chamber.
2. Current is supplied to the thermionic cathode, which is an auxiliary source of electrons.
3. As a result of heating, thermionic emission occurs.
4. Inert gas is supplied to the chamber. Argon is the most popular.
5. A voltage arises between the anode and thermionic cathode, which initiates the formation of a plasma glow charge.
6. A powerful charge is applied to the cathode.
7. Positive ions affect the sputtered target material.
8. Sputtered atoms are deposited on the workpiece in the form of a thin coating.

Ion plasma deposition is used as decorative or protective coatings, which are characterized by high density and strength, as well as the absence of changes in the stereochemical composition.

To change the color of the product, reactive gases are added to the technological cycle: oxygen, acetylene, nitrogen or carbon dioxide.

Plasma spraying

One of the most effective is the diffusion metallization method. Features of the technological process:

1. The operating temperature of the plasma can reach 6000 ºC. This contributes to a high rate of deposition of the composition on the surface. The duration of the process is tenths of a second.
2. It is possible to change the structural composition of the workpiece surface. Together with the hot plasma, individual chemical elements can diffuse into the upper layers of the product.
3. The plasma jet is characterized by constant pressure and temperature. This has a positive effect on the quality of spraying.
4. Due to the short processing time, the workpiece is not exposed to harmful surface factors such as overheating or oxidation.

A spark, pulse or arc discharge is used as an energy source for plasma formation.

Laser deposition

Laser metal deposition is used to achieve the following goals:

• increasing the strength of the surface layer;
• restoration of product geometry;
• reducing the friction coefficient;
• protection against corrosion processes.

Unlike other metallization methods, the heat source is laser radiation energy. High focusing accuracy allows energy concentration to be concentrated exactly in the work area. This reduces the thermal effect on the workpiece, which avoids changes in the geometry of the product and makes it possible to spray virtually any material.

Due to the high cooling rate, structures with high hardness are formed in the surface layer of the metal, which increases the performance characteristics of the part.

Vacuum spraying

Vacuum metal deposition is an effective and universal method of surface metallization. Using this method you can process almost any product. During the technological cycle, a number of transformations occur with the material:

• evaporation;
• condensation;
• adsorption;
• crystallization.

The performance of the process depends on many factors: the structure of the workpiece, the type of material being applied, the flow rate of charged particles and many others.

Vacuum units differ in their operating principle. There are continuous, semi-continuous and batch equipment.

Chemical chromium plating method

Chemical reagents are used as an active component to implement such spraying. The classic composition includes chromium chloride, sodium, acetic acid, and water with a solution of sodium hydroxide. The spraying process is carried out at a temperature of about 80 °C. Work begins with preparing the material. Typically, chrome plating is used to treat metal surfaces, in particular steel. Before the operation itself, the material is subjected to a primary coating with a copper layer. Next, chemical chrome plating is carried out using a sandblasting machine connected to a compressor unit. After completing the procedure, the product is washed in clean water and dried.

Chemical vapor deposition (CVD).

The method allows you to obtain high-purity chromium on the surface of the product. Coating deposition occurs at temperatures above 1000˚C. Because of this, only products made of hard alloys or ceramics with high heat resistance can be coated.

The essence of the process: when a gas mixture is burned, it decomposes into active radicals, ions and electrons. Radicals and ions arrive at the surface to be coated, react and form a polycrystalline or amorphous structure. By adjusting the composition of the combustible mixture and the temperature of the process, it is possible to set the required characteristics of the coating.

Unique plasma-chemical deposition

(PECVD) made it possible to obtain amorphous and polycrystalline chromium films on the surface of the product at a process temperature lower than in the simple chemical vapor deposition (CVD) process.

The uniqueness of the coating also lies in the fact that it is possible to obtain a “sandwich” coating, each layer of which will have different characteristics. For example, the bottom layer of the coating will have a hardness of 5 GPa, and the top layer will have a hardness of a whopping 35 GPa.

The disadvantage of the method is its high cost.

Wires and cord materials

Continuous electrodes in the form of wires of various designs are used primarily for metallization of surfaces. Spraying the metal of a continuous electrode requires its mandatory melting and transition into a liquid state. When metallizing, wire materials with a diameter of 0.5-5.0 mm are used, which are divided into the following groups: 1) solid wire; 2) flux-cored wires with a metal sheath; 3) flux-cored wires with an organic sheath.

Solid wires, usually from pure metals or alloys based on them, are produced by drawing methods. This type of wire materials is most widely used in metallization. Wire preparation before spraying most often involves degreasing and etching. Organic contaminants are removed by degreasing; etching - oxide films. The compositions of the baths and processing modes are determined by the brand of wire. In many cases, wire abrasive blasting, electropolishing and other processing methods are effective.

Of the iron-based wires, carbon and low- and medium-alloy wires are most widely used for metallization. Carbon and low-alloy steels are most appropriately used for restoration repairs by flame spraying or electric arc metallization. Sprayed coatings have a fairly high hardness. Refurbished products are as worn as the original ones. For spraying, wire made from St3 steels is mainly used; U 7; 40X; 50HFA, etc. When spraying coatings from steel U 7, the microhardness of the metal fluctuates between 2100-7750 MPa. High-carbon steels and cast irons, when sprayed, form brittle coatings that are practically unsuitable for use.

Flux-cored wires with a metal sheath are promising for spraying composite coatings. Flux-cored wires are produced by rolling a strip into a tube with simultaneous filling of dispersed charge into the resulting cavity. With subsequent drawing of the filled tube, wire of various diameters is obtained. In this case, the ratio between the mass of the powder and the shell is easily adjusted. Various combinations are possible in the arrangement of the shell and powder (Fig. 3.30).

Cord materials are flux-cored wires with an organic shell, which are used mainly for flame spraying and less often for plasma spraying.

Rice. 3.30. Flux-cored wires (I-III) and cord materials with an organic sheath (IV).

The preparation of flux-cored wires before spraying is carried out by degreasing them by wiping them with strong solvents (gasoline, acetone, etc.) or by abrasive blasting.

Rods and tubular electrodes

Solid cross-section rods are made by casting. Powder rods are formed from crushed materials such as oxides and then sintered. Typically their diameter is 3-6 mm and length 500-600 mm.

Flame spraying and metallization

Heating of the filler material during flame spraying and metallization is carried out due to the heat released as a result of the combustion of various flammable gases (acetylene, propane-butane, natural gas, etc.) in an oxygen environment. Of the combustible gases, acetylene is most widely used, the combustion of which in a mixture with oxygen allows one to obtain a flame temperature of the order of 3100-3200 °C, which is 500-800 °C higher than the temperature of its substitutes ( Table 3.5 ).

Flame types

Depending on the ratio of combustible substance and oxygen, gas flames are divided into:

• oxidative - with excess oxygen;
• normal - with a parity ratio of combustible substance and oxygen;
• reducing - with an excess of flammable gas.

The type of gas flame used for spraying is selected depending on the chemical composition of the metal being sprayed ( Table 3.6 ).

Table 3.5. Thermodynamic characteristics of gas mixtures.

The flame power is selected depending on the size of the part. When spraying steel parts, a reducing (normal) or carburizing (with a slight excess of acetylene) flame is used. Before spraying begins, the part is heated to a temperature of 50-100 °C. During the spraying process, it is necessary to ensure that the surface of the sprayed part does not heat above 250 °C. Temperature can be controlled using heat sensitive pencils.

Based on the type of filler material, flame spraying and metallization are divided into:

• metallization with rod filler materials;
• powder coating.

Wire Sprayers

The first flame wire sprayer was developed in 1913 by M.U. Shoop. The rod filler material is directed through the central hole of the burner through the central opening of the burner into the high-temperature flame zone, where it is heated to the melting point. The resulting drop of liquid metal is sprayed from its end with compressed air and transferred in the form of small particles to the surface of the part ( Fig. 3.6 ).

Table 3.6. Characteristics of flame spraying.

Rice. 3.6. Diagram of a wire spray: 1 - air nozzle; 2 - gas nozzle; 3 - rod; 4 - guide tube.

Rods, wires and cord materials are used as rod filler material.

Rod materials are used in ceramic spraying. The rods are made from metal oxides or carbides with a liquid glass binder with a diameter of up to 8.0 mm. The content of solid particles in the rod can reach 95%. When the rod is heated, the binder burns out, and grains of the solid phase are fed to the surface of the product. The main disadvantage of using ceramics is the intermittency of the process, which affects the quality of the coating surface. Along with rod materials, tubular hollow rods filled with grain relit are also used.

The atomizer for bar materials has an additional air nozzle that directs air in a radial direction into the melting zone of the ceramic rod, where the axial velocity of the particles is low. “Bending” air crushes relatively large (100-160 microns) molten particles into smaller ones (20-40 microns) and directs them at an angle of 45-50° to the surface of the product. The spraying distance is 50 mm. The axial location of the sprayer and the short spraying distance made it possible to apply coatings to the inner surface of a pipe with a diameter of 100 mm. Wire for spraying is used with a diameter of 0.8 to 2.0 mm and is made of various materials (corrosion-resistant and carbon steels, brass, bronze, babbitt, Al, Cu, Mo, Zn, Sn, Pb, nickel and cobalt alloys basics). The productivity of spraying and metallization with wire from non-ferrous metals is up to 15 kg/h, from steel and alloys - up to 9 kg/h. Oxygen consumption - 50 l/min, acetylene or propane consumption - up to 20 l/min. Air pressure - 0.5 MPa.

With gas flame wire spraying, the resulting coating contains less oxides than with powder spraying. This is especially important for obtaining corrosion-resistant coatings with low porosity. To reduce the degree of oxidation of the filler material, the combustion chamber is brought closer to the nozzle outlet. However, the relatively low speed of particle movement during flame spraying with wire does not ensure the formation of a high-density coating.

In recent decades, along with wires, cord filler materials have become increasingly used. The strength and elasticity of flexible cords allows them to be used in the same way as wire and to apply coatings using wire-type gas-flame apparatus.

Cord materials consist of an organic binder that makes up the shell and a powder filler, including high-hard components and compounds that ensure the occurrence of exothermic reactions and the synthesis of new phases during the spraying process. This allows you to increase the adhesion and cohesive strength.

Cord materials use powder fillers based on self-fluxing alloys of the Ni(Co)-Cr-B-Si systems and in mixtures with tungsten carbide or oxides of aluminum, titanium, chromium, and zirconium. Cords are produced in diameters ranging from 4.0 and 7.0 mm and cast tungsten carbide grain sizes ranging from 0.1 to 2.5 mm, with special combinations of fine and coarse tungsten carbide used for specific wear applications. The uniform distribution of carbide grains in the powder cord ensures their most favorable location on the sprayed surface, which leads to increased wear resistance of the deposited layer ( Fig. 3.7 ).

The matrix of the deposited layer, which is a nickel self-fluxing alloy of the Ni-Cr-B-Si system, provides good wetting of carbide grains, has a low melting point (950-1050 ° C), has high fluidity and is highly resistant to acids, alkalis and other corrosive environments.

Rice. 3.7. Technology of manual gas-flame surfacing of cord material “Sfecord-HR”.

Powder Sprayer

Spraying with powders allows you to control the composition of the applied coatings within a wide range. Depending on the location of the powder supply to the burner and its transportation to the flame zone, gas-powder spraying is divided into two methods.

1. The powder from the feeder ( Fig. 3.8 ) enters the central channel of the burner, is captured by the transport gas and fed into an oxygen-acetylene flame, the jet of which is melted and directed to the surface of the part, forming a given coating layer.

Rice. 3.8. Scheme of gas-flame spraying with the introduction of powder into the flame zone by a transporting gas: 1 - nozzle; 2 - flame; 3 - coating; 4 - detail; 5 - oxygen and flammable gas; 6 - transport gas; 7 - sprayed powder

The powder jet is surrounded by a ring of flame. When mixing flame jets and gas-powder suspension, heat exchange occurs. The particles are heated to the melting point and transferred to the surface of the part.

1. Powder from the hopper ( Fig. 3.9 ) is fed from the outside of the nozzle into the flame zone, where its particles are melted and directed by a gas flow to the surface of the sprayed part.

Using the first method of spraying a transport gas, usually inert, to supply the powder makes it possible to reduce its oxidation, but the supply circuit and design of the gas burner become more complicated. The second method is characterized by greater simplicity of equipment and easier selection of the optimal mode.

The highest quality coatings are obtained by first spraying a sublayer with a thermosetting powder 0.05-0.15 mm thick, and then the main layer with a wear-resistant powder alloy 2 mm thick. The sublayer and the main layer are applied using the same spraying modes:

• oxygen pressure 0.35-0.45 MPa;
• acetylene pressure 0.03-0.05 MPa;
• oxygen consumption 960-1100 l/h;
• acetylene consumption 900-1000 l/h;
• the distance from the cut of the mouthpiece nozzle to the surface to be deposited is 160-200 mm;
• longitudinal feed 3-5 mm/rev;
• powder consumption 2.5-3 kg/h.

Rice. 3.9. Scheme of flame spraying with external powder input.

The flame spraying process can be carried out with simultaneous melting, which is only possible with a gas flame. Due to the intense uneven heating of the sprayed layer, the plasma jet does not provide a high-quality coating. Spraying with simultaneous melting is recommended to be performed in the following sequence:

• heat the entire part to a temperature of 250-300 °C;
• Spray layers 0.2-0.3 mm thick onto the surfaces to be restored to protect them from subsequent oxidation;
• heat the sprayed area of ​​the surface to the state of “fogging”, which is typical for the melting process;
• Apply a new one to the pre-melted layer, bringing it to the state of melting.

During the melting process, it is important to prevent overheating of the sprayed layer to the state of a liquid bath, and after melting, ensure slow cooling of the part (in sand, asbestos, oven). Overheating leads to metal dripping and the formation of pores, and rapid cooling leads to cracks and peeling. To restore parts using this method, it is most rational to use powder alloys PG-YUN-01, PG-YUN-03, PG-SRZ, PG-SR4. The thickness of the sprayed layer can reach up to 3 mm.

High speed spraying

High-speed gas flame spraying appeared in the early 80s of the last century and is characterized by gas flow speeds of up to 1000 m/s. The coating density reaches 99%. Increasing the speed of particles at a lower temperature made it possible to reduce the level of oxidation of the sprayed metal and increase the density of the powder coating. Powders of carbides, metal carbides, alloys based on Ni, Cu, etc. are used as the applied material. To increase the speed of particles, the flow rate of combustion products is increased by increasing the pressure in the combustion chamber to 1.0-1.5 MPa. In Fig. Figure 3.10 shows a diagram of a high-speed sprayer of the VSN system.

Rice. 3.10. Diagram of a high-speed powder sprayer: 1 - powder supply (axial); 2 — oxygen supply; 3 — fuel supply; 4 — powder supply (radial); 5 - trunk.

Rice. 3.11. Spray nozzle: a - cylindrical; b - expanding (Laval nozzle)

In the VSN powder sprayers of the first and second generations, a cylindrical nozzle was used ( Fig. 3.11, a ), and then a Laval nozzle began to be used in the design of the nozzle apparatus ( Fig. 3.11, b ).

For first generation systems, the pressure in the combustion chamber was 0.3-0.5 MPa, the particle speed was 450 m/s for powder mixtures of the WC-Co system with a granulation of 10-45 microns.
For second generation systems, the pressure in the combustion chamber increased to 0.6-1.0 MPa, which led to an increase in the speed of particle movement to 600-650 m/s. The powder consumption was 10 kg/h. In third-generation systems using expanding Laval profile nozzles, powder consumption reaches 18 kg/h. You may also be interested in the following articles:

• Combined method of metal protection
• Electroless nickel plating
• Protection of ferrous metals from corrosion
• Hot galvanizing of metals. What does Wikipedia say?
• Galvanizing of VGP pipes

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Metal protective coatings

Anodic metal coatings are metals whose electrochemical potential is lower than that of the materials being processed. On the contrary, it is higher for cathode ones.

Cathodic coatings prevent the penetration of aggressive media into the base metal due to the formation of a mechanical barrier. They better protect surfaces from negative influences, but only if they are undamaged.

Depending on the method of application, metal coatings are divided into the following types.

Electroplating

Galvanization is an electrochemical method of applying a metal protective coating to protect surfaces from corrosion and oxidation, improve their strength and wear resistance, and give an aesthetic appearance.

Galvanic coatings are used in aircraft and mechanical engineering, radio engineering, electronics, and construction.

Depending on the purpose of specific parts, protective, protective-decorative and special galvanic coatings are applied to them.

Protective ones are used to isolate metal parts from aggressive environments and prevent mechanical damage. Protective and decorative are designed to give parts an aesthetic appearance and protect them from destructive external influences.

Special galvanic coatings improve the characteristics of the treated surfaces, increasing their strength, wear resistance, electrical insulating properties, etc.

Thermal spraying

It represents the transfer of molten particles of material to the surface being treated by a gas or plasma stream. Coatings formed by this method are distinguished by heat and wear resistance, good anti-corrosion, anti-friction and extreme pressure properties, electrical insulating or electrical conductivity. The materials to be sprayed are wires, cords, powders made of metals, ceramics and metal-ceramics.

The following methods of thermal spraying are distinguished:

• Flame spraying: the simplest and most inexpensive method used to protect large surface areas from corrosion and restore the geometry of parts
• High-speed flame spraying: used to form dense cermet and metal coatings
• Detonation spraying: used for applying protective coatings and restoring small damaged surface areas
• Plasma spraying: used to create refractory ceramic coatings
• Electric arc metallization: for applying anti-corrosion metal coatings to large surface areas
• Spraying with fusion: used when there is no risk of parts deformation or it is justified

Immersion in melt

When using this method, the workpieces are dipped into molten metal (tin, zinc, aluminum, lead). Before immersion, the surfaces are treated with a mixture of ammonium chloride (52-56%), glycerin (5-6%) and chloride of the metal being coated. This allows you to protect the melt from oxidation, as well as remove oxide and salt films.

This method cannot be called economical, since the applied metal is consumed in large quantities. At the same time, the thickness of the coating is uneven, and it is not possible to apply the melt into narrow gaps and holes, for example, on threads.

Thermal diffusion coating

This coating, the material for which is zinc, provides high electrochemical protection of steel and ferrous metals.
It has high adhesion, resistance to corrosion, mechanical stress and deformation. The thermal diffusion coating layer has the same thickness even on parts of complex shapes and does not peel off during operation.

Cladding

The method is the application of metal using a thermomechanical method: by plastic deformation and strong compression. Most often, protective, contact or decorative coatings are created in this way on parts made of steel, aluminum, copper and their alloys.

Cladding is carried out through the process of hot rolling, pressing, extrusion, stamping or explosion welding.

Cold spray equipment

There are two options for protecting metals from the negative effects of external and working factors - alloying and sputtering with vacuum equipment. That is, atoms of chemical elements are added to the alloy, giving the products the required characteristics, or a protective coating is applied to the base surface.

Most often in the metallization industry they use the technology of applying galvanic coatings, use methods of immersing parts in the melt, use a vacuum environment in processing processes, and use magnetron equipment.

Sometimes detonation gas spraying is used, which accelerates particles to incredible speeds. Plasmatrons, electric arc metallization, gas-flame processing, and ion sputtering are widely used. Industry challenges dictate their conditions, and engineers have faced the need to create inexpensive, easy-to-use equipment that can use the properties of heated compressed air.

The concept of powder metallization appeared with the addition of finely dispersed ceramics or hard metal particles to the metal powder. Used to work with aluminum, nickel, copper.

The results of the experiments exceeded expectations, allowing us to solve the following problems:

• Heating of the compressed air in the chamber leads to an increase in pressure, which causes an increase in the rate of flow of deposits from the nozzle in installations.
• When metal particles are collected in a high-speed gas environment, they hit the substrate, soften and stick to it. And ceramic particles compact the resulting layer.
• The use of powder technology is suitable for metallization of ductile metals - copper, aluminum, nickel, zinc. After spraying, the products can be processed mechanically.

Thanks to the successful work of engineers, it was possible to create a portable device that allows metallization of coatings at all industrial enterprises and at home. Requirements for successful operation of the equipment are the presence of a compressor unit (or air network) with a compressed air pressure of five to six atmospheres and power supply.

The table below shows data for chrome plating aluminum at home. Before applying galvanic coating, it is necessary to “put” an intermediate metal layer on the part, and then spray aluminum.

Table 1. Chrome plating of aluminum

The use of advanced equipment for metallization of products makes it possible to solve technical issues related to increasing anti-corrosion, strength, and performance characteristics, as well as giving machines, parts and mechanisms the required properties for operation in difficult operating conditions.

Laser welding (2 videos)

Spraying process and working settings (24 photos)

Surfacing of surface layers

Surfacing of surface layers is the process of applying a coating to a workpiece using electric welding (in a protective gas environment, electroslag, under a layer of flux) or a plasmatron. Using surfacing, you can restore the dimensions of a worn part or apply a hardening coating to the workpiece. To obtain the coating, materials of different physical states are used: metal powder, cored wire, metal wire, pieces of chopped wire (grits), flexible cord materials.

Rice. 6. Metallization of steel structures in the field

Surfacing installations consist of two parts - electromechanical and energy. Electromechanical equipment ensures the application of a new strengthening layer of metal to the desired part of the workpiece, and energy equipment ensures the melting of the additive and its connection to the workpiece. Based on this, the composition of the equipment is determined. The general diagram of the energy part of the equipment for surfacing with a plasmatron is similar to that shown in Fig. 4. In many cases, metal-cutting machines are used as an electromechanical part. When surfacing on cylindrical surfaces, it can be a lathe; when hardening flat surfaces, it can be a milling machine, etc. When surfacing large flat surfaces, it is most advisable to use multi-electrode machines or strip electrodes.

When surfacing complex surfaces, manipulations with the gun, torch and holder are carried out manually, sometimes in semi-automatic mode and less often in automatic mode in the presence of additional devices or special manipulators. A general view of the installation for mechanized plasma powder surfacing of cylindrical parts is shown in Fig. 7, a, in Fig. 7, b - manual surfacing of a strengthening coating on the punch.

A mixture of flammable gas (propane, propane-butane, propylene, natural gas) is burned in the catalytic combustion chamber of the gun, generating a high-speed jet of combustion products. Coating material in the form of an alloy or composite powder is supplied here. It is heated in the combustion chamber and accelerated in the jet, forming a coating when the impact of particles on the substrate. The AC-HVAF gun, for example, accelerates powder particles up to speeds of 700. .800 m/s and produces a jet with a diameter of more than 16 mm and a length of over 250 mm, which is much longer than the spraying distance, which is usually 125. . .180 mm. The diameter of the flow of sprayed particles in the jet is usually 3. . .5 mm. The thickness of the surface-hardened metal layer, formed by one or several layers, can be different: usually 0.5. . .10 mm, but a thicker layer can be applied, and the mass of deposited metal is 3.5. . . 4.5 t, as when restoring rolling rolls.

Rice. 7. Installation diagram for surfacing with metal powder and performing manual surfacing: 1 - gearbox; 2 - clamping chuck; 3 - powder feeder; 4 - workpiece; 5 — spray burner (gun); 6 — supporting rollers; 7 — air preparation device; 8 — air receiver; 9 — cylinders with working gases; 10 - compressor; 11 — pipeline for supplying powder to the burner.

To restore parts with high wear, electric arc surfacing is used with a consumable electrode under a layer of flux using additional filler material. The layout of the installation is similar to that shown in Fig. 6 7, but the energy part includes electric welding equipment instead of gas supply equipment

Enterprises in the Moscow region

Moscow region, Sergiev Posad, Red Army Ave., 212B, bldg. 8

Experience (years): 11

Employees:
20
Area (m²):
1400
Machines:
30
Slotting Tool sharpening Countersinking holes Gear shaping Gear hobbing Gear grinding work Jig boring work Thread rolling Thread cutting Surface grinding work Reaming holes Locksmith work Automatic lathe work Electrical discharge machining Hardening ka HDTV Volume hardening Aluminizing Anodizing Gas-dynamic spraying Oxidation Cementation Laser cutting Plasma cutting Gas welding Gas-press welding Diffusion welding Arc-press welding Resistance welding Forge welding Robotic welding Manual arc welding Submerged arc welding Thermite welding Powder coating Working with stainless steel Ultrasonic testing

Moscow region, Istra, st. Panfilova, 11

Experience (years): 61

Employees:
500
Area (m²):
10000
Machines:
86
Horizontal boring work Countersinking of holes Jig boring work Cylindrical grinding work Mechanical processing on a machining center Thread rolling Thread cutting Surface grinding work Broaching Reaming holes Thread grinding work Drilling holes on CNC machines drilling holes on universal machine tools Locksmith work Turning on CNC machines Turning on universal machines Automatic lathes Milling on CNC machines Milling on universal machines Honing Slitting milling Electrical discharge machining Dispersion hardening High-frequency hardening Normalization Volume hardening Metal annealing Metal tempering Surface hardening Sorbitization Metal improvement Boroalitizing Gas dynamic spraying Gas thermal spraying Electroplating with copper (copper plating, copper plating) Electroplating with nickel (nickel plating) Electroplating with chromium (chrome plating) Electroplating with zinc (zinc plating, galvanizing) Carbonitration Nitrocarburizing Thermal diffusion galvanizing Metal etching Chemical phosphating Chromoalitizing Chromosiliconization Laser cutting ka Shaped cutting of pipes Rolling of sheet metal metal Profile rolling Rolling of rod metal Profile bending Bending of rod metal Pipe bending Argon (argon arc) welding Gas welding Gas press welding Contact welding Metal cutting Sheet stamping Metal perforation Metal punching Rolling Manufacturing of parts according to customer drawings Manufacturing of non-standard metal structures Laser engraving Aluminum processing Titanium processing Painting brush painting spray gun Powder painting Working with stainless steel Working with galvanized steel

Moscow region, Mytishchi district, Krasnaya Gorka village, st. Shkolnaya, 38

Experience (years): 6

Employees:
?
Area (m²):
?
Stankov:
?
Turning on CNC machines Disperse hardening High-frequency hardening Cryogenic treatment Normalization Volume hardening Metal annealing Metal tempering Surface hardening Sorbitization Metal improvement Nitriding Aluminizing Anodizing Boriding Boronating Gas dynamic spraying Gas thermal spraying Electroplating with copper (copper plating, copper plating) Electroplating with nickel (nickel plating) Electroplating chrome (chrome plating) Electroplating with zinc (zinc plating, galvanizing) Carbonitration Multi-layer plating with copper and nickel Multi-layer plating with copper, nickel and chromium Nitrocarburizing Oxidation Plating Siliconizing Thermal diffusion galvanizing Metal pickling Chemical phosphating Chromo-alitizing Chromo-silicining Cementation Cyanidation Electrochemical metal polishing Gas/gas-flame /oxygen cutting Waterjet cutting Laser cutting Plasma cutting Transverse cutting of coiled steel Longitudinal cutting of coiled steel Longitudinal-transverse cutting of coiled steel Cutting reinforcement Cutting on a band saw Cutting with press shears Cutting on guillotine shears Figured cutting of pipes Rolling of sheet metal Profile rolling Rolling of bar metal Pipe rolling 3D bending pro sheet metal bending dies Press bending Profile bending Rod metal bending Pipe bending Argon (argon arc) welding Gas welding Gas press welding Diffusion welding Arc press welding Contact welding Forge welding Laser welding Surfacing Soldering Semi-automatic arc welding Robotic welding Manual arc welding Reinforcement welding Explosion welding Semi-automatic welding flux layer Friction welding Welding pipes Thermite welding Ultrasonic welding Chemical welding Cold welding Electron beam welding Drawing Metal cutting Forging Sheet stamping Volume stamping Metal perforation Straightening of flat rolled metal Metal pressing Metal punching Metal rolling Rolling-drawing Rolling-pressing Punching Rolling Metal cutting on a coordinate-punching press Khudozhe natural forging Visually -measurement control Manufacture of parts according to customer samples Manufacture of parts according to customer drawings Manufacture of non-standard metal structures Manufacture of standard metal structures Inspection by penetrating substances Laser engraving Magnetic particle testing Plasma marking Aluminum processing Tumbling drum processing Titanium processing Rewinding of metal rolls Sandblasting Brush painting Spray gun painting Powder forged painting Working with reinforcement Working with stainless steel Working with galvanized steel Development of 3D models from drawings Ultrasonic thickness gauging Ultrasonic testing Chemical analysis

Gas dynamic spraying

The main goal of gas-dynamic metal spraying is to impart certain properties to the surfaces of workpieces. This procedure is performed not only for metal workpieces, but also for other materials. It is aimed at increasing strength characteristics, electrical and thermal conductivity. This technology provides protection against corrosion and restores geometric dimensions. Enterprises providing gas-dynamic metal spraying services in Moscow

, cope with this task perfectly, because they have high-tech equipment at their disposal.

In most cases, surfaces are metallized, and the applied coatings have excellent adhesive properties. The adhesion to the base is as reliable as possible, and the products gain additional strength. Only metal powders or substances containing, in addition to metal, also a ceramic component in certain dosages can be sprayed. This significantly reduces the cost of the powder coating formation technique and does not affect its characteristics. The essence of the cold gas-dynamic spraying technique is to apply and fix solid metal particles or mixtures of materials on the surface of elements. Their size is 0.01-50 microns. They accelerate to the required speed in air, ozone or helium. Such material is called powder.

These are aluminum particles, nickel particles, combinations of aluminum with zinc. The medium used to mix the material can be hot or cold. In the first case, the maximum heating is 700 degrees. When interacting with the surface of the product, a lamellar transformation takes place, the kinematic energy is transformed into adhesive and thermal energy. Due to this, a durable surface layer is formed. The powder is applied not only to metal surfaces, but also to concrete, glass, ceramic, and stone. This significantly expands the scope of use of the technique for forming surfaces with specific properties.

Gas dynamic spraying can be high or low - this depends on the pressure level. In the first case, the working medium that moves the powder is nitrogen and helium. Moving metal particles have a pressure of over 15 atm. In the second case, compressed air is used, supplied at a pressure of no more than 10 atm. The differences between these types also lie in the heating power and the consumption of the working medium. Spraying is performed in several stages, including:

1. Preparing the surface for applying powder (using a mechanical or abrasive method).
2. Heating the working environment to the required temperature.
3. Supply of heated gas into a special nozzle under the required pressure (gas is supplied along with the powder).
4. The powder acquires enormous speed and comes into contact with the surface of the product.

The cost of gas-dynamic metal spraying services in the Moscow region is quite affordable.

Adhesive coating methods

Adhesive (galvanic coatings) are obtained by deposition of the required metal onto the surface of a part from an electrolyte solution with or without the application of electric current. Galvanic processes have a number of advantages:

• allow the application of thin coatings of uniform thickness from 0.05 to 0.5 mm with varying hardness and wear resistance;
• do not deteriorate the structure of the base metal, since it remains practically cold during the build-up process;
• allow you to simultaneously process a large group of parts.

At the same time, these methods have a number of disadvantages, such as significant complexity and a large amount of work when performing technological processes for restoring parts, low speed of electrolytic deposition, reduced fatigue resistance of parts, and environmental pollution from production waste. Galvanic coatings are distinguished by the adhesive nature of their connection with the base. This determines their low adhesion strength to the surface of the part.

The most widely used processes are chrome plating and iron plating, nickel plating, used for the external and internal surfaces of parts with wear not exceeding 0.2-0.5 mm, high surface hardness and with non-rigid requirements for the adhesion strength of the coating to the base metal.

You may also be interested in the following articles:

• Steel production in Russia
• Factors affecting the uniformity of galvanic coatings
• Lead and its electrolytes
• Flushing or drainage during galvanizing process
• Research on hot-dip galvanizing baths

Materials and equipment used

Chemical metallization, as mentioned above, can be done with your own hands and in a home workshop. Moreover, products that are small in size and simple in shape are processed using this method even without the use of special equipment. If you have such equipment at your disposal, then you can chemically apply a layer of metal even to large parts of complex configurations.

When performing this procedure yourself, you should be extremely careful, as this involves using chemicals that are hazardous to health. If you properly prepare the equipment and materials for performing chemical metallization, then you can obtain coatings on various products with your own hands at home, the quality of which is practically no different from those formed at the factory.

Reagents for chemical metallization

The kit for chemical metallization must contain reagents that have the properties of an activator and a reducer. To perform this procedure, you will also need a primer, which is applied to the surface to be treated, and a varnish that protects the finished coating from the negative influence of external factors. To apply the final varnish coating, you should choose a material that has high hardness and wear resistance.

To color the applied metal layer in the desired color, you can use a special color toner. The primer mentioned above is necessary in order to improve the adhesion of the applied metal layer to the material from which the product being processed is made. The result of do-it-yourself chemical metallization may not always be of high quality. However, the applied coating can be removed using special removing solutions.

The chemical metallization unit is designed for coating any hard surface

Vacuum spraying method

In this case, we are talking about a group of methods that involve the formation of thin films in a vacuum under the influence of direct vapor condensation. The technology is implemented in different ways, including through thermal effects, evaporation by electron and laser beams. Vacuum deposition is used to improve the technical qualities of parts, equipment and tools. For example, such processing allows the formation of special “working” coatings that can increase electrical conductivity, insulating properties, wear resistance and corrosion protection.

The technology is also used to create decorative coatings. In this case, the technology can be used in operations that require high precision. For example, vacuum deposition is used in the manufacture of gold-plated watches, to give an aesthetic appearance to eyeglass frames, etc.

Scope of application of CGN

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

Elimination of defects, restoration of surfaces and sealing

All of this is work that even small businesses can do. For example, in small workshops you can repair parts made of light alloys (parts of an automobile structure, for example), primarily aluminum and aluminum-magnesium. Moreover, defects that arose both during the production process and during operation are easily eliminated.

And the absence of strong heating and 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 restoring sealing (when the use of liquid sealants is impossible) in pipelines, heat exchangers or vessels for working gases and liquids.

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

CGN

very effective in repairing complex products that require precise restoration of geometric parameters, elimination of hidden defects, but at the same time maintaining all operational characteristics, as well as presentation. That is why this method is actively used in the military-industrial complex, railway and aviation industries, agriculture, gas pumping, etc.

You cannot do without this technology in creating contact pads. Prices for metal spraying equipment? 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 copper plating processes, creating power current-carrying networks, applying current leads, making sublayers for soldering, etc.

Anti-corrosion treatment and elimination of deep defects

Spraying a so-called anti-friction coating is a highly effective way to get rid of local damage (deep chips, burrs, scratches). This allows you to avoid the procedure of complete refilling or even replacing the product, which, of course, is not economically profitable.

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 DYMET ®

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 using the gas-dynamic method, which provide protection up to 1000-1100 degrees Celsius. Electrical conductivity is on average 80-90% of the electrical conductivity of the bulk material. Corrosion resistance depends on the characteristics of the aggressive environment.

The operation of DYMET equipment, developed and mass-produced by the Obninsk Powder Spraying Center (OTsPN LLC), is based on the effect of fixing metal particles, if they move at supersonic speed, on the surface upon impact with it, gas-dynamic spraying of metals DYMET. The technology makes it possible to apply metal coatings not only to metals, but also to glass, ceramics, stone, and concrete. To date, DYMET technology makes it possible to apply coatings of aluminum, zinc, copper, tin, lead, babbitt, nickel and apply them not only to metals, but also to glass, ceramics, stone, and concrete.

Plakart specialists produce coatings using the gas-dynamic method 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 plants (setup, service, training).

Depending on the composition of the consumable material (powder) and changes in its application modes, you can obtain a homogeneous or composite coating with a solid or porous structure and its own functional purpose. This can be: restoration of 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 exposure to chemically active environments, high thermal loads, etc.

In the description of Browning's invention, these problems are discussed, but not resolved. A way out of this situation is the 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 high-speed collision with a hard surface was expressed in Shestakov’s invention 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 mode, new proposals for the design of the nozzle assembly were needed.

Gas-dynamic spraying is a technology for applying a metal coating to various materials and products for protective or decorative purposes, in which the formation of a surface layer occurs due to the impact of particles of the applied substance on the surface of the coated workpieces. Gas-dynamic spraying can be cold (CGN) and pulsed (IGN). In the first case, the particles are not heated, and their acceleration is ensured using a supersonic gas flow. In the second, average heating and acceleration of particles occurs by a series of shock waves of a fixed frequency.

Powder mixture coatings: molybdenum + self-fluxing alloy

Along with nickel, cobalt and copper, molybdenum is one of the most “grateful” metals for thermal spraying due to the fact that its oxides are reduced at a relatively high partial pressure of oxygen (the metal is well cleared of oxides even in a neutral acetylene-oxygen flame). In addition, molybdenum is very ductile at high temperatures, which is critical for sealing the coating during spraying.

An important feature of molybdenum is its high melting point (2620°C), which, on the one hand, requires sputtering devices with a high flame temperature, but, on the other hand, provides very high thermal energy to the molten molybdenum particles. This energy is released in the forming coating as the particles cool and leads to heating of neighboring particles, as well as the surface of the substrate, which, if used wisely, can provide a significant increase in bond strength

Molybdenum coatings have advantages when used for friction pairs (excellent anti-scuff and anti-friction properties). Currently, in the global automotive industry, chrome-plated piston rings are being replaced by rings with plasma-sprayed molybdenum coatings, which have a longer service life. Despite the incomparably higher price of molybdenum coatings, their implementation in this area turns out to be economically profitable.

There are two main types of molybdenum coatings: pure molybdenum coatings (atmospheric plasma spray powder or flame spray wire) and coatings made from a powder mixture of molybdenum and self-fluxing nickel alloy. Gas-flame spraying with molybdenum wire is an older, cheaper and more common process, but combined plasma-sprayed coatings of molybdenum plus a self-fluxing alloy have significantly better quality.

Let's consider the mechanism of operation of this combined coating:

As is known, the melting point of self-fluxing alloys is about 1050°C, while molybdenum melts at 2620°C. When such a powder mixture is heated in a flame to a temperature sufficient to melt molybdenum particles, highly superheated droplets of a self-fluxing alloy melt are automatically formed. When such droplets land on the surface of a substrate, they must have enough energy to reduce the oxide films, form a metallurgical bond with the substrate material, and spread over the surface before they crystallize. Due to the high melting point, molybdenum particles crystallize immediately,

arriving at the substrate, the function of these frozen particles is no less important: the cooling solid particles of molybdenum maintain the particles of the self-fluxing alloy longer in the molten state and create mechanical obstacles to the formation of a continuous film of the melt, which is absolutely necessary to reduce shrinkage tensile stresses in the coating (see Fig. previous chapter).

Thus, the following fundamental advantage of a two-phase coating from a mixture of molybdenum powders and a self-fluxing alloy is obtained: When thermally spraying molybdenum powders or a self-fluxing alloy separately onto a cold substrate, it is impossible to obtain dense coatings, whereas this is possible from a mixture of them! Figures 47 and 48 show electron micrographs of a transverse section of a Mo + NiCrBSi coating on a titanium substrate obtained by atmospheric plasma spraying (single-cathode plasmatron A60 from Thermico GmbH).

The light phase is molybdenum, the darker phase is a self-fluxing alloy.

Rice. 47. Mo + NiCrBSi coating on titanium substrate.

Rice. 48. Enlarged fragment of the same coating.

To compare the porosity and quality of bonding with the substrate, we present an optical micrograph of a plasma-sprayed coating of pure molybdenum based on the results (Figure 49):

Rice. 49. Pure molybdenum coating.

Individual processing

Each individual material is processed individually.

Material processing requires individual adjustment of gas temperature and pressure. The combination of these two physical parameters determines the particle speed and the quality of the coating. The range of optimal spray velocity, limited by the critical velocity and the erosion rate, is called the deposition range. Within this range, the quality of coating application is influenced by parameters.

Candidates of physical and mathematical sciences O. KLYUEV and A. KASHIRIN.

When the first metal tools appeared, it turned out that, although hard and durable, they often deteriorated under the influence of moisture. As time passed, people created mechanisms and machines, and the more advanced they became, the more difficult conditions their metal parts had to work in. Vibrations and alternating loads, enormous temperatures, radioactive radiation, aggressive chemical environments - this is not a complete list of “tests” to which they are subjected. Over time, people have learned to protect metal from corrosion, wear and other phenomena that shorten the service life of parts. Essentially, 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 applying them are varied: some are common and relatively uncomplicated, others are very subtle, allowing the creation of coatings with unique properties. And restless engineers continue to invent new coatings and come up with ways to obtain them. The fate of these inventions can be happy if the coating is much superior to its predecessors in useful properties or if the technology provides a significant economic effect. The development of physicists from Obninsk combined both of these conditions.

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

Diagram (above) and general view (below) of an apparatus for spraying metal coatings.

Using the device, you can apply coatings in any room and even in the field.

A negative pressure zone appears behind the critical section of the nozzle, and 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 - crack in an automatic transmission; b - cavity in the cylinder head.

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

TEMPERATURE PLUS SPEED

Of the methods for metallizing surfaces in modern technology, the most commonly used are galvanic deposition and immersion in a 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 applied metal is melted, sprayed into tiny droplets and transferred to a substrate with a gas stream.

Metal is melted with gas torches, electric arcs, 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 flame spraying process, a metal rod, wire or powder is melted and sprayed in the flame of a burner operating on a mixture of oxygen and flammable 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 generated by plasmatrons of various designs is used to heat and spray the material. Detonation gas spraying occurs as a result of an explosion that accelerates metal particles to enormous speeds.

In all cases, 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 small compared to thermal energy, and the nature of their connection with the substrate and among themselves is determined by thermal processes: melting, crystallization, diffusion, phase transformations, etc. Coatings are usually characterized by good adhesive strength to the substrate (adhesion) and, unfortunately, low uniformity, since the spread of parameters across the cross section of the gas flow is large.

Coatings created using 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, it can “lead.” Finally, the use of flammable gases and high temperatures in the work area complicate measures to ensure worker safety.

The detonation-gas method stands somewhat apart. During an explosion, the particle speed 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 the stability of the result is almost impossible to guarantee.

SPEED PLUS TEMPERATURE

The desire to create more advanced technology has been around for a long time. 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, coatings should be formed mainly due to the kinetic energy of metal particles (the particles should not be allowed to melt: this will prevent heating of the part and oxidation of the substrate and coating particles), and, secondly, the particles should acquire high speed not due to explosion energy, as in the detonation-gas method, but in a jet of compressed gas. This method was 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 gas flow speed in a supersonic nozzle is proportional to the speed of sound in the corresponding gas. In light gases (hydrogen was not considered due to 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 kinetic energy sufficient to attach to the target. It was believed that the use of heavier gases, including air, was doomed to failure.

The work of experimental sputtering installations gave good results: 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 using light gases would inevitably be expensive and could only be used in enterprises that produce high-tech products (only there there are lines with compressed helium). And compressed air lines are available in almost every workshop, every car service center, and repair shops.

Numerous experiments with compressed air seemed to confirm the worst expectations of the developers. However, intensive search allowed us to find 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 was 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 therefore the flow rate from the nozzle also increases. Metal particles that have gained enormous speed in a gas stream are softened when they hit the substrate and welded to it. Ceramic particles play the role of microscopic sledgehammers - they transfer their kinetic energy to the underlying layers, compacting them, reducing the porosity of the coating.

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

THE MAIN CONDITION IS SIMPLICITY AND RELIABILITY

The efforts of technologists will be in vain if 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 is a supersonic nozzle and a small-sized electric compressed air heater capable of raising 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 light gas systems. We are talking about introducing sprayed powder into a gas stream. 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 a high-pressure area. Purely technical difficulties were aggravated 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 coating application modes. In the design of the apparatus with an air jet, engineers used 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, its speed in a bottleneck increases, and the static pressure drops and may even be below atmospheric pressure. The channel through which the powder came from the feeder was located in just 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:

to operate the device, you only need an electrical network and an air line or a compressor providing a compressed air pressure of 5-6 atm and a flow of 0.5 m 3 /min;

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

coatings have high adhesion (40-100 N/mm 2) 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 a sprayer itself weighing 1.3 kg, which the operator holds in his hand or secures in a 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.

We also had to work hard to create consumables. Industrially produced powders have too large particle sizes (about 100 microns). A technology has been developed that makes it possible to obtain powders with grains of 20-50 microns in size.

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 during repair work, when it is necessary to restore areas of products, for example, to repair a crack or a sink. Thanks to the low temperatures of the process, it is easy to restore thin-walled products that cannot be repaired in any other 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 parts of complex shapes, such as gearbox housings, engine cylinder blocks, etc.

Sputtering devices are already used in the aerospace and electrical industries, at nuclear power facilities and in agriculture, in auto repair plants and in foundries.

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

Restoration of worn or damaged surface areas.

Using spraying, parts of gearboxes, pumps, compressors, lost wax casting molds, and molds for the production of plastic packaging that are damaged during operation are restored. The new method has become a great help for workers at auto repair plants. Now, literally “on their knees,” they repair 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 coatings makes it possible to eliminate leaks in pipelines and vessels when sealing compounds cannot be used. The technology is suitable for repairing containers operating under pressure or at high and low temperatures: heat exchangers, car radiators, air conditioners.

Application of electrically conductive coatings.

Sputtering makes it possible to apply copper and aluminum films to a metal or ceramic surface. In particular, the method is more cost-effective than traditional methods for copper plating of current-carrying busbars, galvanizing of contact pads on grounding elements, etc.

Anti-corrosion protection.

Films made of aluminum and zinc protect surfaces from corrosion better than paint and varnish 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. By 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.

Babbitt liners are usually used in plain bearings. Over time, they wear out, the gap between the shaft and the bushing increases and the lubricant layer is damaged. Traditional repair technology requires either replacing the liner or welding defects. And spraying allows you to restore the liners. In this case, ceramics cannot be used to compact the layer of sprayed metal. Solid inclusions will cause the bearing to fail within a few minutes after the start of operation, and the surfaces of both the bushings and shaft will be damaged. I had to use a special design nozzle. It allows the coating of pure babbitt to be applied in the so-called thermokinetic mode. Powder particles immediately beyond the critical section of the nozzle are accelerated by a supersonic air flow, then the flow speed 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 are deformed, partially melt and adhere well to the underlying layer.

NOTE FOR SPECIALISTS

Literature

Kashirin A.I., Klyuev O.F., Buzdygar T.V. Device for gas-dynamic application of coatings from powder materials.

RF patent for invention No. 2100474. 1996, MKI6 S 23 S 4/00, publ. 12/27/97. Bulletin No. 36.

Kashirin A.I., Klyuev O.F., Shkodkin A.V. Method for producing coatings.

RF patent for invention No. 2183695. 2000, MKI7 C 23 C 24/04, publ. 06.20.02. Bull. No. 17.

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

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By the way, Obninsk engineers have already developed several modifications of DIMET installations. Considering the wide demand for this equipment, both manual and automated cold gas dynamic spraying devices are now being 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. To operate the complex (in addition to the material for spraying), only compressed air is required (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 let's talk about the advantages and disadvantages of the method. Metal spraying equipment from China? Firstly, unlike the gas-thermal method, CGN can be effectively used at normal pressure, in any temperature range and humidity level.

Secondly, it is absolutely environmentally safe. Thirdly, due to its high speed, it can also be used for abrasive cleaning of surfaces. Well, the only drawback of the technology is the ability to apply coatings only from relatively ductile metals, such as copper, aluminum, zinc, nickel, etc.

Powder coating methods

1. Electrostatic spraying method is a commonly used powder coating method. The paint particle adheres to the surface due to electrostatic interaction. Powder that does not stick during the painting process can be used again: there is special equipment in the painting booth to collect it. 2. Another method of applying powder paint is a directed air flow (fluidized bed). The particles are evenly distributed over the product to be painted, preheated in the chamber. The more accurately the optimal heating temperature is determined, the better the quality of the coating. Automatic application of powder paint in a “fluidized bed” is used in Moscow in conveyor production. The method was developed for thermoplastic paints, since the coating is quite thick. Meshes or large flat products are painted in this way. Air is supplied under pressure into a bath with a porous bottom, resulting in the formation of a fluidized layer of paint. The products to be painted are heated to a temperature exceeding the melting point of the coloring material itself. The exposure time and temperature determine the thickness of the coating. If the product is large, it accumulates a sufficient amount of heat for the coating to complete the curing process. If this does not happen, for example, when painting metal-intensive equipment, the product is sent to a polymerization chamber for post-curing. Advantages of the method: obtaining a thick-layer coating in just one application cycle. 3. The third method of applying powder paint is the use of an open flame (flame spray). Powder coating is applied with a gun equipped with a propane torch. When they enter the burner flame, the particles melt and appear semi-liquid on the surface to be painted. The product itself does not need to be preheated. The flame dyeing method is used to create thermoplastic coatings. Paint passed through burning propane forms a durable layer on the surface. Since direct heating of the painted product does not occur, the method can be used not only for metal, but also for rubber, stone, and composites. It is successfully used for large-sized or permanently fixed objects.

Physical vapor deposition (PVD).

PVD is vacuum deposition. The coating is applied under high pressure in a vacuum using ion bombardment. Essentially, the chromium simply condenses onto the surface of the product, forming a coating. The vacuum allows the operating temperature of the process to be reduced.

The coating is relatively cheap. Widely used around the world to give a decorative appearance to watches, cases, needles, etc.

The downside is the low available thickness. As a rule, it is applied up to 2-3 microns. Accordingly, the coating will not have wear resistance. Corrosion resistance is moderate, but many times lower than the resistance of galvanic hard chromium.

List of used literature

1. Borisov Yu.S. Gas thermal coatings from powder materials / Yu.S. Borisov, Yu.A. Kharlamov. – Kyiv: Naukova Dumka, 1987. – 210 p. 2. Vityaz P.A. Theory and practice of flame spraying / P.A. Vityaz, V.S. Ivashko, E.D. Manuilo. – Minsk: Science and technology, 1993. – 295 p. 3. Kudinov V.V. Plasma application of refractory coatings / V.V. Kudinov, V.M. Ivanov. – M.: Mechanical Engineering, 1981. – 192 p. 4. Rykalin N.N. Physical and chemical problems of joining dissimilar materials / N.N. Rykalin, M.X. Shorshorov, Yu.L. Krasulin. // Inorg. materials. – 1965. – T.1. – P. 29 – 36. 5. Terekhov D.Yu. Method of surface preparation before thermal spraying / D.Yu. Terekhov, B.M. Soloviev // Copyright certificate of the USSR No. 1638198 AI С23С 4/02 08/30/91 Bull. No. 32. – All-Union Scientific and Production Association for the Restoration of Parts “Remdetal”. 6. Nadolsky V.O. Method for preparing the surface of parts / V.O. Nadolsky, A.N. Navoznov // Copyright certificate of the USSR No. 1758082 AI S23S 4/02 08/30/92. Bull. No. 32. 7. Medvedev Yu.A. On the influence of roughness and the degree of hardening on the adhesion strength of plasma coatings / Yu.A. Medvedev, I.A. Morozov // Physics and chemistry of materials processing. – 1975. – No. 4. – pp. 27-30. 8. Popovkin B.A. Progressive technology and equipment for shot blasting metal / B.A. Popovkin // Technology, organization of production and management. – 1978. – No. 10. – pp. 31-35. 9. Ivashko V.S. Adhesion strength of coatings made of self-fluxing hard alloys / V.S. Ivashko // Mechanical Engineering. – 1979. – Issue. 2. – pp. 103-105. 10. Kudinov V.V. Production of coatings by high-temperature spraying / V.V. Kudinov., L.K. Druzhinin. – M.: MIR, 1973. – 85 p. 11. Kupriyanov I. L. Gas-thermal coatings with increased adhesion strength / I. L. Kupriyanov, M. A. Geller. – Minsk: Science and technology, 1990. – 176 p. 12. Masino M.A. Organization of restoration of automobile parts / M.A. Masino. – M.: Transport, 1981. – 176 p.

General information about metallization technologies

Among modern methods of surface metallization, galvanic deposition and immersion in melts are most often used. Traditional technology also involves vacuum deposition processing, which has its own classifications depending on the active media used. One way or another, any metal spraying involves processing the base of the material in order to obtain certain protective qualities. This may be the formation of an anti-corrosion layer, restoration of a lost structure, or repair of operational wear.

In this case, the working surface itself is in most cases subjected to heat treatment. Before applying metal particles, it is melted by torches, inductors or through exposure to low-temperature plasma. In this way, a base with optimal physical and chemical properties is prepared, on which metals are subsequently sprayed in powder form

It is important to note that the main material can be the same metal, glass, plastics or some types of wood and stones