The load-bearing surface of a part sometimes requires modification: changes in the structure or properties of mechanical and physical parameters. This transformation can be carried out using plasma spraying. The process is a type of diffusion in which metallization of the outer layer of the product occurs. To carry out such processing, special equipment is used that is capable of converting metal particles into plasma and transferring it to the object with high precision.

The properties of coatings obtained by this method are of high quality. They have good adhesion to the base and practically form a single whole with the latter. The versatility of the method lies in the fact that absolutely any metals can be applied, as well as other materials, such as polymers.

It is possible to obtain spraying using the plasma transfer of particles only in the conditions of production workshops in factories and factories.

The essence of the plasma spraying process is that a dosed amount of metal particles is fed into a plasma jet, which has ultra-high temperatures and is directed at the object being processed. The latter melt and, carried away by the jet, settle on the surface of the part. Plasma spraying is used in the following cases:

1. Creating a protective layer on the product. This can be mechanical reinforcement, when a stronger metal is applied to a weaker base. Using diffusion metallization, it is also possible to increase the resistance of a part to corrosion by applying a film of oxides or metals that are less susceptible to oxidation.
2. Restoration of worn parts. In this case, due to a new layer of coating, surface destruction defects can be removed to give the product its original condition. The coating material used here is a metal identical to the base material.

Plasma spraying differs from other types of spraying in a number of features:

1. Due to the fact that plasma acts on the original base using ultra-high temperatures (5000–6000 degrees Celsius), the process proceeds in an accelerated mode. Sometimes fractions of seconds are enough to obtain the desired spray thickness.
2. Diffusion metallization allows you to apply both a monolayer to the surface and do a combined deposition. Using a plasma jet, it is possible to supplement the diffusing metal with gas elements necessary to saturate the layer with elementary particles of the desired chemical elements.
3. With plasma spraying there is practically no effect of additional oxidation of the base metal. This is due to the fact that the reaction occurs in an environment of inert gases without the involvement of oxygen.
4. The final coating is of high quality due to ideal homogeneity and uniform penetration of atoms of the sprayed metal into the base layer.

Using plasma-type diffusion metallization, it is possible to obtain layers with thicknesses ranging from several millimeters to microns.

Technology and spraying process

When using gas plasma spraying of metals, the basis of the working gaseous medium is the inert gases nitrogen or argon. Additionally, as required by the technological process, hydrogen can be added to the main gases. An arc occurs during operation between the cathode, which is an electrode in the form of a pointed rod inside the burner, and the anode, which is a water-cooled copper nozzle. It heats the working gas to the required temperature, which acquires the state of a plasma jet.

At the same time, metal material in powder form is fed into the nozzle. This metal, under the influence of plasma, is transformed into a substance with a high ability to penetrate into the surface layer of the workpiece. The melting material sprayed under pressure settles on the base.

Modern plasma torches have an efficiency of 50–70%. They allow you to work with any metals, including refractory alloys. Plasma spraying is a fully controlled process that allows you to regulate the plasma supply speed, power and shape of the jet.

In the case of restoring the shape of a part by plasma spraying, the technological process has the following stages:

1. Preparation of sprayed material. The essence of the process is to dry the powder in special cabinets at a temperature of 150–200 degrees Celsius. If necessary, the powder is also sifted through a sieve to obtain granules of uniform size.
2. Preparing the substrate or base. At this stage, all foreign inclusions are removed from the surface of the part. These can be oxides or various contaminants with oily substances. For better adhesion, the base can be subjected to an additional roughening process. If there are areas on the product that should not be sprayed, they are covered with special screens.
3. and operations for final processing of the resulting surface.

The sprayed material can reach the substrate in a solid state, in a plastic form, or in a liquid form. This is determined by the technological process mode.

Equipment used

The standard plasma spraying installation kit includes:

1. Electrical power source. Its purpose is to power the high-voltage discharge formation circuit and all systems.
2. Discharge formation block. Depending on the design of the circuit, it can generate spark discharges, pulsed high-frequency voltages, or a continuous electric arc.
3. Gas storage tanks are most often ordinary gas cylinders.
4. The chamber where the deposition directly occurs. The workpiece to be processed and a plasma torch are placed inside such a sealed tank.
5. Vacuum type installation with pump. The tasks of this unit include creating the required vacuum in the chamber and generating a traction flow to supply the working medium.
6. A plasma torch is a device that is equipped with a nozzle for supplying a working medium and a drive system for moving the nozzle in space.
7. Sprayed powder dosing system. Serves to accurately supply the required amount of sprayed material per unit of time.
8. Cooling system. The task of this element is to remove excess heat from the area of ​​the nozzle through which the hot plasma passes.
9. Hardware part. It includes a computer that controls the entire plasma spraying process.
10. Ventilation system. It serves to remove exhaust gases from the working chamber.

Modern diffusion metallization installations have special software that allows, by entering specified parameters, to carry out a completely autonomous operation of processing the product. The operator’s tasks include installing the part into the chamber and setting the exact conditions for the process.

Dear site visitors: plasma spraying specialists and technologists! Support the topic of the article in the comments. We will be grateful for constructive comments and additions that will expand the issue under discussion.

Plasma spraying is one of the methods of gas-thermal coating. This process is based on heating the sprayed material to a liquid or plastic state, transferring it to the substrate with a high-temperature plasma jet, followed by the formation of a coating layer.

In plasma spraying, powders, wires, and rods are used as spraying materials. Powder spraying is the most widely used. The scheme of plasma spraying using powder materials is shown in Fig. 1. In a plasma torch consisting of a water-cooled cathode assembly (cathode 2 and housing 3) and an anode assembly, a plasma arc 8 is excited using a constant welding current source 9, which is stabilized by the walls of the nozzle channel and the plasma-forming gas entering through inlet 1. The powder is supplied from powder feeder 6 with the help of gas supplied through inlet 7.

The temperature of the plasma jet reaches 5000-55000 °C, and the outflow speed is 1000-3000 m/s. In the plasma jet, the powder particles melt and acquire a speed of 50-500 m/s. The flight speed of powder particles depends on their size, the density of the material, the strength of the welding arc current, the nature and flow rate of the plasma-forming gas, and the design of the plasma torch. The powder is introduced into the plasma jet below the nozzle exit, at the nozzle exit, or directly into the nozzle. Heating of sprayed parts does not exceed 100-200 °C.

Rice. 1. Scheme of plasma powder spraying:

1 - supply of plasma-forming gas; 2 - plasmatron cathode; 3 - cathode body; 4 - insulator; 5 - anode body; 6 - powder feeder; 7 - gas supply transporting the powder; 8 - plasma arc; 9 - power supply.

The advantages of the plasma spraying method include the possibility of obtaining coatings from most materials that melt without decomposition and restrictions on the melting temperature. The productivity of plasma spraying is quite high: 3-20 kg/h for plasma torches with a power of 30-40 kW and 50-80 kg/h for plasma torches with a power of 150-200 kW.

Plasma spraying is used to apply coatings to both flat surfaces and bodies of rotation and curved surfaces. The coating is characterized by a layered structure with high heterogeneity of physical and mechanical properties (Fig. 2). The type of bonds between the coating and the part (substrate), as well as between the coating particles, is usually mixed - mechanical adhesion, the force of physical and chemical interactions. The adhesion strength of the coating to the substrate is usually 10-50 MPa when tested for normal peeling.

The physical features of the formation of coatings determine the appearance of open and closed porosities. As the thickness of the applied layer increases, the open pores close and the porosity of the coating decreases. Therefore, the density of plasma coatings differs from the density of the material and ranges from 80-97%. Typically, the porosity of plasma coatings is 10-15%.

The thickness of the coating is practically unlimited by the capabilities of the method itself. However, due to the physical characteristics of the coating formation process, as the thickness of the applied layer increases, internal stresses in it increase, which tend to tear the coating away from the substrate. Therefore, usually the coating thickness does not exceed 1 mm. The structural load is carried by the material of the part, and the coating material imparts properties to the surface of the part such as hardness, wear resistance, etc.

Argon, high-purity nitrogen, hydrogen, helium, as well as mixtures of these and other gases are used as plasma-forming gases. In recent decades, plasma spraying processes using a mixture of air and flammable hydrocarbon gas (methane, propane-butane) as a plasma-forming gas have been successfully developed.

Rice. 2. Diagram of the structure of the plasma coating:

1 - boundary between particles of sprayed material;

2 - boundary between layers;

3 - boundary between the coating and the part;

4 - particle of sprayed material;

5 - surface of the part.

Rice. 3. Micrograph of plasma coating.

Various plasma torches are used to generate plasma. The range and level of specific powers implemented in a specific design characterize the efficiency of converting the electrical energy of the arc into thermal plasma jets, as well as the technological capabilities of the plasma torch.

The task of developing a technological plasma torch always comes down to the creation of a relatively simple, repairable design that ensures stable long-term operation in a wide range of changes in the welding arc current, flow rate and composition of the plasma gas, as well as generating a plasma jet with reproducible parameters, which makes it possible to effectively process materials with different properties.

In spraying practice, both homogeneous powders of various materials (metals, alloys, oxides, oxygen-free refractory compounds) and composite powders, as well as mechanical mixtures of these materials, are used.

The most common powder materials are:

metals - Ni, Al, Mo, Ti, Cr, Cu;

alloys - alloy steels, cast iron, nickel, copper, cobalt, titanium, including self-fluxing alloys (Ni-Cr-B-Si, Ni-B-Si, Co-Ni-Cr-B-Si, Ni-Cu-B-Si);

oxides of Al, Ti, Cr, Zr and other metals and their compositions;

oxygen-free refractory compounds and hard alloys - carbides Cr, Ti, W and others and their compositions with Co and Ni ;

composite clad powders - Ni -graphite, Ni -А l, etc.;

composite conglomerated powders - Ni-Al, NiCrBSi-Al
and etc.;

mechanical mixtures - Cr 3 C 2 + NiCr, NiCrBSi + Cr 3 C 2, etc.

In the case of using composite powders in thermal spray technology, the following goals are pursued:

use of the exothermic effect of interaction of components ( Ni - Al, Ni - Ti, etc.);

uniform distribution of components in the volume of the coating, for example, such as cermets ( Ni - Al 2 0 3, etc.);

protection of the particle core material from oxidation or decomposition during spraying ( Co - WC, Ni - TiC, etc.):

formation of a coating with the participation of a material that does not independently form a coating during thermal spraying ( Ni -graphite, etc.);

improving the conditions for coating formation by increasing the average particle density, introducing components with high enthalpy.

Powders used for spraying should not decompose or sublime during the spraying process, but must have a sufficient difference between the melting and boiling points (at least 200 ° C).

When choosing powder materials for obtaining various plasma coatings, the following points must be taken into account.

The particle size distribution of the powder materials used is of paramount importance, since the productivity and utilization rate, as well as the properties of the coatings, depend on it. The particle size of the powder is selected depending on the characteristics of the thermal energy source, the thermophysical properties of the sprayed material and its density.

Usually, when spraying a fine powder, a more dense coating is obtained, although it contains a large amount of oxides resulting from heating of the particles and their interaction with the high-temperature plasma flow. Excessively large particles do not have time to warm up, so they do not form a sufficiently strong bond with the surface and with each other, or simply bounce off upon impact. When spraying a powder consisting of a mixture of particles of different diameters, smaller particles melt in the immediate vicinity of the point where they are fed into the nozzle, melt the hole and form nodules, which from time to time break off and fall in the form of large drops onto the sprayed coating, deteriorating its quality. Therefore, spraying should preferably be carried out with powders of one fraction, and all powders should be subjected to dispersion (classification) before spraying.

For ceramic materials, the optimal powder particle size is 50-70 microns, and for metals - about 100 microns. Powders intended for spraying must have a spherical shape. They have good flowability, which facilitates their transportation to the plasma torch.

Almost all powders are hygroscopic and can oxidize, so they are stored in closed containers. Powders that have been in an open container for some time are calcined in a stainless steel drying oven with a layer of 5-10 mm at a temperature of 120-130 °C for 1.5-2 hours before spraying.

The powder for spraying is selected taking into account the operating conditions of the parts being sprayed.

Possible defects of the plasma-arc coating method are peeling of the sprayed layer, cracking of the coating, the appearance of large drops of the coating material, drops of copper on the surface, as well as variations in the thickness of the coating (above the permissible).

In order to increase the adhesive and cohesive strengths and other quality characteristics, plasma coatings are subjected to additional processing in various ways: rolling in rollers under current, cleaning the sprayed surfaces from scale and removing particles weakly adhered to the base or to the previous layer with metal brushes during the spraying process, jet-abrasive and ultrasonic treatment, etc.

One of the most common ways to improve the quality of coatings made of self-fluxing alloys is their reflow. For melting, induction or furnace heating, heating in molten salts or metals, plasma, gas flame, laser, etc. are used. In most cases, preference is given to heating in inductors with high frequency currents (HF). Sprayed coating systems Ni - Cr - B - Si - C subjected to melting at 920-1200 0 C in order to reduce the initial porosity, increase the hardness and strength of adhesion to the base metal.

Technological process plasma spraying consists of preliminary cleaning (by any known method), activation treatment (for example, abrasive jet) and direct coating by moving the product relative to the plasmatron or vice versa.

Literature:

Lashchenko G.I. Plasma hardening and sputtering. – K.: “Ecotechnologist i i", 2003 – 64 p.

Plasma welding of aluminum and its alloys is very similar in technology to argon welding. Its essence lies in melting the metal in the right place under the influence of a plasma flow - ionized atoms and molecules. The entire process is carried out in a protective gas cloud, which prevents the penetration of a mixture of gases contained in the atmosphere into the weld pool. At the same time, plasma welding of aluminum has its own specific features:

• During operation, refractory aluminum oxide is formed, having a melting point of 2050 C. It has a density greater than that of aluminum, and therefore it is difficult to melt the edges material, and the seam becomes dirty oxide particles.
• High turnover molten aluminum prevents uniform distribution of metal inside the weld pool. It seeps through the root of the joint and destroys the hard metal around the tub. With the help of ceramic, graphite or steel pads this problem is partially solved.
• The aluminum welding process uses hydrogen. Its use causes the occurrence of porosity, which reduces the ductility and strength of the workpiece. To prevent this it is necessary thoroughly degrease welded parts. Reducing porosity can also be achieved by preheating the material to 150-240 degrees.
• Aluminum has high coefficient of thermal expansion and reduced elasticity, that leads to deformations during welding. This disadvantage is minimized by using various welding modes.
• The use of additional heat sources and preheating of aluminum allows reduce heat loss coefficient, which is initially high for this metal.

Video

Plasma welding of aluminum with reverse polarity

This type of welding of aluminum parts is used to combat the oxide film. A compressed arc of alternating and direct current of reverse polarity destroys the oxide and then it is removed. With the use of this method it appears a range of technological advantages:

1. Work productivity increases by 50-60%.
2. Argon consumption decreases by 4–6 times.
3. Quality of welded joints much higher than when using conventional arc welding.
4. Heating efficiency rises to 60-70 percent. With conventional argon arc welding, the efficiency is 40-45%.
5. Consumption is reduced filler wire up to 50%.
6. The seams are noticeably narrower than with classical welding.
7. Welding of parts without preliminary etching is possible.

For your information! Welding with reverse polarity current is especially widely used when working with cold-worked surfaces and thermally compacted alloys. By reducing the total energy supplied, the percentage of poor-quality seam areas is reduced and the plasma jet penetrates deeper into the material. This allows you to weld thick aluminum parts.

Features and Benefits

• The choice of welding technology and mode parameters is determined brand of alloy, dimensions and shape of the product, type of seams, thickness of the elements being connected, spatial position and configuration of the seams, their length, production conditions and some other factors.
• Maximum efficiency plasma welding of aluminum alloys can be achieved with automatic welding butt seams and using advanced technologies. The efficiency of using manual plasma welding of aluminum in the production and repair of large structures in workshop conditions and installation situations is also high.
• The plasma welding process, thanks to its compressed arc, allows concentrate high energy in the heating spot, due to which this type of welding has become promising for joining aluminum and its alloys.
• The main advantage of plasma welding in high speed, significant reducing the thermal impact zone And process stability, due to which it is not necessary to strictly control and maintain a constant arc length, which makes manual welding easier.
• aluminum provides deep penetration, which sharply increases the amount of base metal when forming a seam. In this case, however, it is necessary to maintain the quality of assembly of the parts for welding and the accuracy of the torch wiring along the joint.
• Using microplasma (low-current compressed arc) can weld aluminum alloys with a thickness of 0.2-1.5 mm current strength 10-100A. When microplasma welding, pure argon (99.98%) is used, pure helium (99.95%) is used as a shielding gas. Helium protects the weld pool from atmospheric gases, hinders the development of the ionization front in the radial direction and, by additionally compressing the arc, makes it stable in space.

Aluminum plasma welding modes

Welding aluminum products has its own characteristics. Constant voltage plasma welding of aluminum with reverse polarity allows solving many problematic aspects of welding aluminum alloys and increasing productivity while maintaining high quality of welded joints of products.

Welding with a consumable electrode

The process occurs in a shell consisting of a protective gas, which is usually argon, helium, or a mixture of both. Parts are welded using special consumable tungsten electrodes using filler wire with a diameter of up to 2.5 mm with reverse polarity current.

Work speed in this mode it can reach 40 m/hour. If the protective cloud consists of a mixture of argon and helium, the thickness of the parts being welded and the width of the seam increases, which is rational when working with thick products.

Automatic arc welding

The process is running using a semi-open plasma arc along a submerged arc, or with a closed arc, then under a submerged arc. It also uses a consumable split electrode and AN-A1 flux for welding work on technical aluminum, and AN-A4 for joining aluminum-magnesium alloys.

The work is carried out over a layer of flux to avoid the occurrence of shunting and disruption of the technological process. The dimensions of the flux layer depend on the thickness of the products being welded and are 20-45 mm in width and 7-15 mm in thickness.

Manual arc

Used for joining parts made of pure aluminum, aluminum-silicon alloys, alloys with magnesium and zinc. In this case, the thickness of the products must be at least 4 mm. Welding work is carried out using DC reverse polarity high speed. There is no lateral displacement. If edge thickness is more than 1 cm, it is necessary to cut edges In this mode it is used butt method only, since with an overlap type of connection a lot of slag can get into the seam and lead to corrosion. This type of work is carried out only after heating the parts to 400 C.

Video

Example of manual welding with a machine:

Electron beam

Produced in a vacuum environment. With this type, aluminum oxides are destroyed by the action of metal vapors on them, as a result of which the oxide decomposes in a vacuum. The vacuum also speeds up the removal of hydrogen from the weld. As a result of the work produces smooth, high-quality seams, the metal practically does not lose its structure at the joint, and the deformation of the workpiece is minimized.

Equipment for plasma welding of aluminum

The aluminum plasma welding machine consists of source alternating or direct current reciprocal and plasmatron - special to generate a plasma discharge.

Plasma torch for aluminum welding Gorynych. Photo from the manufacturer’s website as-pp.ru/gorynych

Power supplies may have different load durations, current values, open circuit voltages and, accordingly, different power consumption.

It has special inlets for plasma-forming and protective gases, as well as for liquid or air cooling of the nozzle walls. for the burner is made of refractory tungsten, hafnium or copper.

There are machines for plasma welding of aluminum from various manufacturers on the market:

As a matter of fact, almost any are suitable for aluminum, they are all designed to work with different metals.

Plasma welding of aluminum and its alloys

In addition to pure aluminum, plasma welding is used for its alloys. Their main types:

1. Heat-strengthened. Such alloys are difficult to weld, so the production of welded products from them is possible only through heat treatment of the product. These include:
   • Aluminum-copper-magnesium (D1, D16, D18, etc.).
   • Aluminum-magnesium-zinc (B92, B92C, etc.).
   • Aluminum-magnesium-silicon and aluminum-magnesium-silicon-copper alloys (AK6 and AK6-1).
   • Aluminum-copper-manganese alloys.
   • And other 5 or more component alloys.
2. Non-heat-strengthened alloys. The most common and excellent for welding work. These are technical aluminum, aluminum-manganese and aluminum-magnesium alloys.

Microplasma welding of aluminum

This type is used for welding work on aluminum with a thickness of 0.2 - 1.5 mm. An alternating voltage source with a current of 10-100 A is used as a power source. The pilot arc receives current from a separate direct current source. The plasma source is argon, and the protective gases are helium and argon.

This type of welding work is characterized by high speed, reaching up to 60 m/h with a mechanized method and 15 m/h with a manual method. The quality of work is also high. The strength of the resulting seams is 0.9.
The main advantage of microplasma welding over argon arc welding is a reduction in material deformation by 25-30%.

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Plasma spraying has a number of advantages compared to flame spraying and electric arc metallization:

• allows you to apply coatings from a wide range of materials (metals, alloys, oxides, carbides, nitrides, borides, plastics and their various compositions) onto a variety of base materials (metals, ceramics, graphite, plastics, etc.);
• plasma torches make it possible to regulate the energy characteristics of the plasma within a wide range, which facilitates the production of coatings with properties determined by the requirements of the technology;
• the use of inert gases and mixtures that do not contain oxygen in plasma torches helps to reduce the oxidation of the sprayed material and the surface of the part;
• Coatings obtained by plasma spraying are superior in physical and mechanical properties to coatings obtained by gas flame and arc spraying methods.

Plasma-arc spraying, based on the type of filler material used, is divided into: powder spraying and wire spraying ( rice. 3.12).

Technological process

Powder sprayers, depending on the properties and sizes of the particles, can supply filler material ( rice. 3.13):

• directly into the plasma jet at the exit from the plasmatron;
• at an angle to the plasmatron nozzle, towards the flow of ionized gas;
• inside the plasma torch nozzle into the post-anode zone or into the pre-anode zone of the plasma arc.

Feeding powder into a plasma jet is used in high-power plasma torches. This supply scheme does not affect the formation of the plasma flow, and plasma torches are characterized by increased power so that the heat of the plasma jet is enough to heat the powder.

Supplying powder to the pre-anode zone is most advantageous from the point of view of heat exchange, but is associated with overheating of particles in the nozzle and clogging of the nozzle with molten particles, which leads to the need to put forward increased requirements for the uniformity of powder supply.

The heating efficiency of powder particles can be increased at the same mode parameters by more uniformly distributing it over the cross section of the hot zone of the plasma jet. This is facilitated by the design of plasma torches, which allow powder to be introduced into the plasma jet not through one hole, but, for example, through three, located at an angle of 120°. In this case, the efficiency of heating the powder varies from 2 to 30%.

Rice. 3.12. Plasma spraying scheme:
a - powder; b - wire. 1 — supply of plasma-forming gas; 2 — plasmatron cathode; 3 — cathode body; 4 - insulator; 5 - anode body; 6 - powder feeder (Fig. a) or wire feed mechanism (Fig. b); 7 — gas supply transporting the powder; 8 — plasma jet; 9 - power supply.

Rice. 3.13. Schemes for feeding powder into the plasmatron:
1 — into a plasma jet; 2 — at an angle to the plasma jet; 3 - into the nozzle.

Application

For spraying wear-resistant coatings, powders with granulation not exceeding 200 microns are used. In this case, the dispersion of powder particles should be within narrow limits with a size difference of no more than 50 microns. If there is a significant difference in particle sizes, it is impossible to ensure their uniform heating. This is explained by the fact that, despite the high temperature of the plasma jet, the large powder does not have time to melt during the short time it is in the plasma jet (10 -4 -10 -2 s), the fine powder partially evaporates, and the bulk of it due to the low kinetic energy is pushed aside by the plasma jet without reaching its central zone. When restoring parts by spraying with wear-resistant powder alloys on a nickel and iron base, the most rational option is to granulate the powder with a particle size of 40-100 microns.

When spraying, as a rule, spherical powder particles are used, since they have the highest flowability. The optimal mode of operation of the plasma torch should be considered one in which the largest number of particles reaches the substrate (base) of the part in a molten state. Therefore, for highly efficient heating and transportation of powder particles, it is necessary that the design of the plasma torch ensures the production of a plasma jet of sufficient power. Currently, installations with a power of up to 160-200 kW have been developed, operating in air, ammonia, propane, hydrogen, in dynamic vacuum, and in water. The use of special nozzles made it possible to obtain a supersonic outflow of a two-phase flow jet, which, in turn, ensured the production of a dense coating. The plasma jet flows out of the plasmatron at a speed of 1000-2000 m/s and imparts a speed of 50-200 m/s to the powder particles.

The increase in the service life of the nozzle apparatus (cathode-anode) of a high-power plasma atomizer (50-80 kW) was hampered due to the low erosion resistance of the copper nozzle in the anode spot area. To increase the durability of the nozzle, tungsten inserts were developed, pressed into the copper nozzle in such a way that heat is effectively dissipated by the copper shell and removed by cooling water. Installations for plasma spraying currently produced by industry are equipped with plasma torches with a power consumption of 25-30 kW at a current strength of 350-400 A.

On the other hand, microplasma torches operating at currents of 15-20 A with a power of up to 2 kW were developed for coating small parts (surfaces), for example, crowns in dentistry, and bandages of gas turbine engine blades in aircraft manufacturing.

The efficiency of heating particles and their flight speed depend on the type of gas used: diatomic gases (nitrogen, hydrogen), as well as air and their mixtures with argon, increase these parameters.

The technological process of restoring parts by plasma spraying includes the following operations: preparation of powder, part surfaces, spraying and mechanical processing of sprayed coatings. The preparation of the surface of the part for spraying is given paramount importance, since the adhesion strength of the powder particles to the surface of the part largely depends on its quality. The surface to be restored should be degreased before treatment. Areas adjacent to the surface to be sprayed are protected with a special screen. Coatings should be sprayed immediately after shot blasting, since after 2 hours its activity decreases due to an increase in the oxide film on the treated surface.

To increase the adhesion strength of the coating to the base, the plasma spraying process is carried out followed by melting. The reflow operation completes the coating process. Melting is carried out with the same plasma torch as spraying, at the same power of the compressed arc, with the plasma torch nozzle approaching the part at a distance of 50-70 mm. Fatigue resistance after reflow increases by 20-25%. The adhesion strength after melting reaches 400 MPa. The mixing zone of the melted and base metals is 0.01-0.05 mm.

Rice. 3.14. Schemes of plasma sprayers:
a - rod; b - wire (“wire-anode”).

Flaws

A significant disadvantage of plasma heating during melting is that the plasma jet, having a high temperature and a significant energy concentration, very quickly heats the coating surface when the surface of the part is not sufficiently heated and thereby often leads to the curling of the melted coating. In addition, as a result of the high flow rate of the plasma jet and significant pressure on the sprayed surface, damage to the coating layer can also occur. Plasma spraying followed by melting is recommended for small-sized parts with a diameter not exceeding 50 mm.

When using wire as a filler material, it is possible to use two schemes for connecting the plasma torch: with a current-carrying nozzle ( rice. 3.14, a) or with a live wire ( rice. 3.14, b).

The wire spraying scheme with a current-carrying wire - anode was developed by V.V. Kudinov in the late 50s of the last century. Then it was possible to obtain unprecedented productivity - 15 kg/h of tungsten with a power of 12 kW. In plasma spraying, rods are used along with wire. So that the heat is effectively dissipated by the copper shell and removed by the cooling water. Installations for plasma spraying currently produced by industry are equipped with plasma torches with a power consumption of 25-30 kW at a current strength of 350-400 A. On the other hand, for coating small parts (surfaces), for example, crowns in dentistry, bandages of gas turbine engine blades In the aircraft industry, microplasma torches were developed operating at currents of 15-20 A with a power of up to 2 kW.

You may also be interested in the following articles:

Plasma spraying based on the use of plasma jet energy for both heating and transfer of metal particles. A plasma jet is produced by blowing a plasma-forming gas through an electric arc and compressing the walls of a copper water-cooled nozzle.
Plasma coatings have the following properties: heat resistance, heat and erosion resistance, thermal and electrical insulation, anti-seize, corrosion resistance, cavitation protection, semiconductor, magnetic, etc.

Areas of application of plasma coatings: rocket, aviation and space technology, mechanical engineering, energy (including nuclear), metallurgy, chemistry, oil and coal industries, transport, electronics, radio and instrument engineering, materials science, construction, machine repair and restoration of parts.

If the cost of flame spraying with wire materials is taken as one, then the cost of plasma and flame spraying of powders will be 1.9 and 1.6, respectively, and electric arc spraying will be 0.85.

The plasma jet is produced in a plasma torch, the main parts of which (Fig. 3.34) are the electrode-cathode /, a water-cooled copper nozzle-anode 4, a steel housing 2, devices for supplying water 3, powder 5 and gas 6. Parts of the housing that interact with the cathode or anode, isolated from each other.
Powdered material is supplied to the feeder using a transport gas. It is possible to introduce powder with plasma-forming gas.
The sprayed material (powder, wire, cord, or a combination thereof) is introduced into the plasma torch nozzle below the anode spot, into the plasma arc column or plasma jet.

High temperatures and jet speeds make it possible to spray coatings from any materials that do not dissociate when heated, without restrictions on the melting temperature. Plasma spraying produces coatings of metals and alloys, oxides, carbides, borides, nitrides and composite materials.

The necessary physical and mechanical properties of coatings are explained by the high temperature of the plasma and its flow rate, the use of inert plasma-forming gases, and the ability to regulate the aerodynamic conditions for the formation of a metal-plasma jet.
There are no structural transformations in the material of the part, it is possible to apply refractory materials and multilayer coatings from various materials in combination of dense and hard lower layers with porous and soft upper ones (to improve the running-in properties of the coatings), the wear resistance of the coatings is high, and full automation of the process is achievable.

When alloying through a wire, surfacing is carried out using high-carbon or alloyed wire under fused flux. This ensures high precision of alloying and stability of the chemical composition of the deposited metal over the coating depth.

Alloying of the deposited metal through flux is performed by surfacing with low-carbon wire under a layer of ceramic flux. The high hardness of the coatings excludes their subsequent heat treatment. However, this alloying method has not found wide application due to the large unevenness of the deposited metal in chemical composition and the need to strictly maintain the surfacing regime.

The combined method of alloying simultaneously through wire and flux has become most widespread.

Rectifiers VS-300, VDU-504, VS-600, VDG-301 and converters PSG-500 with a flat-sloping or rigid external characteristic are used as power sources. Special installations are used as part rotators (UD-133, UD-140, UD-143, UD-144, UD-209, UD-233, UD-299, UD-302, UD-651, OKS-11200, OKS- 11236, OKS-11238, OKS-14408, OKS-27432, 011-1-00 RD) or decommissioned turning or milling machines. For wire feeding, heads A-580M, OKS-1252M, A-765, A-1197 are used.

The main technological parameters of surfacing: composition of the electrode material and flux, arc voltage U, current strength / and polarity, surfacing speed vH and feed vn of the electrode material, surfacing pitch S, electrode displacement from the zenith e, diameter d3 and electrode stickout. Approximate modes of surfacing under a layer of flux for cylindrical parts are given in Table. 3.52.

Surfacing under a layer of flux has the following varieties.

Surfacing with a lying electrode (rod or plate) made of low-carbon or alloy steel is used to restore planes. Part of the flux is poured onto the surface to be restored (3...5 mm thick), and part - onto the electrode (the thickness of the flux layer reaches 10...15 mm). Flux mixtures are used. In one place, the electrode is connected to a part to excite an arc, which, when burning, wanders in the transverse direction. The current density is 6...9 A/mm voltage 35...45 V. To carry out the process there is an OKS-11240 GosNITI installation.

Increased productivity and a higher content of alloying elements in the coating are provided by multi-electrode submerged arc surfacing on parts with significant wear over a large area (Fig. 3.23). A stray arc burns between the part and the electrode closest to it.

Trapping a layer of powder (6...9 mm thick) under a flux increases the productivity of the process and ensures the production of thick coatings of the desired composition.
The scope of application of mechanized surfacing with a layer of flux extends to the restoration of parts (with a diameter of more than 50 mm) made of carbon and low-alloy steels, requiring the application of a layer with a thickness of > 2 mm with high requirements for its physical and mechanical properties. Shaft journals, surfaces of rollers and rollers, bed guides and other elements are fused.

Mechanized surfacing under a layer of flux has the following advantages:

An increase in labor productivity by 6...8 times compared to manual electric arc surfacing with a simultaneous reduction in energy consumption by 2 times due to higher thermal efficiency;

High quality of the deposited metal due to saturation with the necessary alloying elements and rational organization of thermal processes;

Possibility of obtaining coatings with a thickness > 2 mm/p.

Argon, helium, nitrogen, hydrogen and their mixtures are used as plasma-forming gases when spraying materials (Table 3.68). Plasma-forming gases do not contain oxygen, therefore they do not oxidize the material and the sprayed surface.

Helium and hydrogen in their pure form are practically not used for economic reasons, as well as due to the destructive effect on the electrode.

Nitrogen and argon are used more often, but gas mixtures, for example Ar + N, and Ar + H2, have the best performance. The type of plasma-forming gas is selected based on the required temperature, heat content and flow rate, its degree of inertness to the sprayed material and the surface being restored. It should be taken into account that the plasma of di- and polyatomic gases, compared to monatomic gases, contains more heat at the same temperature, because its enthalpy is determined by the thermal motion of atoms, ionization and dissociation energy.

When spraying powder or cord materials, electrical voltage is applied to the electrodes of the plasma torch. When spraying wire materials, voltage is applied to the burner electrodes; in addition, it can be applied to the sprayed material, i.e. the wire may be current-carrying or not. The sprayed part is not included in the load circuit.

Powders for plasma spraying should not create blockages in transport pipelines, but should be uniformly fed into the plasma stream and move freely with the gas flow. These requirements are met by spherical powder particles with a diameter of 20...100 microns.

At the Institute of Electric Welding named after. E.O. Paton NAS of Ukraine developed flux-cored wires. AMOTEC. consisting of a steel shell and powder filler. These materials are intended for applying wear- and corrosion-resistant coatings using gas-flame, electric arc and plasma spraying. A special feature of the materials is the possibility of amorphizing the structure of sprayed coatings. The presence of an amorphous component in the structure of coatings provides a complex of increased service properties (wear and corrosion resistance, strength of connection with the base).

To protect particles of the sprayed material from oxidation, decarburization and nitriding, gas lenses (annular flow of inert gas), which are like a shell of a plasma jet, and special chambers with an inert environment in which the spraying process takes place are used.

Let us give examples of the use of plasma spraying in the processes of restoring parts.

Several varieties of the process of restoring the main supports of cylinder blocks have been mastered. The first researchers of the method recommended low-carbon steel wire Sv-08 as the applied material to ensure a uniform, finely dispersed structure of the coating and increase the strength of its connection to the base. Later, powdered materials were recommended. Composite powders and bronze powders have become widespread. Bronze powders are applied to the surfaces of both cast iron and aluminum alloy parts. A thermoresponsive Al-Ni sublayer must first be applied.

When restoring the main bearings in cast iron cylinder blocks, a cheaper powder with a granulation of 160...200 microns of the composition: Fe (base) is used. 5% Si and 1% AI. Coating mode: plasma arc current 330 A, voltage 70 V, plasma gas (nitrogen) flow rate 25 l/min, plasma torch nozzle diameter 5.5 mm, plasma torch oscillation frequency 83 min', part feed 320 mm/min, powder consumption 7 kg/h.

The process of applying plasma coating to the surfaces of holes in aluminum alloy parts includes:

1) drying powders at a temperature of 150..20 °C for 3 hours;

2) preliminary boring of holes to a size exceeding the nominal hole size by 1 mm;

3) installation of protective screens;

4) degreasing the sprayed surfaces with acetone;

5) coating in two operations;

6) removal of protective screens;

7) preliminary and final boring;

8) flash removal.

In the first operation, a sublayer of PN-85Yu15 is applied, in the second, a main layer of PMS-N copper powder is applied. Coating application modes: current 220...280 A, nitrogen flow 20...25 l/min at a pressure of 0.35 MPa. distance from the nozzle to the part is 100... 120 mm, coating time is 15 minutes. The coating is applied on a stand. Plasma-forming equipment consists of a power source IPN 160/600 n installation UPU-ZD or UPU-8.

Plasma spraying is used to apply coatings to the planes of silumin cylinder heads. The technology includes preliminary milling of the worn surface, coating and subsequent processing. Aluminum powder and 40...48% Fe are used as coating materials. Coating mode: current 280 A, distance from nozzle to part 90 mm. consumption of plasma-forming gas (nitrogen) 72 l/min.

In order to reduce the cost of the process and increase its productivity, the process of electric arc spraying of planes from Sv-AK5 wire with a diameter of 2 mm was introduced. A VGD-301 current source and an EM-12 metallizer are used. Spraying modes: current 300 A, voltage 28... 32 V, spray air pressure 0.4...0.6 MPa, distance from nozzle to part 80... 100 mm. A coating 5 mm thick is applied in 8... 10 minutes.

When restoring aluminum alloy pistons, a plasma coating of PR-Br bronze powder is applied. AZHNMts 8.5-4-5-1.5 (8.5% AI, 4% Fe, 4.8% Ni. 1.4% Mn, the rest Cu). They use the UPU-8 installation. Application mode: current 380 A, distance from nozzle to part 120 mm. The plasma-forming gas is a mixture of argon and nitrogen.

When restoring crankshafts made of high-strength cast iron, a plasma coating from a composition of powders is applied to a thermoresponsive base made of PN-85Yu15 material. Composition: 50% PGSR, 30% PZh4 and 20% PN85Yu15.

Process modes: I = 400 A, distance from nozzle to workpiece 150 mm. nitrogen flow 25 l/min. According to the author's certificate for the invention of the USSR No. 1737017, the purpose of which is to increase the adhesive and cohesive strength of coatings, the applied material contains (in wt.%): self-fluxing alloy of the Ni-Cr-B-Si system 25...50, iron powder 30...50 and nickel -aluminum powder 20…25.

Microplasma spraying is used when restoring parts of parts with dimensions of 5... 10 mm in order to reduce losses of sprayed material. Low power plasmatrons are used (up to 2...2.5 kW), generating a quasilaminar plasma jet at a current strength of 10...60 A. Argon is used as a plasma-forming and shielding gas. With microplasma spraying, it is possible to reduce the diameter of the metal-plasma jet to 1...5 mm. The process is characterized by a low noise level (30...50 dB) and a small amount of exhaust gases, which allows spraying to be carried out indoors without the use of a working chamber. The MPN-001 microplasma spraying installation has been created.

Technological modes of plasma spraying are determined by: the type and dispersion of the material, the current of the plasma jet and its voltage, the type and flow rate of the plasma-forming gas, the diameter of the plasma torch nozzle and the distance from the nozzle to the sprayed surface.

The dispersion of material particles, the current of the plasma jet and the flow rate of the plasma-forming gas determine the heating temperature of the particles and their speed of movement, and therefore the density and structure of the coating.

Greater uniformity of coating properties is ensured at a higher speed of movement of the plasma torch relative to the part and a smaller layer thickness. This speed has little effect on the material utilization rate and has a significant impact on the productivity of the process.

The distance from the nozzle to the surface to be restored depends on the type of plasma-forming gas, the properties of the sprayed material and varies within 120...250 mm (usually 120...150 mm). The angle between the axis of the particle flow and the surface to be restored should approach 90°.

The optimal combination of the heat content of the plasma flow, the residence time of particles in this flow and their speed ensures the production of coatings with high physical and mechanical properties.

The properties of plasma coatings are significantly improved when they are melted. In this case, the most fusible part of the material melts, but the heating temperature must be sufficient to melt borosilicates, which reduce metals from oxides and form slags.

The materials to be melted must meet the following requirements: the melting temperature of the low-melting component of the alloy should not exceed 1000... 1100 °C. The alloy in a heated state should well wet the surface of the workpiece and have the property of self-fluxing. Nickel-based powder materials having a melting point of 980...1050 °C and containing fluxing elements: boron and silicon have such properties. Insufficient heating temperature of the coating leads to the formation of metal drops on the surface. The liquid state of part of the coating promotes intensive diffusion processes, while the material of the part remains in a solid state.

As a result of melting, the strength of the connection between the coating and the base significantly increases, cohesive strength increases, porosity disappears and wear resistance improves.

Melted coatings have machinability close to that of monolithic heat-resistant steels and alloys of similar chemical composition.
Coatings are melted: with a gas torch (oxy-acetylene flame), in a thermal furnace, with an inductor (high-frequency currents), with an electron or laser beam, with a plasma torch (plasma jet), by passing a large current.

Reflowing with a gas torch is the simplest method that allows you to visually control the quality of reflow. The disadvantages of this method are one-sided heating of the part, which can lead to warping, and greater labor intensity when processing massive parts.

Furnace melting ensures heating of the entire volume of the part, so the likelihood of cracks is reduced. However, the areas of the part adjacent to the coating become covered with scale, and their physical and mechanical properties deteriorate. The negative influence of an oxidizing atmosphere on the properties of coatings when heated is eliminated in the presence of a protective environment.

Good results are obtained by induction reflowing, which provides greater productivity without disrupting the heat treatment of the entire workpiece. Only the coating and the adjacent thin layer of base metal are subjected to heating. The thickness of the heated metal depends on the frequency of the current: as the latter increases, the thickness decreases. High heating and cooling rates can lead to cracks in the coating.

Melting coatings with an electron or laser beam practically does not change the properties of the areas associated with the coating and the core of the part. Due to their high cost, these methods should be used when restoring critical, expensive parts whose coatings are difficult to melt using other methods.

Melted coatings from nickel-based alloys PG-SR2. PG-SRZ and PG-SR4 have the following properties:

Hardness 35...60 HRC depending on boron content;

Increased wear resistance by 2...3 times compared to hardened steel 45, which is explained by the presence of hard crystals (borides and carbides) in the coating structure;

Increased 8...10 times the strength of the connection between the coating and the base compared to the strength of the connection of unfused coatings;

Increased fatigue strength by 20...25%.

The area of ​​application of plasma coatings with subsequent melting is the restoration of surfaces of parts operating under conditions of alternating and contact loads.

Melted coatings have a multiphase structure, the components of which are borides, excess carbides and eutectic. The type of microstructure (dispersity, type and number of components) depends on the chemical composition of the self-fluxing alloy, heating time and temperature.

The best wear resistance of parts in loaded joints is provided by coatings made of self-fluxing alloys. The structure of the coating is a highly alloyed solid solution with inclusions of dispersed metal-like phases (primarily boride or carbide) with a particle size of 1...10 microns, uniformly distributed in the base.

For plasma spraying of metal and non-metallic coatings (refractory, wear-resistant, corrosion-resistant), the following installations are used: UN-115, UN-120, UPM-6. UPU-ZD. UPS-301. APR-403. UPRP-201.

Various plasma torches are used to generate plasma. The range and level of specific powers implemented in a specific design characterize the efficiency of converting the electrical energy of the arc into thermal plasma jets, as well as the technological capabilities of the plasma torch.

The task of developing a technological plasma torch always comes down to the creation of a relatively simple, repairable design that ensures stable long-term operation in a wide range of changes in the welding arc current, flow rate and composition of the plasma gas, as well as generating a plasma jet with reproducible parameters, which makes it possible to effectively process materials with different properties.

In spraying practice, both homogeneous powders of various materials (metals, alloys, oxides, oxygen-free refractory compounds) and composite powders, as well as mechanical mixtures of these materials, are used.

The most common powder materials are:

metals - Ni, Al, Mo, Ti, Cr, Cu;

alloys - alloy steels, cast iron, nickel, copper, cobalt, titanium, including self-fluxing alloys (Ni-Cr-B-Si, Ni-B-Si, Co-Ni-Cr-B-Si, Ni-Cu-B -Si);

oxides of Al, Ti, Cr, Zr and other metals and their compositions;

oxygen-free refractory compounds and hard alloys - carbides Cr, Ti, W, etc. and their compositions with Co and Ni;

composite clad powders - Ni-graphite, Ni-А l, etc.;

composite conglomerated powders - Ni - Al, NiCrBSi - Al
and etc.;

mechanical mixtures - Cr 3 C 2 + NiCr, NiCrBSi + Cr 3 C 2, etc.

In the case of using composite powders in thermal spray technology, the following goals are pursued:

use of the exothermic effect of interaction of components (Ni - Al, Ni - Ti, etc.);

uniform distribution of components in the volume of the coating, for example, such as cermets (Ni - Al 2 0 3, etc.);

protection of the particle core material from oxidation or decomposition during spraying (Co - WC, Ni - TiC, etc.):

formation of a coating with the participation of a material that does not independently form a coating during gas-thermal spraying (Ni-graphite, etc.);

improving the conditions for coating formation by increasing the average particle density, introducing components with high enthalpy.

Powders used for spraying should not decompose or sublime during the spraying process, but must have a sufficient difference between the melting and boiling points (at least 200 ° C).

When choosing powder materials for obtaining various plasma coatings, the following points must be taken into account.

The particle size distribution of the powder materials used is of paramount importance, since the productivity and utilization rate, as well as the properties of the coatings, depend on it. The particle size of the powder is selected depending on the characteristics of the thermal energy source, the thermophysical properties of the sprayed material and its density.

Usually, when spraying a fine powder, a more dense coating is obtained, although it contains a large amount of oxides resulting from heating of the particles and their interaction with the high-temperature plasma flow. Excessively large particles do not have time to warm up, so they do not form a sufficiently strong bond with the surface and with each other, or simply bounce off upon impact. When spraying a powder consisting of a mixture of particles of different diameters, smaller particles melt in the immediate vicinity of the point where they are fed into the nozzle, melt the hole and form nodules, which from time to time break off and fall in the form of large drops onto the sprayed coating, deteriorating its quality. Therefore, spraying should preferably be carried out with powders of one fraction, and all powders should be subjected to dispersion (classification) before spraying.

For ceramic materials, the optimal powder particle size is 50-70 microns, and for metals - about 100 microns. Powders intended for spraying must have a spherical shape. They have good flowability, which facilitates their transportation to the plasma torch.

Almost all powders are hygroscopic and can oxidize, so they are stored in closed containers. Powders that have been in an open container for some time are calcined in a stainless steel drying oven with a layer of 5-10 mm at a temperature of 120-130 °C for 1.5-2 hours before spraying.

The powder for spraying is selected taking into account the operating conditions of the parts being sprayed.

Possible defects of the plasma-arc coating method are peeling of the sprayed layer, cracking of the coating, the appearance of large drops of the coating material, drops of copper on the surface, as well as variations in the thickness of the coating (above the permissible).

In order to increase the adhesive and cohesive strengths and other quality characteristics, plasma coatings are subjected to additional processing in various ways: rolling in rollers under current, cleaning the sprayed surfaces from scale and removing particles weakly adhered to the base or to the previous layer with metal brushes during the spraying process, jet-abrasive and ultrasonic treatment, etc.

One of the most common ways to improve the quality of coatings made of self-fluxing alloys is their reflow. For melting, induction or furnace heating, heating in molten salts or metals, plasma, gas flame, laser, etc. are used. In most cases, preference is given to heating in inductors with high frequency currents (HF). Sprayed coatings of the Ni - Cr - B - Si - C system are subjected to melting at 920-1200 0 C in order to reduce the initial porosity, increase hardness and adhesion strength to the base metal.

The technological process of plasma spraying consists of preliminary cleaning (by any known method), activation treatment (for example, abrasive jet) and direct coating by moving the product relative to the plasmatron or vice versa.

Lashchenko G.I. Plasma hardening and sputtering. – K.: “Ecotechnologist I”, 2003 – 64 p.