Plasma spraying has a number of advantages in comparison with gas-flame spraying and electric arc metallization:
- makes it possible to apply coatings from materials of a wide composition (metals, alloys, oxides, carbides, nitrides, borides, plastics and their various compositions) on a variety of base materials (metals, ceramics, graphite, plastics, etc.);
- Plasma torches make it possible to regulate the energy characteristics of the plasma over a wide range, which facilitates the production of coatings with properties determined by the requirements of the technology;
- the use of inert gases and oxygen-free mixtures 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 terms of physical and mechanical properties to coatings obtained by gas-flame and arc spraying methods.
Plasma-arc spraying according to the type of filler material used is divided into: powder spraying and wire spraying ( rice. 3.12).
Technological process
Powder atomizers, depending on the properties and particle size, can supply filler material ( rice. 3.13):
- directly into the plasma jet at the outlet of the plasma torch;
- at an angle to the plasma torch nozzle, towards the flow of ionized gas;
- inside the plasma torch nozzle into the anode zone or into the preanode zone of the plasma arc.
The supply of powder into the plasma jet is used in high power plasma torches. Such a supply scheme does not affect the formation of the plasma flow, and plasma torches are characterized by an overestimated power so that the heat of the plasma jet is enough to heat the powder.
The supply of powder to the pre-anode zone is most advantageous in terms of heat transfer, but is associated with overheating of the 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 uniform distribution of powder over the cross section of the hot zone of the plasma jet. This is facilitated by the design of plasma torches, which make it possible to introduce the powder 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 powder heating varies from 2 to 30%.
Rice. 3.12. Plasma spraying scheme:
a - powder; b - wire. 1 - supply of plasma gas; 2 - plasma torch cathode; 3 - cathode body; 4 - insulator; 5 - anode body; 6 - powder feeder (Fig. a) or wire feeder (Fig. b); 7 - supply of gas transporting the powder; 8 - plasma jet; 9 - power supply.
Rice. 3.13. Schemes of powder supply to the plasma torch:
1 - into the plasma jet; 2 - at an angle to the plasma jet; 3 - into the nozzle.
Application
For spraying wear-resistant coatings, powders with a 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 μm. With a significant difference in particle size, it is impossible to ensure their uniform heating. This is explained by the fact that, despite the high temperature of the plasma jet, coarse powder does not have time to melt during the short time it is in the plasma jet (10 -4 -10 -2 s), fine powder partially evaporates, and its main mass due to the low kinetic energy is pushed aside by the plasma jet, without reaching its central zone. When restoring parts by spraying with powder wear-resistant nickel- and iron-based alloys, the most rational is powder granulation 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 the one in which the largest number of particles reaches the substrate (base) of the part in the molten state. Therefore, for highly efficient heating and transportation of powder particles, it is necessary that the design of the plasma torch ensures that a plasma jet of sufficient power is obtained. Currently, installations with a capacity of up to 160-200 kW have been developed, operating on air, ammonia, propane, hydrogen, in dynamic vacuum, in water. The use of special nozzles made it possible to obtain a supersonic outflow of a two-phase flow jet, which, in turn, provided a dense coating. The plasma jet flows out of the plasma torch at a speed of 1000-2000 m/s and imparts a speed of 50-200 m/s to the powder particles.
An increase in the resource of the nozzle apparatus (cathode-anode) of a high-power plasma sputter (50-80 kW) was hampered due to the low erosion resistance of the copper nozzle in the anode spot zone. In order to increase the durability of the nozzle, tungsten inserts were developed, pressed into the copper nozzle in such a way that the heat was effectively removed by the copper sheath and removed by the cooling water. Plasma spraying installations currently produced by the 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 applying coatings on small parts (surfaces), for example, crowns in dentistry, shroud shelves of GTE blades in the aircraft industry, microplasma burners operating at currents of 15-20 A at a power of up to 2 kW were developed.
The efficiency of particle heating 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.
Technological process restoration of parts by plasma spraying includes the following operations: powder preparation, part surfaces, spraying and machining of sprayed coatings. The preparation of the surface of the part for spraying is of 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 must 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 already 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 process of plasma spraying is carried out with subsequent reflow. The reflow operation completes the coating process. Melting is carried out by 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 reflow reaches 400 MPa. The zone of mixing of melted and base metals is 0.01-0.05 mm.
Rice. 3.14. Plasma spray patterns:
a - bar; b - wire ("wire-anode").
Flaws
A significant disadvantage of plasma heating during reflow is that the plasma jet, having a high temperature and a significant energy concentration, very quickly heats the surface of the coating with insufficient heating of the surface of the part and thereby often leads to the collapse of the melted coating. In addition, as a result high speed The outflow of the plasma jet and significant pressure on the surface being sprayed can also cause damage to the coating layer. Plasma spraying with subsequent reflow is recommended for small parts with a diameter not exceeding 50 mm.
When using wire as a filler material, it is possible to use two schemes for connecting a plasma torch: with a current-carrying nozzle ( rice. 3.14, a) or with a current-carrying wire ( rice. 3.14b).
The scheme of wire sputtering with a current-carrying wire - the anode was developed by V. V. Kudinov at the end of the 50s of the last century. Then it was possible to obtain unprecedented productivity - 15 kg / h of tungsten at a power of 12 kW. In plasma spraying, rods are used along with wire. In such a way that the heat is effectively removed by the copper sheath and removed by the cooling water. Plasma spraying installations currently produced by the 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, shroud shelves of GTE blades in the aircraft industry, microplasma burners were developed that operate at currents of 15-20 A at a power of up to 2 kW.
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Plasma spraying is based on the use of plasma jet energy both for heating and for transporting metal particles. The plasma jet is obtained by blowing the 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 strength, thermal and electrical insulation, anti-seizure, corrosion resistance, cavitation protection, semiconductor, magnetic, etc.
Applications of plasma coatings: rocket, aviation and space technology, mechanical engineering, energy (including nuclear), metallurgy, chemistry, oil and coal industry, transport, electronics, radio and instrumentation, materials science, construction, machine repair and restoration of parts.
If the cost of flame spraying with wire materials is taken as a unit, then the cost of plasma and flame spraying of powders will be 1.9 and 1.6, respectively, and electric arc - 0.85.
A plasma jet is obtained in a plasma torch, the main parts of which (Fig. 3.34) are the electrode-cathode /, water-cooled copper nozzle-anode 4, steel case 2, devices for supplying water 3, powder 5 and gas 6. Parts of the case interacting with the cathode or anode, isolated from each other.
Powdered material is fed by a feeder with the help of a carrier gas. It is possible to introduce powder with plasma gas.
The material to be sprayed (powder, wire, cord, or a combination thereof) is introduced into the plasma torch nozzle below the anode spot, into the plasma arc column, or into the plasma jet.
The high temperature and speed of the jet make it possible to spray coatings from any materials that do not dissociate when heated, without restrictions on the melting point. Plasma spraying produces coatings from metals and alloys, oxides, carbides, borides, nitrides and composite materials.
The necessary physical and mechanical properties of the coatings are explained by the high temperature of the plasma and the speed of its outflow, the use of inert plasma-forming gases, and the possibility of controlling 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 layers (to improve the running-in of coatings), the wear resistance of coatings is high, and full automation of the process is achievable.
When alloying through a wire, surfacing is carried out with a high-carbon or alloyed wire under a fused flux. This ensures high doping accuracy and stability. chemical composition deposited metal by coating depth.
Alloying of the deposited metal through the flux is performed by surfacing with a 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 non-uniformity of the deposited metal in terms of chemical composition and the need to strictly maintain the surfacing mode.
The combined method of alloying simultaneously through the wire and flux is the most widely used.
As power sources, rectifiers VS-300, VDU-504, VS-600, VDG-301 and converters PSG-500 with a gently dipping or rigid external characteristic. In the role of rotators of parts, special installations are used (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. Heads A-580M, OKS-1252M, A-765, A-1197 are used for wire feeding.
Main technological parameters surfacing: composition of the electrode material and flux, arc voltage U, current strength / and polarity, deposition rate vH and feed vn of the electrode material, deposition step S, electrode offset from the zenith e, diameter d3 and electrode stick-out. Approximate modes of surfacing under a flux layer of cylindrical parts are given in Table. 3.52.
Surfacing under a flux layer has the following varieties.
Surfacing with a lying electrode (rod or plate) 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 is poured 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 initiate an arc, which wanders in the transverse direction when burning. The current density is 6…9 A/mm voltage 35…45 V. To perform the process, there is an OKS-11240 GosNITI unit.
An increase in 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 workpiece and the electrode closest to it.
Setting on a layer of powder (6…9 mm thick) under flux increases the productivity of the process and ensures the production of thick coatings of the desired composition.
The field of application of mechanized hardfacing with a flux layer extends to the restoration of parts (with a diameter of more than 50 mm) from carbon and low alloy steels that require a layer with a thickness of > 2 mm with high demands to its physical and mechanical properties. Shaft journals, surfaces of rollers and rollers, frame guides and other elements are welded.
Submerged arc mechanized surfacing has the following advantages:
An increase in labor productivity by 6...8 times compared to manual electric arc surfacing with a simultaneous reduction in power consumption by 2 times due to a higher thermal efficiency;
High quality of the deposited metal due to saturation with the necessary alloying elements and rational organization 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 such as 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 to be restored. It should be taken into account that the plasma of two- and multi-atomic gases, compared with one-atomic 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 material being sprayed, i.e. the wire may or may not be a current conductor. The part to be sprayed is not included in the load circuit.
Powders for plasma spraying should not create blockages in transport pipelines, but should be evenly fed into the plasma jet 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. E.O. Paton of the National Academy of Sciences of Ukraine developed flux-cored wires ser. AMOTECH. consisting of a steel shell and powder filler. These materials are intended for application of wear- and corrosion-resistant coatings by means of flame, electric arc and plasma spraying. A feature of the materials is the possibility of amorphization of the structure of the sprayed coatings. The presence of an amorphous component in the structure of the coatings provides a complex of enhanced service properties (wear and corrosion resistance, bonding strength with the base).
To protect the particles of the sprayed material from oxidation, decarburization and nitriding, gas lenses (annular flow of inert gas) are used, which are like a shell of a plasma jet, and special chambers with an inert medium in which the spraying process takes place.
Let us give examples of the use of plasma spraying in the process of restoring parts.
Several varieties of the process of restoring the main bearings 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 fine structure of the coating and increase the strength of its connection with the base. Powdered materials were later recommended. Composite powders and bronze powders have become widespread. Bronze powders are applied to the surfaces of both cast-iron parts and parts made of aluminum alloy. A thermosetting Al-Ni undercoat must be applied first.
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% C and 1% AI. Coating mode: plasma arc current 330 A, voltage 70 V, plasma gas (nitrogen) consumption 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 on the surface of holes in aluminum alloy parts includes:
1) drying of 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 - the main layer of copper powder PMS-N. Coating modes: current strength 220…280 A, nitrogen consumption 20…25 l/min at a pressure of 0.35 MPa. distance from nozzle to workpiece 100…120 mm, coating time 15 min. The coating is applied on the stand. Plasma-forming equipment consists of a power source PPN 160/600 and a UPU-ZD or UPU-8 unit.
Plasma spraying is used when coating the planes of cylinder heads from silumin. The technology includes preliminary milling of the worn surface, coating and subsequent processing. Aluminum powder and 40...48% Fe are used as the coating material. Coating mode: current strength 280 A, distance from nozzle to part 90 mm. consumption of plasma gas (nitrogen) 72 l/min.
In order to reduce the cost of the process and increase its productivity, a process of electric arc deposition of planes from Sv-AK5 wire with a diameter of 2 mm was introduced. Apply current source VGD-301 and metallizer EM-12. Spray 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 with a thickness of 5 mm is applied in 8 ... 10 minutes.
When restoring pistons made of aluminum alloy, a plasma coating is applied from bronze powder PR-Br. AZHNMts 8.5-4-5-1.5 (8.5% AI, 4% Fe, 4.8% Ni. 1.4% Mn, the rest is Cu). They use the UPU-8 installation. Application mode: current 380 A, distance from the nozzle to the part 120 mm. plasma gas - a mixture of argon and nitrogen.
When restoring crankshafts made of high-strength cast iron, a plasma coating is applied from a composition of powders to a thermosetting underlay made of PN-85Yu15 material. The composition of the 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 May.%): self-fluxing alloy of the Ni-Cr-B-Si 25 ... 50 system, iron powder 30 ... 50 and nickel - aluminum powder 20…25.
Microplasma spraying is used in the restoration of sections of parts with dimensions of 5 ... 10 mm in order to reduce the loss of the sprayed material. Plasma torches of low power (up to 2 ... 2.5 kW) are used, generating a quasi-laminar plasma jet at a current strength of 10 ... 60 A. Argon is used as a plasma-forming and protective 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 indoors without using a working chamber. A microplasma spraying unit MPN-001 was created.
The 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 gas, the diameter of the plasma torch nozzle and the distance from the nozzle to the sprayed surface.
The dispersity of the particles of the material, 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 hence the density and structure of the coating.
Greater uniformity of coating properties is provided 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 utilization of the material and significantly affects the productivity of the process.
The distance from the nozzle to the surface to be restored depends on the type of plasma gas, the properties of the sprayed material and varies within 120...250 mm (usually 120...150 mm). The angle between the particle flow axis and the surface to be restored should approach 90°.
The optimal combination of the heat content of the plasma flow, the residence time of the particles in this flow, and their velocity 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 low-melting part of the material is melted, however, the heating temperature must be sufficient to melt borosilicates, which reduce metals from oxides and form slags.
The melted materials must meet the following requirements: the melting point of the fusible component of the alloy must not exceed 1000 ... 1100 °C. the alloy in the heated state should wet the surface of the workpiece well and have the property of self-fluxing. Such properties are possessed by nickel-based powder materials having a melting point of 980...1050 °C and containing fluxing elements: boron and silicon. Insufficient heating temperature of the coating leads to the formation of metal droplets on the surface. The liquid state of a part of the coating contributes to the intensive flow of diffusion processes, while the material of the part remains in a solid state.
As a result of reflow, the bonding strength of the coating to the base is significantly increased, the cohesive strength increases, porosity disappears, and wear resistance improves.
Fused coatings have machinability close to that of monolithic heat-resistant steels and alloys of similar chemical composition.
Coatings are melted: by a gas burner (oxy-acetylene flame), in a thermal furnace, by an inductor (high-frequency currents), by an electron or laser beam, by a plasma burner (plasma jet), by passing a large current.
Reflow with a gas burner is the simplest way to visually control the quality of reflow. The disadvantages of the method are one-sided heating of the part, which can lead to warping, and high labor intensity when processing massive parts.
Furnace reflow provides heating of the entire volume of the part, so the likelihood of cracks is reduced. However, the areas of the part associated with the coating are covered with scale, their physical and mechanical properties deteriorate. Negative influence oxidizing atmosphere on the properties of coatings during their heating is excluded in the presence of a protective environment.
Good results are obtained by induction reflow, which provides greater productivity without disturbing the heat treatment of the entire workpiece. Only the coating and a thin layer of base metal adjacent to it are subjected to heating. The thickness of the heated metal depends on the frequency of the current: with an increase in the latter, the thickness decreases. High heating and cooling rates can lead to cracks in the coating.
The melting of 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 the high cost, these methods should be used in the restoration of critical expensive parts, the coatings on which are difficult to melt by other methods.
Fused coatings from alloys based on nickel PG-SR2. PG-SRZ and PG-SR4 have the following properties:
Hardness 35…60 HRC depending on the content of boron in them;
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 by 8 ... 10 times the strength of the connection of the coating with the base compared to the strength of the connection of unmelted coatings;
Fatigue strength increased by 20...25%.
The area of application of plasma coatings with subsequent reflow is the restoration of the surfaces of parts operating under conditions of alternating and contact loads.
Fused coatings have a multiphase structure, the components of which are borides, excess carbides and eutectic. The type of microstructure (dispersion, type and number of components) depends on the chemical composition of the self-fluxing alloy, time and temperature of heating.
The best wear resistance of parts in loaded interfaces 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 µm, evenly distributed in the base.
For plasma spraying of metal and non-metal 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 power density implemented in a specific design characterize the conversion efficiency electrical energy arcs into a thermal plasma jet, as well as technological capabilities plasma torch.
The task of developing a technological plasma torch always comes down to creating a relatively simple, maintainable design that ensures stable long-term operation in a wide range of changes in the arc welding 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 the practice of spraying, both homogeneous powders of various materials (metals, alloys, oxides, oxygen-free refractory compounds), and composite, 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- Cr, Ti, W, etc. carbides and their compositions with Co and Ni;
composite clad powders - Ni-graphite, Ni-A 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 the technology of thermal spraying, the following goals are pursued:
the use of the exothermic effect of the 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 sputtering (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.);
improvement of the conditions for the formation of coatings by increasing the average density of particles, the introduction of components with high enthalpy.
The powders used for spraying should not decompose or sublimate during the spraying process, but should 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 provisions should be taken into account.
The granulometric composition of the powder materials used is of paramount importance, since the productivity and utilization factor, as well as the properties of the coatings, depend on it. The particle size of the powder is chosen depending on the characteristics of the source of thermal energy, thermophysical properties of the sprayed material and its density.
Usually, when spraying a fine powder, a denser coating is obtained, although it contains a large amount of oxides resulting from the heating of particles and their interaction with a high-temperature plasma flow. Excessively large particles do not have time to warm up, therefore they do not form a sufficiently strong bond with the surface and between themselves, 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 place of their supply to the nozzle, seal the hole and form sags, which from time to time come off and fall on the sprayed coating in the form of large drops, deteriorating its quality. Therefore, spraying should preferably be carried out with powders of the same fraction, and all powders should be subjected to dispersion (classification) before spraying.
For ceramic materials, the optimal particle size of the powder is 50-70 microns, and for metals - about 100 microns. Powders intended for spraying should 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 sprayed parts.
Possible defects of the plasma-arc coating method are delamination 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 the difference in thickness of the coating (above the allowable one).
In order to increase the adhesive and cohesive strengths and other qualitative characteristics, plasma coatings are subjected to additional processing in various ways: rolling with 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 itself, jet-abrasive and ultrasonic treatment, etc.
One of the most common ways to improve the quality of self-fluxing alloy coatings is their melting. For reflow, induction or furnace heating, heating in molten salts or metals, plasma, flame, laser, etc. are used. In most cases, preference is given to heating in inductors with high-frequency currents (HFC). The sprayed coatings of the Ni - Cr - B - Si - C system are subjected to melting at 920-1200 0 С in order to reduce the initial porosity, increase the hardness and strength of adhesion 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 plasma torch or vice versa.
Lashchenko G.I. Plasma hardening and sputtering. – K.: Ecotechnologist i Ya, 2003 – 64 p.
Plasma spraying
The method of applying coatings using a plasma flow is superior in its capabilities to metal deposition methods using an oxy-acetylene flame and arc welding. The advantage of this method over others lies in the possibility of melting and applying multilayer coatings to materials made of refractory metals, regardless of the melting temperature of the latter, which makes it possible to restore parts that have out of all repair sizes.
Like other methods of high-temperature spraying of coatings, plasma spraying does not cause warping of the part and changes in the structure. The wear resistance of plasma coatings is 1.5...3 times higher, and the coefficient of friction is 1.5...2 times lower than that of hardened steel 45.
The plasma jet is used for surfacing and coating products made of steels, aluminum and its alloys and other materials by melting filler wire or metal powders. Plasma is used for cutting and surface treatment of various materials, heating for soldering and heat treatment. The use of neutral gases for plasma formation and protection - argon, nitrogen and their mixtures - ensures minimal burnout of alloying elements and oxidation of particles. Plasma spraying improves the properties of metal coatings, however, its widespread use is limited by the low adhesion strength of the coating to the surface of the restored part and the reliability of plasma torches, high noise and brightness of the arc. The plasma arc is a high-intensity source of heat, consisting of molecules of atoms, ions, electrons and light quanta in a highly ionized state, the temperature of which can reach 20,000 °C or more.
The plasma jet has a brightly glowing core, the length of which can vary from 2...3 to 40...50 mm depending on the size of the nozzle and channel, gas composition and flow rate, current value and arc length.
The power supply circuit of the installation consists of two sources: one of them is designed to power the plasma arc, and the second - to maintain the main arc. Plasma-forming gas is supplied from the cylinder through the gas equipment located in the control panel. A carrier gas is used to feed the filler powder. Gas equipment consists of cylinders, reducers, flow meters, a mixer, fuses and electromagnetic valves.
For surfacing, it is advisable to use plasma torches in which two arcs burn simultaneously: one is plasma-forming, and the second serves to melt the base metal and melt the filler. When spraying, burners are recommended in which the filler and base metals are heated by a part of the plasma flow that has passed through the hole in the nozzle.
Niresist and bronze powders are used for spraying antifriction coatings. Powders of self-fluxing alloys PG-SRZ, SNGN-50, stainless steel are used in mixtures for spraying wear-resistant coatings, as well as for restoring shafts and bearing seats.
Intermetallic powders (chemical compound of metal with metal) PN55T, PN85Yu15 are used as a sublayer (0.05...0.1 mm) to increase the adhesion strength of coatings and as a component of a powder mixture to increase the cohesive strength of the coating. Plasma coatings have sufficiently high adhesive strength values with a layer thickness of up to 0.6 ... 0.8 mm.
For spraying the main and connecting rod journals of the crankshaft of the ZIL-130 engine, you can use a mixture of powders - 15 ... 25% (by weight) PN85Yu15 + 35 ... 40% PG-SRZ + 35 ... 50% P2X13. For economic reasons, it is advisable to spray with mixtures, the main components of which are cheap powders (ni-resist, stainless steel, bronze). 10…15% powder PN85Yu15 is introduced into their composition.
Powders PR-N70Yu30 and PR-N85Yu15, produced by NPO Tulachermet, can serve as a sublayer and main coating layer in combination with high-carbon powders.
The quality of the coating during plasma spraying largely depends on the power of the burner, gas flow, electric mode, powder supply, spraying conditions (the distance of the burner from the product, the spraying angle is set experimentally for each specific case.
Rice. 1. Scheme of installation for plasma surfacing:
1 - main current source; 2 - current source for excitation; 3 - plasma torch; 4 - gas cylinder transporting welding powder; 5 - gas reducer; 6 - dispenser; 7 - cylinder with plasma gas; 8 - rotameter; 9 - mixer.
Rice. 2. Schemes of plasma torches for surfacing (a) and for spraying (b):
1 - tungsten electrode (cathode); 2 - insulating gasket; 3 - nozzle (anode); 4 - plasma; 5 - deposited layer; 6 - base metal; 7 - channel for supplying welding powder; 8 - channels for cooling water; 9 - sprayed layer.
To restore parts of the “shaft” type (gear shafts, hollow and solid shafts and axles, cardan crosses and differentials) with a wear of not more than 3 mm, the OKS-11231-GOSNITI installation is used by plasma surfacing with hard-alloy materials.
The diameter and length of the welded parts are 20…100 and 100…800 mm, respectively. Applied powders: sor-mite, charged with aluminum powder ASDT; US-25 with aluminum; T-590 with aluminum; PG-L101 with aluminum; gas - argon, compressed air. The hardness of the applied metal is up to 66 HRC3. Overall dimensions of the machine 2225X1236X1815 mm.
According to GOSNITI, the annual economical effect from the implementation of the installation will be more than 9 thousand rubles.
At the OKS-11192-GOSNITI installation, the chamfers of the valve discs of diesel engines of all brands are successfully restored with PG-SR2 powder material. Its productivity is 80…100 valves per shift.
High reliability in operation was shown by the small-sized plasma torch VSKHIZO-Z, which, in combination with the converted UMP-5-68 installation, is recommended for the restoration of crankshafts of YaMZ-238NB, SMD-14 and A-41 engines using the following compositions: wire Sv-08G2S-80 …85% + PG-SR4-15…20% powder (SMD-14 and A-41) and 15GSTYUTSA-75…80% wire + PG-SR4-20…25% powder. The hardness of the shaft journals in the first case is 46.5 ... 51.5 HRC3, in the second - 56.5 ... 61 HRC3. The wear resistance of the journals and liners is at the level of the crankshaft.
The problem of ensuring the necessary strength of adhesion of the metal coating to the product, the search for new cheap materials and effective ways preparation of worn surfaces of parts before plasma spraying.
The first can be solved by introducing an additional operation - melting of the sprayed coating, which is performed by a plasma or oxy-acetylene torch immediately after coating, as well as by heating with high-frequency currents. After the coating is melted, its physical and mechanical properties are improved, and the adhesion strength increases by 10 times or more.
The technological process of restoring parts in this way includes cleaning the surface of the product from impurities and oxides (if necessary, preliminary grinding to give the correct geometric shape of the part), its degreasing and abrasive blasting (creates hardening, destroys the oxide film, increases roughness), spraying the part with melting coating and then machining of the product.
Compressed air pressure during abrasive blasting - 0.4 ... 0.6 MPa, blowing distance 50 ... 90 mm, abrasive jet attack angle 75 ... 90 °. The duration of treatment depends on the abrasive (powder of white electrocorundum 23A, 24A or black silicon carbide 53C, 54C with a grain size of 80 ... 125 microns GOST 1347-80, steel or cast iron shot DSK and DCHK No. 08K; No. 1.5K GOST 11964-69), the material of the part and its hardness and the area of the machined surface. The time between preparation and spraying should be as short as possible and not exceed 1.5 hours.
The distance from the nozzle exit to the surface of the part during plasma melting is reduced within 50 ... 60 mm.
For cylindrical parts, melting is carried out during their rotation with a frequency of 10 ... 20 min-1.
As a rotator for plasma spraying, installations 011-1-01, 011-109 or a screw-cutting lathe can be used.
When choosing the final layer thickness, one should take into account shrinkage during flashing (10...20%) and machining allowance (0.2...0.3 mm per side).
Plasma coatings sprayed with metal powders are processed on screw-cutting or grinding machines with a standard cutting tool. Grinding with synthetic diamond wheels is especially effective.
The conducted studies have shown that it is possible to restore critical autotractor parts of any shape (plates and pusher rods, chamfers of plates and valve stems, crankshafts, water pump rollers) by plasma spraying with coating reflow, which should be taken into account by specialists when developing technological processes for the restoration of these parts.
The use of plasma spraying is advisable in the restoration of wearable working parts of agricultural machines (in this case, the application of carbide powders is desirable). It can be used to apply heat-resistant anti-corrosion coatings for parts operating at high temperatures.
At the same time, the problem of sprayed coatings has not been completely solved yet. For example, control in the process of spraying the thickness of coatings, mechanical processing of sprayed coatings. It is necessary to further improve the existing technology of high-temperature spraying and equipment for its implementation, in-depth and versatile studies of the possibilities and advantages of this technology, and the development of scientifically based recommendations for the use of flux-cored materials on specific parts.
To Category: - Advanced Repair Methods
In the plasma method of coating, the sprayed material is heated to a liquid state and transferred to the surface to be treated using a high-temperature plasma flow. The material to be sprayed is available in the form of rods, powders or wires. The powder method is the most common.
The uniqueness of the plasma spraying method lies in the high temperature (up to 50 thousand degrees Celsius) of the plasma jet and the high speed (up to 500 m/s) of particles in the jet. The heating of the sprayed surface is small and does not exceed 200 degrees.
The productivity of plasma spraying is 3-20 kg/h for plasma generators with a capacity of 30...40 kW and 50-80 kg/h for equipment with a capacity of 150...200 kW.
The adhesion strength of the coating to the surface of the part is on average 10-55 MPa for separation, and in some cases up to 120 MPa. The porosity of the coating is in the range of 10...15%. The coating thickness is usually no more than 1 mm, since when it increases, stresses arise in the sprayed layer, tending to separate it from the surface of the part.
Plasma-arc spraying in combination with simultaneous surface treatment with a rotating metal brush makes it possible to reduce the coating porosity to 1-4%, and increase the total spraying thickness to 20 mm.
Plasma-forming gases are nitrogen, helium, argon, hydrogen, their mixtures and a mixture of air with methane, propane or butane.
Plasma spraying uses wire, including powder type, powders from ferrous and non-ferrous metals, nickel, molybdenum, chromium, copper, metal oxides, metal carbides and their compositions with nickel and cobalt, metal alloys, composite materials (nickel-graphite, nickel-aluminum, etc.) and mechanical mixtures of metals, alloys and carbides. The regulation of the spraying mode makes it possible to apply both refractory and low-melting materials.
Metals and non-metals (plastic, brick, concrete, graphite, etc.) can serve as the basis for plasma spraying. To apply coatings on small surfaces, a microplasma spraying method is used, which saves the loss of the sprayed material (spraying width 1-3 mm).
Plasma torch details
In order to increase the adhesion of sprayed coatings, protect against oxidation, and reduce porosity, the method of plasma spraying is used in a protective environment (vacuum, nitrogen, a mixture of nitrogen with argon and hydrogen) and with the use of special nozzles that close the area between the sprayer and the treated surface. A promising direction in plasma spraying technology is supersonic spraying.
The plasma spraying process includes 3 main stages:
1) Surface preparation.
2) Spraying and additional coating treatment to improve properties.
3) Machining to achieve finishing dimensions.
The preliminary dimensions of the surfaces to be sprayed must be determined taking into account the thickness of the spray and the allowance for subsequent machining. Surface transitions should be smooth, without sharp corners, in order to avoid peeling of the coating. The ratio of the groove width or hole diameter to its depth must be at least 2.
Parts must be thoroughly cleaned and degreased before spraying. Repair parts with oily grooves or channels should be heated in an oven at a temperature of 200-340 degrees. for 2-3 hours to evaporate the oil.
Next, the surface is activated - giving it a certain roughness to ensure adhesion. Activation is carried out by blowing the part with compressed air with an abrasive or cutting a torn thread.
The abrasive is chosen with a grain size of 80 ... 150 according to GOST 3647, or iron / steel shot DChK, DSK No. 01 ... 05 according to GOST 11964 is used.
Metal shot is not used for processing heat-resistant, corrosion-resistant steels and non-ferrous metals and alloys, since it can cause their oxidation.
The surface roughness for plasma spraying should be 10...60 Rz, the surface should be matte.
Surfaces that are not subject to abrasive treatment are protected by screens. The airflow area must be 5+/-2 mm larger than the nominal size of the sprayed surface.
Thin parts are fixed in fixtures to prevent them from warping during processing.
The distance from the nozzle to the workpiece during abrasive blasting should be within 80 ... 200 mm, smaller values are taken for harder materials, larger ones for soft ones. After that, the parts are dedusted by blowing with compressed air.
The time interval between cleaning and spraying should be no more than 4 hours, and when spraying aluminum and other rapidly oxidizing materials - no more than an hour.
Torn thread cutting instead of abrasive blasting is used for parts with the shape of bodies of revolution. The thread is cut into lathe with a conventional threaded cutter, offset below the axis of the part. The thread is cut without cooling in one pass. The thread pitch is selected according to table 1.
For plasma spraying, powders of the same fraction should be used, the shape of the particles is spherical. The optimal particle size for metals is about 100 microns, and for ceramics - 50...70 microns. If the powders were stored in leaky containers, they must be calcined at a temperature of 120 ... 130 degrees for 1.5-2 hours in an oven.
Those parts of the part that are not sprayed are protected by asbestos or metal screens, or by coatings.
The preliminary heating of the part before spraying is carried out by a plasma torch to a temperature of 150 ... 180 degrees.
Processing modes are determined empirically. The average values of plasma spraying modes are as follows:
1) The distance from the nozzle to the part is 100...150 mm.
2) Jet speed — 3...15 m/min.
3) The speed of rotation of the part is 10 ... 15 m / min.
4) Spray angle - 60...90 degrees.
The total thickness of the coating is gained in several cycles with overlapping of the deposition strips by 1/3 of the diameter of the deposition spot.
After deposition, the part is removed from the plasma torch, protective screens are removed and cooled to room temperature.
Figure 1 - Schematic diagram of plasma powder spraying: 1 - plasma gas supply, 2 - plasma torch cathode, 3 - cathode housing, 4 - insulator, 5 - anode housing, 6 - powder feeder, 7 - powder carrier gas supply, 8 - plasma arc, 9 - power source.
Figure 2 - Schematic diagram of plasma spraying using wire: 1 - plasma gas supply, 2 - plasma torch cathode, 3 - cathode case, 4 - insulator, 5 - anode case, 6 - wire feed mechanism, 7 - solid or flux-cored wire, 8 - plasma arc, 9 - power source.
Figure 3 - The structure of the coating sprayed by the plasma method
To improve the quality of sprayed coatings, the following methods are used:
1) running in rollers under electric current;
2) spraying with simultaneous processing with metal brushes;
3) melting of coatings from self-fluxing alloys. Reflow is carried out using furnaces, high-frequency current, heated molten salts and metals, plasma, laser or gas-flame methods. The melting temperature of the nickel-chromium-boron-silicon-carbon coating is 900..1200 degrees.
The final dimensions of parts after plasma spraying are obtained by turning and grinding with cooling with aqueous solutions and water-oil emulsions. Grinding wheels are selected from electrocorundum grade E on a ceramic bond, grain size 36 ... 46, hardness CH. Grinding modes are as follows: wheel rotation speed 25...30 m/s, wheel feed 5...10 mm/rev, workpiece rotation speed 10...20 m/min, workpiece feed 0.015...0.03 mm/ dv.h.
Further produce final control, if there are cracks, delaminations, risks, blackness on the surface of the sprayed part, the finishing dimensions are not maintained, then the part is returned for defect correction (no more than 1 time), while the spraying area should be increased by 10...15 mm around the perimeter.
The production of metal products is modernized as advanced technologies develop. The metal is more susceptible to moisture, therefore, in order to ensure a long service life and give parts, working mechanisms and surfaces the required properties, metal spraying is widely used in modern industry. Powder treatment technology consists in applying a protective layer to the base metal base, which provides high anti-corrosion characteristics of the sprayed products.
The metal surface after powder treatment acquires important protective properties. Depending on the purpose and field of application, metal parts are given refractory, anti-corrosion, wear-resistant characteristics.
The main purpose of spraying the metal base base is to ensure a long service life of parts and mechanisms as a result of the impact of vibration processes, high temperatures, alternating loads, and the influence of aggressive environments.
Metal spraying processes are performed in several ways:
- vacuum processing- the material, when strongly heated in a vacuum environment, is converted into vapor, which, in the process of condensation, is deposited on the surface to be treated.
- Plasma or gas-plasma spraying of metal– the processing method is based on the use of an electric arc formed between a pair of electrodes with inert gas injection and ionization.
- Gas dynamic processing method– a protective coating is formed upon contact and interaction of cold metal microparticles, the speed of which is increased by an ultrasonic gas jet, with the substrate.
- Spraying with a laser beam– the process is generated using optical-quantum equipment. Local laser radiation allows processing complex parts.
- Magnetron sputtering– is carried out under the influence of cathode sputtering in a plasma medium to deposit thin films on the surface. Magnetrons are used in magnetron processing technology.
- Protection of metal surfaces by ion-plasma method- based on the spraying of materials in a vacuum environment with the formation of condensate and its deposition on the base being processed. The vacuum method does not allow metals to heat up and deform.
The technological method of spraying parts, mechanisms, metal surfaces is selected, depending on the characteristics that need to be given to the sprayed base. Since the method of bulk alloying is economically expensive, in industrial scale widely use advanced technologies of laser, plasma, vacuum metallization.
Sputtering in magnetron installations
The metallization of surfaces using magnetron sputtering technology is based on the melting of the metal from which the magnetron target is made. Processing occurs in the process of impact action by ions of the working gaseous medium formed in the discharge plasma. Features of the use of magnetron installations:
- The main elements of the working system are the cathode, the anode, and the magnetic medium, which contributes to the localization of the plasma jet near the surface of the sputtered target.
- The action of the magnetic system activates the use of constant field magnets (samarium-cobalt, neodymium) mounted on a base of soft magnetic materials.
- When voltage is applied from the power source to the cathode of the ion setup, the target is sputtered, and the current must be maintained at a consistently high level.
- The magnetron process is based on the use of a working medium, which is a combination of inert and reaction gases of high purity, fed into the chamber of vacuum equipment under pressure.
The advantages of magnetron sputtering make it possible to use this processing technology to obtain thin metal films. For example, aluminum, copper, gold, silver products. Semiconductor films are formed - silicon, germanium, silicon carbide, gallium arsenide, as well as the formation of dielectric coatings.
The main advantage of the magnetron method is the high rate of target sputtering, particle deposition, the accuracy of chemical composition reproduction, the absence of overheating of the workpiece, and the uniformity of the applied coating.
The use of magnetron equipment for sputtering makes it possible to treat metals and semiconductors with a high rate of particle deposition, to create thin films with a dense crystalline structure and high adhesive properties on the sputtered surface. The main list of works on magnetron plating includes chromium plating, nickel plating, reactive deposition of oxides, carbo- and oxynitrides, high-speed copper surfacing.
Technology of ion-plasma surfacing
To obtain multimicron coatings on metal products, the method of ion-plasma spraying is widely used. It is based on the use of a vacuum environment and physical and chemical properties materials evaporate and disperse in an airless space.
Technologically complex process allows solving important technical problems for the metallization of products through the use of an ion-plasma spraying unit:
- Increase in wear resistance parameters, exclusion of sintering when operating products at high temperatures.
- Improving the corrosion resistance of metals during operation in aggressive aqueous, chemical environments.
- Giving electromagnetic properties and characteristics, operation within the infrared and optical range.
- Obtaining high-quality galvanic coatings, giving decorative and protective properties to products, processing parts and mechanisms used in various industries.
The process of ion-plasma spraying is based on the use of a vacuum environment. After ignition of the cathode, spots of the first and second levels are formed, which move at a high speed and form a plasma jet in the ion layer. The jet obtained as a result of eroding the cathodes passes through a vacuum medium and interacts with the condensed surfaces, depositing a densely crystalline coating.
The use of ion-plasma spraying makes it possible to apply protective coatings at a cathode ignition temperature of up to 100°C; it is distinguished by a fairly simple scheme for obtaining layers up to 20 μm thick.
With the help of ion-plasma spraying on the metal, it is possible to impart the required properties structurally. complex products non-standard geometric shape. After processing, the metal surface does not need to be covered with a finishing layer.
Features of plasma metallization
Along with ion-plasma spraying and magnetron metal processing methods, another method is used - plasma metallization. The main task of the technology is to protect products from oxidative processes in aggressive environments, improve performance, harden the treated surface, and increase resistance to mechanical stress.
Plasma spraying of aluminum and other metals is based on high-speed acceleration of metal powder in a plasma flow with the deposition of microparticles in the form of a coating layer.
Features and advantages of plasma spraying technology on metal:
- The high-temperature method of applying a protective layer to the treated surface (about 5000-6000 °C) takes place in a fraction of a second.
- Using the methods of controlling the gas composition, it is possible to obtain a combined saturation of the metal surface with atoms of powder coatings.
- Due to the uniformity of the plasma jet flow, it is possible to obtain an equally porous, high-quality coating. The end product is superior to traditional plating methods.
- The duration of the spraying process is low, which helps to achieve one hundred percent economic efficiency use of plasma equipment in different production scales.
The main components of the working installation are a high-frequency generator, a sealing chamber, a gas medium reservoir, pumping unit for pressure supply, control system. It is allowed to use the technology of plasma spraying on metal at home if available necessary equipment with a vacuum chamber - exposure to oxygen leads to the oxidation of hot metal surfaces and targets.
In the video: restoration of details by spraying.
Laser processing process
Surfacing of metals by the laser method makes it possible to restore parts and mechanisms with light fluxes generated from optical-quantum equipment. Vacuum laser deposition is one of the most promising methods obtaining nanostructured films. The process is based on the sputtering of a target by a light beam, followed by the deposition of particles on a substrate.
Advantages of the technology: ease of implementation of metallization, uniform evaporation of chemical elements, obtaining film coatings with a given stoichiometric composition. Due to the narrow focus of the laser beam in the place of its concentration, it is possible to obtain surfacing of the product with any metals.
Mechanisms for the formation of liquid-drop phases:
- Large droplets of molten target particles are formed by the action of a hydrodynamic mechanism. In this case, the diameter of large droplets varies in the range of 1–100 μm.
- Drops of medium size are formed due to the processes of volumetric vaporization. The droplet size ranges from 0.01-1 µm.
- When the target is exposed to short and frequent pulses of the laser beam in the erosive torch, target particles of small size - 40-60 nm are formed.
If all three mechanisms of the working process (hydrodynamics, vaporization, high-frequency pulse) act simultaneously in a laser installation when surfacing metals on a target, the acquisition of the required characteristics by the product depends on the magnitude of the influence of a specific surfacing mechanism.
One of the conditions for high-quality laser processing is the exposure of the target to such an irradiation mode as to obtain laser torches with the smallest inclusion of liquid-drop particles at the output.
Cold Spray Equipment
There are two options for protecting metals from the negative effects of external and working factors - alloying and deposition 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, the metallization industry uses the technology of applying electroplated coatings, using methods of immersing parts in a melt, using a vacuum medium in processing processes, and using magnetron equipment.
Sometimes detonation gas spraying is used, which accelerates particles to incredible speeds. Widely used plasmatrons, arc metallization, flame treatment, ion sputtering. The challenges of the industry dictate their own conditions, and the need arose for engineers to create inexpensive, easy-to-use equipment for which the properties of heated compressed air can be used.
The concept of powder metallization appeared with the addition of finely dispersed ceramics or particles to the metal powder solid metal. It is used for work with aluminum, nickel, copper.
The result of the experiments exceeded expectations, allowing to solve the following tasks:
- The heating of the compressed air in the chamber leads to an increase in pressure, which causes an increase in the flow rate of the deposit from the nozzle in installations.
- When metal particles are collected in a high-velocity gas medium, they hit the substrate, soften and stick to it. And ceramic particles compact the formed layer.
- The use of powder technology is suitable for plating ductile metals - copper, aluminium, nickel, zinc. After spraying, the product can be machined.
Thanks to the successful work of engineers, it was possible to create a portable apparatus that allows coatings to be metallized on all industrial enterprises and at home. The requirements for the 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 the data for aluminum chrome plating at home. Before applying electroplating, it is required 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 the metallization of products allows us to solve technical questions associated with an increase in anti-corrosion, strength, operational 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 installations (24 photos)
The main differences between plasma metallization and other melting methods are higher temperature and higher power, which provides a significant increase in process productivity and the ability to apply and melt any heat-resistant and wear-resistant materials (Fig. 4.8). For plasma spraying, argon and nitrogen gases are used to provide the jet temperature. For plasma metallization, UPU and UMN installations are widely used, which include a rotator, a protective chamber, a powder dispenser, a power source and a control panel.
The main part of the installation is a plasma torch, the service life of which is determined by the resistance of the nozzle. The period of operation of the plasma torch is short; therefore, its wearing parts are made replaceable. Current sources are welding generators PSO-500 or rectifiers AND PN-160/600.
Rice. 4.8. Scheme of the plasma spraying process:
1 - powder dispenser; 2 - cathode; 3 - insulating gasket; 4 - anode; 5 - carrier gas; 6 - coolant; 7 - plasma gas
As a plasma-forming gas, argon or less scarce and cheap nitrogen is used. However, it is more difficult to strike an arc in a nitrogen environment and a much higher voltage is required, which is dangerous for service personnel. A method is used in which the arc is ignited in an argon medium with an arc excitation and burning voltage less, and then they switch to nitrogen. The plasma-forming gas is ionized and exits the plasma torch nozzle in the form of a jet of small cross section. Compression is facilitated by the walls of the nozzle channel and the electromagnetic field that arises around the jet. The temperature of the plasma jet depends on the current strength, type and flow rate of the gas and varies from 10,000 to 30,000 °C; gas flow rate 100-1500 m/s. Argon plasma has a temperature of 15,000-30,000 °C, nitrogen - 10,000-15,000 °C.
In plasma metallization, a granular powder with a particle size of 50-200 microns is used as the applied material. The powder is fed into the arc zone by a carrier gas (nitrogen), melted and transferred to the workpiece. The flight speed of powder particles is 150-200 m/s, the distance from the nozzle to the surface of the part is 50-80 mm. Due to the higher temperature of the applied material and the higher velocity of the sprayed particles, the bond strength of the coating with the part in this method is higher than in other metallization methods.
Plasma metallization, occurring at a high temperature of the plasma jet, allows you to apply any material
sala, including the most wear-resistant, but this raises the problem of subsequent processing of superhard and wear-resistant materials.
The use of pulsed laser radiation, the duration of which is milliseconds, makes it possible to obtain minimal heat-affected zones that do not exceed several tens of microns. The minimum volumes of the melt and the minimum heat input into the welded part make it possible to reduce longitudinal and transverse deformations and thereby keep the precision dimensions of the part within the tolerance field of several microns. The pointing accuracy and locality of the laser beam action makes it possible to weld strictly defined geometric sections of the part, providing a minimum machining allowance, which is 0.2-0.5 mm. Since the zones of thermal influence are very small during pulsed laser cladding, the substrate remains practically cold, and the cooling rate of the liquid phase of the metal melt reaches 102–103 °C/s. Under these conditions, the process of self-hardening takes place, which leads to the formation of an extremely fine structure with increased wear resistance.
When compared, almost all fundamental technical differences between the technology of electric arc surfacing and pulsed laser surfacing are the result of the fact that the arc is a concentrated welding energy source, and the laser beam is a highly concentrated energy source. Pulsed laser surfacing, compared to arc surfacing, is characterized by minimal melt volumes, heat-affected zones, and, accordingly, significantly lower transverse and longitudinal shrinkages.
After arc surfacing, allowances can reach several millimeters, which necessitates subsequent machining. The use of an electric arc as an energy source is accompanied by its force action on the liquid phase of the metal melt, as a result, undercuts are formed that do not occur during laser cladding. Electric arc surfacing requires preliminary and concomitant heating of surfacing sites and subsequent heat treatment and “and type from laser cladding.
Laser cladding technology can be used to restore worn out molds, dies and eliminate various defects that form during the manufacturing process. molds and stamps. Types of defects that can be eliminated using laser cladding: HRC hardness test points, cracks, nicks, burrs, cavities and pores, fire cracks, adhesive setting points. The technological process of laser cladding is a simultaneous supply to the defect site of laser radiation and filler wire in an inert gas environment. The filler material, melting, fills the place of the defect. After laser cladding, minimal mechanical processing is required compared to traditional cladding methods. The high accuracy of pointing the laser beam to the defect site, the locality of the action of laser radiation makes it possible to weld strictly defined areas of defective parts (Fig. 4.9).
The short duration of the process, the duration of the laser radiation pulse, which is a few milliseconds, as well as the exact dosage of energy, ensure minimal heat-affected zones and the absence of a part leash. Laser cladding can significantly reduce the labor intensity of tooling repair and, as a result, the cost due to the exclusion from the process of preheating, subsequent heat treatment, the need to remove the chromium coating from the surface and then apply it if the part is chrome plated. The advantages of laser cladding are listed in Table. 4.2.
To prevent oxidation of the molten metal, the welding zone is protected with inert gases, for example, a mixture of argon and helium. For surfacing of large units (up to several meters long), solid-state laser systems equipped with fiber-optic systems are used. A technology has been developed for eliminating defects in the form of hot and cold non-through cracks formed during stick-electrode arc welding using pulsed laser radiation from solid-state lasers.
Welding of several cracks using pulsed laser radiation makes it possible to implement the so-called "cold" welding mode, in which there is no heating weld repaired area, which allows you to maintain the mechanical strength of the welded joint and avoid metal tempering in the seam.
The use of a fiber optic system with a length of several meters allows repairs to be made in the most inaccessible geometries. This technology can be used to eliminate various defects formed during electric arc welding - cracks, both cold and hot, shells, craters, fistulas, undercuts.
By the nature and conditions of operation, the side surface of high-pressure turbine blades is subjected to microdamages of mechanical, chemical and thermal influence. Damage analysis shows that about 70% of their total number are parts with surface defects up to 0.4-2.0 mm deep. The use of fiber-optic systems for delivering a laser beam to a defect opens up the possibility of repairing a turbine blade without dismantling it. The size of the heat affected zone does not exceed 15 µm. The structure of the deposited layer is finely dispersed.
Rice. 4.11. Cross section in the place of the non-soldered tube of the refrigerator section
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Rice. 4.12. Polished section of a defect processed in the welding-soldering mode
In the process of manufacturing water sections, defects in the form of non-solders may occur. A technology has been developed to eliminate leaks in sections by the method of pulsed laser soldering and welding (Figures 4.11 and 4.12).
To eliminate leaks in a solder joint, pulsed laser radiation from a solid-state laser is used. A television system built into the laser emitter using target designation based on an He-Ne (helium-neon) laser makes it possible to accurately direct the laser beam to the defect site. Equipping the laser with a fiber optic system makes it possible to eliminate defects in hard-to-reach places and make a quick transition from one defect to another.