Some of the parts and components of modern machines and equipment operate in such conditions that they must be both mechanically strong and stable when exposed to high temperatures and chemically aggressive media, etc. To carry out such products from the same material is almost impossible and economically unfeasible. It is much more profitable and easier to manufacture the item, such as structural steel meeting the requirements of mechanical strength, and its surface is covered by a more expensive heat-resistant, wear-resistant and acid resistant alloy. Using a variety of protective coatings on the composition of metallic and nonmetallic materials can Prida Vat detail required in the general mechanical, thermal, dielectric and other properties. The most versatile and sophisticated methods of applying protective coatings are surfacing and plasma arc spraying [1, 2]. The coating material, a specially prepared in the form of powder or wire melkogranulirovannogo, served in the plasma jet flow, and heated and melted in this thread, moved with him to obrab Pipeline product. Simultaneously, the plasma jet fuels product. Advantages of plasma coating method over the other (electroplating, vacuum, oxygen, acetylene, etc.) are as follows:
- high temperature plasma stream can melt and cause the most refractory materials;
- plasma flow makes it possible to obtain alloys of different material properties or to be multi-layered coatings of various alloys. This opens up a wide possibility of obtaining coatings that combine a variety of protective properties;
- possibilities of this method is not limited to the shape and size of the workpiece;
- Plasma arc - the most flexible source of heat, allowing a wide range to adjust its energy characteristics.
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| The experimental setup for plasma welding metal powder 1 - power supply direct action of the arc, 2 - ballasts, 3 - supply arc indirect, 4 - 5 Oscillator - nozzle for a plasma-forming gas, 6 - burner housing 7 - hole for shielding gas 8 - feeder for feeding powder 9 - tube through which the gas supply, carrying the powder |
protection layer deposited from the environment provided by a stream of inert gas surrounding the arc and submitted to the outer tip of the plasma torch. Filler powder is also an inert carrier gas from a special powder feeder. using plasma deposition metal powders can be heat-resistant and most durable coating of nickel-based alloys and cobalt. This method allows to obtain thin, uniform coating with a smooth non-porous surface, often requiring no additional machining. In current-carrying plasma surfacing filler wire arc burns between the cathode plasma torch and wire, the anode being uniformly supplied into the space between the nozzle and the workpiece. With this method provides a high performance welding process at a small depth of penetration of the base metal, but the possibility of obtaining thin and uniform layer with this method of deposition is limited. In addition, the use of filler material in powder form can be used for virtually any alloy cladding, it is difficult to implement when using wire as a filler material. When the plasma as the plasma surfacing, protective and transporting gas is commonly used argon. The gas flow rate and the range of operating currents and voltages during surfacing about the same as that of a plasma welding. In contrast to the surfacing of the deposition process is characterized by a greater concentration of heat flow and high flow velocity of the plasma jet. The appearance of this difference due to the fact that during plasma spraying as a coating material applied refractory metals (tungsten, molybdenum, tantalum, etc.) or metal oxides (Al2O3, MgO, ZrO2), silicides (MoSi2), carbide (B4C, SiC) , borides (ZnB2, HfB2), ie, non-metallic materials with very high melting point. These materials are prepared in the form of melkogranulirovannogo powder (particle size 40-70 mm), passing through the plasma jet, time to warm up in the main only to the plastic state. However, due to high velocity plasma jet powder particles acquire a large kinetic energy and the collision with the sprayed surface of flattened root in it and fill irregularities. Pr1 The kinetic energy of the particles released in the form of heat, the temperature rises, which provides strong adhesion between the particles and the surface of the product. To use the plasma arc spraying indirect, burning between the water-cooled tungsten cathode and a copper nozzle (anode) and is blown through a nozzle in the form of a plasma torch. Scheme of the plasma torch for the deposition shown in Fig. 10. At dosoplovom and vnutrisoplovom areas of plasma torch is. The powder, together with its transporting gas is fed into a small hole near the exit of the nozzle, that is blown into the most high-temperature region of the plasma jet. The heating of the powder is in the area, which starts from the anode spot of the plasma torch and ends. The efficiency of heating of the powder particles by the time of their stay in the plasma, ie, the distance from the nozzle to the product and the power of the plasma jet. Improve the capacity can be achieved by the use of diatomic gases with high heat content, such as N2 and H2. Due to the high thermal conductivity of hydrogen increases the length of the high-temperature flame, which makes it possible to raise the temperature of the powder by removal of a plasma torch on the workpiece. However, the velocity of the plasma jet with distance from the nozzle is reduced. Therefore it is necessary to maintain an optimal distance from the nozzle to the workpiece surface, the magnitude of which depends on the profile, deposition from the coating material and products, and varies from 4 to 20 mm. Power plasma torch used for spraying, you can also increase with increasing length and dosoplovogo vnutrisoplovogo sections of the arc column, but the excessive increase vnutrisoplovogo plot arc column arc difficult arousal, usually produced by high-frequency breakdown. Increasing the length of the nozzle channel over a certain limit leads to the phenomenon of shunting of the arc column and reduce the efficiency of the plasma torch. Usually in the plasma torches for spraying nozzle diameter is 5 - 6 mm, length dosoplovogo site - 4-8 mm, and length of the channel nozzle - 10-18 mm. Increasing capacity by increasing the plasma torch arc current is limited to resistance nozzle (anode). When the nozzle erosion appears not only in danger of destruction, but also the possibility of contamination of the deposited material, which can dramatically reduce the quality of the coating. In the plasma torch for the deposition of the rotation of the anode spot on the inner wall of the nozzle is created either by entering the vortex system of the working gas, or with the help of a magnetic field formed by the DC coil placed over the nozzle. When using hydrogen as the working gas in order to reduce the heat flux directed from the arc to the nozzle, the hydrogen is used in mixtures with argon, which provides thermal insulation from the nozzle of the arc column. Usually in the plasma torches for spraying the current does not exceed 400 and the voltage by using a mixture of nitrogen and hydrogen with argon, depending on their consumption varies in the 60-100. Thus, the power does not exceed 40 kW. The productivity of the deposition process, depending on the coating material is 2-3 kg / h The quality of surface treatment with plasma spraying is determined by the maximum adhesion of the coating material to the product and the minimum porosity of the coating. High quality coverage is provided for under the physical properties of materials, such as in the case of close values of their coefficients of thermal expansion. Improving the quality achieved with careful surface preparation products to the process (degreasing, sand blasting, drying, etc.) and proper choice of deposition parameters of the regime. These issues are discussed in detail in the literature.
See also:
Plasma Welding
Plasma Welding Introduction
Plasma Welding Technique
Microplasma Welding
Gases for plasma processing of materials
Separation of plasma jet cutting
Compression of the arc
The energy properties of the plasma arc
Rationalization of plasma welding
Plasma welding and spraying
The plasma melting and remelting
Plasmatron. Requirements for plasmatron
Plasmatron. Schemes, classification
Classification by type of electrode plasma torches
Classification of torches by the nature of the current
Structure of the plasmatron basic units
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