In view of the electrode - the cathode plasma torches DC can be divided into two groups: plasma torches with rod and plasma torches with distributed katodom.V plasma torches with rod cathode, the cathode spot is fixed on the end of the electrode, and plasma torches with a distributed cathode - intensely moved by the gas-vortex or magnetic rotation of the developed surface of the electrode.
In plasma torches for metal used mainly rod cathodes, subdivided into three main types: consumable, and gazozaschischenny plenkozaschitny (Scheme 1, 2, 3).
Expended, usually graphite, the electrode was shown in the diagram of the plasma torch with water stabilized. Despite the fact that graphite has a high melting temperature, when heated to this temperature, it does not melt, but sublimes, and this is due to its increased consumption.
Gazozaschischenny tungsten electrode - the most common of all types of electrodes. When you work in an inert (argon, helium) and reducing (nitrogen, hydrogen) environments, the cathode of a refractory tungsten rod with a load of 15-20 A/mm2 almost spent. Tungsten cathode as compared to graphite is much stronger and has many times more conductive.
To improve the working conditions of cathode arc plasma torches are made of tungsten with a small amount of thorium oxide or lanthanum (1.5-2%). It is believed that thoriated tungsten electrodes have a radiation power, so in recent years increasingly used lantanirovannye tungsten electrodes. The impurities of thorium oxide or lanthanum increases the emission properties of tungsten cathode (electron work function is lowered from 4.5 to 2.63 eV), which provides the best conditions for ignition, a higher allowable current density and stability of the arc. In addition, tungsten with these additives is the recrystallization temperature of 600 ° K higher than that of pure tungsten and, consequently, saves the fibrous structure and plasticity to a higher temperature. However, the resistance of the tungsten electrode by adding oxygen to the atmosphere due to the formation of volatile compounds is sharply reduced. For example, in the plasma torches used for cutting, using technical nitrogen containing 3-5% O2, on a tungsten cathode after 2-3 h of operation at a current of a crater formed by a 300-400, the offset is relative to the center axis of the nozzle causes a corresponding displacement of the arc column and leads to the phenomenon of double arcing. Therefore, when using oxygen-plasma-forming medium is fed into the primary nozzle argon, protects the tungsten electrode from exposure to oxygen-containing working gas fed into the secondary nozzle. A system with a dual gas flow has significant drawbacks. If you use a cheap working gas such as air, is still a need to use scarce argon. In this complicated construction of the plasma torch and heat the working gas is deteriorating, as the most efficient gas heated near the cathode region. More recently, a new type of cathode - plenkozaschitny rod cathode that has high resistance to the gas containing oxygen (in air, carbon dioxide, nitrogen, technically). It is a rod of zirconium or its alloys, pressed into a copper holder. The mechanism of such an electrode should be studied in detail yet, but we can no longer be considered established, lennym that the relatively high thermal stability of zirconium, which has a relatively low melting point (2125 ° K), largely due to the formation of stable films of its refractory oxides and nitrides, which protects pure zirconium by evaporation. Refractory compounds, forming a film at ordinary temperatures are insulators, while at temperatures near the melting point (3200 ° K), and lose their insulating properties, and become agents of the ionic conductivity (σ> = 1 ohm-1 cm-1 *). Thus, the resistance of zirconium cathode is determined by its interaction with the thermochemical plasma-forming medium. If required conditions, providing high resistance of the cathode, is the presence of gaseous oxygen and nitrogen environment, as well as the intensity of such cooling, the temperature at which the cathode spot would not exceed the decomposition temperature of the fusible-tight connections. Zirconium cathode erosion increases sharply at a higher percentage of oxygen than in air (20%) and especially in the presence of hydrogen in the gas phase. Of particular importance from the viewpoint of increasing the resistance of zirconium cathode has its design and cooling system. At present in the plasma torches for air plasma cutting achieved quite satisfactory resistance of zirconium cathode currents at work on a 400. Application plenkozaschitnogo electrode AC plasma torches is excluded because of its active destruction of half-periods of reverse polarity. The peculiarity of the zirconium inserts a gradual deepening of the lower base in a copper holder to the extent of erosion. Zirconia inset consumed mainly in the inclusion of the arc, apparently due to thermal stress fracture of the film. At a certain deepening of the bottom surface of the insert followed by ignition of the arc with a copper holder, so the displacements of the cathode spot on the zirconium inserts and rigid fixation should be applied to gas-vortex or magnetic stabilization of the arc, providing a strict alignment of the arc column to the electrode and the nozzle of the plasma torch. When using the zirconium electrode may be a large current density, reaching 80-100 A/mm2 than using a tungsten electrode. When using a plasma torch with an oxidizing plasma-forming medium to high currents (1000 A and above) uses a variety of types of cathodes are distributed, the most common of which are hollow, disc and ring (Scheme 4, 5 and 6). The disadvantages are the complexity of distributed cathodes of their design, the difficulty of uniform movement of the cathode spot on the entire surface of the electrode and low arc stability, the increase in the cathode region of the arc voltage and the associated increase in power losses in the electrode. Therefore, the plasma torches for metal cathodes are distributed have not found practical application. There is a classification of plasma torches and plasma-forming medium. The composition of the plasma-forming medium is dictated by the process and in turn is the determining factor when choosing a scheme of the plasma torch.
By chemical attack on the workpiece and the electrode of the plasma torch all plasma-forming medium can be divided into three major groups: inert, reducing and oxidizing. Physical properties and evaluation of the role of each of the plasma-forming gases have already been discussed above.
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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