Plasma arc welding and plasma arc cutting is formed on the basis of ARGON tungsten arc welding, which is a promising advanced technology in welding field. Using the high temperature of plasma arc, it can weld metal materials that arc welding can weld, and even solve the problem of very thin metal welding that argon arc welding can not solve. Plasma arc welding and cutting has become another important processing technology for alloy steel and non-ferrous metals. The technology is now widely used.
Generally, welding arc is carried out without external constraints. The diameter of arc column changes with the arc current and voltage. The energy is not highly concentrated and the temperature is limited to 5730 ~ 7730℃, so it is called “free arc”. If the arc column of the free arc is forced to “compress”, the conductive cross section can be shrunk to a smaller size, so that the energy is more concentrated and the gas in the arc column is fully ionized, as shown in 《Schematic diagram of plasma arc generating device》. Such arc is called plasma arc.
1—tungsten electrode ; 2—intake-tube ; 3—water inlet ; 4—exhalent siphon; 5—nozzle; 6—plasma arc; 7—weldment ; 8—high frequency generator
Besides solid, liquid and gas, plasma is the fourth existing form of substances ionized by gaseous substances. It is a gas-like substance with good electrical conductivity composed of positively charged ions, negatively charged electrons and some neutral atoms that are not ionized. Plasma arc is a kind of compressed arc. The transverse section of arc column decreases and the ionization degree increases with the increase of current density. Plasma arc has more concentrated energy and higher column temperature than ordinary free arc. Choosing proper specification can make plasma arc flame flow with high velocity and produce great mechanical scouring force, so it becomes a more ideal energy for welding and cutting. The forced compression on the arc column of free arc is generally called compression effect. There are three forms of compression effect:
Mechanical compression effect As shown in figure 《Compression effect of plasma arc》a, in a very (cathode) and weldment between (the anode) and a high voltage, by stimulating make gas ionization form arc at this point, the gas with certain pressure 3000 force ACTS on the arc column, forcing the pores by water-cooled nozzle, arc column was mechanical compression, arc column cross-sectional area was reduced, known as the mechanical compression effect.
thermal contraction effect When arc by water-cooled nozzle, and by constantly to the plasma gas flow (argon, nitrogen, hydrogen, etc.) of cooling effect, make the arc column periphery to form a low temperature air layer, degree of ionization fell sharply, forcing arc column conductive section narrowed, further improve the current density, the shrinkage is called thermal contraction effect of arc column.(《Compression effect of plasma arc》b)
Magnetic contraction effect The current density of plasma arc at nozzle is greatly increased after mechanical compression and thermal contraction effect. If the arc is regarded as a bundle of parallel current lines in the same direction, the electromagnetic force generated by its own magnetic field attracts each other, thus generating electromagnetic contraction force. Such magnetic contraction force forces the arc to be further compressed, as shown in Figure 《Compression effect of plasma arc》C.
(a）Mechanical compression correspondence; (b)Thermal pinch effect; (c)Magnetic pinch effect
1—tungsten electrode ; 2—arc ; 3—workpiece ; 4—nozzle; 5—cooling water flow; 6—cooling draught
Under the action of the above three effects, the arc column is compressed to a very fine degree, and the gas in the arc column is highly ionized. The temperature is up to 16000-33000 ℃, and the energy density increases sharply. Moreover, the arc stiffness is good, and the arc has strong mechanical scour force, forming the plasma arc of high-energy beam.
The characteristics of plasma arc are as follows:
1) High temperature and energy concentration Due to the compression of the arc column of the plasma arc, the gas reaches a high degree of ionization, resulting in high temperature. The energy concentration of the plasma arc at the center temperature of the arc column is 18000-24000K, and its energy density can reach 105-106W /cm2. The center temperature of the free argon tungsten arc column is 14000-18000K and the energy density is less than 105W/cm2. Therefore, plasma arc used for cutting, can cut any metal, such as good thermal conductivity of copper, aluminum, and high melting point of platinum, tungsten, all kinds of alloy steel, cast iron, low carbon steel and stainless steel.
2) In the arc column of plasma arc with good electric conductivity and thermal conductivity, charged particles are often in the accelerated electric field and have high electric conductivity and thermal conductivity. So in a smaller section can pass through a larger current, more heat conduction.
3) The arc stiffness is good and the stability is strong. After compression, the shape of the plasma arc has changed greatly and becomes cylindrical. Compared with the general arc, the divergence Angle of the arc column is only 5°, while the tungsten-argon arc in free state is 45°. Therefore, the plasma arc has better stability, less sensitivity to the change of arc length, better stiffness, and less influence of external air flow and magnetic field on the plasma arc
Small, less arc blowing and drift phenomenon.
4) Large impact force under the action of mechanical compression, thermal contraction and magnetic contraction three kinds of compression effects, the plasma arc has a small section, large current density, high temperature rise, the internal has a great expansion force, forcing charged particles from the nozzle high-speed ejecta, flame flow speed can reach 300m/s above. Therefore, can produce a lot of impact force, used for welding, can increase the melting depth; Used for cutting, can blow off slag; Used for spraying, can spray powder and so on.
The plasma arc can be divided into three types: non – transfer arc, transfer arc and combined arc.
The non-transfer arc tungsten pole is connected to the negative pole of the power supply, and the nozzle is connected to the positive pole of the power supply. A plasma arc is generated between the tungsten pole and the inner surface of the nozzle (Figure. 《Plasma arc form》a). Continuous plasma gas is sent through the arc space to form a plasma flame ejected from the nozzle. This kind of plasma arc is generated between tungsten pole and nozzle. The workpiece itself is not energized, but is indirectly heated and melted. The effective utilization rate of heat is not high, so it is not suitable for welding and cutting thick materials.
Transfer arc tungsten pole is connected to the negative pole of the power supply, and the workpiece and nozzle are connected to the positive pole of the power supply. First, a small arc is ignited between the tungsten electrode and the nozzle, then the circuit between the tungsten electrode and the workpiece is connected, and then the circuit between the nozzle and the tungsten electrode is cut off. At the same time, the arc between the tungsten electrode and the nozzle is extinguished, and the arc is transferred to the direct combustion between the tungsten electrode and the workpiece. This kind of arc is called the transfer arc (Figure 《Plasma arc form》b). This kind of plasma arc can directly heat workpiece and improve the effective utilization rate of heat, so it can be used for welding and cutting workpiece of medium thickness or above.
Combined arc a plasma arc with both transfer arc and non-transfer arc is called combined arc (Figure.《Plasma arc form》c). The two arcs of the combined arc are respectively supplied by two power sources. The plasma arc between tungsten pole and workpiece is produced by the main power supply, which is the main welding heat source. Another power source is added between the tungsten pole and the nozzle to produce a small arc, called a maintenance arc. The function of maintaining arc is to maintain gas ionization during the whole welding process, that is, when the plasma arc is interrupted under the influence of some factors, the plasma arc can be reignited immediately by maintaining arc. Combined arc is mainly used for micro arc plasma welding and spray welding of powder materials.
（a）nontransferred arc ；（b）transferred arc；（c）Associative arc
1—tungsten electrode；2—plasma gas;3—nozzle；4—Coolant hole；5—workpiece；6—Non-transferable arc；7—transferable arc
There are many applications of plasma arc. With the continuous maturity and development of plasma arc technology, it will be widely used in all aspects of production. At present, its main application has the following aspects：
1) Plasma arc welding can weld alloy steel, stainless steel, nickel and nickel alloy, titanium and titanium alloy, aluminum and aluminum alloy with high melting point (Figure 1-39). Argon – filled plasma arc welding can also weld tungsten, molybdenum, niobium, tantalum, zirconium and their alloys.
2) Plasma arc cutting Plasma arc can cut stainless steel, cast iron, titanium, molybdenum, tungsten, copper and copper alloy, aluminum and aluminum alloy and other difficult to cut materials (Figure 1-40) using non-transfer plasma arc, but also can cut granite, silicon carbide and other nonmetals.
3) Plasma arc surfacing can be divided into powder plasma arc surfacing and wire filled plasma arc surfacing (Figure 1-41). Plasma arc surfacing uses plasma arc as primary heat source and non-transfer arc as secondary heat source, which is characterized by high deposition speed, low dilution rate, and easy control of dilution rate and surface shape.
4) Plasma spraying Plasma spraying is a process method in which the powder spraying material is heated and accelerated by plasma flame flow (i.e., non-transferred plasma arc) as the heat source and sprayed to the workpiece surface to form a spraying layer (Figure 1-42).
The characteristics of plasma arc make it have many important applications in metallurgy, chemical industry and space technology. Plasma arc has high temperature, high energy concentration, fast airflow speed, can use various working media, and its power and various characteristics have a wide range of adjustment, these characteristics make the practical application of plasma arc has a very broad prospect.
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