Among all types of plasma processing of materials plasma cutting is most popular, because in modern engineering are increasingly using special alloys, stainless steels, nonferrous metals and their alloys, for which the gas-oxygen cutting or other types of practice are of little use. Plasma cutting provides better performance compared with the oxygen and the cutting of ferrous metals and alloys.
The essence of the process of separating plasma cutting is intense local melting of the metal in the volume of the cavity cut heat generated by the compressed arc, and the removal of liquid metal from the cutting zone of a high-speed plasma stream emanating from the plasma torch nozzle channel.
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| 1 - cathode holder, 2 - cathode, 3 - the case of the plasma torch 4 - interelectrode insert, 5 - nozzle - anode, 6 - 7 plasma flow - the product. |
The greater the length of the nozzle channel, the higher the velocity of the plasma jet and better cut quality, as this is an increase in heat output and temperature of the plasma jet. However, for very long channels (more than 12 mm), reliability is reduced due to the destruction of the plasma torch nozzle thermal plasma flow or the formation of a double arc. The optimal length of the nozzle channel must be greater than the diameter of the nozzle is 1.5 ... 1.8. The best material for the nozzles is copper.
The nozzle and cathode are electrically isolated from each other. The material is an insulator must possess the following properties:
a) a high dielectric strength, as the duty arc excited by high-frequency discharge of the oscillator;
b) high mechanical strength;
c) high density, impermeability, as pass through channels to the plasma gas and cooling water.
Generated by the compressed plasma torch cutting is an arc transmitter of electrical energy into heat. Therefore, it is as part of the circuit is characterized by the electrical parameters (current, voltage), and as a source of heat - thermal (temperature, heat content). Compressed arc voltage depends on the structural dimensions of the plasma torch (diameter and length of the nozzle channel) on the current composition and plasma gas flow rate and distance from the end of the nozzle to the surface of the material being cut. The plasma temperature is the heat source parameter of the plasma torch. It varies as the cross section of the arc column, and along its axis. Temperature, as well as voltage, depends on many parameters of the regime. Determinants of which are current, and the flow of plasma gas, plasma arc column diameter (compression ratio of the arc).
An important parameter of the compressed heat of the arc is its heat content (enthalpy), ie the amount of heat contained in a unit volume or mass of the jet. Enthalpy of molecular gases (N 2 , H 2 ) is much higher monohydric (Ar, He), and their use as plasma-forming environment in the energy of more profitable. In addition, the reduced heat losses by radiation into the environment and in the walls of the plasma torch nozzle. In addition to these parameters is characterized by a compressed arc flow velocity of the plasma flow. Due to thermal and mechanical effects arc column penetrates and plunges into the thick metal. Because of this metal from the cut cavity melted and blown.
In contrast to the oxy-fuel cutting, in which singles out the flames a little heat, has a relatively low temperature and penetration into the metal you want to spend some time on the local heating of the cut metal up to its ignition temperature in a stream of oxygen, the plasma arc is due to the high temperature and flow velocity of the plasma cut into the metal is almost instantaneous, regardless of the nature of the material and its thermophysical properties.
At the optimum ratio of the thickness of the cut metal, power, compressed arc cutting speeds of P and the arc column penetrates the entire thickness of the metal and the anode spot is located in the lower part of it. Under these conditions, is provided a nearly vertical edges of the cut without burrs. The increase in cutting speed helps fixing the anode spot above the lower plane of the cut, resulting in a backlog of melting front at the bottom and narrow cut in it. Excessive increase in cutting speed leads to incomplete cutting of metal. By reducing the cutting speed lower than the optimum width of the cut at the bottom increases sharply.
The choice of plasma gas should be carried out based on the characteristics and type of plasma torch with a tungsten, hafnium, or other type of cathode. Argon is not appropriate to use in plasma cutting in terms of both quality and cutting performance and condition of the high cost of argon. Plasma cutting in the medium of technical nitrogen is a reliable, cost-effective and high-protsessamd and is recommended for cutting almost all construction materials. For example, when cutting stainless steels up to 40 mm thickness is approximately equal to the performance of process performance by using compressed air and a 2 ... 3 times higher than with argon. With increasing thickness of the material being cut cutting performance when using nitrogen is higher than when using compressed air. This is achieved by increasing the allowable current for the given parameters of the cathode and the plasma torch nozzle. Plasma arc cutting in air has several advantages. These include: lack of cost of production of plasma-forming gas and increase productivity while cutting carbon and low alloy steels. The disadvantages process are: low resistance electrodes of zirconium and hafnium, and the possibility of saturation of the surface of the cutting gases that make up the air.
To make plasma cutting of metal separation, it is necessary to melt a certain amount of it along the cutting line and then removed from the cavity of the cutting speed plasma flow. For melting the desired amount of metal on the cut line, bring a certain amount of heat. This heat enters into the material of the plasma arc column and is called the effective power of the arc q.
The value of q has a specific value for a given material, below which the cutting is not possible.
The molten heat of the plasma arc metal is formed on the frontal surface of the cutting is removed fast flow of the plasma jet. The flow rate of the plasma increases with increasing plasma gas flow rate and decreases with increasing diameter of the nozzle. Velocity of the molten metal from the cutting zone depends on the plasma flow velocity at the interface between the molten metal - plasma stream at the bottom of the cut metal. Velocity of the plasma flow can reach 800 m / s at the current value of 250 A. In this case, the cutting of the metal thickness of 5 = 5. .20 Mm at a speed vp = 1 ... 6 m / min and a cutting width of 4.6 mm, the velocity of the molten metal from the bottom of the cut is 20 ... 40 m / s.
Under the influence of the plasma jet on the front wall of the cut can be divided into three characteristic regions, which has its own mechanisms of interaction of the heat flow of the plasma jet to cut material. At the 1st section (cut from the upper surface of the metal to the underside of the anode spot) melting of the metal is by the thermal energy of the plasma arc column. Regulation of the heat flow through the thickness of the metal is due to the lag axis from the front of the plasma arc melting. At the 2nd part of the formation of the heat flux is due to the increase in thermal conductivity of plasma-forming gas with a decrease in its temperature, which decreases sharply with distance considered section from the end of the compressed arc plasma torch. In this region the thermal energy of the plasma flow is added to the energy from the anode spot of the arc, resulting in slightly ahead of the melting front in relation to other parts of it. However, this energy is much smaller than the energy of the plasma flow on a 3-m section of the formation of the heat flux is carried out by reducing the width of the cut in its lower part. The molten metal is removed from the front of fusion power flow of the plasma jet.
Plasma arc cutting of aluminum and its alloys can be performed using a plasma gas of compressed air or oxygen. When cutting with the rate of oxygen is reduced by about 10%. Profiles of plasma cutting can vary widely depending on the desired quality of cut, diameter and length of the channel of the plasma torch nozzle, plasma gas flow rate and other parameters.
In plasma cutting of titanium and its alloys hold special technological measures in order to obtain the cut surface, requiring no further machining, which is characterized by high labor intensity and low adaptability. The difficulties arise primarily because of oxidation and gas saturation of the surface layers of cut. Nitrogen, oxygen and hydrogen, penetrating into the metal, form a solid implementing solutions that have high hardness and low ductility and toughness. This feature of titanium and its alloys leads to maintenance of the cutting process with the greatest possible speed in order to ensure its minimum duration. If this is not the place to protect an additional cutting of an inert gas, or are cut by using argon as the plasma gas.
Air Plasma Cutting mild steel up to 80mm and non-ferrous metals Teschin to 60 mm is an inexpensive and effective way of cutting. Limiting the thickness of the cut metal is practically impossible to determine because it depends on the process technology and quality requirements for cut. Currently, the maximum thickness of the cut metal thickness of 160 mm limit. Due to the high temperature arc column plasma cutting process is universal, since the properties of the cut metal is practically no influence on the process of cutting.
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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