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The method of cutting by using the thermal energy of the plasma arc is called plasma arc cutting. The principle of plasma cut is to use a high-temperature and high-speed plasma arc as a heat source to partially melt the workpiece to be cut, and use the mechanical scour force of the compressed high-speed airflow to blow away the melted metal or non-metal to form a narrow incision. As shown in Figure Schematic diagram of plasma cut principle.
Plasma arc is an ideal cutting heat source, it can cut aluminum, copper, nickel, titanium, cast iron, stainless steel and high alloy steel, etc. metals and nonmetals. Moreover, the cutting speed is fast, the production efficiency is high, the deformation of the heat-affected zone is small, the cut is narrow, smooth, neat, non-sticky, and of good quality.
Plasma arc cutting adopts a DC power supply with steep external characteristics, which requires high no-load voltage and working voltage. Generally, the no-load voltage is between 150 and 400 V. There are two types of general power supply: one is a dedicated arc welding silicon rectifier power supply; the other can be used to connect more than two ordinary arc welding generators in series. The electrodes are thoriated tungsten or cerium tungsten. The working gas uses nitrogen, argon, hydrogen and their mixed gases, and nitrogen is commonly used.
The process parameters of plasma arc cutting mainly include no-load voltage, cutting current, working voltage, gas flow, cutting speed, distance from nozzle to cutting piece, distance from tungsten electrode to nozzle end face and nozzle size, etc. The selection method of process parameters is as follows: first, select the appropriate power according to the thickness of the cutting piece and the material properties, select the cutting current according to the power, then determine the nozzle aperture and electrode diameter, and then select the appropriate gas flow and cutting speed to obtain good quality Slit.
Plasma cut principle
The principle of Plasma cut is essentially different from that of general oxygen-acetylene flame cutting. It mainly relies on the high-temperature and high-speed plasma arc and its flame flow to locally melt and evaporate the cut material and blow it away from the substrate, forming a narrow kerf with the movement of the Plasma cut torch.
The temperature of the plasma arc column is high, far exceeding the melting point of all metals and non-metals, so the Plasma cut process does not rely on oxidation reaction, but to cut materials by melting, so it is much more applicable than the oxyacetylene flame cutting method. Capable of cutting most metal and non-metal materials.
When the transfer type plasma arc is used to cut metal materials, the heat source comes from three aspects: the radiation energy of the plasma arc column in the upper part of the incision, the energy of the anode spot (active spot) in the middle of the incision, and the heat conduction energy of the plasma arc flame in the lower part of the incision ( Fig. Schematic diagram of energy distribution during cutting), in which the energy of the anode spot has the strongest thermal effect on the cut.
Classification of Plasma cut
Plasma cut is divided into three types: ordinary Plasma cut, water recompression Plasma cut and air Plasma cut.
Ordinary Plasma cut
Ordinary Plasma cut can be divided into transferred arc and non-transferred arc, and non-transferred arc is suitable for cutting non-metallic materials. Figure Schematic diagram of plasma arc cutting principle is a schematic diagram of the principle of Plasma cut. The ion gas and cutting gas of Plasma cut share one gas, so the structure of the cutting torch is simple. In order to increase the plasma arc energy, the cutting gas should be diatomic gas. A small current plasma arc (micro-beam plasma arc) can be used to cut thin plates.
Water recompression Plasma cut
In addition to the cutting airflow, the water recompression plasma arc also ejects high-speed water flow from the nozzle. The high-speed water flow has three functions: increasing the cooling of the nozzle, thereby enhancing the thermal contraction effect of the arc; part of the compressed water is evaporated and decomposed into hydrogen and oxygen. Participate in the formation of cutting gas; due to the presence of oxygen, especially when cutting low-carbon steel and low-alloy steel, it causes a violent oxidation reaction, which enhances the combustion and melting of the material.
Water recompression Plasma cut is usually carried out in water, which not only reduces the thermal deformation of the cutting parts, but also absorbs cutting noise, arc ultraviolet rays, dust, smoke, splashes, etc., thus greatly improving the working environment. Figure Schematic diagram of water recompression plasma arc cutting principle-a and b show two forms of compressed water injection respectively, among which the radial water injection type has a stronger compression effect on the arc. The disadvantages of water recompression Plasma cut are: since the cutting torch is placed in water, the water in the torch body must be drained first when starting the arc, so the ion gas flow rate increases and arc starting is difficult, so the no-load voltage of the power supply must be increased; Arc striking high-frequency electricity has a strong absorption effect, so in order to enhance the isolation between the torch body and water in the structure of the cutting torch, the power of the high-frequency oscillator must be increased; the resistance of water is much smaller than that of air, so double-arc phenomenon is prone to occur .
Air Plasma cut
There are two types of air Plasma cut. The ion gas and cutting gas shown in Figure Schematic diagram of air plasma arc cutting principle-a are both compressed air, so the structure of the torch is simple, but the compressed air is highly oxidizing, so tungsten electrodes cannot be used, but tungsten electrodes should be used. Pure zirconium, pure hafnium or their alloys are used to make mosaic electrodes. The plasma gas shown in Figure Schematic diagram of air plasma arc cutting principle-b is an inert gas, and the cutting gas is compressed air, so the structure of the torch is complicated, but tungsten electrodes can be used. The temperature of the air plasma arc is (18000±1 000) ℃, and the oxygen after decomposition and ionization will have a strong oxidation reaction with the cutting metal, so it is suitable for cutting carbon steel and low alloy steel.
Air Plasma cut abandons the traditional inert gas as the ion gas, and uses the inexhaustible dry air to be directly connected to the nozzle as the working gas after being compressed. Air is a mixture of nitrogen (about 80% by volume) and oxygen (about 20% by volume), and its cutting performance is between nitrogen plasma arc and oxygen plasma arc. Therefore, it can be used not only for cutting stainless steel and aluminum alloy, but also for cutting carbon steel and low alloy steel.
Since the plasma arc contains oxygen, when cutting carbon steel, the exothermic reaction between oxygen and iron in the incision provides additional heat, and at the same time generates FeO slag with low surface tension and good fluidity, which improves the flow characteristics of the molten metal in the incision , so not only the cutting speed is fast, but also the cutting surface is smoother, the off-line of the incision basically does not stick to slag, and the inclination angle of the cutting surface is also small (generally below 3°). However, air will produce an oxidation reaction to tungsten in a high-temperature state. For this reason, zirconium-lead or its alloys are used as electrodes. In order to improve the life of the electrode, the electrode is generally made into a mosaic shape with direct water cooling. When the current is small, water cooling is not required.
The main disadvantages of the air Plasma cut method are:
There is an oxide layer on the cutting surface, and pores will be generated in the weld seam during welding. Therefore, the cutting edge used for welding needs to be polished with a grinding wheel, which is time-consuming.
Electrodes and nozzles are prone to wear and tear, have a short service life and need to be replaced frequently.
Due to the low cost of compressed air, especially the cutting speed of carbon steel and low alloy steel, which are most used in the processing industry, is fast and the thermal deformation is small, it is highly valued by the industrial sector. When cutting stainless steel and aluminum alloy, oxygen reacts with chromium in aluminum and stainless steel to form high melting point oxides, so the cutting surface is rough.
The cutting speed comparison between air Plasma cut and gas cutting is shown in Fig. Comparison of air plasma arc cutting speed and gas cutting speed. It can be seen from the figure that the air Plasma cut speed is several times faster than that of the low pressure diffusion cutting nozzle.
Air Plasma cut is generally divided into high current cutting method and low current cutting method according to the working current used. For high current air Plasma cut, its working current is above 100A, and practically it is mostly 150~300A. It adopts a water-cooled cutting torch structure, and its application is not very wide.
Low current air Plasma cut, its working current is less than 100 A. Because the cutting current is low, the heat of the nozzle and electrode is reduced. Generally, it is not necessary to use water cooling, but air cooling is sufficient, so that the structure of the cutting torch is simplified, the weight is reduced, and the volume is reduced. It can even be made into a miniature pen-like cutting torch. Figure Schematic diagram of small current air plasma arc cutting shows the principle diagram of low current air Plasma cut.
Because the small cutting torch can be used for hand-held cutting or installed on various small cutting machines, the power consumption is also small, and it can be used to cut carbon steel, stainless steel and non-ferrous metals with the same cutting torch, which has good adaptability and is especially suitable for It is used by small and medium-sized enterprises with multi-variety and small-batch production.
The technological characteristics of small current air Plasma cut are:
It can cut thin metals with a thickness of 0.1 mm, including galvanized sheets and pre-coated color sheets, without affecting the quality of the coating layer after cutting. Therefore, in the cutting of sheet metal and thin plate parts, it can replace mechanical cutting methods such as shearing and sawing, improve the processing accuracy of parts, and solve difficulties such as curved edges and internal openings that are difficult to process by mechanical cutting.
The cutting quality is good, the incision width is small, and the slag adheres less. When using the contact process to cut thin plates, its quality is even better than that of gas cutting, and the cutting deformation is greatly reduced.
Contact cutting can be implemented. By properly selecting the nozzle aperture and gas flow, some cutting torches with a cutting current lower than 70A can cut the nozzle directly against the workpiece (that is, contact cutting) without double arc phenomenon, which greatly improves the cutting performance and Operability and safety.
See Table Comparison of operating performance between contact and non-contact cutting processes for the performance comparison between low-current plasma arc contact and non-contact cutting.
Plasma cut Features
Due to the concentration of plasma arc energy, high temperature, great mechanical impact, and stable arc, Plasma cut has the following characteristics:
All current metals can be cut. Including ferrous metals, non-ferrous metals and various high-melting point metals, such as stainless steel, heat-resistant steel, cast iron, tungsten, molybdenum, titanium, copper, aluminum and their alloys. Cut stainless steel, aluminum, etc. up to 200mm or more. The non-transferred plasma arc can also cut various non-metallic materials, such as refractory bricks, concrete, granite, silicon carbide, etc.
Fast cutting speed and high productivity. For example, when cutting a 10mm thick aluminum plate, the speed can reach 200~300m/h, and when cutting a 12mm thick stainless steel plate, the speed can reach 100~130m/h.
High cutting quality. The incision is narrow, clean and tidy, the deformation and heat-affected zone of the incision are small, and the change in hardness and chemical composition is small. Usually, it can be welded directly after cutting, and there is no need to process and clean the groove.
The ability to cut thick plates is not as good as that of gas cutting, and the incision width and the bevel angle of the cutting surface are larger. However, when cutting thin plates, special cutting torches or processes can be used to obtain near-vertical cutting surfaces.