Plasma arc cutting knowledge
Cutting using the thermal energy of the plasma arc is called plasma arc cutting. The principle of plasma arc cutting is that a high-temperature and high-speed plasma arc is used as a heat source, and the parts to be cut are used to create a scour force by using its high-temperature method, and the process of mechanically manufacturing melted metal or non-metal is blown to form different parts. process, as shown in Figure Schematic diagram of plasma arc cutting.
Plasma arc is an ideal cutting heat source, it can cut aluminum, copper-nickel, titanium, cast iron, stainless steel and high-alloy steel that cannot be cut by oxy-acetylene flame and ordinary arc or are difficult to cut, and can cut any difficult. Molten metals and non-metals. Moreover, the cutting speed is fast, the production efficiency is high, the deformation of the heat affected zone is small, the incision is narrow, smooth, neat, non-sticky, and the quality is good.
Plasma arc cutting adopts DC power supply with steep drop 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 connected in series with more than two ordinary arc welding generators. The electrode adopts thorium tungsten electrode or cerium tungsten electrode. The working gas is nitrogen, argon, hydrogen and their mixtures, 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: first select the appropriate power according to the thickness and material properties of the cutting piece, 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, you can obtain good quality. kerf.
Plasma arc cutting principle
The principle of plasma arc cutting is essentially different from that of general oxyacetylene flame cutting. It mainly relies on the high temperature and high speed plasma arc and its flame flow to partially melt and evaporate the material to be cut and blow it away from the substrate, forming a narrow slit with the movement of the plasma arc torch.
The temperature of the plasma arc column is high, far exceeding the melting point of all metals and non-metals, so the plasma arc cutting process does not rely on oxidation reaction, but on melting to cut materials, so it is much more applicable than the oxygen-acetylene flame cutting method. , capable of cutting most metal and non-metal materials.
When a transferred 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 ( Figure Schematic diagram of energy dispersion during plasma arc cutting), in which the energy of the anode spot has the strongest thermal effect on the incision.
L1 – the length of the arc column cutting area;
L2 – the length of the active puncta cleavage region;
L3 – the length of the plasma flame cutting zone;
1-Arc column action area;
2-Active spot action area;
3- Plasma flame zone
Plasma arc cutting classification
Plasma arc cutting is divided into three types: ordinary plasma arc cutting, water recompression plasma arc cutting and air plasma arc cutting.
Ordinary plasma arc cutting
Ordinary plasma arc cutting is divided into transfer arc and non-transfer arc, and non-transfer 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 arc cutting. The high gas and cutting gas of plasma arc cutting 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 low-current plasma arc (microbeam plasma arc) can be used to cut thin plates.
Water Recompression Plasma Arc Cutting
In addition to cutting the 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 shrinkage effect of the arc: Part of the compressed water is evaporated and decomposed into hydrogen and oxygen to participate Forming cutting gas: Due to the presence of oxygen, especially when cutting low carbon steel and low alloy steel, a violent oxidation reaction is caused, which enhances the burning and melting of the material.
Water recompression plasma solitary cutting 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, b respectively represent two spray forms of compressed water, of which the radial water spray type has a stronger compression effect on the arc. The disadvantage of water recompression plasma arc cutting is: because the cutting gun is placed in water, the water in the gun body must be drained before arc ignition, so the flow of ion gas increases, arc ignition is difficult, and the no-load voltage of the power supply must be increased; The high-frequency electricity of arc ignition has a strong absorption effect, so the isolation of the gun body and water must be enhanced in the structure of the cutting gun, and the power of the high-frequency oscillator must be increased; the resistance of water is much smaller than that of air, so double arcs are prone to occur.
Air plasma arc cutting
Air plasma arc cutting is divided into two forms. The ion gas and cutting gas shown in Figure Schematic diagram of the principle of air plasma arc cutting a are both compressed air, so the structure of the cutting torch is simple, but the oxidizing property of the compressed air is very strong, so the tungsten electrode cannot be used. Pure zirconium, pure lead or their alloys are made of inlaid electrodes. The plasma gas shown in Figure Schematic diagram of the principle of air plasma arc cutting b is inert gas, and the cutting gas is compressed air, so the structure of the cutting torch is complicated, but tungsten electrodes can be used. The temperature of the air plasma arc is (18000 ± 1000) C, and the decomposed and ionized oxygen will have a strong oxidation reaction with the metal to be cut, so it is suitable for cutting carbon steel and low alloy steel.
Air plasma arc cutting abandons the traditional inert gas as the ion gas, and uses the inexhaustible dry air to directly connect 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 that of nitrogen plasma arc and oxygen plasma arc. Therefore, it can not only be used for cutting stainless steel and aluminum alloy, but also suitable for cutting carbon steel and low alloy steel.
Since the plasma arc contains oxygen, when cutting carbon steel, the exothermic reaction of oxygen and iron in the kerf 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 kerf , So not only the cutting speed is fast, but also the cutting surface is smoother, the lower line of the incision is basically free of slag, and the inclination angle of the cutting surface is also small (generally below 3°). However, air will oxidize tungsten in a high temperature state. For this reason, zirconium, hafnium or their 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 arc cutting method are:
(1) There is an oxide layer on the cut surface, and pores will be generated in the weld during welding. Therefore, the cutting edge for welding needs to be ground with a grinding wheel, which is labor-consuming.
(2) Electrodes and nozzles are easy to wear and have a short service life and need to be replaced frequently.
Due to the low cost of compressed air, especially the fast cutting speed and small thermal deformation of carbon steel and low-alloy steel, which are the most widely used in the processing industry, they are highly valued by the industrial sector. When cutting stainless steel and aluminum alloys, oxygen reacts with chromium in aluminum and stainless steel to form high melting point oxides, so the cutting surface is rough.
The comparison of cutting speed between air plasma arc cutting and gas cutting method is shown in Figure 2-16. It can be seen from the figure that the cutting speed of the air plasma arc is several times faster than that of the low pressure diffusion cutting nozzle.
1- 250 A air plasma arc cutting (cutting speed);
2- -250 A air plasma arc cutting (practical cutting speed);
3- High pressure diffusion cutting nozzle (cutting oxygen pressure 1.57 MPa) gas cutting;
4-Low pressure diffusion cutting nozzle (cutting oxygen pressure 0.69 MPa) gas cutting
Air plasma arc cutting is generally divided into high current cutting method and small current cutting method according to the working current used. Large electric aviation gas plasma arc cutting, its working current is more than 100 A, practically 150-300 A, using water-cooled cutting torch structure, its application is not very wide.
Small current air plasma arc cutting, its working current is less than 100 A. Due to the low cutting current, the heating of nozzles and electrodes is reduced. Generally, there is no need to use water cooling, but air cooling, which simplifies the structure of the cutting torch, reduces the weight, and reduces the volume, and can even be made into a miniature pen-shaped cutting torch. Figure Schematic diagram of small current air plasma arc cutting shows the principle diagram of low current air plasma arc cutting.
Since the small cutting torch can be used for hand-held cutting and can be installed on various small cutting machines, the power consumption is also small, and the same cutting torch can be used to cut carbon steel, stainless steel and non-ferrous metals, with good adaptability, especially suitable for many Variety, small batch production of small and medium-sized enterprises.
The process characteristics of low current air plasma arc cutting are:
(1) Thin metal with a thickness of 0.1 mm can be cut, including galvanized sheet and pre-painted color sheet, and the quality of the coating layer will not be affected 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 machining accuracy of the parts, and solve the difficulties of curved edges and internal openings that are difficult to be processed by mechanical cutting.
(2) The cutting quality is good, the incision width is small, and the slag adhesion is less. When the thin plate is cut by the contact process, the quality is even better than that of gas cutting, and the cutting deformation is greatly reduced.
(3) Contact cutting can be performed. By properly selecting the nozzle aperture and gas flow rate, some cutting torches with a cutting current lower than 70 A can cut the nozzle directly against the workpiece (ie, contact cutting), without the double arc phenomenon, which greatly improves the cutting performance. as well as operability and safety.
Plasma arc cutting Features
Due to the concentration of plasma arc energy, high temperature, great mechanical impact force, and stable arc, plasma arc cutting has the following characteristics:
(1) All 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. Cutting stainless steel, aluminum, etc. up to 200 mm or more. Non-transferred plasma arcs can also cut various non-metallic materials, such as refractory bricks, concrete granite, silicon carbide, etc.
(2) The cutting speed is fast and the productivity is high. For example, when cutting 10mm thick aluminum plate, the speed can reach 200~300m/h, and when cutting 12mm thick stainless steel plate, the speed can reach 100~130m/h.
(3) High cutting quality. The incision is narrow, smooth and neat, the deformation of the incision and the heat-affected zone are small, and the hardness and chemical composition change is small. Usually, it can be welded directly after cutting, and there is no need to process and clean the groove.
(4) The ability to cut thick plates is not as good as gas cutting, and the width of the incision and the bevel of the cutting surface are larger. However, when cutting thin plates, a special cutting torch or process can be used to obtain a nearly vertical cutting surface.