A machine that uses the thermal energy of a plasma arc to achieve cutting is called a plasma cutting machine. The principle of plasma arc cutting is to use a high-temperature and high-speed plasma arc as a heat source to partially melt the parts to be cut, and use the mechanical scouring force of the compressed high-speed airflow to blow away the molten metal or non-metal to form a narrow incision process.As shown in Figure Plasma cutting schematic.
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 oxyacetylene flame and ordinary arc or are difficult to cut, and can cut any refractory steel. 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 cutting machines all use DC power supplies with steep drop characteristics, which require 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 cutting machine 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.
The cutting principle of the plasma cutting machine is essentially different from the general oxygen-acetylene flame cutting principle. 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 oxy-acetylene flame cutting method. , capable of cutting most metal and non-metal materials.
When the 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 distribution during cutting), in which the energy of the anode spot has the strongest thermal effect on the incision.
The classification of plasma cutting machine cutting is divided into three types: ordinary plasma arc cutting, water recompression plasma arc cutting and air 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 cutting principle is a schematic diagram of the principle of plasma arc cutting. The plasma arc cutting ion gas and cutting gas share the same 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.
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 Participate in the composition Cutting gas; due to the presence of oxygen, especially when cutting mild steel and low alloy steel, it causes a violent oxidation reaction, which enhances the burning and melting of the material.
Water recompression plasma arc 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. Figures Schematic diagram of water recompression plasma arc cutting principle-a and b respectively show two spray forms of compressed water, among 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 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. Pure zirconium, pure hafnium 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 an inert gas, and the cutting gas is compressed air, so the structure of the cutting torch is complicated, but a tungsten electrode can be used. The temperature of the air plasma arc is ( 18 000 ± 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, tungsten, tungsten 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 arc cutting method are:
There is an oxide layer on the cutting 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.
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 cut surface is rough.
The comparison of cutting speed between air plasma arc cutting and gas cutting method is shown in Figure Comparison of air plasma arc cutting speed and gas cutting speed. It can be seen from the figure that the cutting speed of air plasma arc is several times faster than that of the low pressure diffusion cutting nozzle.
Air plasma arc cutting is generally divided into high current cutting method and low current cutting method according to the working current used. The high-current air plasma arc cutting has a working current of more than 100 A, and is practically 150 to 300 A. It adopts a water-cooled cutting torch structure, and 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. It 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.
Because 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 varieties , The process characteristics of small-scale production of small and medium-sized enterprises using 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.
See Table Process operation performance comparison between contact and non-contact cutting for the comparison of process operation performance between low-current plasma arc contact and non-contact cutting.