Electric locomotive and urban rail vehicle manufacturing technology C2 laser cutting machine cutting technology and application Guo Shulin (CSR Zhuzhou Electric Locomotive Co., Ltd., Zhuzhou 412001, Hunan).
With the development of laser technology, CO2 laser cutting is widely used in modern processing plants. Compared with traditional plasma and flame cutting technologies, CO2 laser cutting has high cutting precision, good surface quality, complex cutting shape and cutting material diversity. Etc.
1CO2 laser cutting principle and process characteristics CO2 laser cutting is to use a focusing mirror to focus the CO2 laser beam on the surface of the material to melt the material, while blowing the melted material with the cutting gas coaxial with the laser beam, and the laser beam and material along the material A certain trajectory is made to move relative to each other to form a slit of a certain shape. The CO2 laser cutting machine is best suited for cutting low carbon steels <12mm thick, <6mm stainless steel, <4mm aluminum sheets.
The cutting precision of CO2 laser equipment guarantees that the quality of its products must meet the requirements of ISO9013-221 when cutting with maximum cutting capacity. It is the highest standard among the three types of thermal cutting. When materials and gases complying with national standards are used, no slag is used; when stainless steel or aluminum alloy is processed, the slits are smooth, the surface is not oxidized, and the sharp corners are good. CO2 laser cutting has a narrow slit width (generally 0.1~0.5mm), high precision (general hole center distance error 0.1~0.4mm, contour size error 0.10.5mm), and rough surface roughness (general Ra is 12.525pm). Process characteristics, the slit is narrow, the slits are parallel on both sides and perpendicular to the surface, and the slits are generally welded without further processing. [. The laser cutting speed is fast, for example, using 3kW laser power, the cutting speed of carbon steel of 8mm thick is 1.6m/min, the cutting speed of stainless steel of 2mm thickness is 3.5m/min, and the heat affected zone of the cutting method is small and the deformation is extremely small. The comparison of laser cutting, oxyacetylene cutting and plasma cutting characteristics is shown in Table 1 (the cutting material is a 6 mm thick low carbon steel plate).
Table 1 Comparison of laser cutting, oxyacetylene cutting and plasma cutting methods Cutting method Cutting/heat affected zone/mm slitting shape Cutting speed Laser parallel fast oxygen acetylene comparison parallel slow plasma wedge shape and fast tilting 2 Main parameters affecting cutting quality CO2 The laser equipment is equipped with a full-featured numerical control system, which can overcome the problem of poor precision of general cutting equipment. Usually, the dimensional tolerance of the product, the verticality (slope) of the cutting surface, and the average roughness are used as the main quality parameter evaluation indicators. Amount, slag, kerf, etc. are used as auxiliary qualitative indicators. It should be emphasized that during the cutting process, the deviation component caused by the verticality (slope) tolerance of the cutting surface must be within the limit tolerance range, so the bottom surface of the workpiece with a small slit should be used as the measurement reference surface. In actual use, the inspection of cutting quality can be referred to. Ignoring the influence of the accuracy of the numerical control equipment itself, the parameters that have a great influence on the laser cutting quality are as follows.
2.1 Nozzle diameter, cutting air pressure nozzle is generally made of copper, small volume, is a vulnerable part, need to be replaced frequently. In use, a certain pressure Pn (gauge pressure Pg) is introduced from the side of the nozzle, and the nozzle pressure is discharged from the nozzle outlet to reach the surface of the workpiece at a certain distance. The pressure is called the cutting pressure P. Finally, the gas expands to the atmosphere. Pressure Pa. As Pn increases, the flow velocity increases and Pc increases.
It can be calculated by the following formula: different gases have different pressure thresholds. When the nozzle pressure exceeds this value, the airflow is a normal oblique shock wave, and the gas flow rate transitions from subsonic to supersonic. This threshold is related to the Pn, Pa ratio and the degree of freedom n of the gas molecule. When n=5, Pn is 1.89 bar. When P>4 bar, the normal oblique shock wave of the airflow becomes positive shock, P. drops, and the airflow The speed is reduced and eddy currents are formed on the surface of the workpiece, which weakens the effect of the airflow to remove the molten material and affects the cutting speed. Therefore, in the case of oxygen cutting, in order to ensure the gas flow rate, the cutting gas pressure is usually below 1.8 bar. In addition, the choice of nozzle diameter and cutting pressure is also related to factors such as the material to be cut and the purity of the cutting gas.
2.2 Lens size and focus position One of the advantages of laser cutting is that the energy density of the beam is high, generally > 10W/cm2. Since the energy density is proportional to 4/nd (d1 is the focal spot diameter), d1 should be as small as possible. In order to produce a narrow slit. At the same time, the å¿’ is also proportional to the focal depth of the lens, and the smaller the focal depth, the smaller it is. However, the cutting has a splash, and the lens is too close to the workpiece to be damaged. Therefore, the focal length of 5"7.5" (127190 mm) is widely used in industrial applications of high-power CO2 laser cutting, and the actual focal spot diameter is between 0.10.4 mm.
For high quality cuts, the effective depth of focus is also related to the lens diameter and the material being cut. It is therefore important to control the position of the focus relative to the surface of the material being cut. Considering the cutting quality, cutting speed and other factors, in principle, the metal material with thickness < 6mm, the focus is on the surface; the carbon steel with thickness > 6mm, the focus is above the surface; the stainless steel with thickness > 6mm, the focus is below the surface. Generally, when cutting materials with a thickness of 4mm or less, a 5" lens is used. When the flying optical path cutter cuts the proximal end and the distal end, the optical path length is different, and the beam size before focusing has a certain difference. The larger the diameter of the incident beam, the larger the diameter of the focal spot. In order to reduce the change of the focal spot size caused by the change of the beam size before focusing, the compensation optical path system of x and direction is added to the flying optical path cutting machine, that is, the compensation optical path is shortened when the optical path of the cutting distal end is increased; When the end optical path is reduced, the compensation optical path is increased to keep the optical path length uniform.
2.3 Laser cutting power The surface quality of laser cutting is mainly determined by the spatial shape of the laser beam, and the spatial shape of the laser beam is determined by the cavity of the laser, and the electromagnetic field has a simple lateral direction in a circular symmetric cavity under given boundary conditions. The spatial shape of the electromagnetic field and the transverse electromagnetic distribution within the cavity are called intracavity transverse modes and are represented by TEM. Among the laser powers below 4 kW, there are mainly two laser beam modes: 00 mode and 01 mode. 00 is called the base film, which can reach the diffraction limit and improve the surface quality of the cut. The 00 mode accounts for a large proportion when the laser output power is small. This is why the lower the laser power, the better the quality of the product. Therefore, in actual operation, the laser cutting power is minimized without affecting the cutting efficiency.
2.4 Cutting speed If the cutting speed is too fast, the lower edge of the incision and even the cutting surface will stick to the slag, and even the workpiece will not be cut through. If the speed is too slow, the efficiency will be low, the cutting surface will not be smooth, and the lower edge of the incision will be slag. In actual use, the cutting should be performed at a relatively high speed as far as possible without affecting the cutting quality.
2.5 cutting gas laser when cutting metal materials, generally can be divided into oxygen combustion cutting and melting cutting, of which oxygen combustion cutting is the most widely used. The cutting method uses a laser as a preheating heat source, and uses an active gas such as oxygen as a cutting gas. The gas blown out on the one hand reacts with the cutting metal to generate an oxidation reaction, which releases a large amount of oxidation heat, and on the other hand, melts the oxide and melts. The material is blown out of the reaction zone to form an incision in the metal. The faster the oxygen-assisted combustion cutting speed, the smaller the heat penetration and the better the cutting quality. When the laser is melted and cut, the metal material is melted by laser heating, and then a non-oxidizing gas (argon, helium, nitrogen, etc.) is blown through a nozzle coaxial with the beam, and the liquid metal is discharged by a strong pressure of the gas to form a slit. Laser melting cutting is mainly used for the cutting of some non-oxidizable materials or active metals. The faster the cutting speed, the easier the slits are to form corrugations, and the worse the cutting quality. In practical applications, cutting carbon steel uses oxygen as a cutting gas, and nitrogen is used as a cutting gas when cutting stainless steel, aluminum and its alloys.
3 Experiences and environmental protection issues that can be used in actual use 3.1 Centralized gas supply When using multiple laser cutting machines, especially when cutting stainless steel and aluminum plates, nitrogen (mainly used for optical path protection and cutting) is used in large quantities, traditional nitrogen The busbar is difficult to meet the gas supply demand. In order to solve the gas supply problem, we use the Tailai Huateng cryogenic liquid cylinder dewar bottle for centralized gas supply. The volume of each dewar is 21 times that of the ordinary gas cylinder. The flow is: low temperature liquid nitrogen cylinder liquid nitrogen busbar Vaporizer regulator outlet valve (multi-channel) multiple cutting equipment nitrogen inlet.
3.2 The utilization rate of unified nesting software materials has a great impact on the cost of cutting materials. Improving the utilization rate of materials and reducing material costs are the top priorities of management and control. There are a variety of nesting materials for laser use, and multiple nesting platforms are used for nesting, resulting in low utilization of side margins. The unified nesting software is used to integrate the nesting platform of the laser cutting machine, so that all kinds of CNC cutting equipment can be used as a nesting software, which can greatly improve material utilization and programming efficiency, and optimize management.
3.3 Laser generator gas pressure control The actual CO2 laser generating gas is high purity (99.999%) carbon dioxide, nitrogen and helium, which is higher than the purity of the laser gas specified by the equipment manufacturer. If the gas contains impurities, it will affect the laser beam, which will affect the quality of the cut. In the past, when the operator was operating the laser, the laser gas was used until the alarm was replaced. At this time, the pressure of the cylinder was about 3 bar. The impurities in the cylinder flowed into the generator tube with the gas, causing contamination of the lens and the generator tube. The quality of the laser beam also brings a lot of inconvenience to the maintenance of the laser generator. When the pressure of the cylinder is about 15 bar, the laser gas is exchanged to prevent gas impurities from flowing into the generator. If a disposable filter is installed at the outlet of the cylinder, the effect will be more significant.
3.4 Employee Health and Environmental Protection Problems The dust, laser radiation and harmful gases generated during laser cutting have a considerable impact on the human body and the environment. Soot is formed by partial vaporization of metal by oxidation or oxidation in the air; laser radiation is always present during cutting; harmful gases are ozone and nitrogen oxides formed by the activation of nitrogen and oxygen in the air, which are more important to the human body and the environment. Big harm. Therefore, it is necessary to properly wear labor insurance products, and carry out strict maintenance on the dust removal facilities to protect employees' health and industrial discharge standards.
4 Conclusion CO2 laser cutting is a new cutting technology with high cutting precision and good surface quality. This paper analyzes the main parameters affecting the cutting quality and the method of improving the cutting efficiency from the perspective of process equipment.
In addition, it is pointed out that factors such as programmers, equipment operators and working environment also have a great impact on improving cutting quality.
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