It has important practical significance for developing laser processing market.
Cut-through energy-balanced slab cutting, only the energy provided by the small and medium power laser is not enough to cut through the workpiece, so the use of aerobic cutting is necessary. Most of the energy required for the oxygen assisted cutting process is provided by the ferrooxidation reaction. The energy required to cut is composed of the energy of the heated workpiece, the energy that melts the workpiece, the energy lost by heat transfer, and the heat carried away by the oxygen flow: 1999 On December 20th, the temperature is received, Tm2 is the melting point temperature of iron oxide, T0 is the temperature before starting the cutting, Hm is the melting heat of steel, C is the specific heat capacity of steel, G> is the specific heat capacity of gas, m is the unit time The mass of the gas ejected from the nozzle is related to the supply pressure, mr is the mass of oxygen reacted with iron per unit time, and R is the Perclet (quasi) number, which is the cutting speed Vc slit width bs and thermal diffusivity K Function: The mass of gas ejected per unit time is determined by: m = n (D / 2) 2 VP (Ts / T) (3) where: D is the nozzle outlet diameter, V is the flow rate at the gas outlet, P is The density under standard conditions, when Ts is the temperature steel plate under standard conditions, the energy required for cutting is calculated to be 5892 W. At this time, Fe combustion becomes FeO, and the energy is Hf (0 is the reaction heat of FeO, XAhhr is iron Burning rate. The calculation shows that iron is 100% burning. The amount is 7536W, which means that 78.2% of iron combustion is enough to supply the speed that all the energy reactions required for cutting can be achieved, that is, the maximum cutting speed that can be used during the cutting process. It is completely determined by this self-sustaining combustion process due to the slitting. The metal itself participates in the reaction and provides the energy needed for cutting. Therefore, if the oxygen can be supplied infinitely, the thickness of the plate that can be cut is theoretically unlimited, which provides a sufficient theoretical basis for the medium and small power laser cutting thick plates. Difficulties in the process of thick plate cutting (1) The quasi-steady-state combustion process is difficult to maintain. During the actual cutting process, the thickness of the plate that can be cut is limited, which is closely related to the fact that the cutting front iron cannot be stably burned. The temperature at the top of the slit must reach the energy released by the ignition reaction of the ignition point alone. In fact, the combustion process cannot be ensured continuously. On the one hand, the oxygen flow from the nozzle is continuously cooled by the slit, which lowers the temperature of the cutting front. On the other hand, the ferrous oxide layer formed by the combustion covers the surface of the workpiece, hindering the diffusion of oxygen. When the concentration of oxygen is reduced to a certain extent, the combustion process will be extinguished. When the laser beam is cut by the conventional convergence beam, the area of ​​the laser beam acting on the surface is small. Because the laser power density is high, not only the laser radiation In the area, the surface temperature of the workpiece reaches the ignition point, and due to heat conduction, a wider area reaches the ignition temperature and the diameter of the oxygen flow on the surface of the workpiece is larger than the diameter of the laser beam, indicating that not only in the laser irradiation region, but also intensely Combustion reaction, and when the laser beam is irradiated on the periphery of the spot, the cutting speed is relatively slow. The surface of the workpiece is burned faster than the cutting head. After burning for a period of time, due to oxygen The concentration drops and the combustion process is extinguished. Only when the cutting head travels to this position, the combustion reaction restarts the cutting front. The combustion process is performed periodically, which causes the temperature of the cutting front to fluctuate and the quality of the cut becomes poor.
(2) When the oxygen purity and pressure in the thickness direction are difficult to maintain a constant thick plate cutting, the decrease in oxygen purity is also an important factor affecting the quality of the incision. The purity of the oxygen stream has a strong influence on the cutting process. When the oxygen flow purity decreases by 0.9%, the ferrite combustion rate will decrease by 10%; when the purity decreases by 5%, the combustion rate will decrease. 37 The decrease of the combustion rate will greatly reduce the energy input into the slit during the combustion process and reduce the cutting speed. At the same time, the content of iron in the liquid layer of the cutting surface is increased, so that the viscosity of the slag is so large that the slag is discharged, so that serious slag is formed in the lower part of the incision, making the quality of the incision unacceptable in order to keep the cutting stable. The purity and pressure of the oxygen flow cut in the thickness direction are kept substantially constant. In the conventional laser cutting process, a common conical nozzle is often used, which can meet the use requirements in the thin plate cutting. However, when the thick plate is cut, the shock wave is easily formed in the flow field of the nozzle as the supply pressure is large, and the shock wave is generated. There are many hazards to the cutting process, reducing the purity of the oxygen flow, affecting the quality of the incision. There are generally two ways to solve this problem: adding a preheated flame layer around the cutting oxygen stream as the main cutting oxygen stream, and the outer layer is buffering oxygen flow, buffering oxygen. The presence of the flow adds the surrounding pressure of the central airflow, so it is not easy to generate shock waves, which improves the purity of the central airflow, thereby improving the cutting speed and quality. However, when the supply pressure is too high, shock waves are generated, and the airflow is generated. The turbulent flow is large, the cutting air pressure varies with the distance between the nozzle and the workpiece, and there is also a large change. Therefore, the supply pressure should not be too high. The inner wall of the nozzle should be designed reasonably to improve the flow field characteristics. The diameter of the inner wall of the supersonic nozzle is firstly shrunk and then enlarged. A supersonic flow is produced at the outlet where the supply pressure can be large but does not generate shock waves. The supersonic airflow speed can quickly blow away the molten metal in the slit, which improves the cutting speed. When the laser cutting is performed by the supersonic nozzle, the cutting quality is also ideal, because the cutting pressure of the supersonic nozzle on the surface of the workpiece is relatively stable. Therefore, it is especially suitable for thick plate cutting.
New process of laser thick plate cutting Low-power laser thick plate cutting The new developments in foreign countries are: preheating, defocusing laser sawing, double focusing, etc. This method differs from traditional laser cutting methods in A preheating flame is added next to the cutting edge. The area of ​​the preheating flame formed on the surface of the workpiece that exceeds the ignition temperature is larger than the area where the oxygen flow acts. It can act on the cutting front or on the airflow around it. The oxyacetylene flame is generated, and the laser beam and the splitting beam can also be used for preheating. The introduction of the preheating flame can maintain the quasi-steady state combustion process, and at the same time, it can preheat the cutting oxygen flow, which is beneficial to maintain the pressure and purity of oxygen. The oxyacetylene flame is used as the preheating flame. The 30 nm thick structural steel has been carried out. The st52-3 cutting method is characterized in that the laser focus is not located on the surface of the workpiece, but inside the nozzle, so that when the beam reaches the surface of the workpiece, it does not converge the beam, but diverges. Reasonable adjustment of the amount of defocus can ensure that the diameter of the beam acting on the surface of the workpiece is larger than the diameter of the oxygen flow to ensure the stability of the reaction. When the device is used to cut 50 mm thick structural steel, the power of the laser beam on the surface of the workpiece is only 1 kW. The gas supply pressure is about 8<10Pa, the distance between the workpiece and the nozzle is 4mm, and the quality of the obtained cut is also ideal. The surface roughness Ra=small is defocusing, the equipment is relatively simple, and the cutting process is stable. The disadvantage is that the cutting speed is low. When cutting 50mm thick plate, the best cutting speed can only reach 160mm/min. (3) Laser sawing method This method uses the reflection convergence system for cutting. The laser sawing processing head is as shown. . 13 is the reflection plane, 2 is the adaptive mirror, and 4 is the curved focusing mirror. When working, with the periodic change of the mirror surface of the adaptive mirror, the focus length follows and changes, so that the focus position vibrates within a few millimeters. Vibration cutting leads to higher cutting quality. In laser sawing, the roughness of the cutting surface decreases with the addition of adaptive mirror control. When the frequency is 50Hz, the surface roughness of laser sawing is lower than that of general laser cutting. .
The disadvantage of this method is that the cutting thickness is limited, generally not more than 30mm. The double focusing method uses a special lens to focus the beam twice in different parts. The cutting device is shown in the figure where Di is the diameter of the central portion of the lens and D2 is the edge of the lens. The radius of curvature of the center of the diameter lens is larger than that of the periphery. When the bifocal shape is actually cut, the lens is generally selected according to the thickness of the plate. The upper focus is on the upper surface of the workpiece, and the lower focus is on the lower surface of the workpiece. When cutting, since the energy density of the upper focus is concentrated on the upper surface, cutting is easy. The lower focus provides a high energy density at the lower edge of the slit, increases the local temperature, facilitates the blowing of the slag, improves the cutting thickness, and the cutting quality is also less sensitive to the focus drift during the entire cutting process. A particularly important disadvantage in the optical path is that the processing of the lens is complicated, and when the thickness of the workpiece changes, the lens is often replaced.
With the increasing demand for thick plate structural parts, improving the traditional laser cutting process, the development of new technology for thick plate cutting has important practical significance for opening up the laser processing market and expanding the application range of small and medium power lasers.
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