The technical difficulties of laser cutting machine cutting thick plates

by:Caodahai     2021-08-30
When cutting thick plates, the cutting speed is quite slow, and the speed of iron oxygen combustion on the surface of the workpiece is faster than the speed of the cutting head. This indicates that not only a strong combustion reaction takes place in the laser radiation area, but also combustion occurs at the same time in the periphery of the laser beam irradiated spot. The diameter of the oxygen flow acting on the surface of the workpiece is larger than the diameter of the laser beam.   There are generally three ways to solve this problem: (1) add a preheating flame around the cutting oxygen flow (2) add an auxiliary oxygen flow around the cutting oxygen flow (3) rationally design the inner wall of the nozzle to improve the characteristics of the air flow field. After the combustion continues for a period of time, the combustion process is extinguished due to the decrease in oxygen concentration. For the combustion process to continue, the temperature at the top of the slit must reach the ignition point. But if you want to cut thicker steel plates, you often need a high-power laser with an output power of more than 5kW, and the cutting quality is also significantly reduced. The purity of the oxygen stream has a strong influence on the cutting process. In the traditional laser cutting process, common cone nozzles are often used, which can meet the requirements for use in thin plate cutting. When using traditional convergent beams for laser cutting, the area where the laser beam acts on the surface is very small. Due to the high laser power density, not only the area where the laser is irradiated, the surface temperature of the workpiece has reached the ignition point, but also due to heat conduction, the area is wider. The area reached the ignition temperature. Because high-power laser equipment is expensive, and its output laser mode is not conducive to laser cutting, traditional laser cutting methods do not have advantages when cutting thick plates. In the actual cutting process of the metal laser cutting machine, the thickness of the plate that can be cut through is limited, which is closely related to the inability of the cutting front iron to burn stably. The metal laser cutting machine has the following technical difficulties in cutting thick plates:    1. It is difficult to maintain the quasi-steady state combustion process. When cutting thick plates with a metal laser cutting machine, the decrease in oxygen purity is also an important factor affecting the quality of the cut. In order to keep the cutting stable, it is required that the purity and pressure of the cutting oxygen flow in the thickness direction of the plate be kept basically constant. However, when cutting thick plates, as the air supply pressure increases, shock waves are easily formed in the nozzle flow field. The shock waves have many harms to the cutting process, reducing the purity of the oxygen flow and affecting the quality of the cut. On the one hand, due to the continuous cooling of the cutting seam by the oxygen stream sprayed from the nozzle, the temperature of the cutting front is reduced; on the other hand, the ferrous oxide layer formed by combustion covers the surface of the workpiece and hinders the diffusion of oxygen. When the concentration of oxygen decreases At a certain level, the combustion process will be extinguished. The decrease in the burning rate will greatly reduce the energy input into the kerf during the combustion process and reduce the cutting speed. At the same time, the iron content in the liquid layer of the cutting surface will increase, which will increase the viscosity of the slag and make it difficult to discharge the slag. Severe dross will appear in the lower part, making the quality of the cut unacceptable. The energy released by the iron-oxygen combustion reaction alone cannot actually ensure the continuous combustion process.  2. It is difficult to maintain a constant oxygen purity and pressure in the thickness direction of the plate. When the purity of the oxygen stream decreases by 0.9%, the iron-oxygen combustion rate will decrease by 10%; when the purity decreases by 5%, the combustion rate will decrease by 37%. The combustion process of the cutting front is carried out periodically, which will cause temperature fluctuations at the cutting front and deteriorate the quality of the cut. The laser cutting machine is no longer a problem for the cutting of steel plates below 10mm thickness. Only when the cutting head travels to this position, the combustion reaction restarts.
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