Difficulty in metal cutting thick plates: difficulty in maintaining quasi steady state combustion process

In the actual cutting process of metal laser cutting machine, the plate thickness that can be cut through is limited, which is closely related to the unstable combustion of iron in the cutting front. The temperature at the top of the slit must reach the ignition point if the combustion process can continue. The energy released by the combustion reaction of ferrite alone can not ensure the continuous combustion process. On the one hand, the cutting seam is continuously cooled by the oxygen flow from the nozzle, which reduces the temperature at the cutting front; on the other hand, the ferrous oxide layer formed by combustion covers the surface of the workpiece, hindering the diffusion of oxygen. When the oxygen concentration is reduced to a certain extent, the combustion process will be extinguished. When the traditional convergent beam is used for laser cutting, the area where the laser beam acts on the surface is very small. Due to the high laser power density, the workpiece surface temperature reaches the ignition point not only in the laser radiation area, but also in a wider area due to heat conduction. The diameter of the oxygen flow acting on the workpiece surface is larger than that of the laser beam. This shows that not only in the laser radiation area, but also in the periphery of the laser spot, the combustion reaction should occur at the same time. When cutting thick plates, the cutting speed is quite slow, and the speed of iron oxide combustion on the workpiece surface is faster than that of the cutting head. After the combustion lasts for a period of time, the combustion process is extinguished due to the decrease of oxygen concentration. Only when the cutting head moves to this position, the combustion reaction starts again. The combustion process at the cutting edge is carried out periodically, which will lead to the temperature fluctuation at the cutting edge and the poor quality of the cut.

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Author: YAN YI FAN