What is difficult about laser cutting thick plates?

What is difficult about laser cutting thick plates?
01
Laser cutting machines are no longer a problem for cutting steel plates up to 10mm thick. But if you want to cut thicker plates, often require the help of high power lasers with an output power of more than 5kW, and the quality of the cut is also significantly reduced. Due to the high cost of high power laser equipment, the output laser pattern is also not conducive to laser cutting, so the traditional laser cutting method in cutting thick plate, does not have the advantage. The following technical difficulties exist in the cutting of thick metal plates.

The quasi-steady combustion process is difficult to maintain

The actual cutting process of the metal laser cutting machine is limited in the thickness of the plate that can be cut through, which is closely related to the fact that the cutting front iron cannot be burned steadily. For the combustion process to be sustainable, the temperature at the top of the cut must reach the ignition point. The energy released by the iron-oxygen combustion reaction alone does not actually ensure that the combustion process can be sustained.

On the one hand, this is due to the fact that the slit is continuously cooled by the oxygen stream emitted 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 and hinders the diffusion of oxygen, and the combustion process will be extinguished when the concentration of oxygen is reduced to a certain level.

When laser cutting with a conventional convergent beam, the laser beam acts on a small area of the surface and, due to the high laser power density, not only does the workpiece surface temperature reach the combustion point in the area of laser radiation, but a wider area reaches the combustion point temperature due to heat transfer. And the diameter of the oxygen flow acting on the surface of the workpiece is larger than the diameter of the laser beam. This means that not only in the area of laser radiation does a strong combustion reaction take place, but also at the periphery of the spot irradiated by the laser beam.

When cutting thick plates, the cutting speed is rather slow and the iron-oxygen combustion on the surface of the workpiece is faster than the speed of the cutting head travel. The combustion process is extinguished after a period of time due to a drop in the oxygen concentration. Only when the cutting head travels to that position does the combustion reaction start again. The combustion process at the cutting front takes place cyclically, which leads to temperature fluctuations at the cutting front and poor kerf quality.


02
Difficulty in maintaining constant oxygen purity and pressure in the direction of plate thickness

A decrease in oxygen purity is also an important factor in the quality of the kerf when cutting thick plates with metal laser cutting machines.

The purity of the oxygen flow has a strong influence on the cutting process. When the purity of the oxygen stream drops by 0. 9%, the iron-oxygen burn rate will drop by 10%; at a 5% drop in purity, the burn rate will drop by 37%. A decrease in combustion rate will greatly reduce the energy input into the cut by the combustion process, reducing the cutting speed, while the iron content in the liquid layer at the cutting surface increases, thus increasing to the viscosity of the slag and leading to difficulties in slag discharge, so that severe slagging occurs in the lower part of the cut, making the quality of the cut unacceptable.

In order to maintain a stable cutting process, it is required that the purity and pressure of the cutting oxygen flow in the direction of the plate thickness be kept essentially constant. In traditional laser cutting processes, common conical nozzles are often used, which can meet the requirements for use in thin plate cutting. However, when cutting thick plates, with the increase in supply pressure, the nozzle flow field is prone to the formation of surge, surge on the cutting process has many hazards, reducing the purity of the oxygen flow, affecting the quality of the cut.

There are three general solutions to this problem.

(1) Add a preheating flame around the cutting oxygen stream.

(2) Adding an auxiliary oxygen stream around the cutting oxygen stream.

(3) Reasonable design of the inner wall of the nozzle to improve the flow field characteristics.

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