Cutting steel plates below 10mm is not a problem with a laser cutter. However, for thicker plates, a high-power laser with a power exceeding 5 kW is often required. This results in a significant decrease in cut quality.
The high cost of high-power laser equipment makes the laser cutting output mode less favorable. As a result, traditional laser cutting methods have no advantage in cutting thick sheets.
The technical challenges in cutting thick sheets with a metal laser cutting machine are:
The quasi-steady state combustion process is difficult to maintain.
The metal laser cutter has limitations on the thickness of the plates that can be cut during the actual cutting process. This is linked to the instability of the combustion of the iron blade.
The temperature at the top of the crack must reach the ignition point to maintain the continuous combustion process. The energy released by the iron oxide combustion reaction alone does not guarantee the continuation of the combustion process.
On the one hand, the temperature of the cutting edge decreases by the constant cooling of the oxygen flow from the cutting nozzle. On the other hand, the iron oxide layer formed after combustion covers the surface of the part, blocking the diffusion of oxygen. When the oxygen concentration decreases to a certain level, the combustion process will be extinguished.
In traditional convergent beam laser cutting, the laser beam is focused on a small area of the surface. The high power density of the laser causes the surface temperature of the part to reach 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 on the surface of the part is larger than the diameter of the laser beam, resulting in a strong combustion reaction not only in the laser radiation area but also outside it.
When cutting thick plates, the cutting speed is slow. The surface of the workpiece burns faster than the speed of movement of the cutting head. After some burning time, the combustion process ends due to the decrease in oxygen concentration. When the cutting head moves to this position, the combustion reaction begins again.
The burning process of the cutting edge occurs periodically, leading to temperature fluctuations and poor incision quality.
The purity and pressure of oxygen in the plate thickness direction are difficult to maintain constantly.
Decreasing oxygen purity also plays a crucial role in determining the cutting quality of thick sheets using a laser cutter. The purity of the oxygen flow has a significant impact on the cutting process.
A decrease in the purity of the oxygen flow by 0.9% leads to a 10% decrease in the iron-oxygen combustion rate. A decrease in purity by 5% results in a 37% decrease in combustion rate. This decrease in combustion rate greatly reduces the energy input into the cut seam and decreases the cutting speed.
Furthermore, the iron content in the liquid layer of the cutting surface increases, causing the viscosity of the slag to increase and making it difficult to discharge the slag. This results in significant slag accumulation at the bottom of the incision, making the quality of the incision unacceptable.
To maintain cutting stability, the purity of cutting oxygen flow in the direction of plate thickness and pressure must be kept constant.
In traditional laser cutting, a conventional conical nozzle is used, which is suitable for cutting thin sheets. However, when cutting thick sheet metal, a shock wave is formed in the nozzle flow field as the feed pressure increases. The shock wave poses several risks to the cutting process, such as reducing the purity of the oxygen flow and affecting the quality of the incision.
There are three solutions to this problem:
(1) Add a preheating flame around the cutting oxygen flow.
(2) Add an auxiliary oxygen flow around the cutting oxygen flow.
(3) Reasonable design of the nozzle inner walls to improve the airflow field.
