3. Nozzle design and airflow control technology: When laser cutting steel, oxygen and focused laser beam are injected through the nozzle to the material to be cut to form an airflow beam. The basic requirement for the gas flow is that the gas flow into the slit is large and the velocity is high so that sufficient oxidation allows the slit material to sufficiently exothermicly react; at the same time, sufficient momentum is applied to eject the molten material. Therefore, in addition to the quality of the beam and its control directly affect the cutting quality, nozzle design and airflow control (such as nozzle pressure, position of the workpiece in the airflow, etc.) are also important factors. At present, the nozzle for laser cutting adopts a simple structure, that is, a tapered hole with a small round hole at the end (Fig. 4). Design is usually done using experimental and error methods. Since the nozzle is generally made of copper and has a small volume, it is a fragile part and needs to be replaced frequently, so fluid mechanics calculation and analysis are not performed. In use, a certain pressure Pn (gauge pressure Pg) is introduced from the side of the nozzle, and the nozzle pressure is called from the nozzle outlet, and reaches the surface of the workpiece at a certain distance. The pressure is called the cutting pressure Pc, and finally the gas is expanded to atmospheric pressure. Pa. Research work shows that as Pn increases, the flow velocity increases and Pc increases.

It can be calculated by the following formula: V=8.2d2(Pg+1)

V-gas flow rate L/min

D-nozzle diameter mm

Pg-nozzle pressure (gauge pressure) bar

There are different pressure thresholds for different gases. When the nozzle pressure exceeds this value, the airflow is a normal oblique shock wave, and the gas flow rate transitions from subsonic to supersonic. This threshold is related to the Pn, Pa ratio and the degree of freedom of the gas molecule (n): n = 5 for oxygen and air, so its threshold Pn = 1 bar × (1.2) 3.5 = 1.89 bar. When the nozzle pressure is higher Pn/Pa=(1+1/n)1+n/2 (Pn; 4bar), the normal oblique shock wave of the airflow becomes a positive shock wave, the cutting pressure Pc decreases, and the airflow speed decreases, and The formation of eddy currents on the surface of the workpiece weakens the effect of the gas stream to remove the molten material and affects the cutting speed. Therefore, the nozzle with a tapered hole with a small round hole at the end has a nozzle pressure of oxygen of 3 bar or less.

In order to further improve the laser cutting speed, according to the aerodynamic principle, the positive shock wave is not generated under the premise of increasing the nozzle pressure, and a zoom nozzle, that is, a Laval nozzle, is designed and manufactured. The structure of Figure 4 can be employed for ease of manufacture. The laser center of Hannover University in Germany used a 500WCO2 laser with a focal length of 2.5 透镜. The test was carried out using a small orifice nozzle and a Laval nozzle, see Figure 4. The test results are shown in Fig. 5: the relationship between the surface roughness Rz of the slit and the cutting speed Vc under different oxygen pressures of NO ₂ , NO ₄ and NO ₅ nozzles, respectively. It can be seen from the figure that the cutting speed of the NO2 small orifice nozzle can only reach 2.75 m/min (carbon steel plate thickness is 2 mm) when the Pn is 400 Kpa (or 4 bar). The NOVA and NO5 Laval nozzles can cut at speeds of 3.5m/min and 5.5m/min when the Pn is 500Kpa to 600Kpa. It should be noted that the cutting pressure Pc is also a function of the distance between the workpiece and the nozzle. Since the oblique shock is reflected multiple times at the boundary of the airflow, the cutting pressure changes periodically.

The first high cutting pressure zone is adjacent to the nozzle outlet, and the distance from the surface of the workpiece to the nozzle outlet is about 0.5 to 1.5 mm, and the cutting pressure Pc is large and stable, which is a common process parameter for cutting hand in industrial production. The second high cutting pressure zone is about 3~3.5mm of the nozzle outlet, and the cutting pressure Pc is also large, and the same effect can be obtained, and the lens is protected and the service life is improved. The other high cutting pressure zones on the curve are too far from the nozzle exit to match the focused beam and cannot be used.

In summary, CO ₂ laser cutting technology is getting more and more applications in China's industrial production, and foreign countries are researching and developing cutting technology and devices with higher cutting speed and thicker steel plates. In order to meet the increasing demands of industrial production for quality and production efficiency, it is necessary to pay attention to solving various key technologies and implementing quality standards, so that this new technology can be more widely used in China.

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