Laser cutting application
Fast axial flow CO2 lasers are mostly used for laser cutting of metal materials, mainly because of their good beam quality. Although the reflectivity of most metals to CO2 laser beams is quite high, the reflectivity of the metal surface at room temperature increases with the increase of temperature and oxidation degree. Once the metal surface is damaged, the reflectivity of the metal is close to 1. For metal laser cutting, a higher average power is necessary, and only high-power CO2 lasers have this condition.
1. Laser cutting of steel materials
1.1 CO2 continuous laser cutting The main process parameters of CO2 continuous laser cutting include laser power, type and pressure of auxiliary gas, cutting speed, focal position, focal depth and nozzle height.
(1) Laser power Laser power has a great influence on cutting thickness, cutting speed and incision width. When other parameters are constant, the cutting speed decreases with the increase of cutting plate thickness and increases with the increase of laser power. In other words, the greater the laser power, the thicker the plate that can be cut, the faster the cutting speed, and the slightly larger the incision width.
(2) Type and pressure of auxiliary gas When cutting low carbon steel, CO2 is used as auxiliary gas to utilize the heat of iron-oxygen combustion reaction to promote the cutting process. The cutting speed is high and the incision quality is good, especially the incision without sticky slag can be obtained. When cutting stainless steel, CO2 is used. Slag is easy to stick to the lower part of the incision. CO2 + N2 mixed gas or double-layer gas flow is often used. The pressure of the auxiliary gas has a significant effect on the cutting effect. Appropriately increasing the gas pressure can increase the cutting speed without sticky slag due to the increase in gas flow momentum and the improvement of slag removal capacity. However, if the pressure is too high, the cut surface becomes rough. The effect of oxygen pressure on the average roughness of the incision surface is shown in the figure below.
The body pressure also depends on the plate thickness. When cutting low carbon steel with a 1kW CO2 laser, the relationship between oxygen pressure and plate thickness is shown in the figure below.
(3) Cutting speed Cutting speed has a significant impact on cutting quality. Under certain conditions of laser power, there are corresponding upper and lower critical values for good cutting speed when cutting low carbon steel. If the cutting speed is higher or lower than the critical value, slag sticking will occur. When the cutting speed is slow, the action time of the oxidation reaction heat on the cutting edge is extended, the width of the cutting is increased, and the cutting surface becomes rough. As the cutting speed increases, the incision gradually becomes narrower until the width of the upper incision is equivalent to the diameter of the spot. At this time, the incision is slightly wedge-shaped, wide at the top and narrow at the bottom. As the cutting speed continues to increase, the width of the upper incision continues to become smaller, but the lower part of the incision becomes relatively wider and becomes an inverted wedge shape.
(5)Focus depth
The depth of focus has a certain impact on the quality of the cutting surface and the cutting speed. When cutting relatively large steel plates, a beam with a large focal depth should be used; when cutting thin plates, a beam with a small focal depth should be used.
(6)Nozzle height
The nozzle height refers to the distance from the end surface of the auxiliary gas nozzle to the upper surface of the workpiece. The height of the nozzle is large, and the momentum of the ejected auxiliary airflow is easy to fluctuate, which affects the cutting quality and speed. Therefore, when laser cutting, the nozzle height is generally minimized, usually 0.5~2.0mm.
① Laser aspects
a. Increase laser power. Developing more powerful lasers is a direct and effective way to increase cutting thickness.
b. Pulse processing. Pulsed lasers have very high peak power and can penetrate thick steel plates. Applying high-frequency, narrow-pulse-width pulse laser cutting technology can cut thick steel plates without increasing laser power, and the incision size is smaller than that of continuous laser cutting.
c. Use new lasers
②Optical system
a. Adaptive optical system. The difference from traditional laser cutting is that it does not need to place the focus below the cutting surface. When the focus position fluctuates up and down a few millimeters along the thickness direction of the steel plate, the focal length in the adaptive optical system will change with the shift of the focus position. The up and down changes in focal length coincide with the relative motion between the laser and the workpiece, causing the focus position to change up and down along the depth of the workpiece. This cutting process in which the focus position changes with external conditions can produce high-quality cuts. The disadvantage of this method is that the cutting depth is limited, generally no more than 30mm.
b. Bifocal cutting technology. A special lens is used to focus the beam twice at different parts. As shown in Figure 4.58, D is the diameter of the center part of the lens and is the diameter of the edge part of the lens. The radius of curvature at the center of the lens is larger than the surrounding area, forming a double focus. During the cutting process, the upper focus is located on the upper surface of the workpiece, and the lower focus is located near the lower surface of the workpiece. This special dual-focus laser cutting technology has many advantages. For cutting mild steel, it can not only maintain a high-intensity laser beam on the upper surface of the metal to meet the conditions required for the material to ignite, but also maintain a high-intensity laser beam near the lower surface of the metal to meet the requirements for ignition. The need to produce clean cuts across the entire range of material thicknesses. This technology expands the range of parameters for obtaining high-quality cuts. For example, using a 3kW CO2. laser, the conventional cutting thickness can only reach 15~20mm, while the cutting thickness using dual focus cutting technology can reach 30~40mm.
③Nozzle and auxiliary air flow
Reasonably design the nozzle to improve the air flow field characteristics. The diameter of the inner wall of the supersonic nozzle first shrinks and then expands, which can generate supersonic airflow at the outlet. The air supply pressure can be very high without generating shock waves. When using a supersonic nozzle for laser cutting, the cutting quality is also ideal. Since the cutting pressure of the supersonic nozzle on the workpiece surface is relatively stable, it is especially suitable for laser cutting of thick steel plates.
Post time: Jul-18-2024
