After the laser generated by the oscillator passes through the lens, it is converged at one point to form a small light spot. By controlling the distance between the lens and the plate, the laser light spot is ensured to be stable at a certain position in the material thickness direction. At this time, due to the convergence of the lens, the light spot gathers laser energy with a very high power density, and the power density can usually reach 106~109W/cm2. The material melts instantly after absorbing the light spot energy. At the same time, the molten material is removed with the help of a high-speed airflow coaxial with the light beam, thereby cutting the workpiece. Laser cutting is one of the thermal cutting methods. Laser cutting can be divided into four types: laser gasification cutting, laser melting cutting, laser oxygen melting cutting and controlled fracture cutting.
Auxiliary principle
The main functions of laser cutting auxiliary gas are: combustion and heat dissipation, timely blowing off the molten stains generated by cutting, preventing the cutting molten stains from rebounding upward into the nozzle, and protecting the focusing lens. According to the different materials to be cut, combined with the power of the laser cutting machine, different laser cutting processes are selected, and the selection of auxiliary gas is also different. The characteristics, uses and application scope of different types of auxiliary gases are as follows:
⑴ When oxygen (O2) is used as an auxiliary gas, while blowing away the molten metal liquid, an oxidation reaction will also occur to promote the metal to absorb heat and melt, thereby achieving the melting of thicker materials. This process will significantly improve the processing ability of the laser. However, due to the presence of oxygen, the cut surface of the material will be significantly oxidized, and the quenching effect will be produced on the material around the cut surface, which will increase the hardness of this part of the material and have a certain impact on subsequent processing.
⑵ When nitrogen (N2) is used as an auxiliary gas, a protective atmosphere will be formed around the molten metal liquid to prevent the material from being oxidized, thereby ensuring the quality of the cut surface. However, since nitrogen has no oxidation ability and cannot enhance heat transfer, it will not help improve cutting ability like oxygen. In addition, since nitrogen is consumed in large quantities when used as an auxiliary gas, the cutting cost is higher than when other gases are used.
⑶ When compressed air is used as auxiliary gas for cutting, nitrogen accounts for about 78% and oxygen accounts for about 21%. Due to the presence of oxygen, the cutting section must undergo oxidation reaction. However, due to the presence of a large amount of nitrogen, the oxidation reaction brought by oxygen is not enough to enhance heat transfer, and the cutting capacity will not be improved. Therefore, the air cutting effect can be understood as between nitrogen cutting and oxygen cutting. The advantage is that the cost of air cutting is very low. The only cost is the power consumption caused by the laser cutting screw air compressor to provide air.
(4) Argon (Ar) Argon is an inert gas that can prevent oxidation and nitridation in laser cutting and can also be used in welding. However, the price of argon is higher than that of nitrogen, and it is not cost-effective to use argon for ordinary laser cutting. Argon cutting is mainly used for titanium and titanium alloys. The end face of the cut cut by argon cutting is white.
Comparison of economic efficiency between compressed air laser cutting machine and laser nitrogen cutting machine
Currently, the liquid nitrogen on the market is about 1400 yuan/ton. Liquid nitrogen used for laser cutting needs to be used in a Dewar tank, which is generally 120kg per tank, and the price of 1kg is more than 3 yuan. Let's calculate it at 1400 yuan/ton.
120x1.4=168 yuan The specific gravity of standard nitrogen is 1.25kg/m3, so the usage of a Dewar tank of liquid nitrogen is about: 120/1.25=96Nm3 The nitrogen cost per Nm3 is converted to: 168/96=1.75 yuan/Nm3
If a Rocky RMZY-20A air compressor is used to provide 16bar compressed air, 1.1m3 can be provided per minute. The full load input power of this type of screw air compressor is 12.6kW.
Industrial electricity charges are calculated at 1.0 yuan/kWh, and the air cost per m3 is: 12.6x1.0/(1.1x60)=0.191 yuan/m3. According to the actual consumption of 0.5m3 gas per minute, the laser cutting air compressor works 8 hours a day. The cost that can be saved by using air cutting compared with nitrogen cutting is: (1.75-0.191)x8x60x0.5=374 yuan per day. If the laser cutting machine works 300 days a year, the gas cost that can be saved in a year is: 374x300=112200 yuan. It can be seen that using compressed air instead of nitrogen cutting is very economical and practical.
Compressed air index
Pressure:
The question of how thick the plate can be cut by the air compressor of the laser cutting machine mainly depends on the laser power. If the laser power is large enough, the compressed air pressure can be lowered to cut; if the laser power is not large enough, the compressed air pressure cannot cut. As long as the laser power is large enough, the higher the compressed air pressure, the better the laser cutting quality and the higher the cutting efficiency. As the power of laser cutting air compressors develops from low power to medium power, high power and even ultra-high power, the pressure requirements of air compressors supporting laser cutting machines have also increased from 8 bar at the beginning to 12.5 bar two years ago, and then to 15 bar now, and are still developing further to 20-30 bar.
Flow rate:
The size of the flow rate mainly depends on the size of the nozzle. The nozzle accounts for 80-90% of the gas consumption. The nozzle of the laser cutting machine has several specifications, and the common ones are 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, etc. The flow rate can be determined by nozzle flow calculation or directly consulting the laser cutting machine manufacturer.
Quality
The quality of compressed air has a very direct impact on the quality of laser cutting. Compressed air contains water mist and oil. If it is not cleaned up, high pressure spraying onto the protective mirror of the laser cutting head will seriously affect the transmission of the laser beam, disperse the focus, and cause the product to not be cut through, resulting in waste. If it is an ultra-high power laser cutting machine, as long as the protective mirror or nozzle surface is attached with a little bit of oil film or water mist, it may cause high-energy laser emission to burn the laser head.
Therefore, the compressed air system of the supporting laser cutting machine is very sensitive to the oil and water content, and requires no oil, water and dust. The difficulty of the compressed air system of the supporting laser cutting machine lies in the drying and purification device of the compressed air.