1 Principle of Laser Cutting
Laser is the product of stimulated radiation. Its production is inseparable from three key elements: working material, pump and resonant cavity. After absorbing energy, low-energy level particles will transition to high-energy level orbits, but the state of high-energy level particles is It is unstable and will fall to a low energy level orbit at any time and emit photons. Laser is generated when many particles emit photons with high consistency at the same time. Laser processing can be divided into hot processing and cold processing according to the size of photon energy. Cold processing is generally used in the laser marking process of plastic parts. The laser completes the separation of molecules and substances by destroying the molecular bonds of the material. During this process, the material will not cause thermal damage, thermal deformation and other problems. Thermal processing is more widely used, such as laser cladding, laser welding, laser cutting, etc. During the laser cutting process, the laser beam forms a small focus on the material surface under the action of the focusing mirror, and the material around the focus absorbs the laser. The energy is quickly heated to melting, and the molten metal separates from the material under the blowing of the auxiliary gas to form a slit, thus completing the entire cutting process.
According to the different physical manifestations of cutting materials, laser cutting can be roughly divided into: laser melting cutting, laser vapor cutting, laser reaction melting cutting and laser controlled fracture cutting.
(1) Laser melting cutting refers to the fact that during the laser cutting process, the focused laser beam causes evaporation inside the material to form holes. The resulting hole appears as a black body, which causes the laser absorption rate to increase rapidly, and the metal wall of the small hole is melted. Surrounded, enlarged and deepened, and then blown out by the action of high-pressure coaxial gas.
(2) Laser gasification cutting means that the material to be cut is heated to vaporize in a very short time, and the vaporized material is blown away from the cutting surface under the action of the auxiliary gas flow. Gas cutting can only be used for materials that are difficult to melt, such as wood, carbon and certain plastic materials. Materials processed in this way can often achieve very high cutting quality.
(3) Laser reaction melting cutting, also called laser oxygen-assisted cutting, is a special melting cutting method that uses oxygen as an auxiliary gas. During the cutting process, the high-energy laser beam heats the material to the ignition point temperature. At this time, the material has not yet melted. However, when the material reaches the ignition point, oxygen will react violently with the material, releasing a large amount of oxidation reaction heat. After that, the material begins to The melting forms a metal molten substance accompanied by the formation of a large amount of oxides. The molten substance is blown away from the surface of the cutting seam by the auxiliary gas flow, and the cutting process is completed. In the laser oxygen cutting process, the main reason for using oxygen as an auxiliary gas is that oxygen is one of the important substances involved in the oxidation and combustion reaction of the material. The combustion reaction will not occur when using other types of auxiliary gases. To complete the cutting, the laser beam is required to provide more More energy to ensure that the metal becomes molten. In the laser oxygen cutting process, the additional energy provided through the combustion reaction can reach 70% of the total cutting energy. In the reactive melting cutting of titanium alloy, it provides nearly 90% of the energy. The iron-oxygen combustion reaction will generate many oxidized melts. These melts generally have low viscosity and are easy to be removed under the action of auxiliary gas. Most of them are ferrous oxide, and there are also a small amount of iron oxide and ferric oxide. Due to the low viscosity of the melt, it is easier to be blown away from the bottom of the material by the auxiliary gas, reducing the possibility of slag adhering to the bottom of the cutting. At the same time, the oxides generated by the combustion reaction increase the laser absorption rate, which is beneficial to absorbing more laser energy.
(4) In laser controlled fracture cutting, cracks are caused by the material absorbing laser energy and heating up. Stress near the laser focus changes due to high temperatures. When the laser beam passes through, these compressive stresses relax to form local residual tensile stresses. Under the action of stress, the cracks will continue to extend in the direction of laser movement, and the material will fracture and separate along the path of the laser beam. This cutting method is particularly suitable for cutting brittle materials such as ceramics.
Fattori che influenzano il taglio laser
(1) Laser power: Laser power is the energy power input to the laser. The intensity of the laser beam can be calculated by dividing the power by the laser spot area. The size of the laser power directly determines the cutting ability and cutting efficiency. However, the bigger the laser power, the better. Too much laser power will lead to an increase in the kerf width and heat-affected zone, and even over-burning of the workpiece, which is serious. Impair processing quality.
(2) Cutting speed: In laser cutting, the cutting speed is directly related to the processing efficiency. The greater the cutting speed, the higher the processing efficiency. However, the cutting speed is restricted by the processing quality. Excessive cutting speed will lead to the inability to cut through the material. , causing the workpiece to be scrapped, but too small a cutting speed will affect the processing efficiency and cause a decrease in factory profitability. Therefore, it is very important to determine the cutting speed when performing laser cutting.
(3) Gas pressure: The main function of the auxiliary gas is to remove the molten metal at the cutting seam to ensure the smooth progress of laser cutting. Generally speaking, the greater the gas pressure, the stronger the removal ability of molten material. However, if the gas pressure is too high, the air flow speed at the nozzle will increase, and the air flow will be disturbed at the slit, which will reduce the ability of the gas to remove molten material and increase the cooling capacity. , thus greatly increasing the probability of slag hanging at the slit. Using nitrogen as an auxiliary gas to cut stainless steel can obtain non-oxidizing, bright cutting edges. For low carbon steel, oxygen can be used as an auxiliary gas to improve cutting efficiency and use lower laser power to complete cutting.
(4) Laser frequency: When the average laser power is constant, the higher the laser frequency, the more laser pulses the laser emits per unit time, and the lower the energy carried by a single laser pulse. When cutting, it is like using a Use a dull knife to cut the material. (5) Laser duty cycle: The laser duty cycle will affect the peak power of the laser. For example, when the laser duty cycle is 10%, the peak power of the laser can reach 10 times the average power. When the average laser power is constant, the laser The larger the duty cycle, the smaller the laser peak power. When the laser duty cycle is 100%, the average laser power is equal to the peak power.
3 Advantages of laser cutting
Compared with conventional cutting methods, laser cutting has the following advantages:
(1) The cutting quality is good. The diameter of the focal point formed by the laser produced by the laser after being focused by the lens is very small. When cutting, the width of the cutting slit is generally ten to twenty filaments. Compared with traditional flame cutting, the accuracy is greatly improved. In addition, the laser machine has a positioning accuracy of five wires and a repeatable positioning accuracy of two wires, and can be used to process various precision parts.
(2) High processing efficiency. As an advanced processing method, the most prominent advantage of laser cutting technology is its high processing efficiency. Generally, the maximum cutting speed can reach 10m/min, and the processing speed is more than ten times that of traditional methods. Due to the small focus diameter, material loss can be reduced during cutting, and it is widely used in precious metal processing.
(3) Contactless processing. Laser cutting is a processing method that does not require mechanical contact. In traditional processing, there is a cutting tool, and there will be processing stress. The workpiece may be deformed or even scratched by the tool, affecting product quality. However, using laser cutting It can ensure no processing stress, and compared with plasma cutting, the heat affected zone during cutting is small, and the workpiece has almost no thermal deformation, which can reduce the subsequent processing steps of the workpiece and save costs.
(4) Wide selection of cutting materials. Laser can cut a wide variety of materials. From the beginning, it could only cut steel plates, but now it can cut most metal and non-metal materials. The principle of laser cutting is to vaporize or melt the material, so the hardness of the processed material can be ignored during cutting. In addition, The cutting precision is high, so it is becoming more and more popular in jewelry processing such as diamonds.
(5) High degree of flexibility. With the maturity of fiber laser technology, the flexibility of laser processing is getting stronger and stronger. The propagation of laser beams requires mirrors to reflect the light. Therefore, the structure of the laser cutting machine is complex and the processing range is limited. Fiber laser cutting using optical fibers for transmission The function can flexibly process parts of any shape, greatly reducing the adverse impact of the shape of the workpiece on cutting.
