Principle of laser cutting
Laser cutting is to focus the laser beam into a small spot, causing it to act on the material. The laser power density can reach more than 106 W/cm2. Under such high power density laser irradiation Under the action, the metal material quickly vaporizes and evaporates, forming holes. The molten metal residue is blown away by the high-pressure gas blown out coaxially, forming a cutting slit. The cutting head moves under a pre-designed program to cut the corresponding shape. In laser cutting, oxygen is used as an auxiliary gas, which is beneficial to the cutting of certain metal sheets. At high temperatures, oxygen will undergo an oxidative exothermic reaction with the material, providing a certain amount of heat for laser cutting. The heat energy of the reaction is absorbed by the material and promotes melting and removal of the material. In this way, the heat source in laser cutting is not only the energy from the laser beam but also the energy from the exothermic reaction of material oxidation. For example, when cutting low carbon steel plates, oxygen cutting is generally used to provide a combustion accelerant.
Characteristics of laser cutting
Compared with other traditional cutting methods, laser cutting has the following advantages:
(1) The cutting gap is narrow, with good cutting accuracy, good cutting surface roughness, and less slag. Laser cutting is a green and environmentally friendly processing method. It causes little harm to operators, does not produce pollutants during cutting, has basically no impact on the environment, and has a high degree of automation.
(2) Laser cutting is highly efficient. The laser beam has high energy density and fast cutting speed. At present, the fastest cutting speed of German Schuler Wanwa double-head laser cutting machine can reach 100m/min, and its single-head cutting speed is 30-50m/ min . Figure 1. 5 is the curve graph between laser power, plate thickness and cutting speed reported in the literature when cutting low carbon steel. It can be seen from the figure that on the premise of ensuring the cutting quality, the cutting parameters can be adjusted to increase the cutting speed. Laser cutting The capability is very high, especially for high-precision and high-speed cutting of medium and low thickness plates.
(3) Laser cutting has a wide range of applications. Laser cutting can be used for almost any material. Non-metallic materials, metallic materials, and even materials with high hardness and high melting point can be cut with laser. If the parameters are appropriately selected, there will be good cutting results.
The main methods of laser cutting.
Whether low-speed or high-speed laser cutting, according to the different cutting materials and parameters, it can be divided into four methods: melting cutting, gasification cutting, oxidation reaction cutting and controlled fracture cutting.
(1) Melting cutting When the power and density of the laser are low, the irradiated material will melt and holes will be formed inside. Under the action of high-concentration nitrogen or other auxiliary gases, the slag around the holes will be blown from the bottom Walk. Melting cutting is often applied to materials that do not react with oxygen, and cutting iron materials can produce oxidation-free cuts. The required power density is generally 107 W/cm2.
(2) When the vaporized cutting material is irradiated by a high-energy-density laser beam, the temperature of the surface of the plate instantly rises to the melting point of the material, causing part of the material to escape in the form of steam, and the other part of the material is sprayed from the bottom of the cutting seam with the help of auxiliary gas. Blow away, and use the relative movement of the cutting head and the workpiece to form a slit to complete the cutting process. This method requires high energy, which is about 10 times that of melting cutting.
(3) Oxidation flux cutting. The auxiliary gas used in oxidation flux cutting is generally an active gas, and the heat generated at high temperature is used as reaction heat. During the laser cutting process, the surface of the workpiece is rapidly heated to the melting point and chemically reacts with oxygen, releasing a large amount of heat. Under the dual action of the two kinds of heat, a small hole containing steam is formed inside. The small hole moves forward with the steam and participates in the conversion between heat. The reaction product is transformed into molten slag, which controls oxygen and metal. The speed of burning. The greater the gas pressure, the faster the oxygen flow rate, the contact area between the material and the gas becomes larger, the chemical reaction rate increases, and the gas blows away the oxides and molten metal generated by combustion, eventually forming a slit.
(4) Controlled fracture cutting When materials that are susceptible to thermal damage are heated by laser beam irradiation, a large thermal gradient will be generated at the spot, which will cause mechanical deformation and form cracks. Controlling fracture cutting is to control a balanced thermal gradient and use a laser beam to guide the development direction of cracks, so that the material can be cut off in a high-speed and controllable manner. This cutting mechanism only requires low laser power. Higher laser power will melt the material and destroy the edges of the kerf, and it is not suitable for cutting sharp angles and corners in brittle materials.
Laser cutting quality evaluation indicators
The quality of plate laser cutting is generally evaluated by the following indicators: slit width; surface roughness; heat-affected zone width; slag hanging height.
(1) Cutting slit width: The size of the laser light source radius is the decisive factor in the slit width. The focus moves downward, causing the laser beam to focus inside the material, so that the upper slit will be slightly larger than the middle slit. The power is high and the heat absorbed by the material is too high, causing the slit width to become wider. When cutting thinner plates, appropriately increasing the cutting speed will make the slit width slightly equal to the laser spot diameter.
(2) Height of slag hanging: After the plate is cut by laser, there will be many burrs attached to the material on the back, which is usually called slag hanging. Some burrs are so small that they are invisible to the naked eye and must be observed through a microscope. However, some burrs are relatively large and can be directly observed with the naked eye. Some burrs are attached to the bottom of the plate and need to be polished to remove them. Some burrs have poor adhesion and are easy to remove.
(3) Heat-affected zone width: At high temperatures, high-intensity laser energy is concentrated on the surface of the plate to melt it. Compared with traditional mechanical cutting, the internal structural properties of the material are easily destroyed during laser cutting, especially at the cutting seams. Oxidation reaction forms heat-affected zone.
(4) Roughness of the cutting surface: The main influencing factors of roughness include cutting speed, laser power, plate thickness, etc. The thicker material will have a lower roughness after cutting than the thinner material. When cutting metal, the roughness of the upper, middle and lower parts of the same cut surface is also different. The roughness is smallest near the upper part of the laser beam, and the roughness is the smallest toward the lower end the greater the roughness.
