Laser is a kind of light, like other natural light, it is produced by the transition of atoms (molecules or ions, etc.). But it is different from ordinary light in that the laser only relies on spontaneous emission for a very short period of time, and the subsequent process is completely determined by the excitation radiation. Therefore, the laser has a very pure color, almost no divergent directionality, and extremely high luminous intensity. And high coherence.
Laser cutting is achieved by applying high power density energy generated by laser focusing. Under the control of the computer, the laser is discharged through pulses, thereby outputting a controlled repetitive high-frequency pulsed laser to form a beam with a certain frequency and a certain pulse width. The pulsed laser beam is guided and reflected by the optical path and focused by the focusing lens group. On the surface of the processed object, a small, high-energy density light spot is formed. The focal spot is located near the surface to be processed, and the processed material is melted or vaporized at an instant high temperature. Each high-energy laser pulse instantly sputters a small hole on the surface of the object. Under computer control, the laser processing head and the processed material perform continuous relative movement according to the pre-drawn graphics, so that the object will be processed into The shape you want.
The process parameters (cutting speed, laser power, gas pressure, etc.) and motion trajectory during slitting are controlled by the numerical control system, and the slag at the slit is blown away by a certain pressure of auxiliary gas.
1. Vaporized cutting.
In the laser gasification cutting process, the speed of the material surface temperature rising to the boiling point temperature is so fast that it is enough to avoid melting caused by heat conduction, so part of the material vaporizes into steam and disappears, and part of the material is sprayed from the bottom of the slit by auxiliary gas The flow blows away. In this case, very high laser power is required.
In order to prevent material vapor from condensing on the slit wall, the thickness of the material must not greatly exceed the diameter of the laser beam. This process is therefore only suitable for applications where the removal of molten material must be avoided. This processing is actually only used in areas where iron-based alloys are small.
This process cannot be used for materials such as wood and certain ceramics that are not in a molten state and therefore are unlikely to allow the material vapor to recondense. In addition, these materials usually require thicker cuts. In laser gasification cutting, the optimal beam focus depends on the material thickness and beam quality. The laser power and the heat of vaporization have only a certain influence on the optimal focus position. In the case of a certain thickness of the sheet, the maximum cutting speed is inversely proportional to the vaporization temperature of the material. The required laser power density is greater than 108W/cm2 and depends on the material, cutting depth and beam focus position. In the case of a certain sheet thickness, assuming sufficient laser power, the maximum cutting speed is limited by the gas jet speed.
2. Melting and cutting.
In laser melting and cutting, the workpiece is partially melted and the molten material is sprayed out with the help of airflow. Because the transfer of the material only occurs in its liquid state, the process is called laser melting and cutting.
The laser beam is matched with a high-purity inert cutting gas to drive the molten material to leave the kerf, and the gas itself does not participate in the cutting. Laser melting cutting can get a higher cutting speed than gasification cutting. The energy required for gasification is usually higher than the energy required to melt the material. In laser melting and cutting, the laser beam is only partially absorbed. The maximum cutting speed increases with the increase of the laser power, and decreases almost inversely with the increase of the thickness of the sheet and the increase of the melting temperature of the material. In the case of a certain laser power, the limiting factor is the air pressure at the slit and the thermal conductivity of the material. Laser melting and cutting can obtain oxidation-free incisions for iron materials and titanium metals. The laser power density that produces melting but not gasification is between 104 W/cm2 and 105 W/cm2 for steel materials.
Melting cutting generally uses inert gas. If it is replaced by oxygen or other active gases, the material is ignited under the irradiation of the laser beam, and a fierce chemical reaction occurs with oxygen to generate another heat source to further heat the material, which is called oxidative melting cutting .
Due to this effect, for structural steel of the same thickness, the cutting rate that can be obtained by this method is higher than that of melting cutting. On the other hand, this method may have worse cut quality compared to fusion cutting. In fact, it will produce wider kerf, obvious roughness, increased heat-affected zone and worse edge quality. Laser flame cutting is not good when processing precision models and sharp corners (there is a danger of burning off the sharp corners). A pulsed laser can be used to limit the thermal influence, and the power of the laser determines the cutting speed. In the case of a certain laser power, the limiting factor is the supply of oxygen and the thermal conductivity of the material.
4. Control fracture cutting.
For brittle materials that are easily damaged by heat, high-speed and controllable cutting is performed by laser beam heating, which is called controlled fracture cutting. The main content of this cutting process is: the laser beam heats a small area of the brittle material, causing a large thermal gradient and severe mechanical deformation in this area, leading to the formation of cracks in the material. As long as a uniform heating gradient is maintained, the laser beam can guide cracks in any desired direction.
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