Common problems in laser cutting

Common Process and Problem Analysis The laser beam input from the melt-cutting perforation technology has high power density and high energy. The inside of the material at the beam irradiation point begins to evaporate, forming a hole. The small hole is surrounded by molten metal, and the auxiliary airflow coaxial with the beam surrounds the hole. The molten material is taken away. The workpiece moves, and the small holes are synchronously traversed in the cutting direction to form a slit to complete the cutting. For thermal cutting technology, it is generally necessary to use a punch to make a small hole in the plate and then use a laser to start cutting from the small hole. Only a few processed workpieces can be processed from the edge of the plate during processing. There are two basic methods for perforating a laser cutter without a stamping device: blasting perforation: a material forms a pit at the center after continuous laser irradiation, and the oxygen flow coaxial with the laser beam quickly removes the molten material to form a hole. .
The processing head and the material to be processed are continuously moved relative to each other according to the pre-drawn pattern, so that the object is processed into a desired shape. When cutting, a coaxial airflow with the beam is ejected by the cutting head, and the molten or vaporized material is blown out from the bottom of the slit (Note: if the blown gas and the material being cut produce a thermal reaction, the reaction will provide cutting The additional energy required; the airflow also cools the cut surface, reduces the heat affected zone and ensures that the focusing mirror is not contaminated). It should be noted that this controlled fracture cutting is not suitable for cutting acute and angular slits. Cutting a large closed shape is also not easy to succeed. Control the fracture cutting speed is fast, do not need too high power, otherwise it will cause the surface of the workpiece to melt and destroy the edge of the slit. The main control parameters are laser power and spot size.

1. Vaporization cutting.
Under the heating of the high power density laser beam, the surface temperature of the material rises to the boiling point temperature so fast that it avoids the melting caused by heat conduction, so that some of the material vaporizes into vapor and some of the material is ejected from the bottom of the slit. The auxiliary gas stream is blown away. Some materials that cannot be melted, such as wood, carbon materials and certain plastics, are cut and formed by this vaporization cutting method.
During the vaporization and cutting process, the steam carries away the molten particles and washes the debris to form holes. During vaporization, approximately 40% of the material is vaporized and 60% of the material is removed by airflow in the form of droplets.

2. Melt cutting.
When the incident laser beam power density exceeds a certain value, the inside of the material at the beam irradiation point begins to evaporate, forming a hole. Once such a small hole is formed, it will absorb all of the incident beam energy as a black body. The aperture is surrounded by the molten metal wall and then an auxiliary gas stream coaxial with the beam carries away the molten material around the hole. As the workpiece moves, the small holes are simultaneously traversed in the cutting direction to form a slit. The laser beam continues to illuminate along the leading edge of the slit, and the molten material is blown away from the slit continuously or pulsatingly.

3. Oxidation melting cutting.
Melting and cutting generally uses an inert gas. If replaced by oxygen or other reactive gas, the material is ignited under the irradiation of a laser beam, and a strong chemical reaction with oxygen produces another heat source called oxidative melting cutting. The specific description is as follows:
(1) The surface of the material is quickly heated to the ignition temperature under the irradiation of the laser beam, and then a fierce combustion reaction with oxygen is performed to release a large amount of heat. Under the action of this heat, a small hole filled with steam is formed inside the material, and the periphery of the small hole is surrounded by a molten metal wall.
(2) The combustion material is transferred into slag to control the burning speed of oxygen and metal, and the speed at which oxygen diffuses through the slag to reach the ignition front also has a great influence on the burning speed. The higher the oxygen flow rate, the faster the combustion of the chemical reaction and the removal of the slag. Of course, the higher the oxygen flow rate, the better, because too fast a flow rate will result in rapid cooling of the reaction product at the exit of the slit, ie, the metal oxide, which is also detrimental to the quality of the cut.
(3) Obviously, there are two heat sources in the oxidative melting and cutting process, namely the laser irradiation energy and the thermal energy generated by the chemical reaction between oxygen and metal. It is estimated that when cutting steel, the heat released by the oxidation reaction accounts for about 60% of the total energy required for cutting.
It is apparent that the use of oxygen as an auxiliary gas results in a higher cutting speed than an inert gas.
(4) In the oxidative melting cutting process with two heat sources, if the burning speed of oxygen is higher than the moving speed of the laser beam, the slits appear wide and rough. If the laser beam moves faster than oxygen, the resulting slit is narrow and smooth.

4. Control fracture cutting.
For brittle materials that are easily damaged by heat, high-speed, controlled cutting by laser beam heating is called controlled fracture cutting. The main content of this cutting process is that the laser beam heats a small area of brittle material, causing a large thermal gradient and severe mechanical deformation in the area, causing the material to form cracks. As long as a balanced heating gradient laser beam is maintained, the crack can be induced to be produced in any desired direction.
An auxiliary gas suitable for the material to be cut is also added during processing to accelerate the melting of the material, blow away the slag or protect the slit from oxidation. Many metal materials, regardless of their hardness, can be laser-deformed without distortion. Most organic and inorganic materials can be laser cut. Among the commonly used engineering materials, except for copper, carbon steel, stainless steel, alloy steel, aluminum and aluminum alloys, titanium and titanium alloys, and most nickel alloys can be laser cut.
After the cone-shaped laser beam is concentrated, the spot becomes smaller and smaller, the slit becomes narrower, and the precision of the laser cutting becomes higher and higher. The smallest spot can reach 0.01mm. The spot size seriously affects the laser cutting accuracy. The composition of the materials is different. The accuracy of the cutting will vary depending on the composition of the material. The same material and the material used locally will vary. The laser cutting accuracy is also affected to some extent by the material of the workpiece. Workbench. Whether it is a simple workbench or a high-precision workbench, it results in high-precision laser cutting. In CNC machine tools, the table can be moved, the accuracy of the table movement track, the flatness of the table, etc. will also have an impact on the cutting accuracy. The size of the hole is related to the thickness of the plate. For thicker plates, the blasting perforation has a large aperture and is not round. It is not suitable for use on parts with high processing precision and can only be used on scrap. In addition, the oxygen pressure used for the perforation is the same as that during the cutting. Pulse perforation: A small peak power pulsed laser is used to melt or vaporize a small amount of material, usually using air or nitrogen as an auxiliary gas. The perforation diameter is small, and the perforation quality is better than the blasting perforation The laser used has a higher output power and requires a more reliable pneumatic control system to achieve gas type, gas pressure switching and perforation time control.

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