Basic knowledge of the hottest laser cutting appli

2022-08-18
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Basic knowledge of laser cutting application

I. main characteristics of laser cutting

1. The slit of laser cutting is narrow, and the workpiece deformation is small

the laser beam is focused into a very small light spot, so that the focus reaches a high power density. At this time, the heat input by the beam is far more than the part reflected, conducted or diffused by the material, and the material is soon heated to the degree of vaporization, which forms holes. With the relative linear movement of the light beam and the material, the holes continuously form a slit with a very narrow width. The heating effect of trimming is very small, and there is basically no workpiece deformation

auxiliary steam suitable for the material to be cut is also added during the cutting process. When cutting steel, oxygen is used as an auxiliary vapor to produce exothermic chemical reaction with molten metal, oxidize materials, and help blow away the slag in the slit at the same time. Compressed air is used to cut polypropylene and inert vapor is used to cut cotton, paper and other flammable materials. The auxiliary steam entering the nozzle can also cool the focusing lens to prevent soot from entering the lens seat to pollute the lens and cause overheating of the lens

most organic and inorganic materials can be cut by laser. In the metal processing industry, which plays a very important role in the industrial manufacturing system, many metal materials, regardless of their hardness, can be cut without deformation. Of course, for high reflectivity materials, such as gold, silver, copper and aluminum alloys, they are also good heat transfer conductors. Therefore, laser cutting is very difficult, or even impossible. It is driven by the new energy vehicle industry and relevant policies

laser cutting has no burr, wrinkle and high precision, which is better than plasma cutting. For many mechanical and electrical manufacturing industries, because the modern laser cutting system controlled by microcomputer program can easily cut workpieces of different shapes and sizes, it is often preferred over punching and molding processes; Although its processing speed is still slower than die punching, it has no die consumption, no need to repair the die, and saves the time of replacing the die, thus saving processing costs and reducing production costs, so it is more cost-effective on the whole

on the other hand, from the perspective of how to make the mold adapt to the changes of workpiece design size and shape, laser cutting can also give play to its advantages of accuracy and good reproducibility. As the preferred manufacturing means of laminated molds, because there is no need for senior mold makers, and the operation cost of laser cutting is not expensive, it can also significantly reduce the mold manufacturing cost. The additional benefit of laser cutting mold is that the cutting edge of the mold will produce a shallow hardening layer (heat affected zone) to inspect the damage and deformation of low-carbon cast iron and improve the wear resistance of the mold during operation. Ian Fuller, the vice president of business development and engineering of the non-contact company of laser cutting, talked to reinforced plastics about the stress free advantages brought by the characteristics to the cutting and forming of circular saw blades, due to the increased service life of saw blades

2. Laser cutting is a non-contact processing with high energy and good density controllability

after focusing the laser beam, a small action point with extremely strong energy is formed, which has many characteristics when applied to cutting. First, laser light energy is converted into amazing heat energy and kept in a very small area, which can provide (1) narrow straight edge slits; (2) The smallest heat affected zone adjacent to the trimming; (3) Minimal local deformation. Secondly, the laser beam does not exert any force on the workpiece. It is a non-contact cutting tool, which means that (1) the workpiece has no mechanical deformation; (2) There is no tool wear, and there is no tool conversion problem; (3) Cutting materials do not need to consider its hardness, that is, the laser cutting ability is not affected by the hardness of the material being cut, and any hardness material can be cut. Thirdly, the laser beam has strong controllability, high adaptability and flexibility, so (1) it is convenient to combine with automatic equipment, and it is easy to realize the automation of cutting process; (2) Because there is no restriction on cutting workpiece, the laser beam has unlimited copying cutting ability; (3) Combined with computer, the whole board can be discharged and materials can be saved

3. Laser cutting has extensive adaptability and flexibility

compared with other conventional processing methods, laser cutting has greater adaptability

first of all, compared with other thermal cutting methods, as a thermal cutting process, other methods cannot act on a very small area like a laser beam, resulting in wide incision, large heat affected area and obvious workpiece deformation. Laser can cut nonmetals, while other thermal cutting methods cannot

(1) oxygen combustible (such as acetylene) cutting. This method is mainly used for cutting low-carbon steel. Because of its large influence of heat input and low cutting speed, it is rarely used to cut materials with accurate dimensions below 20mm

(2) plasma cutting. The cutting speed is significantly faster than oxyacetylene cutting, but the cutting quality is poor. The top of the cutting edge is round head, and the cutting edge is obviously wavy. It is also necessary to prevent ultraviolet radiation generated by the arc. It is slightly superior to laser cutting in that it is suitable for cutting thicker steel plates and aluminum alloys with high reflectivity to light beams

(3) die stamping. The die stamping method for mass production of parts has the advantages of low cost and short production cycle. However, its adaptability to design changes is very poor. New molds need to be designed for a long time and cost high. For medium and small-scale production, the characteristics of laser cutting will be fully displayed. In addition, laser program-controlled cutting facilitates the close arrangement of workpieces and saves materials, while die stamping requires materials to be reserved around each workpiece

(4) complex parts are punched in sections. In general, the punch often cuts workpieces that are much larger than the size of the die, and some workpieces are very complex, which leads to many small shell shaped cutting edges, which need a second preparatory processing and renovation. In addition, the punch will form a much wider incision than laser cutting, resulting in a large number of iron filings

(5) sawing. The speed of cutting thin metal is obviously slower than that of laser cutting, and as a flexible non-contact and profiling cutting tool, laser can cut from any point of the material to any direction. It is difficult to do this by sawing

(6) electrical processing. Generally, there are two methods, EDM and electrochemical machining, which use electric corrosion or melting effect, for fine machining of hard materials. The notch roughness is good, but the cutting speed is several orders of magnitude slower than the laser cutting speed

(7) water cutting. Many metal materials can be cut, but the cost is very high

II. Main process of laser cutting

1. Vaporization cutting

under the heating of high-power density laser beam, 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 some materials vaporize into steam and disappear, and some materials are blown away from the bottom of the slit as ejecta by the auxiliary gas flow. Some materials that cannot be melted, such as wood, carbon materials and some plastics, are cut and formed by this vaporization cutting method

in the process of vaporization cutting, the steam carries away the molten particles and scouring debris, forming holes. In the process of vaporization, about 40% of the material turns into steam and disappears, while 60% of the material is driven away by the air flow in the form of droplets

2. Melting cutting

when the power density of the incident laser beam exceeds a certain value, the material at the beam irradiation point will evaporate and form holes. Once the hole is formed, it will absorb all the incident beam energy as a blackbody. The small hole is surrounded by the molten metal wall, and then the auxiliary air flow coaxial with the light beam takes away the molten material around the hole. As the workpiece moves, the small hole moves horizontally synchronously according to the cutting direction to form a cutting seam. The laser beam continues to irradiate along the front edge of the crack, and the molten material is continuously or Pulsatively blown away from the crack

3. Oxidation melting cutting

melting cutting generally uses inert gas. If oxygen or other active gas is replaced, the material will be ignited under the irradiation of laser beam, and another heat source will be generated due to intense chemical reaction with oxygen, which is called oxidation 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 it has a fierce combustion reaction with oxygen, releasing a lot of heat. Under the action of this heat, small holes filled with steam are formed in the material, and the surrounding of the small holes are surrounded by molten metal walls

(2) the transfer of combustion substances into slag controls the combustion rate of oxygen and metal. At the same time, the speed of oxygen diffusion through slag to the front of ignition also has a great impact on the combustion rate. The higher the oxygen flow rate, the faster the combustion chemical reaction and slag removal. Of course, the higher the oxygen flow rate is, the better, because too fast the flow rate will lead to the rapid cooling of the reaction product at the slit outlet, that is, the metal oxide, which is also detrimental to the cutting quality

(3) obviously, there are two heat sources in the oxidation melting cutting process, namely, the laser irradiation energy and the heat energy generated by the chemical reaction between oxygen and metal. It is estimated that when cutting steel, the heat released by oxidation reaction accounts for about 60% of the total energy required for cutting. Obviously, compared with inert gas, using oxygen as auxiliary gas can obtain higher cutting speed

(4) in the oxidation melting cutting process with two heat sources, if the combustion speed of oxygen is higher than the moving speed of laser beam, the slit appears wide and rough. If the laser beam moves faster than the combustion speed of oxygen, the resulting slit is narrow and smooth

4. Control fracture cutting

for brittle materials that are easy to be damaged by heat, high-speed and controllable cutting through laser beam heating is called controlled fracture cutting. The main content of this cutting process is: the laser beam heats a small area of brittle material, causing a large thermal gradient and serious mechanical deformation in the area, resulting in the formation of cracks in the material. As long as the uniform heating gradient is maintained, the laser beam can guide the crack to produce in any desired direction

it should be noted that this controlled fracture cutting is not suitable for cutting sharp degradable plastics to obtain large development angles and corner edges. It is not easy to succeed in cutting super large closed shapes. Control the fracture cutting speed, and do not need too high power, otherwise it will cause the workpiece surface to melt and damage the cutting edge. The main control parameters are laser power and spot size

III. laser cutting machine and process control parameters

laser cutting machine system is generally composed of laser generator, (external) beam transmission components, workbench (machine tool), microcomputer numerical control cabinet, cooler and computer (hardware and software)

1. Laser generator

for the purpose of laser cutting, except for a few occasions when YAG solid-state lasers are used, most of them use CO2 gas lasers with high electric optical conversion efficiency and high power output. Because laser cutting requires high beam quality, not all lasers can be used for cutting

2. CNC cutting machine tool

it is composed of three parts, namely, workbench (generally precision machine tool), beam transmission system (sometimes called external optical path, that is, the optical and mechanical components of beam transmission in the whole optical path before the laser beam reaches the workpiece) and microcomputer numerical control system. According to the relative movement mode between the cutting cabinet and the workbench, it can be divided into the following three types:

(1) during the cutting process, the light beam (emitted by the cutting torch) and the workbench move. Generally, the light beam moves in the Y direction and the workbench moves in the X direction

(2) in the cutting process, only the beam (cutting torch) moves, and

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