Laser cutting principle, classification and characteristics

Laser cutting principle, classification and characteristics

Laser cutting uses a focused high-power-density laser beam to illuminate the workpiece, causing the irradiated material to rapidly melt

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 (1) Principle of laser cutting

 
Laser cutting uses a focused high-power-density laser beam to illuminate the workpiece, causing the irradiated material to rapidly melt, vaporize, ablate, or reach a flash point. At the same time, the molten material is blown off by a high-speed airflow coaxial with the beam, thereby cutting the workpiece. open. Laser cutting is one of the hot cutting methods. The principle of laser cutting is shown in the figure below.
 
(2) Classification of laser cutting
 
Laser cutting can be divided into four types: laser vaporization cutting, laser melting cutting, laser oxygen cutting, laser scribing and controlled fracture.
 
1) Laser vaporization cutting
 
The workpiece is heated by a high energy density laser beam to rapidly rise the temperature, reach the boiling point of the material in a very short time, and the material begins to vaporize to form a vapor. These vapors are ejected at a high rate, and a slit is formed in the material while the vapor is ejected. The heat of vaporization of the material is generally large, so a large power and power density is required for laser vaporization cutting.
 
Laser vaporization cutting is often used for cutting very thin metal and non-metallic materials such as paper, cloth, wood, plastics and rubber.
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2) Laser melting cutting
 
When the laser is melted and cut, the metal material is melted by laser heating, and then a non-oxidizing gas (Ar, He, N, etc.) is blown through a nozzle coaxial with the beam, and the liquid metal is discharged by a strong pressure of the gas to form a slit. Laser melt cutting does not require complete vaporization of the metal, and the required energy is only 1/10 of the vaporization cut.
 
Laser melting and cutting is mainly used for the cutting of some non-oxidizable materials or active metals, such as stainless steel, titanium, aluminum and their alloys.
 
3) Laser oxygen cutting
 
The principle of laser oxygen cutting is similar to oxyacetylene cutting. It uses a laser as a preheating heat source and uses an active gas such as oxygen as a cutting gas. On the one hand, the injected gas acts on the cutting metal to cause an oxidation reaction to release a large amount of heat of oxidation; on the other hand, the molten oxide and the melt are blown out from the reaction zone to form a slit in the metal. Since the oxidation reaction during the cutting process generates a large amount of heat, the energy required for laser oxygen cutting is only 1/2 of the melt cutting, and the cutting speed is much larger than the laser vaporization cutting and melting cutting.
 
Laser oxygen cutting is mainly used for easily oxidizable metal materials such as carbon steel, titanium steel and heat-treated steel.
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4) Laser dicing and control fracture
 
Laser dicing is to scan the surface of the brittle material with a high energy density laser, so that the material is evaporated to a small groove by heat, and then a certain pressure is applied, and the brittle material is cracked along the small groove. Lasers for laser scribing are generally Q-switched lasers and CO2 lasers.
 
Controlled fracture is a steep temperature distribution created by laser engraving, creating local thermal stresses in the brittle material that cause the material to break along the small slots.
 
Laser cutting features
 
Compared with other thermal cutting methods, laser cutting is characterized by high cutting speed and high quality. Specifically summarized as the following aspects.
 
(1) Good cutting quality
 
Laser cutting can achieve better cutting quality due to the small laser spot, high energy density and fast cutting speed.
 
1 The laser cutting slit is narrow, the slits are parallel and perpendicular to the surface, and the dimensional accuracy of the cut parts can reach ±0.05mm.
 
2 The cutting surface is smooth and beautiful, the surface roughness is only a few tens of microns, and even laser cutting can be used as the last process. No machining is required, and the parts can be used directly.
 
3 After the laser is cut by the laser, the width of the heat-affected zone is small, the performance of the material near the slit is almost unaffected, and the deformation of the workpiece is small, the cutting precision is high, the geometry of the slit is good, and the shape of the slit cross-section is relatively regular. Rectangular. The comparison of laser cutting, oxyacetylene cutting and plasma cutting methods is shown in Table 1. The cutting material is a 6.2 mm thick low carbon steel plate.
 
(2) High cutting efficiency
 
Due to the transmission characteristics of the laser, the laser cutting machine is generally equipped with multiple numerical control worktables, and the entire cutting process can realize numerical control. During operation, it is only necessary to change the numerical control program to apply the cutting of different shapes of parts, which can perform two-dimensional cutting and three-dimensional cutting.
 
(3) Fast cutting speed
 
Cutting a 2mm thick low carbon steel plate with a power of 1200W, the cutting speed can reach 600cm/min; cutting a 5mm thick polypropylene resin board, the cutting speed can reach 1200cm/min. The material does not need to be clamped and fixed during laser cutting, which saves the fixture and saves the auxiliary time of loading and unloading.
 
(4) Non-contact cutting
 
There is no contact between the torch and the workpiece during laser cutting, and there is no wear of the tool. To machine parts of different shapes, there is no need to change the "tool", just change the output parameters of the laser. The laser cutting process has low noise, low vibration and no pollution.
 
(5) There are many types of cutting materials
 
Compared with oxyacetylene cutting and plasma cutting, there are many types of laser cutting materials, including metal, non-metal, metal-based and non-metal matrix composites, leather, wood and fiber. However, for different materials, due to their different thermophysical properties and different absorption rates of laser light, different laser cutting adaptability is exhibited. The laser cutting performance of various materials is shown in Table 2 using a CO2 laser.
 
(6) Disadvantages
 
Laser cutting Due to the limitation of laser power and equipment volume, laser cutting can only cut medium and small thickness plates and tubes, and as the thickness of the workpiece increases, the cutting speed decreases significantly.
 
Laser cutting equipment is expensive and has a one-time investment.
 
Laser cutting application range
 
Most laser cutting machines are controlled by a numerical control program or made into a cutting robot. Laser cutting is a sophisticated machining method that cuts almost all materials, including two-dimensional or three-dimensional cutting of thin metal sheets.
 
In the field of automobile manufacturing, the cutting technology of space curves such as car roof windows has been widely used. Volkswagen AG uses a 500W laser to cut complex body sheets and various curved parts. In the aerospace industry, laser cutting technology is mainly used for the cutting of special aviation materials, such as titanium alloy, aluminum alloy, nickel alloy, chrome alloy, stainless steel, cerium oxide, composite materials, plastics, ceramics and quartz. Aerospace components processed by laser cutting include engine flame tube, titanium alloy thin wall machine, aircraft frame, titanium alloy skin, wing long stern, tail siding, helicopter main rotor, space shuttle ceramic heat insulation tile, etc.
 
Laser cutting forming technology also has a wide range of applications in the field of non-metallic materials. It can cut not only high hardness and brittle materials, such as silicon nitride, ceramics, quartz, etc., but also flexible materials such as cloth, paper, plastic sheets, rubber, etc., such as laser cutting for clothing, saving material 10 %~12%, improve the efficacy by more than 3 times.
 
Overview of laser cutting technology and precision of laser cutting
 
The laser beam is focused into a small spot with a minimum diameter of less than 0.1 mm, so that a high power density at the focus can exceed 106 W/cm2. At this time, the heat input by the beam (converted by light energy) far exceeds the portion of the material that is reflected, conducted or diffused, and the material is quickly heated to vaporized humidity to evaporate to form a hole. As the beam moves relatively linearly with the material, the holes are continuously formed into slits having a narrow width (e.g., about 0.1 mm). The heat effect of trimming is small and there is almost no deformation of the workpiece. An auxiliary gas suitable for the material to be cut is also added during the cutting process. When steel is cut, oxygen is used as an auxiliary gas to form an exothermic chemical reaction oxidizing material with the molten metal, and at the same time, it helps to blow away the slag in the slit. A type of plastic for cutting polypropylene uses compressed air, and flammable materials such as cotton and paper are cut to use an inert gas. The auxiliary gas entering the nozzle also cools the focusing lens, preventing smoke from entering the lens holder and contaminating the lens and causing the lens to overheat.
 
Most organic and inorganic materials can be cut with a laser. In the metal processing industry, which has a heavy weight in industrial manufacturing, many metal materials, regardless of their hardness, can be shaped without deformation. (The thickness of the industrial steel can be cut with the most advanced laser cutting system. 20mm). Of course, for high reflectivity materials such as gold, silver, copper and aluminum alloys, they are also good heat transfer conductors, so laser cutting is difficult or even impossible to cut (some difficult materials can be cut using a pulsed laser beam, Due to the extremely high peak power of the pulse wave, the absorption coefficient of the material to the beam is instantaneously increased sharply).
 
Laser cutting without burrs, wrinkles, high precision, better than plasma cutting. For many electromechanical manufacturing industries, modern laser cutting systems due to microcomputer programs can easily cut workpieces of different shapes and sizes (workpiece drawings can also be modified), which is often preferred over die cutting and molding processes; despite its processing speed Slower than die punching, but it does not have mold consumption, no need to repair the mold, and saves time to replace the mold, thereby saving processing costs and reducing product costs, so it is economically more cost-effective.
 
On the other hand, laser cutting can also take advantage of its accuracy and reproducibility from the perspective of how to adapt the mold to the size and shape of the workpiece. As a priority manufacturing method of the laminated mold, since the high-level mold making worker is not required, the laser cutting operation cost is not expensive, and the mold manufacturing cost can be remarkably reduced. The additional benefit of laser cutting dies is that the die trim produces a shallow hardened layer (heat affected zone) that improves the wear resistance of the die during operation. The non-contact characteristics of laser cutting give the circular saw blade a stress-free advantage, which increases the service life.
 

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