Standard notation
LBC - Laser Beam Cutting - Cutting with a laser beam
The essence of the process
Unlike conventional light beam to the laser beam is characterized by properties such as orientation, monochromaticity, and coherence.
Due to the orientation of the laser beam is focused on a relatively small area. Thus, according to its focus the laser beam is thousands of times greater than the spotlight.
The laser beam compared to conventional light is monochromatic, ie, has a fixed wavelength and frequency. This makes it easier to focus the optical lens.
The laser beam has a high degree of coherence - a coherent course of time some wave processes. Coherent oscillations cause the resonance to amplify the power of radiation.
Thanks to these properties of the laser beam can be focused on a very small surface of the material and create her energy density sufficient to heat and destroy the material (for example, the order of 10 8 W / cm 2 for melting metal).
The impact of laser radiation on metal when cutting is characterized by the general provisions related to the absorption and reflection of radiation, the absorbed energy distribution in terms of the material by conduction, etc., as well as a number of specific features.
In the field of the laser beam heats the metal up to temperature first fracture - melting. With the further absorption of radiation is melting the metal and the melting phase boundary moves into the material. At the same time, the energy impact of the laser beam leads to a further increase in temperature, reaching the second temperature destruction - the boiling point, at which the metal begins to actively evaporate.
Thus, two mechanisms of laser cutting machine - melting and evaporation. However, the latter mechanism requires high energy and feasible only for sufficiently thin metal. Therefore, in practice carry out fusion cutting. At the same time in order to significantly reduce energy costs, increasing the thickness of the treated metal and speed of cutting is applied auxiliary gas injected into the zone of cutting to remove the decay products of the metal. Usually the gas is used as an auxiliary oxygen, air, inert gas or nitrogen. This is called gas laser cutting.
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| The laser cutting process |
- initially promotes pre-oxidation of the metal and reduces its ability to reflect the laser beam;
- then the metal ignites and burns in a stream of oxygen, resulting in additional heat is released, the reinforcing action of laser radiation;
- oxygen jet blows and blows from the region of the molten metal cutting and its combustion products, providing a simultaneous flow of gas directly to the front of the combustion reaction.
In the second mechanism for cutting the material does not burn, but melts and the gas jet removes molten metal from the cutting area. This mechanism is used for metals and alloys with low thermal effect of combustion, as well as for those who have interaction with oxygen to form refractory oxides. For example, alloyed and high-carbon steel, aluminum, copper, etc.
Types of lasers
A laser usually consists of three main components:
- energy source (pumping mechanism or system);
- active (working) the body which is subjected to "pump", which leads to its stimulated emission;
- optical resonator (mirror system), which provides amplification of stimulated emission of the active body.
- Solid-state and
- gas - a longitudinal or transverse flow of gas gap, and gas dynamics.
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| solid-state laser |
In gas lasers as an active body is a mixture of gases, usually carbon dioxide, nitrogen and helium. In lasers with longitudinal pumping gas mixture of gases coming from the tanks, is pumped by a pump through the discharge tube. Electrical discharge between the electrodes connected to a power source is used for the excitation energy of the gas. The edges of the tube and has a translucent reflective mirror.
More compact and powerful lasers are the transverse flow of gas. Their total capacity of up to 20 kW and above.
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| lasers with longitudinal and transverse gas flow |
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| laser slit |
The most powerful lasers - gasdynamic (100-150 kW and above). Gas heated to a temperature of 1000-3000 K, occurs at supersonic speed through the Laval nozzle (constricted in the middle of the channel), with the result that it expands and cools adiabatically in the area of the optical resonator. Upon cooling, the excited molecules of carbon dioxide are emitted coherent radiation. The pumping laser can be an auxiliary laser or other powerful source of energy.
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| gas dynamic laser |
Solid state lasers are poorly treated non-metals, as a number of such materials is fully or partially transparent to radiation with a wavelength of about 1 micron, such as plexiglass. The laser beam is more sensitive to the uneven surface of the material. However, when cutting aluminum alloys, copper and brass, solid-state lasers have the advantage in comparison with carbon dioxide, since the absorption of radiation by the surface of these metals is much higher at the wavelength solid-state laser.
Carbon dioxide lasers are more versatile and are used for the treatment of almost all metals and nonmetals. In addition, they have a very low beam divergence, which makes it possible to place the radiation source is far away from the treatment area without loss of beam quality.
Cutting of various materials
For cutting metal is required in the main laser power of 450-500 W and above, non-ferrous metal - from 1kW and above.
Cutting carbon steels most often performed with the use of oxygen as an auxiliary gas. The interaction of oxygen with the heated metal beam exothermal oxidation of iron is usually with the release of 3-5 more heat than light from the laser. The quality of the cut end surface - high. The lower edge of the cut is characterized by the formation of a slight burr.
The biggest problem is the possibility of transition of the cutting process, running on very low speeds (typically less than 0.5 m / min), an unmanaged autogenous mode, in which the metal begins to heat up to temperatures of combustion outside the influence of the beam, which leads to an increase in the width of cut and an increase in its roughness.
In some cases, such as cutting of parts with sharp corners and holes of small diameter, instead of oxygen is preferred to use an inert gas at high pressure.
Laser cutting stainless steel, especially the large thickness, the process is complicated by the presence of slagging cut in the metal alloying elements affect the melting temperature of the metal and its oxides. So, perhaps the formation of refractory oxides, preventing the cart to the laser processed material. Complicating the process of cutting and low fluidity raplavlennyh oxides, for example, typical for stainless chromium-nickel and high-chromium steels.
For the best cut using high-purity nitrogen served at elevated pressures (typically up to 20 atm). When cutting a thick stainless steel requires penetration of the beam in the focal spot of the cut metal. As a consequence, increases the diameter of the inlet and increases the flow of gas into the metal in the melt zone.
For laser cutting of aluminum and its alloys, copper and brass require a higher light output, due to the following factors:
- low absorptive capacity of these metals with respect to the laser, especially with a wavelength of 10.6 micron carbon dioxide laser, and therefore the solid-state lasers are preferred;
- high thermal conductivity of these materials.
When cutting aluminum used an auxiliary gas at pressures above 10 atm. The structure of the end surface of the cut - with a porous easily removable burr on the bottom edge of the cut. With increasing thickness of the metal quality of the end surface of the cut deteriorates.
When cutting brass cut end surface has a rough porous structure with an easily removable grata at the bottom of the cut. With increasing thickness of the metal quality of the end surface of the cut deteriorates.
TABLE. The typical thickness of cut sheets of laser power P = 5 kW
| Name | Thickness (mm) |
| Carbon and alloy steel | to 40 |
| Stainless steel | 25 |
| Copper | 5 |
| Brass | 12 |
| Aluminum alloys | 12 |
When cutting the foam should observe heightened fire because it can ignite. Due to the ignition is impossible or very difficult to cut thick foam board (thickness 10 mm).
Impossible or extremely difficult laser cutting of materials such as PCB, glass fiber, getinaks, cellular polypropylene, polycarbonate, polycarbonate. Difficult to cut materials that are prone to cracking, such as ceramics or glass.
TABLE. The typical thickness of cut sheets of laser power P = 1.5 kW
| Name | Thickness (mm) | Cutting speed (m / min) |
| Plastic | 25 | 2 |
| Plywood | 10 | 3–4.5 |
| Dried pine | 20 | 2 |
| CPD | 20 | 1.5 |
| Fibreboard | 5 | 6 |
| Glass | 1–8 | 5–0.5 |
| Asbestos, metalloasbest, paronite | 4 | 1.5 |
The main technological parameters of laser cutting are as follows:
- radiated power;
- cutting speed;
- auxiliary gas pressure;
- diameter of the focused spot, etc.
- pulse repetition frequency;
- pulse duration;
- average output power.
Quality of cut is determined by the roughness of its surface. It is different for different zones across the thickness of the metal. The best quality is typical for the upper layers of the cut metal, the worst - for the lower.
Advantages, disadvantages, and comparative analysis of laser cutting
Focused laser radiation allows to cut almost any material, regardless of their thermophysical properties. It is possible to obtain high quality and narrow cuts (0.1-1 mm wide) with a relatively small zone of thermal influence. Laser cutting there are minimal deformation, as time in the processing of the workpiece, and the residue after complete cooling. As a result of cutting is possible with a high degree of accuracy, including the non-rigid and legkodeformiruemyh products. Due to the relatively simple operation with a laser beam can perform automatic processing of flat and three-dimensional parts of the complex contour.
Laser cutting is especially effective for steel up to 6 mm, providing a high quality and accuracy at a relatively high speed cutting. However, the metal thickness of 20-40 mm, it is used much less oxygen and plasma cutting, and metal thickness over 40 mm - almost never used.
TABLE. Comparison of laser cutting with oxygen, plasma and waterjet cutting
| Name | Characteristics of laser cutting in relation to | ||
| oxygen | Plasma | Waterjet | |
| The typical width of cut (mm) | less than in times and ten times | ||
| Quality | greatly exceeds | greater than | inferior |
| The zone of thermal influence | less | less | more |
| The restriction on the maximum thickness of the metal | very much inferior to | significantly inferior to the non-ferrous metals, yields on other metals | much lower than |
| Cutting performance thin steel (up to 6 mm, batch cutting) | greater than | comparable | greatly exceeds |
| Cost of equipment | much higher | above | comparable |
| Cost of service | above | comparable | comparable |
See also:
CNC laser cutting
laser welding
waterjets
Cutting technology:
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