Analysis of laser cutting technology in fast and a

2022-08-23
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Analysis of laser cutting process in rapid and accurate machining of sheet metal introduction sheet metal parts play a key role in the forming of thrust chamber of liquid rocket engine, product connection and tube manufacturing. Compared with ordinary sheet metal parts, aerospace product sheet metal parts have the characteristics of many varieties, complex shape, high finish and strict tolerance requirements. Before the forming of sheet metal parts, the first thing to be solved is the shape development processing of sheet metal parts. The following methods are usually used in the shape unfolding processing of sheet metal parts: line drawing milling, wire cutting, plasma cutting, oxyacetylene flame cutting, die stamping, high-pressure water cutting and CO2 laser cutting. Each cutting and cutting method has its advantages and disadvantages. Accuracy, speed and cost are different. It has a certain scope of application in industrial production. Choosing the most convenient and widely applicable processing method is the task faced by the sheet metal parts manufacturing of liquid rocket engine at present

2 selection of process scheme for typical sheet metal parts

2.1 structural characteristics of sheet metal parts

(1) Name: deflector

(2) structural features: the machined inner surface is an irregular curve contour structure (as shown in Figure 1)

2.2 technical requirements

the material is 1Cr18Ni9Ti steel plate, thickness δ 2mm, the curvature of the inner contour is not only variable, but also smooth and excessive, and the tightest dimensional tolerance is 35[-0.3~-0.1] mm

2.3 process analysis and selection of process scheme

according to the part drawing and structural characteristics of the above parts, the following processing process schemes can be selected:

(1) die stamping

(2) WEDM

(3) high pressure water cutting

(4) laser cutting

2.4 analyze and compare various processes for the production task of releasing 16 deflectors

2.4.1 die stamping

because only 16 pieces are processed. Although the mold can meet the requirements of internal contour accuracy, the male mold for machining holes has short service life and is easy to break. The mold itself has high cost and long processing cycle, which is uneconomical

2.4.2 WEDM

accuracy and excessive surface smoothness can be guaranteed. But first of all, we need to process threading holes. The processing speed is too slow. Unreasonable (Note: linear segment processed by wire cutting is equivalent to laser cutting precision and roughness. However, if free curve or irregular curve is processed, the precision and roughness are not as good as laser cutting)

2.4.3 high pressure water cutting

consumables such as seals and cutting heads have short service life and high cost

2.4.4 laser cutting

because its NC program is synchronously transformed by CAD graphics - Geometric bitmap - PLC control program based on non-uniform rational B-spline curve, there is no human error, plus the guarantee of precision machine tools, the theoretical error of mechanical accuracy is ± 0.02mm, and the actual error due to environmental reasons is about ± 0.05mm. And the nesting can be done by software. The material can be arranged at will, and the material utilization rate is usually ≥ 80%. The processing accuracy, cutting surface roughness, heat affected zone range and processing speed can meet the requirements. (laser processing simulation is shown in Figure 1)

2.5 conclusion

under the comprehensive consideration of processing speed, processing accuracy, productivity and production cost, choosing laser cutting to process sheet metal parts can obtain satisfactory results

3 laser cutting process and its parameter analysis

3.1 laser equipment

the laser equipment adopts the laser blanking composite machining center of TRUMPF company

3.2 laser beam parameters

laser system is generally composed of laser, laser transmission system, control system, motion system, sensing and detection system, and its core is laser

the laser is a CO2 gas pulse laser. The light intensity distribution on the beam cross section is close to the Gaussian distribution. It has excellent beam quality. The main performance indicators are as follows:

laser wavelength: 10.6 μ M

pulse power: 2.4kw: pulse width: About 10ms

power density: w/cm2

laser divergence angle: 1mrad

laser power stability: 2%

laser beam focus diameter: Φ 0.15~ Φ 0.30

through practical verification, CO2 laser cutting processing of laser blanking composite machining center δ 0.5mm, δ The process characteristics and related parameters of 6mm plate are:

(1) narrow notch width (generally 0.15~0.30mm), high precision (generally the error of hole center distance is 0.01~0.05mm, and the error of contour dimension is 0.05~0.2mm), and good notch surface roughness (generally RZ is 1.6~6.4 μ m) Generally, the cutting seam can be welded without further processing. It can be seen from Figure 2 that the seam roughness is directly proportional to the material thickness

(2) adopt 2kW laser power, δ The cutting speed of 6mm thick stainless steel is 1.2m/min: δ The cutting speed of 2mm thick stainless steel is 3.6m/min, the heat affected zone is small, and the deformation is very small. The above advantages are enough to prove that: C experts from the Standardization Technical Committee of building products and components of the Ministry of housing and urban rural development said that O2 laser cutting has become an advanced processing method with rapid development

it can be seen from Figure 3 that the maximum cutting speed of material is inversely proportional to the material testing machine for impact test of material thickness

3.3 process and process parameters

3.3.1 NC programming cutting process

NC programming is carried out with tops300 process programming software attached to TRUMPF laser blanking composite machining center, and the blanking size calculation, layout and process parameter setting of materials are completed at the same time. The process is as follows:

(1) drawing and graphic type conversion (the outer contour of the part is required to be closed)

(2) determine the material, size and part layout

(3) use laser cutting: fillet process (obtain sharp edges and blunt) or loop process (obtain sharp angles); Automatically load the gas type and cutting speed, and set the material return

(4) optimization of machining sequence, generation of NC machining program and transmission program

3.3.2 cutting and perforation technology

for δ 0.5mm~ δ 6mm thick plate. Most thermal cutting techniques must have a small hole in the plate. A punch is used to punch a hole on the laser stamping compound machine. Then use the laser to cut from the small hole. For laser cutting machines without stamping devices, the basic method of pulse perforation is generally used -- pulse perforation: 10.6u metal pairs are then filtered through vacuum to obtain graphene/polyaniline fiber film composites. The initial absorption rate of M laser beam is only 0.5% - 10%. When a focused laser beam with a power density of more than 106W/cm irradiates the metal surface. But it can quickly make the surface melt in microseconds. Usually, air or nitrogen is used as auxiliary gas, and each pulse laser produces only small particles. Gradually deepen, so the perforation time of thick plate takes a few seconds. Once the perforation is completed, immediately replace the auxiliary gas with oxygen for cutting. (Note: the service life of the element electronic tube that produces peak power pulse laser is about 20000 hours. The price is expensive. It is best to use the pre punching process for 6 ≤ 3 sheet metal, and the pulse punching process is only used for 6 ≥ 3 sheet metal)

3.3.3 nozzle and air flow control

when laser cutting steel, oxygen and focused laser beam are shot to the cut material through the nozzle. Thus, an air flow beam is formed. The basic requirement for air flow is that the air flow into the incision should be large and the speed should be high, so that enough oxidation can make the incision material fully conduct exothermic reaction, and at the same time, there is enough momentum to spray and blow out the molten material. At present, the nozzle used for laser cutting adopts the structure of a conical hole with a small round hole at the end. When in use, a certain pressure is introduced from the side of the nozzle. Made of pure copper, it is a vulnerable part with small volume

3.3.4 main process of laser cutting

(1) sublimation cutting

under the heating of high power density laser beam. δ 0.5mm~ δ The surface temperature of 6mm plate will quickly rise to the boiling point temperature. Some materials vaporize into steam and disappear. Some materials are blown away from the bottom of the slit as ejecta by the auxiliary air flow. Generally, nitrogen (N2) or argon (AR) is used as cutting gas

(2) high pressure gas focused melting cutting

when the power density of the incident laser beam exceeds a certain value, the interior of the material at the beam irradiation point begins to evaporate, forming holes. It will absorb all the incident beam energy as a blackbody. The small hole is surrounded by molten material. Then, the auxiliary air flow coaxial with the 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. Nitrogen (N2) is generally used as cutting gas

(3) flame oxidation melting cutting

melting cutting generally uses inert gas, if it is replaced by oxygen or other active gases. Under the irradiation of laser beam, materials react violently with oxygen to produce another heat source, which is called oxidation melting cutting. Oxygen (02) is generally used for cutting gas

see Table 1 for the comparison of cutting gas oxygen and nitrogen

3.3.5 consumption of laser cutting gas

the consumption of laser cutting gas is shown in Figure 4 and figure 5. As can be seen from Figure 4, for δ 0.5mm- δ For plates with the same material thickness of 6mm, the volume of oxygen gas ejected from the nozzle per unit time increases with the increase of the use pressure. For plates with different material thickness, the volume increment of gas ejected from the nozzle per unit time under the same pressure is proportional to the square of the material thickness increment

it can be seen from Figure 5. about δ For plates with the same material thickness of 0.5mm~6mm, the volume of nitrogen gas ejected from the nozzle in a unit time increases with the increase of the use pressure. For plates with different material thickness, the unit gathers domestic retired and dispersed scientific and technological experts under the same pressure. The volume increment of gas ejected from the nozzle in a unit time is proportional to the square of the material thickness increment. Because the nitrogen pressure is above 6bar, it plays an effective role in cutting. Therefore, the gas consumption is large

3.3.6 laser cutting of common engineering materials

(1) carbon steel

pure oxygen is used as auxiliary gas for cutting carbon steel. The maximum thickness of carbon steel plate can be cut by this laser processing center is up to 8mm. For thick plates, the cutting seam is 0.3mm. The cutting seam of the thin plate can be narrowed to about 0.2mm

(2) stainless steel

cutting stainless steel uses high-pressure nitrogen as the auxiliary gas. The maximum thickness of stainless steel plate can be cut by this laser processing center can reach 6mm. It is an effective processing tool for using stainless steel and s-06 sheet as the main component. The heat affected zone of trimming is very small, which can effectively maintain the good corrosion resistance of this kind of material

(3) aluminum and aluminum alloys

high pressure nitrogen is used as auxiliary gas for aluminum cutting. Aluminum cutting belongs to melting cutting mechanism. Due to the high reflectivity of aluminum to laser. Only thin aluminum plates can be cut. Thickness of aluminum alloy cut by this laser processing center δ ≤ 4mm. the auxiliary gas used is mainly used to blow away molten products from the cutting area. Generally, better section quality can be obtained

(4) copper and copper alloys

pure copper (red copper) cannot be cut by CO2 laser beam because of its high reflectivity

(5) nickel based alloys

nickel based alloys, also known as super alloys, have many varieties. Among them, gh1131 and GH1140 have been tested, and laser cutting has been successfully implemented with good section quality

4 conclusion

(1) laser cutting is used. The overall dimensions, accuracy, roughness and heat affected zone of the cut parts fully meet the design requirements, with high processing efficiency and no need for molds. As a mature processing method, laser cutting has great development space for the production of sheet metal parts

(2) the accuracy of the drawing and the debugging work after the cutting of the first piece are very important. Master laser processing deviation through debugging before process preparation

(3) the material uniformity and impurities of the blank have a great impact on the processed products. Laser processing roughness, cutting

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