Analysis of laser cutting technology in fast and a

Analysis of laser cutting process in rapid and accurate machining of sheet metal parts

1 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

various cutting and cutting methods have their 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 1Crl8Ni9Ti steel plate, with a thickness of 82mm. The internal contour curvature is not only variable, but also smooth and excessive, and the tightest dimensional tolerance is 1mm。

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 ensure the accuracy requirements of the inner contour, the machining Φ The male mold with 1 hole 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

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: 10.61xm

pulse power: 2.4kw; Pulse width; About l0ms

power density: 107w/cm2; Laser divergence angle: 1mrad

laser power stability: 2%

laser beam focus diameter: Φ 0.15- Φ 0.30

it is verified by practice that the laser blanking composite machining center co improves the friction consumption and 2 laser cutting processing δ 0.5mm- δ The process characteristics and related parameters of 6mm plate are:

(1) narrow notch width (generally 0..30mm), high precision (generally 0..05mm hole center distance error, 0..2mm contour dimension error), and good notch surface roughness (generally RZ is 1..41 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

Figure 2 Relationship between seam roughness and material thickness in oxygen cutting carbon steel

(2) with 2kW laser power, 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 CO2 laser cutting has become an advanced processing method with rapid development

it can be seen from Figure 3 that the maximum cutting speed of the material is inversely proportional to the material thickness

Figure 3 the relationship between the maximum cutting speed and material thickness of several common materials

3.3 process and process parameters

3.3.1 NC programming cutting process

use the sensor of TRUMPF public tensile testing machine to carry out NC programming with the tops300 process programming software attached to the laser blanking composite machining center of American force sensor company, and complete the calculation of material cutting size, layout and process parameter setting 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 L. For laser cutting machines without stamping devices, the basic method of pulse perforation is generally used - pulse perforation: the initial absorption rate of metal to 10.6um laser beam is only 0.5% - 10%. When a focused laser beam with a power density of more than 106w/cm2 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 it takes a few seconds for the thick plate to pass through the hole. Once the perforation is completed, immediately replace the auxiliary gas with oxygen for cutting. (Note: the service life of the element gas electronic tube that produces peak power pulse laser is about 20000 hours, which is expensive, and it is not suitable for δ ≤ 3 thin plates are preferably pre punched, δ Only plates ≥ 3 adopt pulse piercing process)

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 and small in size, it is a vulnerable part similar to household appliances

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, and some materials are blown away from the bottom of the slit as ejecta by the auxiliary air flow. The cutting gas is generally nitrogen or argon

(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 is generally used for 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 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 directly proportional to the square of the material thickness increment

Figure 4 oxygen consumption figure

Figure 5 nitrogen consumption figure

can be seen from Figure 5. about δ 0.5mm- δ For plates with the same material thickness of 6mm, the volume of nitrogen 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 directly 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 laser processing center can cut carbon steel plates with a maximum thickness of 8mm, and the cutting seam of thick plates 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 an auxiliary gas. The laser processing center can cut stainless steel plates with a maximum thickness of 6mm, which is an effective processing tool for using stainless steel and s-06 thin plates as main components. 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 alloy

high pressure nitrogen is used as auxiliary gas for cutting aluminum. Aluminum cutting belongs to melting cutting mechanism, because aluminum has high reflectivity to laser. Only thin aluminum plates can be cut. Thickness of aluminum alloy cut by this laser processing center δ