1 Introduction
In the liquid rocket engine thrust chamber forming and product connection, catheter manufacturing, sheet metal parts play a key role. Compared with ordinary sheet metal parts, aerospace sheet metal parts have many features, such as multiple types, complex shapes, high finish, and strict tolerance requirements. And sheet metal parts before molding. The first thing to be solved is the development of the shape of the sheet metal part. The following methods are commonly used for the unfolding process of sheet metal parts: line milling, wire cutting, plasma cutting, oxyacetylene flame cutting, die stamping, high pressure water cutting, and CO2 laser cutting.
Various cutting methods have their advantages and disadvantages. Accuracy, speed, and cost are all different. There is a certain scope of application in industrial production. The selection of the most convenient and widely adaptable processing method is the task currently faced by liquid rocket engine sheet metal parts manufacturing.
2 Selection of typical sheet metal parts process solutions
2.1 Structural Features of Sheet Metal Parts
(1) Name: deflector.
(2) Structural features: The inner profile of the processing is an irregular curved profile structure (Figure 1).
2.2 Technical Requirements
The material is 1Crl8Ni9Ti steel plate with a thickness of 82mm. The curvature of the inner contour is not only changeable but also must be smooth and excessive, and the strictest dimension tolerance is 35-0.1mm.
2.3 Process Analysis and Process Options Selection
Based on the above parts of the parts drawings and structural features. There are several types of processing solutions to choose from:
(1) mold stamping;
(2) Wire EDM;
(3) high-pressure water cutting;
(4) Laser cutting.
2.4 Analysis and comparison of various processes for the production of 16 deflector production tasks
2.4.1 Die Stamping
Because only 16 pieces are processed. Although the mold can guarantee the accuracy of the inner contour, the life of the male mold for processing the Φ1 hole is short and easy to break, the cost of the mold itself is high, and the processing cycle is long and uneconomical.
2.4.2 Wire EDM
Accuracy and surface smoothness are guaranteed. But first need to process the threading hole. Processing speed is too slow. Unreasonable (Note: The linear section of the wire cutting process is equivalent to laser cutting accuracy and roughness. However, if the free curve or irregular curve is processed, the accuracy and roughness are not as good as laser cutting).
2.4.3 High pressure water cutting
Short life and high cost of consumables such as seals and cutting heads.
2.4.4 Laser Cutting
Because the NC program is synchronously transformed from the CAD graph-geometry bitmap-based PLC control program based on the non-uniform rational B spline curve, there is no human error, plus the precision machine tool guarantee, the mechanical accuracy theoretical error is ± 0.02mm, the actual error is about ±0.05mm due to environmental reasons. And the nesting can be arranged in software. As far as the discharge is concerned, the material utilization rate is usually ≥80%, and the machining accuracy, cutting surface roughness, heat affected zone range and processing speed can all meet the requirements.
2.5 Conclusion
In the comprehensive consideration of processing speed, processing accuracy, productivity, and production costs, the laser cutting of sheet metal parts can achieve satisfactory results.
3 laser cutting process and its parameter analysis
3.1 Laser equipment
The laser equipment uses Trumpf's laser blanking compound machining center.
3.2 Laser beam parameters
Laser systems are generally composed of lasers, laser transmission systems, control systems, motion systems, sensing and detection systems, and their cores are lasers.
The laser is a CO2 gas pulsed laser. The light intensity distribution on the beam cross-section is close to the Gaussian distribution. With 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: Φ0.15-Φ0.30
Through practical verification, the process characteristics and related parameters of the CO2 laser cutting of δ0.5mm-δ6mm plates by the laser blanking compound machining center are:
(1) narrow incision width (usually 0.15-0.30mm), high precision (normal hole center distance error is 0.01-0.05mm, contour size error is 0.05-0.2mm), incision surface roughness is good (generally Rz is 1.6- 6.41μm), slits generally do not need to be processed to weld.
It can be seen from Figure 2 that the roughness of the kerf is proportional to the material thickness.
Fig.2 The relationship between the roughness of cut section and material thickness of oxygen-cut carbon steel
(2) With a 2kW laser power, the cutting speed of stainless steel 6mm thick is 1.2m/min; the cutting speed of stainless steel δ 2mm thick is 3.6m/min, the heat affected zone is small, and the deformation is minimal. 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.
Fig. 3 Relationship between maximum cutting speed and material thickness of several common materials
3.3 Process and Process Parameters
3.3.1 CNC cutting process
Trupsf's TOPS300 process programming software attached to the laser blanking combined machining center was used for numerical control programming. Material blank size calculation, layout, and process parameter settings were also completed. The process is as follows:
(1) Conversion of drawings and types of graphics (requirements for the external outline of parts to be closed);
(2) determine the material, size and part layout;
(3) Use laser cutting: rounding process (obtaining sharp edges and blunt) or loop process (obtaining acute angles); automatically loading the gas type, cutting speed, and setting the return material;
(4) The processing sequence is optimized to generate NC machining programs and transmission programs;
3.3.2 Cutting Perforation Technology
For δ 0.5mm-δ6mm thick plate. Most thermal cutting techniques must wear a small hole in the board. On the laser stamping compound machine, a punch is punched out first. Then use a laser to start cutting at the small L. For a laser cutting machine without a stamping device, the basic method of pulse piercing is generally used. Pulse piercing: The initial absorption of metal to a 10.6 um laser beam is only 0.5% to 10%. When a focused laser beam with a power density of more than 106 W/cm2 strikes a metal surface. But it can quickly melt the surface in microseconds. Air or nitrogen is commonly used as an assist gas, and each pulsed laser produces only a small particle spray. Gradually deeper, so it takes a few seconds for the plank to wear. Once piercing is complete, the auxiliary gas is immediately replaced with oxygen for cutting. (Note: The lifetime of the vital part electron tube producing a high peak power pulsed laser is about 20,000 hours, which is expensive. It is better to use a pre-punching process for δ ≤ 3 thin plates, and a pulse piercing process is used for δ ≥ 3 plates).
3.3.3 Nozzle and Airflow Control
When laser cutting steel, oxygen and focused laser beams are directed through the nozzle to the material being cut. A stream of air is thus formed. The basic requirement for gas flow is that the gas flow into the incision is large and the velocity is high, so that sufficient oxidation makes the incision material fully exothermic, and at the same time there is sufficient momentum to blow out the molten material. At present, the nozzle for laser cutting adopts a structure with a small hole at the end of a tapered hole. In use, a certain pressure is applied from the side of the nozzle. Made of pure copper, it is a small part and is a fragile part.
3.3.4 The main process of laser cutting
(1) Sublimation cutting
Under high power density laser beam heating. The surface temperature of the δ 0.5 mm to δ 6 mm sheet will rapidly rise to the boiling temperature. Some of the material vaporizes and disappears into vapor, and some of the material is ejected from the bottom of the slit by the auxiliary airflow as an ejected material. Cutting gas is generally nitrogen or argon.
(2) High pressure gas focused melt 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 incident beam energy as a black body. The pores are surrounded by molten material. then. Auxiliary air flow coaxial with the beam takes away the molten material around the hole. As the workpiece moves, the small holes are traversed synchronously in the cutting direction to form a slit. Cutting gas is generally nitrogen.
(3) Flame oxidation melting cutting
Melt cutting generally uses inert gas if it is replaced with oxygen or other reactive gases. The material undergoes a violent chemical reaction with oxygen under the irradiation of a laser beam to generate another heat source called an oxidative melt cutting. Cutting gas generally uses oxygen. The cutting gas oxygen and nitrogen are shown in Table 1.
3.3.5 Laser Cutting Gas Consumption
The consumption of laser cutting gas is shown in Figure 4 and Figure 5. As can be seen from Figure 4, for the same sheet thickness of δ0.5mm-δ6mm, the volume of oxygen gas emitted from the nozzle per unit time increases with the increase in the use of pressure, for the same thickness of the sheet material, in the same The volume of gas ejected from the nozzle per unit time under pressure is proportional to the square of the increment of material thickness.
Figure 4 Oxygen consumption map
Figure 5 Nitrogen consumption map
Can be seen from Figure 5. For δ 0.5mm-δ6mm sheets of the same material thickness, the volume of nitrogen gas emitted from the nozzles per unit time increases with the increase of the use pressure, for different material thickness sheets. The volume of gas ejected from the nozzle per unit time under the same pressure is proportional to the square of the increment of material thickness. Since nitrogen pressure is above 6 bar, 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
Cutting carbon steel uses pure oxygen as an auxiliary gas. The maximum thickness of the carbon steel plate can be cut by the laser processing center up to 8 mm, and the slit thickness of the thick plate is 0.3 mm. The slits on the sheet can be as narrow as about 0.2 mm.
(2) Stainless steel
Cutting stainless steel uses high-pressure nitrogen as an auxiliary gas. The laser processing center can cut the maximum thickness of the stainless steel plate up to 6mm, which is an effective processing tool using stainless steel and S-06 sheet as the main component. The trimming heat affected zone is small and can effectively maintain the good corrosion resistance of such materials.
(3) Aluminum and aluminum alloys
Cutting aluminum uses high-pressure nitrogen gas as an auxiliary gas. Aluminium cutting is a melting and cutting mechanism because aluminum has a high reflectivity to laser light. Only thin aluminum plates can be cut. The thickness of the aluminum alloy cut by the laser processing center is δ<4mm. The auxiliary gas used is mainly used to blow away the molten product from the cutting zone. Generally better cut quality can be obtained.
(4) Copper and copper alloys
Pure copper (copper) cannot be cut with a CO2 laser beam because of its high reflectance.
(5) Nickel base alloy
Nickel-based alloys are also called super alloys and they come in many varieties. Among them, GHll3l and GHll40 have been subjected to process tests. Laser cutting has been successfully performed and the section quality is good.
4 Conclusion
(1) Using laser cutting, the dimensions, accuracy, roughness, and heat affected zone of the cut parts are fully in line with the design requirements, and the machining efficiency is high and no mold is needed. As a mature processing method, laser cutting has a lot of room for the development of sheet-type parts.
(2) The accuracy of the drawing and the debugging work after the first piece is cut are very important. Master laser machining deviations before commissioning.
(3) Material uniformity and impurities of the blank are processed.
The product has a greater impact. The relationship between laser processing roughness, cutting speed, gas consumption and material thickness is: the cutting surface roughness is proportional to the material thickness, the cutting speed is roughly inversely proportional to the material thickness, and the gas consumption is proportional to the square of the material thickness increment.
Dongguan Leda Metal Co.,Limited
