1.Preface
Aluminum alloy thin frame structural parts have the characteristics of light weight, pressure resistance and corrosion resistance, and are widely used in aerospace spare parts.[1-3]in order to achieve the goal of reducing the overall quality of the aircraft model and improving flight performance. However, due to the large size of structural parts and high requirements for surface quality, the cutting force and clamping force during machining will form residual stresses in the material, which will lead to changes in the size of the parts.[4-7]it is difficult to meet the requirements for product characteristics. Current existing processing methods require the use of high-end and high-precision equipment to reduce the amount of cutting and perform multiple passes.[8-10]low processing efficiency and high production cost. This article takes a high-precision aluminum alloy thin-walled part with a complex shape for aerospace as an example. Depending on the size of the workpiece, a blank with a chuck is designed and a reasonable process is used for efficient completion. cold working, heat treatment and wire cutting. Arrange to avoid dimensional changes caused by stress in thin-walled parts and control the extent of processing deformation.
2. Processing difficulties
Thin-walled parts are made of high-strength 2D14 cemented carbide. The overall volume is relatively large and the walls are thin. The requirements for dimensional accuracy and geometric tolerances are high. The workpiece milling process mainly involves cavity milling and contour milling. Parts are fixed using pressure plates, combination fixtures and machine tool worktables. After the parts are processed, the stress release caused by the tightening of pressure plates and other fasteners. will cause dimensional deviations. The size of the final processed thin-walled parts changes and cannot meet the characteristic high-precision requirements of aerospace parts.
3. Process Layout
3.1 Overall process path
According to the appearance characteristics and processing difficulties of the parts, the process sequence is reasonably organized, which involves the use of knowledge and equipment related to cold processing, electrical processing and heat treatment. The overall process layout is shown in Figure 1, and the shape and structure of the parts are shown in Figure 2.
Figure 1 Overall process layout
Figure 2 Appearance and structure of the part
According to the appearance characteristics of thin-walled aluminum alloy parts, when cutting, leave a 30-50mm chuck at each of the left and right ends. The chucks at both ends are tightened and fixed, as well as a two-edged φ16. A φ20mm carbide tool is used to roughen the blank. It is 6000 to 7000 rpm, leaving a margin of 3 to 5 mm on one side. After rough machining, the parts are subjected to the first stabilization and aging treatment, and then a φ10~φ16mm three-edged carbide tool is used to semi-finish the parts. The mandrels at both ends are tightened to correct the shape and interior cavity. , leaving 0.5 on one side. The semi-finished parts undergo a second stabilization aging treatment, then the clamps at both ends are clamped by pressure plates, and φ6~φ8mm three-edged carbide tools are used to finish the parts, and the Outer shape and inner cavity size of the parts are processed and formed. After the processing of the cavity and contour of the workpiece is completed, wire EDM is used to remove the mandrels at both ends, because there are traces of wire cutting removal on thin-walled workpieces and the chuck after wire EDM, to ensure proper operation. integrity of the overall surface quality of thin-walled parts, use fine sandpaper to polish the wire cutting chuck of the workpiece. Use tools of different diameters during rough machining, semi-finishing and finishing. Double-edged tools can quickly remove material, and three-edged tools can refine the surface quality to ensure that there are no traces on the milled surface of the workpiece. at the same time, two heat treatments can reduce cold treatment. To release the stress of the material during machining, the non-contact wire EDM processing method can effectively avoid the dimensional changes caused by the residual stress of the material during machining and the elastic deformation of the material during clamping.
3.2 Heat treatment
The implementation of an aging stabilization treatment is crucial. For the first stabilization aging, the roughly machined part is placed in an artificial aging furnace, heated to 250-290°C, kept for 2-4 hours and then cooled in air. For the second stabilization aging, the semi-finished part is placed in an artificial aging oven, heated to 250-290°C, kept for 1-2 hours, and the parts are subjected to thermal cycling. The thermal cycle treatment step for aluminum alloys consists of placing the parts in a low temperature container between -70 and -50°C for 1 to 2 hours. In order to improve the effect of thermal cycle treatment, it can be cooled in liquid nitrogen. The cooling rate of cold processing does not have a substantial impact on thermal cycling processing. Once the cold treatment of the product is completed, take it out of the container at low temperature. After the formed frost melts, place it in the heating equipment to heat it to the specified temperature, or dry it first at about 50°C for 1 hour. at 2 o’clock. If the room cannot be heated immediately after drying due to working conditions, an interruption of up to 20 hours is allowed. After the parts are heated and kept warm, they are cooled to room temperature and then placed in a low temperature container. Here, the number of cycles should be selected based on the processing requirements of the part.
3.3 Cold working
When CNC milling thin-walled aluminum alloy parts, in order to avoid deformation, the processing process is divided into three stages: rough machining, semi-finishing and finishing. When rough machining, the tool speed is 6000-7000 rpm. The high speed can effectively remove material, form the overall contour of the workpiece in a short time, and improve processing efficiency. At the same time, a margin of 3 to. 5 mm is left on one side of the part for semi-finishing; During semi-finishing, the tool speed is controlled between 2000 and 2500 rpm. Low speeds can be effectively maintained. It can check the surface roughness of parts after processing, and at the same time, the processed tool marks can be thinned to further improve the surface quality of parts, and control to leave a margin of 0.5-1mm on one side for finishing. During finishing, the tool should be reduced appropriately. Rotation speed, control the tool speed at 1500 ~ 1800 rpm, remove excess and ensure surface quality.
3.4 Electrical machining
After the cavity and contour processing of aluminum alloy thin-walled parts is completed, the processing chucks are left at both ends of the parts to avoid the stress on the thin-walled parts caused by the removal of the parts. chucks from the cold. working machine and resulting in product deformation, wire EDM technology is used. Wire EDM machining is non-contact EDM machining. There is no significant cutting force between the tool electrode and the workpiece, and no error will occur due to mechanical deformation. According to the properties of the aluminum alloy, the positive polarity processing method is adopted (the workpiece is connected to the positive electrode and the electrode wire is connected to the negative electrode, the processing current is selected between 3 and 5 A, the pulse). the width is selected between 30 and 50 μs and the duty cycle is 1:7 to 1:5.
The joint between the workpiece and the chuck after wire EDM processing is polished with fine sandpaper to ensure the overall surface integrity of thin-walled parts and ensure that the parts as a whole meet the requirements of high precision and high aerospace performance.
4.Conclusion
Based on the difficult-to-machine characteristics of aluminum alloy materials, this article optimizes the process of high-precision thin-walled parts with complex shapes. Through the reasonable arrangement of cold processing, heat treatment and electric machining, and the selection of different tools and processing methods according to rough machining, semi-finishing and finishing methods, the quality of Parts and processing efficiency are effectively guaranteed, and we get rid of the conventional thinking of relying on high-end machine tools. It has been verified by actual processing that the overall layout of the process route is reasonable and the process layout is scientific and compact, which not only avoids changes in the machining dimensions of parts, but also reduces the parts turnover time and improves production efficiency. .
Expert commentary
The article takes high-precision aluminum alloy thin-walled parts with complex shapes as an example. According to the shape and size of parts, it focuses on optimizing cutting methods and clamping methods by adding blank chucks and adopting reasonable process routes. heat treatment, cold treatment and wire cutting are organized efficiently. In other processes, appropriate tools and processing methods are selected to avoid dimensional changes caused by the processing stress of thin-walled parts, and the experience accumulated in controlling the deformation of these parts.
The highlight of the article is to change ideas, find new paths, break conventional thinking and bypass technical bottlenecks. Through reasonable process arrangements, we can gradually solve the processing difficulties and get rid of the dependence on high-end machine tools in the processing of high-precision aluminum alloy thin-walled parts. Depending on the shape characteristics of the parts, we will apply them globally. the knowledge related to heat treatment, cold treatment and electrical machining, and through the overall layout and process improvements, have solved the problem of processing deformation of complex parts.
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