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Research on Precision Machining Technology of Titanium Alloy TC11
Discussion on Fine Cutting Technology of Titanium Alloy TC11
Titanium alloy has the characteristics of low density, high strength, higher specific strength than ultra-high strength steel; and good thermal stability, good corrosion resistance, and high high temperature strength; at 300 ~ 500 ℃, its strength is about 10 times higher than that of aluminum alloy. , has been widely used in aerospace, aviation and missile engine products. In particular, (α+β) titanium alloy can be quenched and aged to strengthen the alloy, and the strength after heat treatment is improved by 50% to 100% compared with the annealed state. And it has excellent low temperature resistance and excellent resistance to seawater corrosion and thermal salt stress corrosion resistance, and is more widely used.
However, because the titanium alloy has a small cutting deformation coefficient (the deformation coefficient is less than or close to 1), the distance of the chips sliding on the rake face increases during the cutting process, which accelerates the tool wear; at the same time, the cutting temperature is high, the cutting force is large, and it is easy to The surface metamorphosis pollution layer occurs, because of the high chemical activity of titanium, it is prone to violent chemical reactions with various gas impurities, such as O, N, H, C, etc., invading the cutting surface of the titanium alloy, resulting in an increase in the hardness and brittleness of the surface layer. In addition, there are TCi and TiN hard surface layers; at high temperatures, the surface layer is formed to form an alpha layer and a hydrogen embrittlement layer and other surface metamorphic pollution layers. The surface layer is unevenly arranged, and some stress is concentrated, which reduces the fatigue strength of the parts. The cutting process also seriously damages the tool, causing nicks, chipping, and falling off. It is easy to bite with the tool material, causing severe sticking to the tool, resulting in severe bonding wear; and the shortcomings of the titanium alloy structure are not stable, which brings many difficulties to cutting, especially fine cutting, so it is also called embarrassing metal processing. Therefore, the technical discussion on the fine machining of titanium alloys is a question that needs to be dealt with urgently.
The tail pipe shell (as shown in Figure 1) is a key functional part of a product in the author’s factory. Because it is necessary to accept high temperature and high pressure during operation, its mechanical performance requirements are tensile strength Rm≥1030MPa, elongation A ≥9, in order to meet its functional requirements, the product planning is made of titanium alloy TC11, which is a typical thin-walled shaft tubular part. After the optimization planning of its fine machining technology, the fine machining of titanium alloy TC11 has been completed.
1 Machining features of titanium alloy TC11
TC11 titanium alloy belongs to (α+β) type Ti alloy. Its structure is composed of α phase of close-packed hexagonal structure and β phase of body-centered cubic structure. Compared with other metals, the texture is more obvious and the anisotropy is stronger, which brings great difficulties to the production and processing of titanium alloys. . Its cutting process features are as follows:
(1) Large cutting force and high cutting temperature. Because the titanium alloy has low density, high strength, large cutting feed shear stress, and large plastic deformation work, so the cutting force is large and the cutting temperature is high.
(2) Work hardening is severe. In addition to plastic deformation, titanium alloy work hardening is also caused by the inhalation of oxygen and nitrogen at high cutting temperatures, the occurrence of void solid solution and the intense conflict effect of high hardness particles on the tool.
(3) Simple stick knife. Titanium alloy has a strong chemical affinity at high temperature, and the large cutting force promotes the bond wear of the tool.
(4) The tool wear is more severe. Gap wear is a distinctive feature of tool wear when cutting titanium alloys.
2 Workpiece analysis
3 technical solutions
3.1 Technology Road
The formulation of the technical road is based on the principle of “roughing first, then fine, first inside and then outside”, reducing deformation during finishing and improving machining accuracy. In the early trial production process, the technical paths are: blanking, turning length, rough turning shape, drilling, rough boring, fine turning inner shape, and fine turning shape.
Because titanium alloy has poor thermal conductivity, low density and specific heat, high cutting temperature, and strong chemical affinity with the tool, it is easy to stick to the tool and make cutting difficult. Experiments have confirmed that the greater the strength of the titanium alloy, the worse its machinability. Therefore, it is necessary to choose tungsten-cobalt cemented carbide with low chemical affinity with titanium alloy, good thermal conductivity and high strength in the processing process.
YG8 is used for rough turning, YG6 for semi-finishing, and YG3X for fine turning. Carbide twist drill (welding YG6 carbide) is used for drilling.
3.2 In doubt
(1) When using a cemented carbide twist drill to drill, the cutting temperature is appropriately high, the drill bit is severely worn, and the thermal stress increases during the processing process, which directly affects the accuracy of subsequent finishing.
(2) The deformation of the workpiece is large, and the machining size is difficult to control.
(3) The condition of coaxiality out of tolerance is severe, the qualified rate of workpiece is low, and the average qualified rate is only 50%.
(4) The production efficiency is not high, the tool wear is large, and the production cost is large.
3.3 Treatment plan
3.3.1 Select a reasonable tool from scratch
After research on materials and processing process, it was decided to use Kennametal HTS-C clip-on drill (jet-suction drill) for drilling; the drill can provide strong cooling and is equipped with indexable PVD-coated solid carbide Inserts and flutes and carbide pilot drills. After experiments, the drill selects KC720 and KC7215 inserts (front and back inserts) specially designed to process difficult-to-machine materials, and drills titanium alloys. The production efficiency is increased by 60%, and the drilled workpiece does not heat up or deform There is no stress effect during processing and no pollution to the surrounding environment, as shown in Figure 2.
3.3.2 Analysis of deformation causes and countermeasures
The main reason for the deformation of the machining process is due to the structural stress of the titanium alloy. In the early trial production process, although the technology adopted the processing technology of first roughing and then fine, first inside and outside, but did not fully consider the elements of unstable titanium alloy structure, forming the appearance of workpiece deformation during machining, and the size of which is difficult to control. How to reduce the deformation control of the titanium alloy machining process to the minimum is a difficult problem.
After repeated experiments, we added an aging annealing process after rough machining of the workpiece. Under the premise of not reducing the mechanical properties of the workpiece, after refining the grains, the refinement structure is achieved, the internal stress is eliminated, and the structure reaches a stable state.
The heat treatment standards are as follows: the aging temperature is 530 °C, and the holding time is 4 to 6 hours. Ensure that Rm≥1030MPa, A≥9%. After several batches of experiments, the tensile strength Rm is generally higher than 1030MPa, and the elongation A is all greater than 9%.
3.3.3 Reasons and countermeasures for out-of-tolerance coaxiality
In view of the low qualified rate of workpieces caused by excessive coaxiality, after further analysis of workpiece materials and processing technology, it is found that the workpiece is a tubular thin-walled part, which is a typical metal that is easily deformed and difficult to machine. As long as the rigidity of all technical systems is improved In order to effectively deal with its processing problems.
(1) When machining the inner hole, the method of reasonably setting technical steps is adopted, and the technical steps with certain rigidity are used as the clamping and positioning benchmark of the workpiece, which effectively solves the problem of deformation of the inner hole during processing, as shown in Figure 3.
(2) During the outer circle machining, the method of filling the vibration-proof material is adopted, that is, during the semi-finishing process of the workpiece, the clamping part is filled with hard pads to prevent deformation of the workpiece; the inner hole of the workpiece is filled with soft pads. The flexible rubber tube or foamed material can be integrated with its inner wall during the processing process, and then the effect of increasing the rigidity of the workpiece can be achieved, as shown in Figure 4.
(3) In order to ensure the coaxiality of the workpiece, a set of over-positioning fixtures is planned in the final finishing process to improve the rigidity of the workpiece, as shown in Figure 5.
Then the coaxiality of the workpiece is formed out of tolerance. Therefore, in the design of the fixture, in order to ensure the rigidity of the workpiece, the positioning device is selected, and all the inner holes of the workpiece are used as the positioning reference. Although the positioning phenomenon has occurred in theory, in practice, it is completely satisfied. The needs of the workpiece . See Figure 6.
According to the characteristics of the above-mentioned TC11 titanium alloy in the cutting process and the mechanism that the alloy is difficult to cut, and the processing methods and experience of difficult-to-machine materials in production practice, the cutting technology path is redrawn as follows: blanking – flat end face – – Drilling – Rough turning inner and outer circles – Aging and mechanical function inspection – Turning benchmarks – Semi-finishing small head inner hole, semi-finishing large head inner hole – Fine turning inner shape – Semi-finishing turning shape ——The total length of the flat, the small end of the fine car—the shape of the fine car.
The tail pipe shell of titanium alloy parts processed by this technical method fully meets the design requirements, and the qualified rate of parts is over 98%. The problem of fine cutting deformation of titanium alloy is effectively dealt with.
The machinability of titanium alloys is very poor, and how to improve and improve its machinability is a difficult problem. This article analyzes the cutting technology of titanium alloy parts tail pipe shell, completes the fine cutting of titanium alloy parts, and effectively deals with the processing difficulties such as turning deformation and tool wear of titanium alloy TC11 thin-walled cylindrical parts. I have further knowledge and understanding of the processing technology of thin-walled titanium alloy parts, and have accumulated certain experience for the processing of titanium alloy parts in the future.
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