Research on Precision Cutting Process of Titanium Alloy TC11

Titanium alloy has the characteristics of low density, high strength, and greater specific strength than ultra-high strength steel; and good thermal stability, 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 alloys can be quenched and aged to strengthen the alloy, and the strength after heat treatment is improved by 50% to 100% compared to the annealing condition. And it has outstanding low temperature resistance and outstanding resistance to seawater corrosion and hot salt stress corrosion resistance, and it is used more and more widely.

However, because titanium alloy has a small cutting deformation coefficient (the deformation coefficient is less than or close to 1), the distance of the chip sliding conflict on the rake face in the cutting process increases, and the tool wear is accelerated; at the same time, the cutting temperature is high, the cutting force is large, and it is easy to The appearance of a degraded pollution layer occurs. Because titanium has high chemical activity, it is easy to undergo violent chemical reactions with various gas impurities. For example, O, N, H, C, etc. invade the cutting surface of the titanium alloy, causing the hardness and brittleness of the surface to increase. There are other hard surface layers of TCi and TiN; at high temperature, the surface layer is composed of an alpha layer and a hydrogen embrittlement layer and other surface degeneration pollution layers. Uneven surface arrangement is formed, some stress concentration occurs, the fatigue strength of the parts is reduced, the cutting process also severely damages the tool, and the appearance of chipping, chipping, and shedding occurs; the affinity is high, when cutting, titanium chips and the cut surface It is easy to bite with the tool data, and the appearance of severe sticking of the tool occurs, which leads to severe bonding and wear; and the disadvantages such as the unstable arrangement of the titanium alloy, bring many difficulties to the cutting process, especially the fine cutting process, so it is also called the embarrassing process of metal. Therefore, technical research on fine cutting of titanium alloy is a question that needs to be dealt with urgently.

The tail pipe shell (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 in the working condition, its mechanical function requirements are tensile strength Rm≥1030MPa, elongation A ≥9, in order to satisfy its functional requirements, titanium alloy TC11 is used in product planning, which is a typical thin-walled shaft tubular part. After optimizing the planning of its fine cutting processing technology, the fine cutting processing of titanium alloy TC11 has been completed.

1 Cutting features of titanium alloy TC11

TC11 titanium alloy belongs to the (α+β) type Ti alloy. The arrangement is composed of close-packed hexagonal structure α phase and body-centered cubic structure β phase. Compared with other metals, the texture is more pronounced and the anisotropy is stronger, which brings greater difficulties to the production and processing of titanium alloys. . The characteristics of the cutting process are as follows:

(1) High cutting force and high cutting temperature. Because titanium alloy has low density, high strength, high cutting feed shear stress, and large plastic deformation work, the cutting force is high and the cutting temperature is high.

(2) The work hardening is severe. In addition to plastic deformation, the work hardening of titanium alloys is also due to the fact that titanium absorbs oxygen and nitrogen at high cutting temperatures, resulting in interstitial solid solution and the intense conflict effect of high-hardness particles on the tool.

(3) Simple sticky knife. Titanium alloy has a strong chemical affinity at high temperatures, coupled with a large cutting force, which promotes the bond wear of the tool.

(4) Tool wear is severe. Divide wear is a significant feature of tool wear when cutting titanium alloys.

2 Workpiece analysis

3 Technical Solution

3.1 Technology road

The technical path is drawn up based on “rough first, then fine, first inside and then outside” to reduce the deformation during finishing and improve the machining accuracy. In the preliminary trial production process, the technical roads 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, simple sticking to the tool makes cutting difficult. Experiments have confirmed that the greater the strength of the titanium alloy, the worse its machinability. Therefore, it is necessary to select tungsten-cobalt cemented carbide with low chemical affinity with titanium alloy, good thermal conductivity, and high strength in the processing process.

Use YG8 for rough turning, YG6 for semi-finished turning, and YG3X for fine turning. Hard alloy twist drill (welded YG6 hard alloy) is used for drilling.

3.2 There is a question

(1) When selecting a cemented carbide twist drill for drilling, the cutting temperature is appropriately high, the drill bit is severely worn, and the increase in thermal stress in the processing process directly affects the accuracy of subsequent finishing.
(2) The workpiece has large deformation, and the machining size is difficult to control.
(3) The concentricity tolerance is severe, the workpiece qualification rate is low, and the uniform qualification rate is only 50%.
(4) The production power is not high, the tool wear is large, and the production cost is high.

3.3 Treatment plan

3.3.1 Choose a reasonable tool from scratch

After discussing the data and processing process, it was decided to choose Kennametal HTS-C machine clamp drill (ejection drill) for drilling; this drill can provide powerful cooling and is equipped with indexable PVD coated solid carbide Blades and chip flutes and carbide guide drills. After experimentation, the drill bit selected KC720 and KC7215 blades (front and back side blades) for processing difficult-to-process materials to drill titanium alloys. The output power increased by 60%, and the workpiece after drilling did not heat or deform. The processing has no stress effect, and no pollution to the surrounding environment, as shown in Figure 2.

3.3.2 Analysis of the causes of deformation and countermeasures

The primary reason for the deformation in the machining process is that the titanium alloy is formed by the stress. In the early trial production process, although the technology adopted the first rough and then refined, first inside and then outside processing technology, but did not fully consider the unstable elements of the titanium alloy arrangement, forming the appearance of deformation of the workpiece during the machining process, and the size of the workpiece 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 function of the workpiece, the grain is refined, and then the refinement arrangement is reached, the internal stress is eliminated, and the arrangement reaches a stable condition.

The heat treatment standard is as follows: the aging temperature is 530℃, and the heat preservation time is 4-6h. Ensure that Rm≥1030MPa, A≥9%. After multiple batches of experiments, the tensile strength Rm is generally higher than 1030 MPa, and the elongation A is all greater than 9%.

3.3.3 Reasons and countermeasures for the out-of-concentricity

In view of the low qualification rate of the workpiece caused by the out-of-concentricity, after further analysis of the workpiece data and processing technology, it is found that the workpiece is a tubular thin-walled part, which is classified as a typical easily deformable and difficult-to-process metal, as long as the rigidity of the technical system is improved Ability to effectively deal with its processing questions.

(1) When machining the inner hole, a reasonable method of setting technical steps is adopted, and the technical step with certain rigidity is used as the clamping and positioning reference of the workpiece, which effectively solves the problem of the deformation of the inner hole in the processing, as shown in Figure 3.

(2) When processing the outer circle, the mechanical processing method of filling the anti-vibration material is adopted, that is, during the semi-finish turning of the workpiece, the clamping part is filled with hard pads to prevent the workpiece from deforming; the inner hole of the workpiece is filled with soft The flexible rubber tube or foaming material is integrated with its inner wall during the processing process, and then the effect of adding rigidity to the workpiece is achieved, as shown in Figure 4.

(3) In order to ensure the coaxiality of the workpiece, a set of positioning fixtures are planned in the final finishing process of turning the shape to improve the rigidity of the workpiece, as shown in Figure 5.

Then the coaxiality of the formed workpiece is out of tolerance. Therefore, in the fixture planning, in order to ensure the rigidity of the workpiece, the positioning device has been selected, and all the inner holes of the workpiece are used as the positioning reference. Although the positioning appearance has occurred in theory, in practice, it is completely satisfied with the needs of the workpiece. . See Figure 6.

Based on the characteristics of the above-mentioned TC11 titanium alloy in the cutting process and the mechanism that the alloy is difficult to cut, and in connection with the processing methods and experience of difficult-to-machine materials in the production practice, the cutting process technology path is drawn up from the beginning as follows: blanking-flat end surface- —Drilling——Inner and outer circle of rough turning — Inspection of aging and mechanical function — Vehicle benchmark —Semi-finished turning small head inner hole, semi-finished turning big head inner hole —Finished turning inner shape —Semi-finished turning shape —— Flat total length, fine car small end—— fine car shape.

The titanium alloy parts and tail tube shells processed by this technical method fully meet the planning requirements, and the qualified rate of parts is more than 98%. Effectively deal with the problem of fine cutting deformation of titanium alloy.

4 Conclusion

The machinability of titanium alloy is very poor, how to improve and improve its machinability is a difficult problem. After analyzing the cutting technology of the tail tube shell of titanium alloy parts, this article has completed the fine cutting of titanium alloy parts and effectively dealt with the machining difficulties such as turning deformation and tool wear of titanium alloy TC11 thin-walled cylindrical parts. He has further knowledge and understanding of the processing technology of thin-walled titanium alloy parts, and has accumulated a certain experience for the processing of titanium alloy parts in the future.