Machining of titanium alloy grille-like parts

2021/08/10



We analyzed the difficulties of processing the lug holes of titanium alloy grille-like parts, the control of the deformation of the parts and the efficient processing of small corners, and innovatively proposed various solutions, and the implementation process was smooth, and finally solved the processing problems of titanium alloy grille-like parts, so that the parts qualified rate reached 100% at the stage.


1 Preface

The titanium alloy grille-like parts of a machine have super-deep lug holes, small corners and weak rigidity due to the newly designed structure form, which brings great difficulties and challenges to the processing and manufacturing. In this paper, we analyze the processability of the part structure, find out the processing difficulties, study each difficulty item by item, and find out the solution to provide reference for the processing of similar structure titanium alloy parts.

2 Analysis of the structure and machinability of grating parts

For a single grille plate, its structure mainly consists of 9 non-uniformly distributed lug holes, 10 non-uniformly distributed U-shaped slots and 6 EDM punching areas.

The grille part is machined from a plate of titanium alloy material, the final web thickness is 4mm and the rib height is 3mm, the part is less rigid and the stress distribution is seriously uneven after EDM machining, which is likely to cause large deformation. Meanwhile, the length of the lug hole is 726mm, and the dimensional accuracy is φ5.1H9, the length-to-diameter ratio is extremely high, so the processing difficulty and risk are extremely high. All the inner shape of the grille and the turning angle of the lug are R2.5mm, so the processing efficiency is low, meanwhile, the tool diameter is small and easy to break.

In view of the above problems, we mainly start the research from the following aspects to solve the problems one by one.

(1) Study on the machining process of the lug hole The lug hole has a length-to-diameter ratio of 142, which is a very large length-to-diameter ratio deep hole. At the same time, the processing accuracy requirement is high, and the processing difficulty is very big. Focus on how to make the accuracy of the lug hole to meet the design requirements.

(2) Research on the control method of part deformation Analyze the internal stress distribution state of the sheet material and design the position arrangement of the part in the stress balance zone of the material; further eliminate the residual stress inside the part through the reasonable arrangement of the thermal process method; reduce the stress generation during the machining process through the reasonable optimization of the CNC machining tool path, and finally achieve the purpose of controlling the deformation of the part.

(3) small corner innovative processing program research Part all the internal shape of the corner are R2.5mm, the minimum diameter of the current general manufacturers tool for 5mm, low efficiency when processing, tool breakage. Research using innovative cycloid milling, small and large diameter tools are processed separately, in order to improve the efficiency of parts processing, while effectively reducing the risk of tool breakage and improve the quality and stability of the parts.

3 Difficulties in the processing of lug holes

Figure 1 shows the grille lug hole, there is no experience in the industry to process such a long-diameter ratio of ultra-slender titanium alloy hinge hole. The processing difficulties are mainly reflected in: ①High hole size accuracy requirements, hole diameter is very easy to exceed the poor. Titanium alloy material is characterized by a certain degree of shrinkage, and the processing process is prone to cause the closure, forming a "flared" hole, with one end exceeding the upper difference and one end exceeding the lower difference. ②The process plan is difficult to arrange. Due to the length-diameter ratio of the lug hole up to 142, currently available information at home and abroad, can not find the processing of such a deep hole program to learn from, the industry has never been such a long deep hole processing precedent. ③ Difficulties in tool design and manufacturing. The length of the drill and reamer needs to be more than 890mm, and the diameter of the tool is 4.8~5.1mm, which requires very high requirements for tool material and machining process, as well as high requirements for runout, straightness and edge accuracy. If the diameter of the cutting edge deviates by more than 0.02mm, it is impossible to process qualified products. ④The tooling is very difficult to manufacture. In order to match the use of drill and reamer, one set of special drilling die and one set of special reaming die have to be designed respectively. The main difficulty is that the accuracy of the drilling die is very high, and the coaxiality requirement is about 0.03mm, which makes the manufacturing of tooling very risky.
Figure 1 Schematic diagram of grille lug holes

4 Optimize the machining plan and adopt special tooling

The structural characteristics of the lug hole determine that the conventional drilling scheme cannot be used, and the following processing scheme was finally decided after repeated arguments.

1)Use special drilling die to drill φ4.8mm bottom hole. The drill bits of different lengths of 300mm and 500mm were used to drill holes from both sides of the part, thus avoiding the problems of drill vibration and deflection caused by the direct use of long drill bits and making full use of the effective length of the drill bit, which is only half of the total length of the lug hole. Reducing the drill bit length by half means reducing the length-to-diameter ratio of the lug hole by half, which can effectively prevent the drill bit from breaking and greatly improve the processability. The disadvantage of this process solution: the lug holes on the left and right sides of the part may not be coaxial, and the 4th and 5th sets of lug diameters will produce a sudden misalignment of the lug hole center axis, making subsequent reaming more difficult.

(2) Reaming to φ4.9mm with a special reaming die. In order to solve the problem of misalignment of the center axis of the lug holes, the front guide of the φ4.9mm reamer is specially designed to be φ4.5mm, in order to ensure that the front guide of the reamer can pass the misaligned lug smoothly, and at the same time, the coaxiality of the lug holes can be corrected to a certain extent after the φ4.9mm reamer (beveled edge, with reaming effect) has been processed.

(3) Reaming to φ5mm, because the reaming volume of this process is small (0.1mm) and the reaming allowance is uniform, the processing stability and processing quality are better, and the deviation of the hole center axis can be further corrected.

4) Reaming to φ5.1H9 to ensure the dimensional accuracy of the final hole. This process mainly depends on the manufacturing accuracy of the final reamer and the manufacturing accuracy of the drilling die. If the parameters and structure of both are reasonably designed and the manufacturing accuracy meets the requirements, the final dimensional accuracy of all the lug holes can be guaranteed.

5 Drill and reamer parameters and materials

The success of processing ear piece holes mainly depends on the accuracy of tools and tooling. The problems and solutions of the tools are as follows.

(1) Drill accuracy problems The design of the drill bit requires a runout of 0.01mm, in fact, the drill bit is placed on the platform and the straightness has reached 3 to 4mm, producing a great deformation. In the process of use, the drill bit produces a great eccentric swing when it rotates with the machine, and the drill bit rod has been thrown into an elliptical trajectory, which directly leads to the bottom hole being drilled not in a straight line, but with a certain deflection "curve". Therefore, the drill bit was reground and subsequently improved at the design level to finally meet the processing requirements.

(2) Reamer parameter design problem The guarantee of the final size of the ear piece hole relied on the φ5.1H9 reamer that was finally used. According to the general design standard, the reamer has an edge diameter of 5.105~5.115mm and a rear guide size of φ5.1f6. However, after testing and verifying several test pieces, the reamer with this size parameter could not produce qualified trunnion holes, and the overrun rate was as high as 88%. After repeatedly resharpening the tool and testing, the actual design parameters of the tool were found to be 5.10~5.11mm for the cutting edge diameter and φ5.1mm for the rear guide diameter, and only reamers with this tolerance range were able to produce qualified lug holes and finally achieved zero overruns.

(3) Tool material problem The original tool material was HSS, which was verified by processing that the strength and wear resistance of HSS material was not high enough, and the tool wear was drastic. Subsequently, in coordination with the design department, the drill and reamer materials were fully changed to carbide.

6 Implementation effect

The feasibility of the process scheme was verified by using 2 pieces of process test pieces, and 6 pieces of ground bench test pieces were used to refine and improve the process method and figure out the design parameters and tool materials for the drill and reamer. The drilling die tooling was repeatedly modified, and finally the one-time processing pass rate of the extra-long lug holes of the grill parts reached 100%. The success of this process solution not only fills the gap in the industry for the development of titanium alloy extra-long aspect ratio slender hole processing, but also reserves the technical capability for the subsequent processing of the same structure parts.

7 Deformation control method research

As there are many factors causing the deformation of the parts, different factors constrain each other and the relationship is intricate and complex, so we should start from many aspects, multi-pronged approach to solve the deformation problem, and finally control the flatness of the parts within 0.3mm.

1) Analyze the stress distribution of the titanium alloy plate, adjust the position of the part in the plate wool, and avoid uneven stress from the source. According to the material standard, the supply state of titanium alloy plate of 30mm thickness is hot-rolled annealed state, and the heat treatment system is: 750~850℃, 15~120min, air cooling. Domestic and foreign scientific research papers and test data on the material properties of TA15M titanium alloy plate show that the stress distribution state: the thickness direction of the center of the tensile stress σb is basically in equilibrium, that is, the stress is symmetrically distributed; while the upper and lower surface direction of the plate, the tensile stress is gradually increasing. Accordingly, the center of the web in the final processing state of the specially designed part is on the central symmetric surface of the thickness direction of the sheet. The position of the part in the stock is shown in Figure 2. In this way, after machining the part, the residual stresses at the web of the part caused by the stock can be basically eliminated, which plays a positive role in the control of flatness.

Fig. 2 The position of the part in the wool is illustrated

2) Arranging heat treatment process and controlling deformation through reasonable arrangement of process scheme. Although the formation of residual stresses during machining can be greatly reduced by the aforementioned means, residual stresses will still inevitably appear during the hot rolling and machining of the wool, which requires the arrangement of heat treatment processes to further eliminate residual stresses after the machining process is completed. After the heat treatment, the EDM process is carried out. Since the EDM process changes the structure of the part tremendously and the stress is redistributed again, the flatness of the part needs to be monitored after the EDM process, and if it is >0.3mm, the heat treatment needs to be carried out again.

8 Innovative treatment solutions for small corners

The inner shape of the grille part and the corner of the lug, U-slot end face and the shape of the turn are all R2.5mm, which requires that only the minimum diameter 5mm tool of the factory can be used to process this part. Since the strength of small diameter tools is very poor, it is very easy to break the tool, resulting in very low machining speed and quality risks.

Dassault Aviation's CATAI V5 software has introduced the Trochoid-Mill cycloidal milling command. The Trochoid-Mill is a good solution to the problem of sudden changes in cutting forces and is very suitable for situations where tool strength and rigidity are poor. The machining trajectory is one circle over another, and the time in the cutting state during the machining process is less, which is very helpful to solve the problem of poor heat dissipation of titanium alloy. Cycloid milling can achieve relatively large depth of cut, small cutting width and large feed, making full use of the effective edge length of the tool, which can achieve full edge length cutting and effectively improve the metal removal rate.

Cycloid machining consists of two motions, namely tool rotation and tool winding. For each revolution of the tool, the tool cuts one unit radially, using circular tangential feed and retreat, and the depth of cut gradually increases from zero to the maximum, and then gradually decreases to zero. At the same time, during the whole process of cycloid machining, the cutting force gradually increases from zero and then decreases to a state of gentle and uniform change all the time. Compared with the layered milling, cycloid milling can increase the tool life by more than 3 times and the machining efficiency by more than 3 times, so the machining advantages are very significant. A comparison of cycloid milling and conventional machining methods is shown in Figure 3.


a)Conventional machining

b) Cycloidal milling

Figure 3 Comparison between cycloid milling and traditional machining methods

Unlike conventional machining methods, the main purpose of cycloid milling is to avoid full immersion milling such as slot milling while fully satisfying the radial depth of cut. This is very beneficial to reduce tool wear and extend tool life. And for the reduction in cutting efficiency that is likely to be brought about by the use of a smaller tool-workpiece envelope angle, a greater axial depth of cut than conventional milling methods can be used in the cycloid milling technique to improve the material removal rate.

Cycloidal milling technology allows the use of larger axial depths of cut, which can replace the need for multiple lay-ups in conventional machining. Cycloidal milling technology is highly effective in the efficient cutting of difficult materials, and the measured results show that tool wear is much lower with cycloidal milling than with conventional machining methods for the same amount of material removal and machining time. Through the application of cycloid milling technology, the processing of small corner structures can achieve twice the result with half the effort, which not only ensures the quality of the installed parts, but also improves the processing efficiency, reduces the production cost, and also better ensures the processing quality of the parts.


9  machining Conclusion

By adopting the above process scheme, the preparation of CNC machining program, as well as the design and manufacture of drill bits, reamers and special drilling dies, the difficult problem of lug hole machining has been solved. The one-time pass rate of titanium alloy deep hole machining reaches 100%, and the flatness of thin-walled structure grill-like parts reaches 0.3mm. compared with the traditional machining method, the efficiency of small corner machining is increased by 3 times. Based on the above innovative breakthroughs in key process technologies, the machining of grill-like parts was successfully completed.