Analysis of Machining Large Titanium Turbo Blades Profile Process
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The fan blades of the large bypass ratio turbofan engine have basically reached more than 500MM in terms of length and size. This large-scale structural feature makes the centrifugal force and vibration stress to be very large during their work, so it has also become a large turbo fan engine Very important parts. |
At present, many turbofan engines still use the more mature titanium alloy damping fan blades. The narrow and long structure of this blade profile makes its weak rigidity in the form of thin-walled structure in the direction of the basin back more prominent. The poor rigidity of the structure and the large surface area of the profile, the nature of the material being difficult to process, have an adverse effect on the traditional machining process, which is intuitively reflected in the contour size accuracy and position accuracy of the profile It is difficult to guarantee, the efficiency of manual polishing is low, the labor intensity is large, and the leaf type is prone to burn and ablation.
The existence of the above problems constitutes a bottleneck for blade production. With the development and application of multi-axis linkage CNC machining technology and the research on the machining technology of this blade profile, the difficulties of this blade profile machining have been gradually broken through, and the machining quality and efficiency level have reached a relatively ideal state.
The main technological route of CNC machining of large titanium alloy fan blade profile
For the machining of the large titanium alloy fan blade profile, considering all aspects involved in the traditional process, its adverse effects have the following aspects.
1. Influence of materials
- ▶ The titanium alloy has a small elastic modulus, which is easy to cause the clamping deformation of the blade machining; the wear of the flank face during machining is prone to increase the cutting force.
- ▶ Poor thermal conductivity, hand-polished dry grinding is easy to cause stress deformation, burns, and ablation.
2. Influence of blade structure
- ▶ The overall machining area of the profile is large, and the precision caused by wear during the entire process of the tool is greatly affected.
- ▶ Due to inconvenience in handling, manual polishing is labor-intensive, and machining accuracy is difficult to guarantee.
3. Influence of wool condition
Due to the influence of materials and specifications, it is difficult to obtain an ideal margin distribution, which results in cutting force fluctuations caused by uneven removal of the profile margin and stress deformation.
4. Influence of machine tool functions
- ▶ The curved structure of the blade profile, the cutting direction of the tool, the actual cutting angle and the cutting parameters are different, resulting in changes in cutting force.
- ▶ Poor cooling conditions, insufficient cooling and no cooling cause thermal stress deformation.
Aiming at the difficult factors of the machining of the blade surface of large titanium alloy fans, based on the comprehensive machining advantages of the multi-axis linkage CNC machining technology, the main machining route determined is:
the machining of the blade tenon and the auxiliary positioning datum → the blade profile CNC rough milling Machining → Stress Relief Annealing → Positioning Benchmark Repair → Numerical Control CNC Blade Milling → Profile Finishing.
The overall process idea established by the above process route is: the surface CNC rough milling process removes most of the margin, and the finish milling process has an ideal margin distribution; the blade profile CNC precision milling process ensures the geometry of the profile And the position accuracy basically meets the final accuracy requirements of the blade; the finishing of the blade profile ensures that the quality of the surface layer of the profile meets the requirements.
The main points of CNC milling of large titanium alloy fan blade profile
According to the overall technological requirements of the blade profile, the milling of the blade profile must ensure that the geometrical position accuracy of the profile basically meets the design requirements and has a certain surface roughness quality. At the same time, the improvement of efficiency in machining is also the focus of profile milling Work one.
According to the understanding of the machining characteristics of the large titanium alloy fan blade profile, it is necessary to comprehensively consider the influence of many factors such as equipment, tools, machining positioning and so on. For milling of large titanium alloy fan blades, it is necessary to select a five-axis machining center. Selecting a mature five-axis linkage blade machining center has both high-efficiency machining considerations and machining accuracy assurance capabilities.
For profile machining with large changes in curvature, the swing angle function of the machine tool spindle can be well adapted to the requirements of consistent cutting force corresponding to the change in profile curvature. The high-pressure cooling system of the machine tool greatly reduces the cutting temperature and avoids rapid tool wear. , So that the profile machining can obtain good machining accuracy and surface machining quality. In order to prevent and reduce torsional deformation during long blade clamping and cutting, it is necessary to ensure that the rotating shafts of the blades at the front and rear ends of the equipment have a synchronous rotation function, and the purpose is to change the one end clamping and one end of the traditional blade machining technology.
Tight positioning clamping method to avoid bending deformation during blade clamping and torsional deformation of the blade profile in the longitudinal direction caused by one end turning and one end following during blade rotation machining. To meet the requirements of blade positioning and clamping, the auxiliary positioning part at the trailing end of the blade has strict position accuracy requirements relative to the tenon positioning reference at the front end.
After roughing of the profile is completed, the front and rear of the blade due to stress deformation The position accuracy error between the end positioning references shall be repaired. After installing the jigs for blade profile machining on the rotary shafts at the front and rear ends of the machine tool, and after determining that there is no concentricity error at the rotary shafts at the front and rear ends of the machine tool, the installation accuracy of the front and rear fixtures is detected and adjusted using a special mandrel. Ensure that the fixtures at both ends have an accurate positional accuracy relationship, so as to avoid the additional torsional stress caused by the synchronous rotation function of the front and back rotary axes of the machine tool due to the poor clamping accuracy of the fixtures. Rough milling of the blade profile is to remove a large margin and leave a uniform machining margin for finishing. Under this premise, the machining of this process should ensure high machining efficiency. The five-axis linkage blade machining center has a wide-row machining function.
The principle is that when milling the blade, the tool centerline is not perpendicular to the tangent of the point or surface being milled, but in the direction of the tool and the point or surface being milled. The normal direction is at a certain angle. This type of milling uses a cylindrical end mill, and the milling path is a wide elliptical arc. Compared with the milling of a ball head, the same profile peak height or surface is milled. In terms of quality, the distance between the generated tool paths is much larger. Therefore, this kind of machining has a high machining efficiency. In actual machining, the rotary machining method that moves from one end to the other end in the length of the blade, that is, the spiral milling method is used. From the perspective of efficiency, the spiral milling method also has higher machining efficiency compared with the longitudinal milling method. Fine milling of the blade profile is to obtain a higher geometric and position accuracy, and at the same time make the profile roughness level meet certain requirements. In order to reduce the impact of “rebound” caused by the machining of titanium alloy materials and the impact of tool wear on the machining accuracy during the machining of large-area profiles, the tool must be sharp and avoid prolonged machining of a tool. For this reason, if possible, use an end mill to perform longitudinal milling of the profile. Longitudinal milling can use several tools to mill the blade back surface, leaf surface, intake edge, and exhaust edge, to avoid the wear caused by a large-scale machining of one tool, and to produce a level of precision in the surface of the blade.
The inconsistency is conducive to the final finishing of the profile. When milling a large titanium alloy fan rotor blade, in order to improve the cutting conditions, all measures to avoid tool wear are necessary. In terms of the selection of tool materials and specifications, the overall hard alloy coated cylindrical ball milling cutter is used to process the inner side of the blade edge plate, the inner side of the edge plate and the profile transition arc, the transition profile close to the edge plate 1. For the intake and exhaust edges, select an end mill with a cylindrical insert and a hard alloy coated blade to process the large-area profile surface of the blade leaf pot and blade back.
The selection of coating materials for machining titanium alloy tools is very important. Avoid using coating materials that have affinity with titanium alloys. At present, PVD coated tools are commonly used for machining titanium alloys. The PVD coating is thin and smooth. When they are attached to the cemented carbide substrate of the tool, they will also produce a residual stress. This stress is conducive to improving the damage resistance of the tool. PVD It can be closely attached to the tool, which is helpful to maintain the sharp cutting edge shape. The PVD tool has good abrasion resistance, stable chemical properties, and is not easy to produce built-up edge. During machining, sufficient coolant should be used to cool the tool and improve the impact of friction, select reasonable cutting parameters, and improve the effect of cutting force.
Characteristics of CNC Finishing of Large Titanium Fan Blade Profile
Blade profile finishing is to ensure that the profile roughness and waviness meet the design requirements, the material structure performance does not change, and the geometric dimensions and position accuracy obtained by milling are basically unchanged during machining.
For actual machining, the finishing of the blade profile is based on removing the remaining tool marks on the milling process to achieve the required roughness and waviness. The amount of metal removal on each side of the molding surface should not be greater than 0.05MM. At present, the use of CNC abrasive belt grinding and polishing machine tools for blade surface finishing is a more mature method for practical machining applications, and the use of CNC diamond grinding wheel grinding machine tools for blade surface finishing is a trial application. A way.
The reason why these machining methods are selected for application is because they have their own characteristics. First of all, for the machining method of CNC abrasive belt grinding and polishing machine tools, it has the following characteristics:
- ▶ The abrasive grain of the abrasive belt is sharp and the grinding efficiency is high, which has reached 10 times of milling and 5 times of ordinary grinding wheel grinding;
- ▶ The friction between the abrasive belt grinding and the workpiece is small, the grinding generates little heat, the abrasive belt circumference is large, and the abrasive particle has a long time interval for heat dissipation. It is easy to obtain complete cooling of air and cutting fluid, which can effectively reduce the deformation of the workpiece Burns and ablation;
- ▶ The softness of the abrasive belt and the rubber body structure on the surface of the work wheel ensure that the abrasive belt is in contact with the workpiece and has a good running-in and polishing effect;
- ▶ Abrasive belt grinding There is a stable abrasive tool size, because the abrasive belt is attached to the work wheel for grinding, the abrasive tool size has better stability;
- ▶ Abrasive belt grinding cannot be processed for a long time with a large amount of removal, and the abrasive belt contains The total amount of abrasives is limited, and long-term machining with large excess removal will quickly consume the abrasives, and it is necessary to interrupt the machining and replace the abrasive belt.
The above-mentioned characteristics of abrasive belt grinding make it possible for large titanium alloy fan blade surface polishing to realize mechanized production under program-controlled conditions. At present, there are two methods to choose from for the CNC belt grinding method used for blade polishing: one is to use a six-axis CNC belt grinding and polishing machine, and the other is to use a robot CNC belt polishing system. machining. The motion function of the six-axis CNC belt grinding and polishing machine is similar to the five-axis CNC machining center during milling.
The structural difference between the belt grinding work wheel and the end mill machining makes it necessary to adapt the profile machining to the blade structure. With swing angle function in 2 directions. Six-axis CNC abrasive belt grinding and polishing machine has the dual functions of profile grinding and polishing. The function transformation depends on the transformation of the power head in the form of rigid grinding and floating grinding.
During the polishing process, the constant pressure floating mechanism is activated, so that the change of the grinding forward pressure can be accurately controlled by the pressure sensor, the grinding power sensor, the constant pressure cylinder and other mechanisms to adapt to the difference in the size of each blade profile within a certain range. Polishing machining without destroying the accuracy of the profile. When performing profile grinding, the contact wheel floating mechanism is locked to allow rigid grinding of the profile.
The rigid grinding process of the profile can supplement or replace the situation when the precision precision of the profile is poor, and the grain size of the abrasive belt used should be changed according to the margin. This machining will change The original dimensional position accuracy, and relative to the milling process, the removal of excessive margins will produce larger stress deformation. Therefore, it is not recommended to use the grinding function under the premise that the milling process has the ability to guarantee accuracy. Robot CNC abrasive belt polishing method is that the robot holds the blade and performs compound motion under program control to perform polishing machining on a fixed abrasive belt machine. The machining uses reverse engineering technology. Before machining, the robot holds the blade tenon part to scan the profile of the blade profile, and then the data machining mechanism generates a machining control program, and finally realizes the polishing of the blade under the program control. At present, because of the limitation of motion accuracy, the robot abrasive belt grinding method is generally only used as a method of profile polishing. The grinding method of CNC diamond grinding wheels belongs to the typical hard and rigid grinding. The machine tool movement mechanism used is basically the same as the five-axis linkage blade milling machining center. The cutting tool used is to change the vertical milling cutter to the surface coated with diamond powder. Cylindrical grinding wheel. During grinding, wide-line machining technology is used. This kind of machining method is hard and rigid grinding. Because the diamond wheel itself has poor air permeability, it cannot achieve the effect of heat dissipation by storing and exchanging the cooling medium, so it is not suitable for grinding the surface of the part with a large amount of removal, and even It is a process that removes a small margin, and it is also easy to burn the grinding of the blade surface of the titanium alloy material.
Therefore, when using this method to process the blade surface of the titanium alloy blade, it is necessary to find out the most suitable cutting parameters and machine tool cooling The way must be very effective. In addition, the hard and rigid grinding properties of the diamond wheel on the profile surface also have a certain “edge” of the cutter. Although it can be improved by adjusting the program to the specification of the grinding wheel, it cannot be completely removed. The impact of blade fatigue performance is unfavorable, so supplementary measures must be taken to eliminate surface “ridges”. It may also be necessary to use CNC abrasive belt grinding and polishing machine tools for supplementary machining under the control of corresponding programs. In addition, the use of free abrasive properties of wet sand blowing method for supplementary machining should also be a feasible method. Due to the above-mentioned characteristics of the CNC diamond grinding wheel grinding method, its machining application is still in the exploratory stage. At present, the CNC abrasive belt grinding and polishing machine tool method is becoming the most suitable method for the polishing of large blade profiles due to its many advantages. Its comprehensive advantage is that it can be used for dry grinding and wet grinding. It can also perform ultra-low temperature grinding under CO2 cooling, which is very beneficial to avoid burns and ablation of the large titanium alloy material blade profile polishing.
The application of CNC grinding and polishing machine tools has changed the large-scale manual polishing of large blade profiles, which has played an important role in improving the production efficiency of large blades. The development and application of multi-axis linkage machining technology has greatly improved the accuracy and quality assurance ability of the key machining link of large engine fan blade profile machining, and also achieved satisfactory results in machining efficiency. I believe that with the process Continuous research and improvement of equipment technology, large-scale fan blade profile machining technology will develop in the direction of mechanization and automation.
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