Aeroengine materials and manufacturing technology are developing towards high temperature, light weight, composite, integration, high efficiency and low cost.
The level of manufacturing technology of complex structural parts of aero-engines, such as casings and blisks, has been improved continuously. Manufacturing technology has also begun to develop from advanced to high-end manufacturing.
The machining technology of aviation complex structural parts involves many aspects, such as CNC machine tools, advanced cutting tools, efficient programming, CNC machining simulation, cutting technology and parameter optimization.
1, Features of machine tools for complicated structured parts of machining
Cutting tools play a vital role in the processing of complex structural parts of materials in aviation, that are difficult to machine. Advanced aviation products require aviation parts to have better performance, lower cost and higher environmental protection. The processing technology requires faster processing speed, higher reliability, high repeatability and reproducibility.
Difficult-to-cut workpiece materials, complex and thin-walled shapes, high-precision dimensions and surface roughness requirements, and large metal removal volumes of aerospace titanium alloys and high-temperature alloy parts put forward higher requirements for tool quality consistency.
Modern high-efficiency and precise machining requires that the tool has the characteristics of high precision, high wear resistance, high impact resistance and high reliability, that is, it should have all the characteristics of high-performance tools. The obvious sign of a high-quality tool solution is that the tool structure and tool material are compatible with the material and structure of the processed part.
CNC machine tool manufacturers spare no effort to develop high-performance CNC machine tools, and further develop research and development for high dynamic response, high precision and high rigidity.
The high-rigidity and high-load-bearing linear guide rails ensure smooth and continuous movement throughout the entire stroke, obtaining high geometric accuracy and surface quality of the workpiece, and ensuring high processing efficiency. The high rigidity of the machine tool reduces the vibration of the processing system and prolongs the service life of the tool.
High-performance tools involve many aspects such as tool materials, tool coating technology, tool structure design and optimization, tool matching technology, and tool applications. The innovation of tool structure is reflected in the optimization of tool structure, reasonable distribution of cutting load, chip breaking geometry and various new indexable insert structures.
The precise machining of parts puts forward new requirements for the clamping of the tool, which requires high clamping accuracy, small radial runout, good clamping rigidity, compact structure and simple operation.
2, Machining of typical complex structural parts of aero-engine
Aeroengines use a large number of difficult-to-machine materials such as titanium alloys and high-temperature alloys and composite materials, and their machinability is extremely low.
Parts such as the casing and the blisk are complex in structure, prone to processing deformation, and difficult to guarantee dimensional accuracy and technical requirements.
1). Case processing technology. Processing deformation control: The casing is a weakly rigid part, and the quality of the blank, the process route, the division of the machining allowance, the clamping and positioning method of the part, the positioning gap and the pressing force, the tool path, the cutting parameters, the rigidity and precision of the machine tool, etc.
It is the influencing factor of casing processing deformation, and the control of casing processing deformation is a systematic project.
Measures taken to control processing deformation:
①. Choose the correct positioning and clamping method. The casing involves machining such as turning, milling and drilling from rough machining to finishing machining. For the process of removing allowance and removing deformation, positioning and clamping should be carried out under the real state of the casing to avoid elastic deformation of parts after positioning and clamping. The radial positioning reference of the fixture should not adopt the whole ring positioning reference, and the radial reference should have a certain gap to ensure that the process reference surface of the casing can be installed freely.
②. Use a clamp with radially adjustable auxiliary supports. The auxiliary support can be hydraulically adjustable or mechanically adjustable. When adjusting the radial auxiliary support, the support point should be just in contact with the casing, so as to play a supporting role to reduce the elastic deformation of the casing.
③.Reasonably plan the cutting route. When the cutting route of the part is different, the consequences of the release of the residual stress of the workpiece are different, and the magnitude and state of the stress generated by cutting are also different. Scientifically and rationally arranging the cutting and cutting route of the casing can reduce the deformation of the casing after machining. The machining allowance should be removed in layers, alternately, and symmetrically, using opposite or opposite tool paths.
The main reasons for the vibration during milling are: the tool is not clamped firmly, the parts are not clamped firmly, and the tool overhang is large.
Improvement measures during milling: reduce the cutting force by reducing the radial and axial depth of cut; choose a milling cutter with a sparse tooth pitch and positive rake angle structure to maximize the clamping force; avoid supporting the part against the cutting force during processing.
By reducing the axial depth of cut, use positive inserts with small corner radii and sharp cutting edges to reduce axial cutting forces. Minimize tool overhang and use coarse and variable pitch milling tools.
Compared with the general CNC machining mode, the ideal state of non-intervention CNC machining refers to unmanned control, unmanned tool change and unmanned measurement during the processing of parts, which is the perfect embodiment of highly intelligent machining of CNC machine tools.
For parts with complex structures, there must be no intervention in the whole program during rough machining; due to factors such as high dimensional accuracy and machining deformation during finishing machining, there are quality risks in direct machining, and process measurement is allowed, and compensation machining methods (no need for separate machining) are adopted, adjust the position of the knife in and out to make it easier to change the blade or measure the size.
2). Whole blisk processing technology. The overall blisk part fixture generally adopts a special fixture, which can make the blisk installation stable and improve the rigidity of the entire processing system during blisk processing.
According to the structure and processing characteristics of the overall blisk, the fixture is required to have the following characteristics: positioning accuracy, clamping stability, clamping uniformity and processing openness, so that the fixture can not only ensure the positioning accuracy and processing accuracy of the parts, but also can meet the requirements of improving productivity and precision machining.
The fixture structure of the turning integral blisk usually uses the disc center hole of the blisk as the radial positioning reference, selects the hub of the blisk and the stepped surface of the blade for axial positioning, and axially compresses the end surface of the annular groove on the disc center hole and the disc. The end face of the connection between the body and the blade, and the fixture is connected with the faceplate of the machine tool.
The machining of the overall blisk airfoil is a typical five-axis CNC milling process. The airflow passage between adjacent blades is narrow, and the surfaces between the blades are prone to interference during machining, which has extremely high requirements for the tool path. Because interference is easy to occur, it is necessary to check the accuracy of the processing program by means of simulation before actual processing.
The simulation steps of the overall blisk machining:
①In the VERICUT environment, call the built five-axis machining center model. Call machine file, CNC control file and tool library file. ②Introduce the blank STL model file of the blisk into the component tree, and set the workpiece coordinate system.
③Call the NC program and define the tool list.
④ Check the correctness of the NC program. Set identification colors such as collision, over travel and interference to check the interference between machine tools, tools and fixtures.
⑤Analyze the simulation results of the impeller, check whether the parts are under-cut or over-cut, and confirm whether the program can be used.
The machining of complex structural parts of aero-engines depends on the innovation of process methods. Its manufacturing level directly determines the performance of the aero-engine.
We must build on a reliable research and development foundation to promote the continuous improvement of aviation manufacturing technology.