Analysis and research of CNC machining of aviation aluminium alloy integral structural parts and machining strategy
- Analysis and research of CNC machining of aviation aluminium alloy integral structural parts and machining strategy
The materials used in modern commercial airliners are mainly aluminium alloys, alloy steels, titanium alloys and composite materials. Although the proportion of composite materials and titanium alloy used in aircraft fuselage has been increasing year by year, however, aluminium alloy is less dense, has a higher specific strength, good corrosion resistance, better formability, mature process, acceptable use data, abundant resources, low cost, and in recent years, the performance has been improving, the cost has been reduced, and a new type of aluminium alloy (aluminium-lithium alloy) has appeared, so the development of aluminium alloy has not been as rapid as expected Withdraw from the stage of aircraft structural materials, but still used in large quantities in the most advanced aircraft such as Boeing 777, A340, A380 and C919. Aluminium alloy is still the indispensable main material in large civil aircraft.
Structural monolithic has a very important impact on the development cycle, production efficiency and manufacturing cost, etc. It can significantly reduce the connection and assembly workload, reduce the number of parts, reduce weight by 10%~30%, and have good sealing performance and structural integrity. Due to the large material removal rate, complex shape and poor overall rigidity of large aeronautical monolithic structural parts, higher requirements are put forward for cutting and machining.
The monolithic aeronautical structural parts are the new generation of large passenger aircraft development trends. They have become an important symbol in modern aircraft design and manufacturing. The processing deformation of integral structural parts involves mechanics, the material forming and processing, cutting and mechanical engineering, which is one of the bottlenecks in the processing technology of aviation products. In this paper, we analyze and study the processing of integral structural parts for aviation aluminium alloy and propose the processing strategy for aviation integral structural parts.
The current situation of aluminium alloy monolithic structure processing
With the increasing performance requirements of modern large commercial airliners, many skeleton parts, especially the main load-bearing structural parts, such as the overall frame, the overall beam, the overall web, long edge strips, etc., are commonly used to be directly “hollowed out” by large block blanks and processed into complex slot cavities, bars, tabs and mitigating holes and other integral structural parts. It is difficult to control the machining quality and precision because of the large volume, thin wall, poor stiffness, easy deformation, large machining allowance, long machining cycle, and high precision, high efficiency, and reliability of cutting for such aerospace structural parts has been an important issue for the aviation manufacturing industry.
Although the aluminium alloy for aviation has good machinability, the aviation manufacturing industry has high requirements for the precision and quality of parts processing efficiency, and the machining accuracy and form error requirements of aviation parts are much higher than those of automobile and other manufacturing industries. Therefore, the research on high-efficiency cutting processing of aviation aluminium alloy, especially high-efficiency milling processing, has been widely concerned. High-efficiency machining is a new process combining high-speed machining technology and cutting process optimization, which is the key technology to solve the problem of large aviation integral structural parts. High-efficiency machining technology is characterized by high material removal rate and short single-part machining time during machining. Machining accuracy and surface quality are ensured by cutting parameter optimization.
The rigidity of large aeronautical monolithic structural parts is poor, and cutting forces, cutting heat and cutting vibration is likely to lead to part deformation and reduce machining accuracy and surface quality. Large passenger aircraft with reinforced integral wall plates, integral frames, integral ribs and beam edge strips are complex-shaped integral structural parts, putting higher requirements for efficient machining systems.
High-speed cutting is the key technology to achieving an efficient machining process. High-speed cutting research has been the relevant government departments, especially the defence sector and the strong support of enterprises, such as the U.S. Air Force in 1979 spent much money commissioned GE and Lockheed and other companies to carry out advanced processing research programs to study nickel-based alloys, titanium alloys and ferrous metals, high-speed cutting problems; France Dassault and Boeing have introduced high-speed milling machine tools.
The weight of aluminium chips produced by Boeing’s annual cutting process is as high as 15,000t, and its overall aircraft frame, beam, edge strip, and wall plate (titanium alloy, aluminium alloy) are now using high-speed milling technology so that production efficiency and product quality are greatly improved. The aluminium alloy wing frame of Boeing C-17 is one of the largest overall structural parts, from 4t to 147kg of the final part. It only takes 100h. min, material removal rate 6000~8000cm3/min, tool life 60~90min.
It can be seen that Europe and the United States, and other developed countries attach great importance to the problem of processing deformation of the overall structural parts of aviation, the United States of the third wave of the company relies on the University of Michigan and several world-renowned universities, with the support of the government and the military-industrial conglomerate, joint research and development can effectively suppress the overall structural parts of CNC machining deformation of process parameters optimization theory and finite element simulation software.
The Paris Institute of Aeronautics and Technology and the French National Aeronautics and Space Administration have jointly established a special strength laboratory for designing and manufacturing integral structural parts of space vehicles and have conducted in-depth research on the process control and safety correction of machining deformation. Due to the secrecy issue, the available literature on the machining deformation of integral structural parts is relatively small, and J. Tlusty et al. proposed a tool path optimization scheme for the deformation of thin-walled parts by effectively using the unmachined part of the part as support, thus making full use of the overall rigidity of the part; Yoyo Iwabe et al. in Japan proposed a parallel dual spindle for the “let tool” deformation of thin-walled parts caused by cutting forces. “Haruki et al. proposed to pour low melting point alloy into the cavity of a thin-walled structure, thus greatly improving the stiffness of the workpiece and effectively suppressing the machining deformation; Ratchev et al. established a cutting force model for cutting weakened parts and proposed an error compensation scheme for the deformation caused by cutting force and cutting heat. Nervisebastian established a mathematical prediction model of machining deformation caused by initial residual stress in the blank, pointing out that the final deformation of the part is closely related to the distribution state of the initial stress in the blank, the position and shape of the part in the blank; KeithA.Young used a combination of numerical simulation and chemical milling to study the residual stress introduced by milling on machining deformation. It is pointed out that the residual stress and deformation introduced by milling are closely related to the radius of the cutting tool tip arc and the radius of the cutting edge blunt circle. It is also pointed out that the wall thickness of many aerospace monolithic structural parts is within 2mm. The influence of residual stress introduced by milling on the processing deformation of the workpiece is not negligible at this time.
Factors affecting the machining of aluminium alloy monolithic structural parts
In the process of precision machining of large aeronautical monolithic structural parts, due to the lack of systematic analysis of cutting and machining deformation mechanism and research on the theory of controlling machining deformation, at present, the processing process is mainly determined by a combination of trial cutting and experience, the processing parameters are unreasonable, and the parameters are selected conservatively. The performance of the high-speed machining centre is not given full play, resulting in deterioration of the processing surface quality and low processing efficiency. The great problems in the machining process mainly manifest in the following aspects.
(1) machining parameters caused by the selection of little cutting chatter, seriously affecting the machining quality, reducing the life of the machine and tool.
(2) The overall structure of the local weak rigid thin-walled parts in the cutting force deformation and large overhang tool deformation under the action of cutting force, resulting in the loss of parts machining accuracy.
(3) The initial residual stress of the blank and the residual stress generated by the strong thermal coupling during the cutting process cause the overall deformation of the overall structural part after redistribution.
In the case that the machining deformation seriously affects the machining accuracy and production efficiency of the aeronautical integral structural parts, it is necessary to study the precise machining process strategy and safe shaping technology of large integral structural parts to seek and explore the law and mechanism of machining deformation and to establish the model for predicting and controlling the machining deformation, which will provide the theoretical basis for optimizing the machining process and achieve the efficient and precise machining of the aeronautical integral structural parts.
There are many reasons for the machining deformation of aerospace monolithic structural parts, which are related to the material of the blank, geometry and stiffness of the workpiece, as well as the machining process method and machining equipment.
After the ANPLLO engineer’s research and analysis, the main factors that cause the processing deformation of integral structural parts are the following.
(1) The material mechanical properties and structural characteristics of the workpiece.
The modulus of elasticity of aviation aluminium alloy is about 70~73MPa, which is about 1/3 of that of steel; due to its small modulus of elasticity and large flexural strength ratio, it is very easy to produce rebound in the cutting process, especially for large thin-walled structural parts, the phenomenon of “let the knife” and rebound is more serious; in addition, the shape of aviation integral structural parts is complex, the geometric structure is asymmetric, thin-walled More parts, their poor stiffness, etc., is also a large deformation of the inherent factors.
(2) The release and redistribution of the initial residual stress of the blank during processing.
The whole structural parts of aviation are usually made of high-strength deformed aluminium alloy thick plates directly milled and processed. To obtain the ideal mechanical properties, the aluminium alloy pre-drawn plate must undergo a series of processes such as rolling, solid solution, stretching, ageing, etc. During these processes, residual stresses are generated in the plate due to uneven temperature fields and elastic-plastic deformation. In the process, with the continuous removal of material, the residual stresses in the plate are released and redistributed, the original stress self-balancing state is destroyed, and the workpiece can only reach a new equilibrium state through deformation. It has been shown that the release and redistribution of the initial residual stress in the blank is one of the important reasons for the processing deformation of the whole structural parts of aviation.
(3) The thermal-force coupling between the tool and the workpiece in the cutting process.
The role of the tool on the workpiece is mainly expressed in the cutting force, cutting heat and machining surface left cutting residual stress. Under the action of cutting force, on the one hand, the workpiece and tool contact part of the elastic-plastic deformation, the material is constantly removed by the tool; on the other hand, the workpiece rebound effect, “let the knife” phenomenon, especially for the thin-walled part, “let the knife” on the machining accuracy The impact of “let tool” on machining accuracy should not be ignored.
In addition, the material to be cut under the action of the tool, elastic and plastic deformation and consumption of work, the chip and the front tool surface, the workpiece and the friction between the back tool surface also consume work, resulting in a large amount of cutting heat, resulting in the uneven temperature of various parts of the workpiece, so that it occurs thermal deformation. The depth of the residual stress layer left by cutting aluminium alloy on the surface of the workpiece is generally within 0.1mm. When the thickness of the workpiece is large, its rigidity is also large. The residual stress generated by the cutting process will hardly deform it; however, for the whole aviation parts, its wall thickness is mostly within 2mm, and the influence of the residual stress on the deformation cannot be ignored at this time.
(4) the workpiece clamping.
The workpiece is deformed under clamping force, forming a clamping stress field inside and generating the corresponding displacement. When the tool-cutting process, there will be an “overcut” or “undercut” phenomenon, resulting in the workpiece surface geometric error deformation; for the rigid thin-walled structural parts, clamping is an important factor causing processing deformation.
Aluminium alloy structural parts processing deformation strategy
The most effective strategy in machining large monolithic structural parts is the active control of machining deformation by optimizing the machining process and improving the clamping system. The general strategies are mainly as follows
1. Optimize tool path
Use the rigidity of the unmachined workpiece material to avoid excessive machining deformation; during the machining process, choose the machining path and machining parameters with less cutting force in the main deformation direction;
2. Select reasonable tool geometry parameters
In the machining process of the parts, the machining process and clamping system optimization are mainly based on the structural features of the parts themselves. Therefore, the optimization strategy can be divided into two categories for different machining features.
(1) Sidewall machining
In machining features for sidewall machining, the radial cutting force has the greatest impact on machining deformation. The radial stiffness of the tool and workpiece greatly impacts machining deformation, and its main control strategies are.
A.Using a layered circular cutting tool path can make the machining process of the part locally maintain high stiffness.
B. Reasonable choice of machining method. According to the different processing situations, reverse milling can avoid the machining error caused by the left tool; smooth milling can avoid the overcutting caused by the tool and workpiece close to each other. Of course, at the same time, it is necessary to consider the impact of the processing method on the quality of the machined surface and tool life.
C. Reasonable selection of tool parameters. Tool fillet has an important influence on the distribution of cutting force. In the process of sidewall machining, the choice of tools with certain fillets can convert the radial force to axial force in the machining process.
(2) Machining features for bottom surface machining
The main control strategies are in the machining features for the bottom surface processing of the parts.
A,Using the centre ring cut tool path can make the machining process of the part locally maintain high stiffness.
B, reasonable selection of tool parameters. Try to use the tool without rounded corners, which can make the machining process of the axial force small.
C, Reasonable choice of the fixture. The choice of vacuum fixture clamping can reduce the bottom surface deformation during processing. Moreover, for large thin-walled parts processing, such as aircraft skin milling processing, a combined flexible multi-point support device is often used for clamping.
Through the above strategies, the processing deformation of large monolithic structural parts can be reduced to a certain extent. To achieve the prediction and control of the deformation of large monolithic structural parts, the cutting process and machining system must be fully analyzed. The finite element method can be used to integrate the machining process, part deformation and clamping system to predict the machining deformation of large monolithic structural parts and to optimize the machining system.
Conclusion
How to achieve efficient and precise machining of aviation monolithic structural parts and solve the problem of machining deformation of aviation monolithic structural parts has become the core key technology in aviation manufacturing. Adopting high-speed cutting technology, selecting reasonable tool parameters, optimizing process parameters and tool paths, and properly clamping parts will be the most effective strategy for deformation control.
However, the foreign technology embargo makes it difficult to introduce advanced cutting technology. Thus the lack of high-speed cutting technology has caused most of the current high-grade CNC machining centres not to play a full role. Therefore, aviation manufacturing enterprises should vigorously carry out independent innovation research through the introduction of foreign advanced high-end CNC machining equipment based on digestion and absorption, combined with the enterprise’s conditions for re-innovation, access to high-speed cutting technology with independent intellectual property rights, to truly achieve efficient and precise processing of the whole structure of aviation.
