| Due to the weak rigid of thin-walled parts, the characteristics of easily deform, and high material removal rate, it has been a problem to machining the thin-walled parts. There are many factors affect the machining accuracy of the thin-walled parts, such as fixtures, tool, process parameters, the heat treatment process, and material properties. Usually, when the machining is completed, the accuracy of thin-walled parts can meet the requirement. However, after the machining of short-term or a period of time the parts are easily failure due to natural large deformation, which is attributed to the unreasonable distribution of residual stress inside the material leads to the cumulative deformation, affecting the long-term stable performance of thin-walled part and bringing the enormous loss of property for the state and enterprises. Therefore, to understand and master the formation mechanism of residual stress is quite important.At present, high speed machining has been widely used in the field of thin-walled parts. In the traditional enterprise application, usually only according to the experience and the existing idle equipment to choose the corresponding processing parameters, which is lack of selection principle for the processing parameters, resulting in miss-undertaken the distribution law of residual stress. Moreover, it lacks of selection principle for controlling the residual stress and the processing precision of thin-walled workpiece in milling under considering multiple factors, which is extremely unfavorable for controlling the deformation of thin-walled parts caused by the residual stress. All the situations, which lead to the research and practice requirement of proposing and putting forward the machining accuracy control method of complex thin-walled parts.Therefore, in this study, combining the finite element method and experimantal method, firstly, discussing the optimization of processing parameters and properties of high speed, optimization principle, and then revealing the distribution of residual stress and the mechanism of deformation. Based on the milling of Uncut Chip Thickness model, the generation mechanism of cutting force and thermal on residual stress is also investigated. And the relevant simulation is evaluated with the experimental analysis. In addition, in order to control the final machining accuracy and residual stress, considering the influences of multi-factors, an aerospace box thin-walled part is selected as the research object. The presented research results provide technical and theoretical support for the machining of thin-walled parts. The main results and innovations of this thesis are as follows:(1) Research the formation mechanism of residual stress affected by the milling force and thermal, it is discovered that the formation mechanism of residual stress for aluminum alloy. The residual tangential stress is determined by the tangential force and thermal gradient. However, the residual radial stress is mainly affected by the cutting radial forces, and less influenced by the thermal gradient than its affect on the residual tangential stress. This is because the heat source is diffuse faster in tangential direction than in the radial direction. These conclusions provide a scientific basis for the interpretation and control of residual stress generation.(2) The high-speed milling characteristics and the principles of processing parameters optimization are researched and revealed. During the high-speed milling process, the characteristics are:1) as the milling linear speed increasing proportionally, the material removal rate is also inclining. Within a certain speed range, the cutting force is a corresponding decline. The surface temperature workpiece can drop.2) With the combination of mutilple factors processing parameters, when the removal rate is the same, configuration the high speed and other process parameters, it can realize the optimal value of the processing quality and efficiency. Based on the above high speed characteristics, two optimization rules can be obtained.1) In the roughing:improving the cutting speed is the first optimization design principle, secondly, the improving priorities of other process parameters are cutting width, depth of cutting and feed rate. The priority combination should be given to the selection of "higher speed, higher cutting width, higher feed rate, smaller depth of cut ".2) In the finishing:the cutting speed is firstly selected, the reducing priorities of other parameters are depth of cut, feed rate and cutting width."Moderate feed, larger cutting width and smaller depth of cut" should be preferred. A bove results provide scientific basis for the further design of high efficiency, high quality and process parameter scheme.(3) Exploring the redistribution mechanism of residual stress generation:From the results using the simulation and experimental methods, it is summarized that controlling the depth of cut in different stages, the subsequent machined subsurface residual stress can be optimized maximum. In addition, compared with the ideal material which contains no initial stress, the redistribution of machined residual stress is reduced when the material contains a certain amount of initial heat treatment (the profile is surface compressive and subsurface tensile residual stress) or processing (the profile is surface tensile and subsurface compressive residual stress) residual stress, which resulting in smaller deformation acused by the residual stress. Moreover, the model and coefficient is constructed and defined for the overlap of the tool path. It is found that taking the control target for the surface residual stress and material removal rate, In order to achieve the perfect unity of quality and efficiency, according to the requirement for the workpiece machining accuracy and chracteristic, the overlap coefficient is sleeted. The above results provide experimental and theoretical basis for controlling the machined residual stress distribution.(4) Taking the engineering application of controlling the machining accuracy of typical Aerospace Thin-walled Box type as study case, reseraching the method to control the machining accuracy of the complex thin-walled parts. The method combines the finite element and experimental method, and comprehensive multi factors optimization control of the processing schemes for thin-wall parts machining. From the results, proposing (1) the selection of milling path for the thin-walled box plane is preferred "from inside to outside",(2) the processing sequence frames should adopt the way of "symmetric cross-from outside to inside".(3) The choice priciple of processing parameters by controlling the depths of cut. Furthermore, the non traditional installation method is proposed in controlling the clamping distortion in the fininshing. In the roughing, using the special fixture to clamping and positioning the machined part, and the selection of depth of cut is larger than the former depth of subsurface machined residual stress tend to stable. Moreover, in the finishing, glue clamping can be used which has no clamping force, the depth of cut is required to choose less than the depth location of the maximum subsurface residual compressive stress. The distortion of the plane machining can be decreased. By adopting this approach, and conducting the engineering test for the aerospace thin-walled part. All the foundings provide methods and theoretical basises for the design of complex thin-walled parts processing precision control.By exploring and investigating the above research content, the authors hope the research results of this topic can provide scientific and theoretical basises for machining accuracy and residual stress control of the thin-walled parts, and provide methods guidance for optimizing the residual stress distribution in the milling process. The author also hopes it can be able to provide clearer direction to solve the problem in the exploration of new scientific problems to the future researchers in the field of thin-walled parts processing. |
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