残余应力对航空整体结构件加工变形的影响分析

基于理论计算和有限元模拟,研究了毛坯的初始残余应力对大型整体结构件数控加工变形的影响,对单向 应力作用的矩形截面梁在剥层过程中的变形挠度值进行了求解。结果表明,理论解与有限元计算值是一致的。面 向工程应用,采用ABAQUS有限元软件模拟了残余应力对隔框类整体结构件加工变形的影响,并进行了试验验证。 有限元仿真结果与试验数值非常吻合。最后,根据工件加工变形的有限元模拟结果,提出了提高整体结构件制造 精度的工艺措施。     

Effects of tool diameters on the residual stress and distortion induced by milling of thin-walled part

Residual stress has a sustained impact on the deformation of thin-walled parts after processing, raising the strict restrictions required in their using procedure. In general, with regard to thin-walled parts, different processing parameters will affect the distortion and residual stress generation of the workpiece, which play the key role in the machining. However, controlling the material removal rate is also quite critical to machining of thin-walled parts. In order to reach these goals, based on the relation between residual stress and uncut chip thickness (UCT), a method is proposed by optimizing the milling tool diameters. The research finding reveals that, by improving the tool diameter, at the same circular position, smaller UCT can be achieved. In addition, take 6 and 12 mm tool diameter as analysis cases; larger tool diameter can reduce the residual tensile stress distribution significantly (the ratio ranges from 13.9 to 34.7 %) and improve the material removal rate. Moreover, a typical thin-walled part is evaluated using different tool diameters (6 and 12 mm) by experiments, as the final distortion can be decreased by 60 % with 12-mm tool diameter. The distribution of machined surface and subsurface residual stress is turning to be more uniform. Hence, it proves that, under the goals of maintaining machining accuracy and material removal rate, also improving the distribution of residual stress, it is possible to achieve by controlling the UCT (tool diameters) in the processing of thin-walled. All these findings can help to enhance the milling precision of thin-walled parts, as well as control and optimize the residual stress distribution.     

航空结构件残余应力释放引起加工变形的数值模拟

航空铝合金结构件毛坯在成形过程中产生很大的残余应力,使其在后续机械加工过程中极易发生翘曲变形,采用有限元数值模拟技术,建立了航空铝合金板材残余应力释放引起加工变形的力学模型,分析了在7075T7351铝合金板材材料去除过程中残余应力释放引起的加工变形规律,并针对7075T7351隔框零件进行铣削加工变形实验研究,并与有限元模拟结果进行比较,验证了数值模拟结果具有较高的精确度。     

Methodology for prediction of distortion of workpieces manufactured by high speed machining based on an accurate through-the-thickness residual stress determination

After high-speed machining of rolled and stretch straightened aluminium sheets, out-of-plane distortion is found. This is especially true for complex machined shapes in thin-gauge and high-strength aluminium alloys. This kind of sheets are mostly found in the aeronautical industry, where thin-weight-optimised aluminium shell designs are used for skin applications. Usually, in the aeronautical sector, low-thickness sections are used wherever possible, and higher thicknesses are used only where additional strength is required. In the present work, a methodology of predicting workpiece distortion based on the residual stress present in the workpiece is presented, which can be applied by machining companies without further investment in measurement equipment. A method for accurately determining through-the-thickness residual stresses was adapted to the special requirements of this industrial sector. The measured residual stresses were used in a finite element model capable of approximately calculating the shape distortion of simple and complex high-speed machined parts. Promising results have been obtained.     

Comparison and analysis of main effect elements of machining distortion for aluminum alloy and titanium alloy aircraft monolithic component

Main effect elements of machining distortion for aluminum alloy and titanium alloy aircraft monolithic component are investigated by finite element simulation and experiment. Based on an analysis of milling process characters, finite element models of machining distortion are developed. Considering the action of initial residual stress, finite element simulation and analysis of machining distortion for aluminum alloy aircraft monolithic component are performed. Initial residual stress, cutting loads, and coupling action of these two effect factors are taken into account, respectively, to perform finite element simulations of machining distortion for titanium alloy aircraft monolithic component. The finite element simulation results are compared with experiment results and found to be in good agreement, indicating the validation of the proposed finite element models. The research results show that the initial residual stress in the blank is the main effect element of machining distortion for aluminum alloy aircraft monolithic component, while cutting loads (including cutting force and temperature) are the main effect element of machining distortion for titanium alloy aircraft monolithic component. To decrease machining distortion of aluminum alloy aircraft monolithic component, the initial residual stress in the blank must be controlled first. Similarly, to decrease machining distortion of titanium alloy aircraft monolithic component, the cutting loads must be controlled first.     

基于正交切削模拟的零件铣削加工变形预测研究

提出了基于正交切削模拟的零件铣削加工变形的预测方法,建立了三维铣削加工的有限元模型。基于正交切削加工模拟结果,利用铣削温度、铣削力的分析模型求解了三维铣削加工的瞬态温度和瞬态切削力,并将其作为动态载荷应用于三维切削加工的有限元模型,模拟了零件的三维铣削加工过程,预测了零件的变形。通过模拟结果与现场加工情况对比,证明该预测方法是行之有效的。     

钛合金TC4退火数值模拟

毛坯残余应力对薄壁件整体加工变形有重要影响。利用大型通用有限元软件ANSYS10.0对钛合金TC4进行退火过程数值模拟研究,通过数值模拟获得了退火过程中温度的变化、残余应力的分布及最终冷却后的残余应力状态。为研究TC4的加工变形规律,提供了具有初始残余应力场的数字化毛坯。     

Numerical modelling of welding distortion redistribution due to the change of self-constraint in a T-joint welded structure

Finite element method (FEM) is a powerful tool for analysing the potential deformation during a material removal process. After the removal of material, re-establishment of equilibrium within the remaining part of the structure causes distortion due to the relief of residual stress in the removed materials. In this study, commercial FEM software (MSC.Marc) was used to simulate material removal, and the accuracy was evaluated by comparison with results from machining experiments. The effect of cutting height on the distortion redistribution and the kerf width in a T-joint welded structure is discussed, and the distortion differences at the centre line of the bottom were compared between the calculated and experimental results. The results demonstrate that the developed model is useful and efficient for simulating the redistribution of welding distortion due to material removal.

     

Influence of Cavitation on Unsteady Vortical Flows in a Side Channel Pump

Double-sided lapping is an precision machining method capable of obtaining high-precision surface. However, during the lapping process of thin pure copper substrate, the workpiece will be warped due to the influence of residual stress, including the machining stress and initial residual stress, which will deteriorate the flatness of the workpiece and ultimately affect the performance of components. In this study, finite element method (FEM) was adopted to study the effect of residual stress-related on the deformation of pure copper substrate during double-sided lapping. Considering the initial residual stress of the workpiece, the stress caused by the lapping and their distribution characteristics, a prediction model was proposed for simulating workpiece machining deformation in lapping process by measuring the material removal rate of the upper and lower surfaces of the workpiece under the corresponding parameters. The results showed that the primary cause of the warping deformation of the workpiece in the double-sided lapping is the redistribution of initial residual stress caused by uneven material removal on the both surfaces. The finite element simulation results were in good agreement with the experimental results.     

飞机整体框类结构件铣削加工的模拟研究

针对航空整体结构件铣削加工变形的复杂制造问题，建立了三维铣削加工有限元模型。深入研究了材料模型、残余应力施加、动态切削载荷、材料去除等铣削加工模拟所涉及的关键技术．并详细论述了铣削加工模拟过程。应用该模型对某框类结构件进行了不同铣削顺序的加工模拟，通过零件变形模拟值与实验加工所得零件变形值的比较，证明该有限元模型可以实现对零件铣削加工变形规律的预测。     

残余应力作用下的航空整体结构件加工变形仿真

铝合金预拉伸板在成型过程中会产生较大的残余应力,在切削过程中毛坯的初始残余应力的释放对整体结构件的宏观变形有重要的影响。在弹塑性力学的基础上,综合运用Hypermesh和ABAQUS建立残余应力单因素作用下的三维铣削仿真加工变形场的有限元模型,利用生死单元技术模拟了材料的去除,分析了铝合金板材材料去除过程中残余应力释放引起的加工变形规律。并且运用Hypermesh提高了有限元前处理的速度,解决了复杂模型的残余应力加载困难与单元去除困难的问题。     

TC4管状零件内壁加工残余应力计算及其有限元分析

针对残余应力对加工零件使用性能有严重影响以及TC4材料难以用X射线法进行残余应力测量的现状,提出了一种通过测量外壁应变来计算TC4管状零件内壁加工残余应力的方法。为验证该方法的正确性,在有限元软件中对模型内壁进行分层施加预定的随深度变化的残余应力,并用生死单元技术将内层单元逐层“杀死”来模拟实际实验中应力层被腐蚀的过程。根据外层不断变化的应变以及推导出的计算公式来计算内壁随深度变化的残余应力,并将计算得到的残余应力与之前所施加的应力进行对比,可发现该方法求得的残余应力值准确度较高,由此可以得出结论：该方法可用于测量实际零件的残余应力。     

Computer simulation and experimental study of machining deflection due to original residual stress of aerospace thin-walled parts

Aerospace thin-walled parts have a complex structure and high accuracy. Factors such as original residual stress, fixing, and machining may make low-rigidity parts deform easily, which is difficult for traditional craftwork to forecast and control. Especially in machining big aerospace parts, original residual stress has a great effect on machining deflection. In this paper finite element model of original residual stress is established to analyze the corresponding deflection by machining aerospace thin-walled parts. Simulation results are validated consistent with experimental results approximately. At last the paper puts forward the corresponding mend methods to control the deflection caused by original residual stress during the actual machining process.     

Investigation of the coupled distribution of initial and machining-induced residual stress on the surface of thin-walled parts

Residual stress on the machined surface and the subsurface is known to influence the distortion of thin-walled parts. Therefore, it is essential to predict the distribution of surface residual stress accurately. In this paper, the coupled distribution law of initial residual stress and machining-induced residual stress is investigated. Firstly, a model with initial residual stress is established and incorporated into thermal mechanical coupled finite element model of 2-D cutting. Then, a tensile fixture is designed to impose initial stress into a thin-walled part of Al-6061, and cutting experiments are carried out. The residual stress distribution is measured by X-ray diffraction/electropolishing method. The results of experiments and simulation show that in the plastic deformation zone, the initial residual stress has no significant influence on the distribution of the machining-induced residual stress. In the elastic deformation zone, the stress that linear grows along depth from zero to initial residual stress is superimposed on machining-induced residual stress. The mathematical model of stress coupling distribution on the surface of thin-walled parts is established by numerical method. Finally, it is found that the effect of coupled stress distribution on distortion is more significant with the decrease of thickness (from 3 to 0.5 mm) of the thin-walled parts.     

切削中残余应力释放对工件加工变形影响研究

为了研究加工过程中毛坯初始残余应力的释放对加工变形的影响,在弹塑性理论的基础上,建立了三维铣削仿真加工变形场的有限元分析模型,利用位移约束转换和"单元生死"技术仿真了加工过程中工件装夹和材料的去除。通过仿真数据与实验数据相比较,结果表明:工件毛坯残余应力在铣削加工过程中对称释放有利于减小工件加工后变形。     

高档数控机床基础件低应力制造特性的研究

围绕高档数控机床基础件的低应力制造问题,研究了残余应力的分布规律,提出从优化制造过程工艺参数出发的降低残余应力方法。明确了铸造、机加工是两个对残余应力的产生起主导作用的工艺环节,采用有限元法对某加工中心床身进行了残余应力分析,得出了该床身在铸造与机加工环节的残余应力大小及分布情况。据此优化铸造残余应力振动时效处理的工艺参数,并分析了铣削加工参数对残余应力的影响。分析结果对数控机床大型基础件的低应力制造提供理论依据,有效保证基础件精度和稳定性。     

残余应力引起的铣削变形有限元分析

应用弹性力学理论推导出了二维连续铣削过程中工件内应力再分布及其引起变形的计算递推公式.解决了有限元建模、初始应力加载、变形评估等仿真关键技术,建立了残余应力引起铣削变形的二维有限元仿真模型.最后对实例进行了仿真计算,分析了残余应力引起的加工变形规律,并用解析法对仿真结果的进行了验证.     

零件结构形式对铣削加工变形的影响研究

在弹塑性理论的基础上,建立了三维铣削仿真加工变形场的有限元分析模型,利用"单元生死"技术仿真了加工过程中材料的去除.研究了零件结构不同,加工过程中毛坯初始残余应力的释放对加工变形的影响,通过仿真数据与试验数据相比较,结果表明,只要建立正确的三维有限元分析模型,完全可以实现对零件加工变形规律的预测;工件毛坯残余应力在铣削加工过程中对称释放有利用减小零件加工后变形.     

Effects of different restraints on the weld-induced residual deformations and stresses in a steel plate

Effects of three different plate boundary constraints on the residual stress field and deformation are investigated numerically during butt-joint welding. For the numerical solution of the heat transfer equations, the finite element method is used to predict the temperature profile as well as residual stress field due to three different plate boundary conditions. The distortion of the welded plate is modeled as a nonlinear problem in geometry and material, adopting a finite element solution based upon the thermo–elastic–plastic large deflection theory. High-strength shipbuilding steel AH36 with temperature-depending material properties and nonlinear stress–strain material properties (bilinear isotropic hardening option uses the von Mises yield criteria) are assumed for the numerical analysis. For verifying the results, the temperature profile is compared with the result obtained in a previous research. In the mechanical analysis, three different boundary conditions are applied. Effects of plate thickness, plate width, and mesh model on the residual stress with boundary constraint are studied.     

Experimental and numerical investigations of material removal process in electrochemical discharge machining of glass in discharge regime

Electrochemical discharge machining (ECDM) can be applied as a non-traditional processing technology for machining non-conductive materials such as glass and ceramics, based on the phenomena of evoked electrochemical discharges around the tool electrode. The material removal mechanism of ECDM is noticeably complex and difficult to experimentally characterize. In this paper, finite element models were proposed to predict the material removal in the ECDM discharge regime. First, the single-pulse discharge on a tapered electrode was modeled. It was found that about 30.5% of the discharge energy is transferred to the workpiece. The continuous discharge on a cylindrical electrode was thereafter modeled according to this phenomenon, in which the removal of a layer of the workpiece material starts from the projected contour of the edge of the electrode end and extends inward during the ECDM processing. The effective discharge ratio for material removal was calculated to be 10.1%. The drilling depths of holes at different applied voltages were predicted by the proposed finite element method. It was found that the predicted values were consistent with the experimental results.