【作者】 路冬；

【导师】 李剑峰；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2007， 博士

【摘要】 整体结构件因其具有重量轻、结构效率高、装配工作量小、可靠性高等显著优点,被广泛应用于航空航天领域。但航空整体结构件大都具有尺寸大、结构复杂、加工周期长等特点,加工过程中因各因素综合作用,加工结束后往往产生弯曲、扭曲或弯扭组合等变形,加工精度难以达到设计要求。引起航空整体结构件变形的因素包括整体结构件的材料属性及结构特征、毛坯初始残余应力、切削力、切削热、夹紧力等。其中夹紧力是影响航空整体结构件变形的重要因素之一,尤其对弱刚度结构件,夹紧力引起的变形更是不容忽视。本文针对航空框类整体结构件的数控加工变形问题,采用理论分析、有限元建模和实验研究相结合的方法,建立了综合考虑毛坯初始残余应力、装夹应力、铣削机械应力及铣削热应力等复杂耦合关系的航空整体结构件变形预测模型。通过模拟夹紧力的施加与释放过程,预测航空整体结构件的变形与应力分布特点。结合提出的遗传算法与有限元法相结合的夹紧点位置及夹紧点数目递推优化方法,优化装夹布局。通过从局部切削仿真到考虑多因素综合影响的整体变形预测以及装夹布局优化的系统研究,为航空整体结构件的加工变形预测、控制和工艺参数优化提供了理论依据。建立了铝合金7050-T7451三维铣削有限元分析模型,获得了单齿顺铣铣削力变化曲线。结果表明,从刀具切入到切出过程中,随切入材料厚度变化,模拟获得的三个方向铣削力先快速增大,达到最大值后缓慢减小,最后快速减小,但达到最大值的时间不同,模拟获得的铣削力变化趋势与实验获得的变化趋势相同。研究了铣削加工过程中的温度、等效应力、等效应变及等效应变率分布规律。铣削最高温度出现在前刀面与切屑接触区。切屑与工件上的温度分布规律为:切屑的温度较高,工件的温度相对较低,这主要是由于高速加工中切削热大部分被切屑带走,只有少部分传入刀具和工件。最大等效应力、最大应变率均发生在第一变形区,这是因为切削过程中第一变形区产生极大的变形。材料应变在第一变形区产生,当形成切屑时达到最大值。模拟切屑形成过程,获得刀具由切入到切出过程中不同时刻切屑形态,模拟得到的切屑形态与相同加工参数下实际铣削加工得到的切屑形态相似。模拟获得的铣削力与实验值较为一致,由此验证了有限元模型的正确性。获得的瞬态铣削力及铣削温度数据可作为初始条件施加到后续整体结构件的变形预测模型中,可大大减少实验量。建立了基于系统刚度变化的工件变形控制模型。系统刚度模型中同时考虑工件刚度、夹具元件刚度及夹具元件与工件间的接触刚度。定义工件—夹具系统约束条件,给出了装夹稳定性的判断准则,预测了加工过程中定位元件与工件间的接触力,预测的法向接触力与实验值比较,误差在15%以内,验证了模型的正确性。分析了随系统刚度变化定位元件与工件间接触力变化规律,给出了不同刚度条件下最小稳定夹紧力计算方法。预测了随系统刚度变化工件变形规律。在保证装夹稳定前提下,通过调整工艺参数实现了对工件变形的控制。建立了航空框类整体结构件模态分析模型,通过对航空框类整体结构件加工过程中的振型分析知:当框类整体结构件壁较薄时,以沿壁厚方向的振动为主,振型为正弦波形,表明壁厚方向是框类整体结构件刚度最弱的方向。随着框类整体结构件壁厚的减小,各阶固有频率均减小。分析了不同装夹方式下框类整体结构件前10阶固有频率及振型。结果表明:采用增加工件侧壁顶面夹紧的方式可以增加工件动刚度,且主振型振幅的一致性比没有增加工件侧壁顶面夹紧时主振型振幅的一致性好。设计了框类整体结构件模态实验,并将模拟获得的前6阶固有频率与实验值比较,误差在6%以内,验证了模型的正确性。对于加工去除量大、刚性小、薄壁的整体结构件,可预测随材料去除系统动态特性变化,及时调整切削参数或装夹方式,以避免共振。建立了综合考虑毛坯初始残余应力、装夹应力、铣削机械应力及铣削热应力复杂耦合关系模型。将框类整体结构件材料模型、毛坯初始残余应力及夹紧力作为初始条件、三维铣削模拟获得的瞬态铣削力及铣削温度作为动态载荷,施加到多应力耦合模型中。动态铣削力及铣削温度按刀具的运动轨迹施加到对应单元节点上,模拟真实的铣削过程,获得加工过程中工件变形及应力分布。模拟了夹具释放过程,获得了夹具释放后框类整体结构件的变形规律。设计了铣削实验,通过将框类整体结构件加工过程中模拟获得的任意两点应变值与对应点的实验值比较,及将模拟获得的框类整体结构件的最终变形与实验获得的最终变形比较,模拟误差皆在16%以内,验证了有限元模型的正确性。为以减小加工变形为目标的装夹布局优化提供依据。提出了航空整体结构件最优夹紧点位置及最少夹紧点数目的确定方法。以加工过程中航空框类整体结构件的最大变形最小为目标函数,采用遗传算法和有限元法相结合的优化方法,建立了航空框类整体结构件夹紧点位置及夹紧点数目递推优化模型。以两个航空框类整体结构件为例对其装夹布局进行优化,通过比较采用优化后的装夹布局与现场中凭经验确定的装夹布局下加工过程中加工点的最大位移,验证了优化模型的有效性。更多还原

【Abstract】 Monolithic components are being widely used in the field of aviation due to their lower height, higher structural efficiency and higher working reliability compared with traditional riveted parts. However, owing to the large dimension and complicated structure, monolithic components deformation during machining is almost inevitable. Material property and structure of monolithic components, initial stress, cutting force, cutting heat and clamping forces affect the deformation of monolithic components, Among these factors, clamping force is one of the main factors, especially for thin-walled components. Both theoretical and experimental analysis methods are used for the Researches of the mechanism of monolithic components deformation. A fixture layout optimization method based on genetic algorithm was proposed. It is possible to predict and control the deformation of monolithic component by using the theoretically optimized results.A finite element model of A17050-T7451 milling process was developed. Cutting forces, temperature distribution along tool-chip contact face, flow stress distribution in cutting zones and plastic strain were obtained during machining. The vary curves of milling force for single tooth milling process were predicted using the FEM model. It shows that milling force increases sharply at the beginning of the milling process, and then decreases slowly as the relative static milling process keeps up and at last goes down fast as the tooth leave the cutting area. The temperature on the chip, workpiece and milling cutter were predicated. It shows that the temperatures on the chip are much higher than that on the workpiece. The reason of this phenomenon is mainly because that the majority of cutting heat is carried off by the chip during high speed machining. The chip formation process was simulated. The simulated chip morphology is similar with that obtained from experiment by using the same parameters as FEM. The cutting forces of FEM model were well agreed with the experimental results. The predicted results were used as the initial conditions for the multi-stress coupled model.A workpiece deformation control model was developed. The system stiffness model of workpiece and fixtures was established. Fixture stiffness, workpiece stiffness and contact stiffness between workpiece and fixtures were considered. The constraint conditions were defined and the criteria of fixturing stability were carried out. The contact forces between workpiece and fixtures were predicted. The normal contact forces predicted were well agreed with the experimental results. The workpiece deformation and contact forces between workpiece and fixtures under different system stiffness were predicted. And the workpiece deformation and contact forces between workpiece and fixtures under different cutting forces and clamping forces were predicted. It is proved that controlling of cutting forces is the basic factor that can reduce the clamping forces and clamping deformation for a given clamping scheme. So, the optimization of cutting parameters must be analyzed based on cutting forces. Selecting higher cutting velocity, smaller axial depth of cut, large radial depth of cut, can reduce workpiece deformation on the premise of cutting efficiency.The modal analysis model for frame-shaped thin-walled monolithic components was developed. Through study on the vibration modes during the milling process, it is indicated that the vibration direction of thin-walled monolithic components is mainly along wall thickness direction. It is can be concluded that the wall thickness direction is the weakest direction of monolithic components that can cause the deformation during milling process. As the wall thickness of monolithic components decreases, every rank of natural frequency decreases. The first ten natural frequency and modes were predicated for different clamping types. It shows that when the top surface is clamped every natural frequency and amplitude decrease comparing with no top clamping. To validate the simulated results, modal experiment was designed and the simulated results of the first six natural frequencies were compared with experimental results. The error between simulation and FEM is within 6%. For thin-walled monolithic components, to avoid resonance, the natural frequency of monolithic components should be predicated and the cutting parameters or clamping types should be adjusted. Multi-stress coupled model including monolithic components initial stress, cutting mechanical stress and clamping stress was developed. Monolithic components material properties, initial stress, clamping forces were added on the multi-stress coupled model as initial conditions. Transient milling forces and heat getting from the 3D FEM milling process were added on the multi-stress coupled model as dynamic loads. The moving track of dynamic loads is the same as the real tool-path in experiment for milling the workpiece. The monolithic components deformation and stress, strain distribution were examined during cutting process. The releasing process of fixture was simulated and the final deformation and distribution of stress and strain of monolithic components were achieved. To validate the multi-stress coupled model, the experimental test of frame monolithic components was designed. The predicted deformation of monolithic components by FEM was well agreed with the experimental results.A determination method of optimal clamping position and clamping amount for frame monolithic components was established. The objective function is to minimize the deformation of monolithic components during milling process. A genetic algorithm method combined with finite element method was proposed. A recurrent optimal model was established to optimize clamping position and clamping amount of frame monolithic components. Two study cases were carried out. Through comparing the deformation of monolithic components during machining using optimal fixture layout and the layout derived by experience, the effectiveness of fixture layout optimization method is verified.更多还原