【作者】 黄志刚；

【导师】 陈新； 郭钟宁；

【作者基本信息】 广东工业大学 ， 机械电子工程， 2007， 博士

【摘要】 现代工业对零件表面精度的要求越来越高，如何实现工件的高精密抛光已经成为加工领域的一个重要研究课题。本文对一种新型的高精度表面加工工艺——流体振动抛光方法进行了研究。该方法由超声振动提供抛光能量，流体分子或悬浮在其中的微细磨粒在振动的驱使下冲击工件表面，另外，超声空化产生的能量集中，可导致更为强烈的材料去除。加工中的复杂性和微观性因素，包括声场分布、超声空化和微观材料去除机理，都难以采用实验手段直接观测。本文采用多种数值仿真方法，从宏观、介观与微观三种尺度，对上述关键问题进行了研究。目前对复杂声场的计算，一般通过用数值方法求解波动方程实现，本文根据驻波的简正分解理论，给出了声压场的直接计算方法。总声压由简正波声压的线性叠加获得，各次简正波的方程根据声源和抛光槽的参数确定。利用此理论对抛光液中的声压场进行了计算，另外，为验证声压场的边界条件，采用有限元流固耦合模型对抛光液的整体流场进行了计算。计算结果表明：驻波的声压波腹一般出现在声源幅射轴与抛光槽侧壁处，声压强度由底向上振荡减弱，声场的基本分布规律由底面超声源决定，侧面超声源只对声场的整体强度有所影响。流场的分布特点与声压场的计算结果吻合，而且符合相应的声学边界条件，不会出现流固振荡。基于整体声场和流场的数值计算结果，采用弹性冲击-接触理论对线性振动引起材料去除进行了估算，发现冲击应力较低，难以形成有效的加工作用。针对耗散颗粒动力学方法难以仿真非稳态过程的缺点，本文提出了非稳态耗散颗粒动力学理论。从求解速度朗之万方程出发，建立了系统的温度-密度条件和压强—密度条件，并给出了具体的计算方法。通过仿真实例对该理论进行了数值验证，仿真结果与理论预测值很好地吻合，表明此理论适用于模拟非稳态过程。本文首次采用耗散颗粒动力学方法研究声致空化现象，探讨空化导致的材料去除，并计算空化射流驱动下的磨粒运动，为冲击过程的仿真提供基础数据。相比目前常用的空化仿真方法，耗散颗粒动力学能够提供更为丰富的气泡动力学和流场演化信息，而且这种方法也可以方便地对群空化以及多相流中的空化进行仿真。气体模型根据本文提出的非稳态耗散颗粒动力学原理建立，解决了原始方法不能模拟强非稳态气泡动力学的难题，而液体模型则结合流体力学相似原理建立，以处理耗散颗粒的柔软性和流体高抗压缩性之间的矛盾。采用独具特色的张力环技术模拟气泡壁，另外，为改善运算的速度和精度，在仿真计算中采用了相分离技术，气体与液体的动力学单独计算。仿真结果表明：空化产生了强烈的能量集中，随着泡壁的快速塌缩，气泡的体积急剧缩小，气泡内温度与压强迅速上升，同时还伴随有明显的冲击波现象。工件表面附近的空化是非对称的，其后果是在周围液体中引发了涡流和高速微射流。声压、气泡的大小和位置等多种因素都将对空化效应产生影响，根据仿真结果对它们的作用进行了量化描述。群空化过程中，单个气泡的空化将相互削弱，气泡的数目越多密度越大，削弱越明显。对于多相抛光液，固相磨粒的存在也将影响空化进程，磨粒在射流通过后可获得一定的运动速度，其大小随磨粒直径的增大而降低。由空化引起的高速射流是导致材料去除的主要原因，射流冲击工件表面后产生了极高的动压，往复冲击可造成疲劳破坏；另一方面，磨粒在射流的推动下冲击或切削工件，可以实现材料的直接去除。流体振动抛光中材料的去除规模可以小到纳米级别，传统的连续介质理论以及基于此的数值方法，不适用于这种尺度范围内现象的处理。本文采用分子动力学这种有效的微观仿真方法，对流体振动抛光的微观材料去除机理进行了研究。加工过程中磨粒的运动是不受约束的，关于自由磨粒冲击的分子动力学仿真目前尚未见到相同的资料。仿真结果发现：冲击打破了工件基体的规则晶格，形成半晶状结构，在此过程中还伴随有基体的弹塑性变形、发热以及振动现象。磨粒在工件表面垂直振动，水平方向做翻滚或摇摆运动，运动过程中磨粒与工件问将出现很强的摩擦效应。磨粒的斜向冲击将在工件表面造成一系列的加工坑，加工坑的形成机制可归结于挤压、撕拉和重结晶等多种效应。冲击能量、冲击角和磨粒尺寸等都将影响加工坑的尺寸和形状，根据仿真数据总结出了加工坑深度与冲击能量间的经验公式。加工表面的最终形貌是由许多磨粒的不断冲击造成的，多次冲击的总体效果近似为单次冲击的线性叠加，工件的表面粗糙度约等于冲击坑的平均深度。多次冲击的表面具有分形特征，且分形维数较低，表明加工后的表面比较光滑。本文除了解释了流体振动抛光的加工机理，所提出的非稳态耗散颗粒动力学方法将在微流体以及复杂流体的仿真中发挥重要作用。目前的数值研究只是针对最基础的物理现象，化学作用的加工原理将是后续研究一个重要方向。另外，研究中未发现空化的选择性，未来可以考虑控制气泡在抛光液中的分布，使加工出现有利的选择性，以提高加工效率与精度。更多还原

【Abstract】 According to the increasing demand for workpiece with high surface quality, high precision polishing technology has now attracted tremendous interest in the field of mechanical engineering. This dissertation will study a novel polishing method termed Polishing Based on Vibrations of Liquid (PVL). In PVL, machining energy is supplied with ultrasonic transducers. Liquid molecules or suspending polishing particles will impact the surface of workpiece when polishing liquid is vibrated by the ultrasonic transducers. Furthermore, energy concentration caused by ultrasonic cavitation will result in the more violent material removal. The complex and microscopic phenomena, including acoustic pressure field, ultrasonic cavitation and microscopic material removal in PVL are all hard to be directly studied by experimental approaches. We will try to solve these key issues with numerical method. According to the specific problems, apply simulations in macro-, meso- or micro- scale.Presently, the calculations of complex acoustic field are generally accomplished through numerical solutions of wave functions. Based on normal mode decomposition theorem derived from linear acoustics, we present a direct calculation method for this problem. The total acoustic pressure is obtained from the superposition of pressures of all normal mode waves, and the wave function for each normal mode is decided by boundary conditions. This method is applied to study the pressure distribution in polishing tank, and in order to validate the boundary conditions, a liquid-solid coupling Finite Element model is employed to calculate the velocity field in polishing liquid. It is found that pressure antinodes of the standing wave generally appear on the radiation axes of ultrasonic transducers or at the boundary of polishing tank. The effective acoustic pressure attenuates undulately from the bottom to the top. Basic features of the pressure field are dominated by the ultrasonic transducers located on the bottom of polishing tank, while the transducers on the side wall will affect the total strength of the field. Velocity distribution of polishing liquid is in accordance with the pressure field obtained before, and the boundary conditions are proofed to be accurate either: liquid-solid resonance will not appear. Based on the numerical results for pressure field and flow field, the damage caused by linear acoustics is estimated with elastic impact-contact theory. A low impact stress is given, which means that linear vibration could hardly result in effective material removal.According to the difficulty of original Dissipative Particle Dynamics (DPD) on modeling non-equilibrium process, we developed a non-equilibrium-DPD theory. Based on the solution of velocity Langevin equation, constraints for temperature-density relation and for pressure-density relation are established, and the detailed calculation method is provided. Validity of the non-equilibrium DPD theory is confirmed numerically with a case study. The results agree well with theoretical predictions, which suggest that this theory is applicable in the simulation of non-equilibrium processes.We employ DPD method to simulate acoustic cavitation for the first time. Cavitation damage is discussed, and the movement of polishing particle in liquid jet caused by cavitation is calculated, so as to provide basic information for the simulation of impact process. Compared to the present numerical methods employed in the simulation of cavitation, DPD method can provide more detailed information for bubble dynamics and liquid flow evolution, moreover, simulating cloud cavitation and cavitation in multiphase environment with DPD is not difficult. Vapor in bubble is modeled with non-equilibrium-DPD method, thus the problem that original DPD method can not simulate bubble dynamics is solved. For the purpose of modeling high bulk modulus, similitude method is employed in addition to DPD method to simulate the dynamics of liquid. Surface tension ring is used to model the movement of bubble wall. In order to improve computational efficiency and accuracy, phase separation scheme is adopted, the dynamics of vapor and liquid are calculated separately. Simulation results suggest that cavitation will result in strong energy concentration. With the fast collapse of bubble wall, bubble volume keeps being compressed and bubble temperature or pressure keeps increasing quickly, at the same time, shock waves appear in the bubble. The cavitation happens near solid boundary is asymmetrical, vortex and high-speed liquid jet is generated consequently. Acoustic pressure, bubble size and the position of bubble in pressure field will all affect the cavitation strength, and the influences of these factors are expressed quantitatively according to the simulation results. Cavitations in a cloud will be weakened by each other, the higher the bubble density is, the more evident weakening effect will be. As for multiphase polishing liquid, the existence of solid polishing particle will affect cavitation process. Polishing particle obtains high velocity when liquid jet passes, and value of the velocity decreases with increasing particle size. It is found that the high-speed liquid jet caused by asymmetrical collapse is mainly responsible for cavitation damage. If liquid jet impact on workpiece, it will result in a very high kinetic pressure, thus repeating impact will bring fatigue damage. Moreover, polishing particle driven by liquid jet will impact or cut workpiece so as to remove material directly.Material removal in PVL might happen at the magnitude of nanometer. Traditional continuum theorem and the numerical method based on it are no longer valid for the solution of this microscopic problem. Thus in this paper, Molecular Dynamics (MD) method, which is powerful for the simulation of microscopic phenomena, is employed to study the microscopic material removal mechanism of PVL. The movement of polishing particle in PVL is not constrained, MD simulation of the impact of free particle on substrate is not found elsewhere. From the simulation result, it is found that the impact of particle will break the regular crystal lattices of workpiece, and amorphous structures will be resulted. At the same time, elastic-plastic deformation, thermal effect and vibration phenomenon will happen. Polishing particle will vibrate at vertical direction, while at tangential direction, the movement of particle will be rocking or rolling as the case may be. During the process, strong friction effect will exist between particle and workpiece. When polishing particle impact workpiece obliquely, a serial of machining dents will be caused on the surface. It is found that press, tear and self-organization effects are responsible for the formation of impact dents. Sizes of the machining dent are affected by incident velocity, particle size and incident angle. The empirical relationship between dent depth and incident energy is provided in the paper. Simulation of multiple impacts suggests that the total effect of multiple impacts is approximately the linear superposition of the effects of all single impacts. Roughness of polished surface roughly equals to the average depth of machining dents. Surface fabricated by multiple impacts has fractal character, and the fractal dimension is low, which means the polished workpiece will be fairly smooth.Based on the numerical studies of the key issues, the machining mechanisms of PVL are revealed. Moreover, the non-equilibrium-DPD method brought about in this paper will be meaningful for the simulation of micro flows and complex flows. The present researches are mostly focused on physical phenomena. As for the future work on PVL, machining mechanism based on chemical effects should be studied equivalently. Another subject that should be considered in the future is cavitation control. With a better cavitation control scheme, both efficiency and precision of PVL could be significantly improved.更多还原