Research Of The Machining Mechanisms Of Polishing Based On Vibrations Of Liquid With Numerical Simulations

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.