Study On Process & Mechanism Of High Efficiency Magnetorheological Polishing Excited By Halbach Array

As an advanced modern high-precision and low-damage processing method,magnetorheological polishing is based on the magnetorheological fluid and its magnetorheological effect to form a flexible magnetorheological pad under the action of magnetic field.It can remove material through shear processing principle under the ductile domain of material to achieve efficient ultra-smooth surface polishing.So magnetorheological polishing is a promising high-precision and low-damage processing technology.In recent years,the optoelectronic industry,LED lighting,optoelectronic devices,and consumer electronics products have been developed rapidly and continued to upgrade.Ultra-smooth surfaces are urgent demanded.The high-end optical field represented by laser nuclear fusion and lithography machines also puts forward higher requirements for the manufacturing efficiency of highprecision devices.The sub-aperture scanning processing mode of wheel-type magnetorheological polishing process is not suitable for high-efficient polishing of ultra-smooth surface.Halbach array is been used as excitation devices for magnetorheological polishing to make full use of the principal advantages of magnetorheological polishing process,ensure the quality of magnetorheological polishing processing and improve the processing efficiency.Halbach array increases the magnetic field strength and gradient,make full use of the magnetic field energy,enhance the magnetorheological effect,and greatly improve the efficiency of magnetorheological polishing.It is proved that Halbach array has significant advantages.Polishing experiments and results have been verified that this new process can greatly improve the efficiency of magnetorheological polishing and quickly realize sub-nanometer ultra-smooth surface polishing.A magnetorheological polishing force measurement system is designed and built to quantitatively analyze the changes of polishing force and friction coefficient during polishing and reveal the polishing removal mechanism.Combined with magnetic field simulation,polishing experiments and polishing force measurement results,a mathematical model of polishing removal is established to deeply understand the characteristics and mechanism of magnetorheological polishing excited by Halbach array.The Halbach array is introduced into the magnetorheological polishing technology as an excitation device,which makes full use of excellent characteristics of the singlesided magnetic flux.The magnetic field strength and gradient are enhanced.The aggregation and magnetorheological effect of magnetic particles,the holding effect of magnetorheological polishing pad on abrasive particles are strengthened.So,higher removal efficiency is achieved.For large-area planar magnetorheological polishing,different grades of large-area linear Halbach permanent magnet arrays with simple structure and small size are designed and manufactured as magnetorheological polishing excitation devices.Thanks to the flexible and variable form of Halbach array,various excitation devices such as ring and spherical Halbach array can also be designed to provide reference for other magnetorheological polishing processes and other magnetic-assisted processing processes.In addition,the high magnetic field utilization of Halbach arrays can reduce the use of rare earth permanent magnet materials.The magnetic field finite element simulation analysis method is used to compare magnetic field distribution of linear air gap permanent magnetic yoke,N-N pole codirectional array,N-S pole alternating array and Halbach array.The force and aggregation behavior of magnetic particles under the action of different excitation devices has also been analyzed by magnetic field simulation.Compared with other excitation devices,the simulation results show that Halbach array has a larger magnetic field area and the highest magnetic induction intensity in the working area,which is more conducive to improving the planar magnetorheological polishing efficiency.The analysis shows magnetic particles are most stressed under the action of the magnetic field of Halbach array,and aggregate toward to the boundary of magnets,which is conducive to the form a stronger and denser magnetorheological polishing pad.Then,the influence of different working gaps and magnetic material grades on the magnetic field of Halbach array is studied.Finally,the accuracy of the magnetic field simulation results is verified by magnetic field measurement.The rheological property of the magnetorheological polishing fluid is tested.The morphology of magnetorheological polishing pad under the action of the magnetic field of different excitation devices is analyzed.Periodic fluctuations are showed on magnetorheological polishing pad under the action of Halbach array,and a denser magnetic cluster structure in the magnetorheological pad is formed.The polishing comparison experiments show that the magnetorheological polishing process excited by Halbach array has the highest removal rate and fastest surface roughness convergence rate,which greatly improves the efficiency of magnetorheological posling.Further upgrades permanent magnets in Halbach array can also further improve polishing removal rates.In addition,the influence of different process parameters on polishing removal efficiency and surface roughness was experimentally studied.The influence of fixed-point polishing at different positions on removal distribution and roughness distribution is also studied.In this paper,a polishing force measuring device and system for large magnetorheological polishing are developed.The polishing force and friction coefficient in three different polishing modes are measured,the analysis results shows that the polishing force and friction coefficient are larger at the boundary of magnet.It shows that under the combined action of magnetic field strength and gradient,a magnetorheological polishing pad with dense structure is formed,which has a strong holding effect on abrasive particles,so the removal rate is higher at here.Compared with force measurement results of wheel-type magnetorheological polishing,the friction coefficient of magnetorheological polishing excited by Halbach array is basically consistent.This phenomenon suggests that they have similar removal mechanisms,both are shearing removal-dominant removal.However,the Halbah array has a greater polishing force,which makes magnetorheological polishing have a stronger removal effect and higher polishing removal efficiency.Different from wheeltype magnetorheological polishing,which has a small area of stable polishing pad and friction coefficient distribution,the area of magnetorheological polishing excited by Halach array is larger,the polishing pad and friction coefficient changes periodically with the position of polishing pad.At last,depend on polishing tests and force measurement results analysis,the Preston formula is improved.The shear stress distribution of effective abrasive particles is described by the relationship between the force distribution of magnetic particles and polishing shear stress which is obtained and corrected by experiments.The relative velocity distribution of polished surfaces is obtained through motion analysis,the polishing removal function model of two typical positions are established finally.The polishing removal function model is calculated by MATLAB and compared with the actual surface contour after polishing,which verifies the validity of the model and the correctness of the aforementioned theoretical analysis.The research of this paper establishes a foundation for subsequent theoretical research.