| Development of modern optoelectronics and integrated circuits industries have put extremely high requirements on ultra-smooth surfaces,which not only requiring surface roughness to reach atomic accuracy,but also requir ing ultra-low subsurface damage.Magnetorheological finishing(MRF)has been proved to be a very promising ultra-smooth surface processing technology,which can meet above requirments.It is very important to explore the damage generation mechanism of MRF process,for which is of great significance to achieve ultra-smooth surface approaching to atomic scale limit.Moreover,realizing the low-cost and high-efficiency planar magnetorheological processing is an urgent demand of many industries.In order to satisfy diversified requirements of above mentioned industries,this paper has carried out a study on polishing damage generation mechanism of planar magnetorheological finishing using a large polishing tool.A mathematical model was established to predict the depth of plastic deformation layer(PDL)after MRF,which has been verified by experiment results.Moreover,the electromagnet excitation device with large polishing tool was developed to obtain sub-nanoscale surface roughness and tens of nanometer damage layer after MRF.According to different application requirements,processing experiments were carried out to verify the ability of the proposed MRF method on ultra-smooth surface processing.The main research points of this paper are listed as follows:(1)Introduction has summarized the current status of ultra-smooth surface processing technology and magnetorheological finishing technology,which focused on analyzing the shortcomings existing among those traditional ultra-smooth surface processing technologies.A MRF process with large polishing tool based on an electromagnet excitation as well as its plastic deformation damage mechanism was investigated in this study.(2)Mechanism of plastic material removal mode of MRF and its processing damage were studied.Generation mechanism of plastic deformation damage layer in MRF was analyzed firstly.Mathematical model of PDL depth during MRF was proposed based on the single abrasive grain cutting model and the effective abrasive grain number model.In order to prove the correctness of the proposed PDL depth model of P-MRF,simulation analyses combined experiment verification w ere carried out.The influence of polishing powder diameter and its concentration,iron powder diameter,normal pressure and effective area on PDL depth are symmetrically studied.Combined with FIB sample preparation technique and TEM high-resolution imaging,the damage layer within tens of nanometer after MRF was first characterized,which the measured depth value has consistent with theoretical mathematical model.A modified pre-thinning procedure of ion-beam thinning for TEM sample preparation was proposed by taking the advantage of the plastic material removal mode of MRF.(3)In order to realize controlling the depth of damage layer in MRF,an electromagnet with U-shaped column and two wedge-shaped magnetic poles combined as iron core was designed and manufactured.Magnetic scalar meth od was adopted to perform theoretical calculations of as-designed electromagnet.The electromagnet is simulated and optimized by Maxwell 16.0 simulation software simultaneously.Finally,the measured value of magnetic flux density combined with simulation values were used to verify that the as-designed electromagnet meets the requirements of the magnetic field strength in MRF with large polishing tool.(4)In order to further improve the performance of the electromagnet,a small-sized array structure with trapezoidal and circular arcs along the air gap direction was proprosed.The dimension of magnetic pole structure was optimized and simulated,and the optimum parameters of magnetic pole have been selected.In order to verify the improvement of electromagnet,simulation and comparison analysis were carried out between as-designed electromagnets.(5)The processing characteristics of permanent magnet yoke and electromagnet were studied.The magnetic field characteristics and stiffned zone controlment between permanent magnet yoke and electromagnet were compared and analyzed.The polishing experiments of permanent magnet yoke,straight line,trapezoidal and arc electromagnet excitation were carried out to show processing characteristics.In order to study the influence of process parameters on effect of magnetorheological finishing with large polishing tool,electromagnets was used as an excitation device to systematically study the process parameters,including the influence of working gap,current,workpiece speed and polishing trough speed on surface roughness and material removal rate.The magnetorheological finishing of zirconia ceramics with high efficiency and low damage was studied by using permanent magnet yoke as excitation system,and the polishing properties of different abrasives were systematically studied. |
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