| With the development of high-power devices such as power electronics and semiconductor lighting（LED）technology,the use of new-generation optoelectronic wafers such as sapphire（Al2O3）is rapidly increasing.In this paper,the unique squeeze strengthening effect of the magnetorheological fluid is combined with the cluster magnetorheological finishing,and a cluster magnetorheological variable gap dynamic pressure finishing method that dynamically changes the machining gap is proposed.During the processing,the variable gap motion squeezes the semi-fixation."Bingham"viscoelastic body’s magnetorheological polishing pad generates dynamic pressure,realizes fluctuations in polishing force during polishing,increases effective abrasive particles,improves material removal process,and achieves the purpose of efficient,high-precision,and without surface/sub-surface defects processing of sapphire wafers.First,the feasibility and effectiveness of the cluster magnetorheological variable gap dynamic pressure finishing is analyzed and verified.By analyzing the mechanism of cluster magnetorheological variable gap dynamic pressure finishing,and using sapphire wafers as the machining object,the polishing test was carried out.It was found that during the cluster magnetorheological variable gap dynamic pressure finishing,the polishing pressure increased and the number of effective abrasive particles increased.The removal effect of the abrasive particles on the surface material of the sapphire wafer was promoted,the material removal rate was increased by 19.5%year-on-year to 2.40nm/min,and the surface roughness Ra was reduced to 0.77 nm.Secondly,study the cluster magnetorheological variable gap dynamic pressure finishing polishing force characteristics.A Kistler 9171A three-way rotary dynamometer was used to set up a cluster magnetorheological variable gap dynamic pressure finishing polishing force measurement test platform.The effects of process parameters such as workpiece disk rotational speed,carbonyl iron powder concentration,squeeze amplitude,squeeze frequency,minimum machining gap and polishing disc rotational speed on polishing force were systematically tested and analyzed.It was found that the polishing positive pressure Fn and the tangential force Ft exhibited the same fluctuations as the dynamic pressure cycle of the workpiece disc.During the dynamic pressure cycle,the positive pressure Fn changed drastically,its magnitude and direction changed,and only the tangential force Ft changed.The related process parameters have a significant effect on the polishing force characteristics.The polishing force positive pressure Fn and tangential force Ft gradually decrease with the increase of the workpiece disk rotation speed,increase with the increase of the concentration of carbonyl iron powder,and decrease with the increase of the minimum machining gap.There is a match between the squeeze speed and the linear speed of the magnetorheological polishing pad.Thirdly,the process of cluster magnetorheological variable gap dynamic pressure finishing was studied and the process parameters were optimized.The effects of different abrasive concentrations,carbonyl iron powder concentrations,polishing disc rotational speed,squeeze amplitude,workpiece disc rotational speed,squeeze frequency,and minimum machining gap on the polishing effect of cluster magnetorheological variable gap dynamic pressure finishing were studied through a systematic single-factor test,And obtained a better process parameter scheme:carbonyl iron powder concentration of 16wt.%,abrasive particle size of 60 nm and concentration of 8 wt.%silica sol,squeeze frequency of 1 Hz,minimum processing gap of 1 mm,squeeze amplitude of 0.5 mm,polishing disk rotational speed of 5 r/min,and a workpiece disk rotational speed of 500r/min,polishing the sapphire wafer for 90 minutes can reduce the average surface roughness from Ra 10 nm to Ra 0.45 nm.and the material removal rate is 3.28 nm/min.Finally,A model of cluster magnetorheological variable gap dynamic pressure finishing material removal rate was established.Combined with the actual processing conditions of cluster magnetorheological variable gap dynamic pressure finishing,the polishing force equation was deduced;the moving state of the workpiece was analyzed to determine the relative shear speed between the polishing pad and the workpiece;The material removal rate model was established based on the classic Preston equation,and the theoretical calculations were consistent with the experimental results. |
