| How to achieve high efficiency and high quality polishing of workpiece surface of difficult-to-process materials is still a major problem in the field of ultra-precision machining.To solve the above problems,this thesis proposes an efficient fluid Polishing method based on non-Newtonian polishing fluid--Weak Magnetorheology Strengthing Thickening Polishing(WMSTP).By combining the theory of magnetorheology,fluid dynamics and surface machining,the weak magnetorheology strengthing thickening polishing technology is based on the flexible fluid polishing mode formed by the weak magnetorheology strengthing thickening effect to realize the high efficiency adaptive ultra-precision fluid polishing of the workpiece surface of difficult-to-process materials.In this thesis,the constitutive characterization method of weak magnetorheology strengthing thickening polishing fluid,material removal mechanism during surface machining,micro-surface morphology evolution mechanism and process implementation method were studied,and the WMSTP processing platform and process optimization strategy were independently established.It lays a solid research foundation in the field of non-Newtonian fluid polishing technology of difficult material surface.The main research contents of this thesis are as follows:(1)The preparation and rheological properties of weak magnetorheology strengthing thickening polishing fluid were analyzed.Polishing liquid with different thickening phases,mass fraction of magnetic particles and abrasive particles was prepared,and its rheological properties were tested and analyzed.The influence of magnetic field on rheological properties of polishing liquid was studied,and the corresponding strengthening and thickening effects of weak magnetorheology were analyzed.Based on Gaussian function,a multi-modal fitting constitutive method was proposed to characterize the polishing fluid,and a high goodness of fitting constitutive equation was established.The results show that the method can achieve high precision fitting characterization of polishing liquid with different preparation schemes under different magnetic field conditions,and the R-square value of goodness of fit is more than 96%.(2)The process design and feasibility of WMSTP method was studied.The experimental platform of WMSTP was built,and the technological parameters of WMSTP were designed based on the rheological characteristics of WMSTPF.The feasibility of the WMSTP method was verified by comparative experiments.The experimental results show that WMSTP method has good surface quality and higher material removal efficiency,with the material removal rate up to 7.82 μm /h,which is156% higher than the traditional STP method.It is a fluid polishing method that can effectively realize ultra-precision machining of difficult material surfaces.(3)The removal mechanism of WMSTP material and the evolution mechanism of microscopic surface morphology were studied.Combined with the constitutive equation of polishing fluid,the numerical distributions of shear rate,shear stress and velocity of polishing fluid in effective polishing region of hemispherical surface were solved.Based on the Preston equation of fluid polishing,the prediction model of material removal rate of WMSTP was established.Based on the material removal mechanism of abrasive collision hypothesis,the surface roughness prediction model of the machined surface was established.Based on the finite element numerical model of polishing flow field,the quality of machined surface can be predicted effectively.The experimental results show that the average relative error of the material removal prediction model is 4.45%.The average relative error of surface roughness prediction method is as low as 8.3%.During the polishing process,the surface quality improvement rate reached 93.9%,the surface uniformity improvement rate reached 73.6%,and the surface roughness Sa was 27.5nm.(4)The analysis and optimization of WMSTP process strategy was studied.Based on the comprehensive test results,the two-factor covariance analysis was used to optimize the polishing process parameters.Considering the evaluation results of workpiece hemispherical surface in machining surface quality,surface quality improvement efficiency and surface uniformity improvement rate,the optimization process strategy of WMSTP method was obtained: WMSTP was processed for 30 min with the combined process parameters of 140 rpm of the polishing pool and 2500 rpm of the workpiece,and then 30 min with the combined process parameters of 50 rpm of the polishing pool and1000 rpm of the workpiece.The experimental results show that the optimized process parameter strategy can obtain better polishing quality.The surface roughness threshold of the effective polishing area was reduced to 69 nm,and the optimal surface quality reached Sa 16.9 nm,which were 38.1% and 38.5% lower than before optimization,respectively. |
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