Fundamental Research On Shear-thickening Polishing Method

The high efficiency, high quality and low cost machining of target workpiece is one of the research hotspots in precision or ultra-precision machining fields. In this thesis, a new ultra-precision machining method- shear-thickening polishing（STP） utilizing the shear thickening mechanism of non-Newtonian power-law fluid based slurry is proposed: A ―flexible fixed abrasive tool‖ is formed due to the strengthening of constraint force of abrasive particles under the conditions of polishing slurry with shear-thickening rheological properties, the increasing of contact area with workpiece and ―tool‖ for having good surface adaptability, which can achieve a high efficiency and precision polishing of workpiece surface.Aiming at investigating the material removal mechanism and technology process of STP method, and centering on exploring some key technology problems including preparation of performance polishing slurry and rheological properties, material removal rate and surface quality, high efficiency and precision machining theory system and processing of STP method with i ndependent intellectual property rights is built. Specific studies include the following aspects:（1） The feasibility of STP machining method is studied. The main factors of the machining process are analyzed, and the effectiveness of the machining mechanism is verified by CFD simulation of the polishing slurry（a non-Newtonian power-law fluid） with shear thickening effect flow field using FLUENT software. Under this basis, the material removal rate（MRR） of GCr15 bearing steel is approximately 10 times larger than that of the control group（slurry with similar initial viscosity）, and the average surface roughness is reduced from Ra 101.33 nm to Ra 7.80 nm, which is far from the control group value（only reduced to Ra 96.81 nm）. The feasibility of STP machining mechanism is proved, which provides the theoretical foundation of further research.（2） The preparation technology and properties of shear-thickening polishing slurry（STPS） are researched. Based on achieving the mechanism of STP method, environment-friendly and suitable shear-thickening based fluid（STBF） with easily obtained raw material is determined, then whose process flow is established. A kind of Al₂O₃-STPS has been prepared by the above preparation technology, which shows shear thinning and reversible shear thickening behavior. The Al₂O₃ ―cluster‖ forms and disappears with varying of the shear stress. Thus, the rheological properties of the polishing slurry can be realized for STP technology.（3） The rheological behavior of STPS is studied, and the numerical calculations of shear-thickening condition and every viscous parameter are determined. It can be found that STPS exhibits three distinct viscosity zones with increase of shear rate: shear-thinning Zone I, shear-thickening Zone II and shear-thinning Zone III. The threshold of shear-thickening conditions of STP is the shear rate g varying from 10 s-1 to 1000 s-1, especially g varying from 200 s-1 to 1000 s-1 in the Zone II, and it may achieve better shear-thickening effect for high efficiency polishing of workpiece.（4） The material removal mechanism of STP process is studied theoretically. Based on the Preston formula, fluid dynamics and shear thickening mechanism, the MRR model is established. The difference between theoretical and experimental MRR results is 6.12%, so the MRR model is verified by experiments. Experimental study on MRR and average surface roughness Ra shows that: MRR becomes large with the increasing of polishing velocity U, the abrasive concentration w%, and with the decrease of grain size d. Surface roughness also improves with an increase of polishing velocity U, abrasive concentration w%, and with the decrease of grain size d. However, the grain size d has slight influence on final variation trend of average surface roughness.（5） The surface roughness predicted model and the surface evolution of the workpiece in STP process are studied. Based on the calculation of the Brinell hardness number（BHN） of the workpiece, the shear-thickening mechanism and plastic indentation on abrasive wear theory, the surface roughness predict ed model of STP is established. Results from the theoretical calculation based on the surface roughness predicted model are closed to that from experimental data, and the maximal difference is only 12.02%. The model was verified by experiments. The established model is helpful to control the surface quality of the plastic workpieces in the actual processing. In addition, In addition, the surface evolution of Cr12Mo1V1 is studied. Especially, the extension research on processing characteristics of single crystal silicon is explored, then the high efficiency and high surface quality processing of single crystal silicon wafer by STP is demonstrated, which provides the basis for predicting the surface roughness of brittle materials.（6） The STP Processing technology of STP is researched. By using orthogonal experiment of Taguchi method, the optimized parameters of the processing conditions are obtained: polishing speed of 300 rpm, abrasive size 4000 #, abrasive concentration of 23 wt % as the best combinations of processing parameters in STP. In the STP test of Cr12Mo1V1（die steel）, the MRR is up to 13.69 m/h, and the surface roughness is reduced from Ra 105.95 nm to Ra 5.1 nm with 0.5 h of processing. STP and both-sides cylindrical lapping combined chemical mechanical polishing（CMP） of cylindrical workpiece are studied comparatively. Results show that The MRR of STP is 2 times more than that of both-sides cylindrical lapping, and is 4 times than that of CMP. The surface roughness of the workpiece is decreased from 125 nm to Ra 5.1 nm after STP process, while the surface roughness of the workpiece is decreased from Ra 76 nm to Ra 16.6 nm by using the both-sides lapping combined CMP. Under the same conditions, the former STP method can obtain the cylindrical workpiece with roundness error RONt 0.376 mm, but the latter both-sides lapping combined CMP method only achieve roundness error RONt 0.390 mm. This shows that STP has good advantages for high efficiency and surface quality polishing of workpiece. For STP of Si₃N₄ ceramics, the surface roughness is reduced from Ra 110.90 nm to 7.20 nm with 150 minutes of processing, and the MRR reaches 2.04 m/h. In addition, On the STP-1 experimental device, the research on the STP machining of the parabolic mirror（HPM38 die steel） with the small polishing tool head is carried out. The surface roughness of workpices is up to Ra 4.73 nm after 120 minutes of processing, and the final form error reaches PV 0.86 mm. Results show that STP is an effective machining method for hard brittle materials polishing, and is the potential and promising processing method for ultra-precision polishing of complex curved surfaces.These results above indicate that the new proposed shear thickening polishing（STP） based on the shear-thickening effect of non-Newtonian power-law fluid, can achieve high efficiency, high surface quality and low cost machining method.