Research On Cutting Mechanism And Processing Technology Of Abrasive Flow Machining

Abrasive Flow Machining (AFM) technology is a special finishing method, in which the surface work piece is cutted by viscoelastic fluid abrasive. AFM possesses excellent adaptability for finishing of complicated surface and inaccessible regions of a component, such as narrow slits of component, micro-voids, cross holes, shaped holes and cavity and so on, in the meanwhile it can complete deburring and rounding. The AFM can be used in a wide range of materials including metals, ceramic and hard plastics. After a processing with the high efficiency of the machining, surface roughness values can reduce grade 1-3. At present it is used widely in hot flow path deburring polishing of mould industry, nozzles of engine industry, polishing of connecting rod and polishing of parts in textile machinery industry.Based on full understanding of the Abrasive Flow Machining technology in domestic and international research, deep theoretical analysis and experimental research has been carried, which focus on the properties of rheology, cutting mechanism and processing technology. Research contents, the methods used and conclusions are as follows:(1) Composition of fluid abrasive and properties were analyzed by applying rheological theory. It has been explained that abrasive has as high viscoelastic as carrier, temperature and shear rate are the main factors that affect the abrasive performance. Based on the principle of measuring rheological parameters by capillary method, abrasive viscosity, the first normals stress difference, viscosity coefficientis have been measured in experiments. Experiments show that the viscosity of the abrasive and the first normal stress difference will reduce with increasing of temperature, abrasive viscosity will decrease with increasing of shear rate of abrasive and first normal stress difference will increase with increasing of shear stress. Because the high viscosity carrier which closely bonds with the abrasive, overall movement of a large number of abrasive is consistent with carrier flow in the process of fluid abrasive. Considering that the flow of the abrasive accords with fluid characteristics, theoretical research can be taken with the power-law fluid equation as the constitutive equations of the flow.(2) Processed surface morphology shows that in the cutting process there are sliding friction, plowing, rolling, and other forms for movement type of side flow layer abrasive, the number of effective abrasive of edge flow layer involved in cutting is relevant to mixing ratio of abrasive particle, namely, the greater abrasive mixing ratio, the more effective number of particle in the same kind of abrasive involved in cutting. The formula of stress of single grits in work piece surface has been established through mechanical analysis of single grit. In addition to the influence of static water pressure, the influence of first normal stress difference was considered in the power analysis of carrier. This will reflect actual situation of force of grits more accurately. Using removing material formula of single grits, the material removal model that abrasive fluid is cutting work piece has been established, material removal depth coefficient of the abrasive been defined. In addition, measurement formula and measuring methods has been given. by the material removal rate model, it is known that the material removal depth and the surface of the method is to force and abrasive on the surface of the wall is proportional to the slip velocity, and the surface of the work piece is inversely proportional to the hardness. The model of material removal rate shows that material removal depth is proportional to both the normal force and the sliding velocity of abrasive in work piece surface and it is inversely proportional to the surface hardness of the work piece.(3) Theoretical analysis and experimental data indicate that the abrasive in process of flow has shown the performance of "plug flow". In view of the flow form, the flow model of abrasive cutting has been established using fluid flow equation on the basis of considering abrasive elasticity. The formula of the slip velocity, the change of the pressure gradient and the suffered cutting force of work piece in the two basic flow forms namely in tube and slit. At the same time, author has given theoretical analysis for abrasive cutting process. Analysis showed that there is a critical point in wall slip in the process of flowing, they are main factors which influence sliding velocity such as flow rate of abrasive cutting, abrasive viscous coefficient, the cutting factors, the first normal stress difference. With the increase of the flow rate, the slip velocity increases rapidly. The slip velocity gradually increases in the direction of the abrasive flow; with the increase of the viscosity coefficient, the slip velocity increases rapidly when the flow is certain. When the viscosity is big enough, in the whole zone of flow, slip velocity is almost equal to the average velocity of the abrasive; In the wall slip zone, the pressure gradient along the flow direction is no longer a constant, but changes in exponential regularity, which is very different from the case of no wall slip.(4) Cutting force, normal pressure, the material removal depth coefficient and temperature changes in cutting were measured when abrasive machining through the experimental apparatus. The experimental results show that the suffered normal pressure in machining process of abrasive is proportional to cutting force. Based on this, cutting coefficient of abrasive was defined and the principles and methods of measurement of the cutting coefficient were given and were measured by experiment. Moreover, abrasive machining process was studied in combination of established theoretical flow model and the actual instance in processing. Result shows that the pressure gradient can decreases by controlling inlet pressure of abrasive when abrasive is flowing in order to reduce the taper of the surface; The results also show that the rational design of process parameters as slit height and tube radius can improve the dimensional accuracy of the work piece through the use of abrasive with different viscosity. From the polishing effect, it was showed that there is a critical value in surface roughness after the same kinds of abrasive are machined; the surface roughness almost does not change with the increase of the size of the workpiece when reaching a critical value; and use of abrasive grits with high mixed ratio can effectively improve the processing efficiency of the abrasive flow. By the impact of temperature on the processing effect, it is known that the viscosity of the abrasive drastically reduces at high temperatures and so as to lose wall slip effect and lose cutting effect on the work piece. Based on the above, it is clear that the high-viscosity elastic of abrasive is a prerequisite of possessing cutting effect.