| Abrasive waterjet (AWJ) polishing and lapping is a newly developed technology in recent years and the research is still at the preliminary stage. In this disseration, the erosion morphology of an oblique AWJ is studied and the pressure field of AWJ in the erosion interface is simulated and analyzed. The curve surface polishing mechanisms of hard-brittle materials eroded by AWJ is analyzed. Moreover, the polished surface roughness model is established.In the case of plane and curve surface eroded by oblique AWJ, the effect of various factors on the erosion zone morphology is investigated. According to the erosion experiment of the plane surface at shallow jet incidence angle, it has been found that the erosion zone was consisted of three parts such as high pressure impacting zone formed by the normal component of jet pressure which is vertical to the workpiece surface, shear zone formed by the tangential componentof jet pressure which is parallel to the tangent of the workpiece surface, and lateral flow zone. It can be seen from the experiments that the area of erosion zone decreases as the pressure and jet incidence angle increases, while decreases with a decrease in the abrasive size. As the increasing of pressure and the jet incidence angle, reducing of abrasive size, the transition from ductile to brittle material removal mechanism is occurred. For the plane surface eroded by the AWJ with a certain oblique angle, the erosion depth increases with an increase in the pressure and the jet incidence angle, and inceases with a decrease in the abrasive size. The asymmetry structure of erosion zone exhibits gradually as the erosion depth increases. From the rim of the erosion zone to the incident line of the jet, the erosion depth is almost becomes larger and the maximum erosion depth is near the bottom of the erosion zone. For the curve surface eroded by the AWJ with a certain oblique angle, the constitution of erosion eroded by theAWJ with a certain oblique angle, the morphology of erosion zone is different from that of the plane surface. The lateral flow is inclined to separating from the workpiece surface once the convex curve surface is eroded by the AWJ with a certain oblique angle, and hence the shear action on the workpicece surface is reduced. In the experiment on the convex curve surface, the erosion depth increases with an increase in the pressure and the jet incidence angle, while inceases with a decrease in the abrasive size. Similar to the case of plane surface, as the increasing of pressure and the jet incidence angle, reducing of abrasive size, the transition from ductile to brittle material removal mechanism is also occurred.Using the FLUENT software, the two-dimensional simulation is carried out for jet flow field inside and outside the nozzle, as well as the pressure field of AWJ in the erosion interface during the erosion process of the plane and curve surface. The multiphase fluid model is selected for the two-dimensional simulation of the jet flow field inside the nozzle with the pressure of 80,100, 120 and 140MPa repectively. The waterjet velocity outside the nozzle and the velocity of abrasive are derived. According to the two-dimensional simulation of free jet outside the nozzle, it is verified that the jet field along with its flow direction consists of three regions such as the initial region, main region and the dissipated region. In the initial region, the jet velocity is equal to that at the nozzle exit and the jet kinetic energy is high. It is suitable to the machining process with large material removal. In the main region, there is some jet spreading and the jet kinetic energy decreases. The jet in this region is suitable for finishing with small material removal. In the dissipated region, the jet has insufficient processing capacity. The dynamic pressure field at the erosion zone interface is simulated for plane surface erosion. As for the plane surface eroded by the jet, the jet incidence angle is less than 90°, the erosion zone consists of three parts such as high pressure impacting zone formed by the normal component of jet pressure which is vertical to the workpiece surface, lateral flow zone and the shear zone formed by the tangential component of jet pressure. The cross section of the erosion zone is asymmetry. The erosion area decreases with an increase in the jet incidence angle, while pressure in the erosion zone increases with an increase in the jet incidence angle. With the jet incidence angle of 90°, the erosion zone consists of two parts that is high pressure zone and lateral flow zone, and the erosion zone structure is symmetrical. The pressure field at the erosion zone interface is simulated for curve surface erosion. The cylinder convex curve surface with an outer radius of 35mm is employed for the simulation. Therefore the normal component of the jet pressure is effective for machining. However, the tangential component has no erosion effect, since the jet is separated from the workpiece surface at the beginning of erosion process. As a result, the erosion zone consists of two parts that is high pressure zone and lateral flow zone. The morphology of erosion zone is symmetry and the erosion area is independent on the jet incidence angle. However, the pressure of erosion zone increases with an increase in the jet incidence angle.The material removal mechanisms of hard-brittle materials eroded by oblique AWJ are investigated. The velocity field and abrasive erosion force of AWJ are analyzed. The surface generating mechanisms of oblique erosion is studied, and it is verified by the experiments. Based on the indentation fracture mechanics model, the brittle fracture and approximate cutting mechanisms of the hard-brittle materials eroded by high velocity abrasive are analyzed. The brittle fracture removal mechanisms are mainly based on the propagation of lateral cracks, while the radial cracks will lead to the subsurface damage. In addition, the workpiece material which is contacted with the front part of abrasive will be fractured. The workpiece is removed by the plastic flow under the shear action. The velocity field model of AWJ is established and simulated. The main region of the jet is suitable for the machining process with little material removal such as polishing. The jet velocity calculation models are developed. The erosion process of workpiece surface eroded by AWJ is studied and the models of normal and tangential component of erosion force are developed. The surface generating mechanisms eroded by oblique AWJ is analyzed. The effect of oblique erosion by AWJ on the workpiece surface can be divided into five steps such as elastic extruded scratching, ploughing, cutting, rebound ploughing and rebounding scatching. The surface generating mechanisms of convex and concave curve surfaces eroded by AWJ are analyzed. It is demonstrated that the profile characteristics difference between convex and concave curve surface has unsignificant effect on the normal impacting, while it has significant effect on the tangential shearing action. The shearing action will be reduced by the convex curve surface, while it will be strengthened by the concave curve surface. The AWJ polishing experiments are carried out on the hard-brittle materials aluminum and monocrystalline quartz. The above theoretical analysis conclusions are verified by the experiment results.The surface roughness model of plane and curve surface for AWJ polishing are developed and verified experimentally. For AWJ surface erosion, the erosion depth increases with an increase in the pressure, abrasive size and jet incidence angle. On the contrary, the erosion area decreases with an increase in the pressure, abrasive size and jet incidence angle. Under higher normal component of the jet energy, the erosion depth increases and the material is more inclined to remove by the brittle mode, as well as the cross section of the erosion zone is asymmetry. In the AWJ polishing, the material is removal in ductile mode and the material removal is little, as well as the erosion depth is small. The workpiece cross section profile expression is developed. Moreover the surface roughness model of ideal plane and practical surface is developed. Based on the study of plane surface polished by AWJ, the surface roughness model of curve surface polished by AWJ is developed. Then the polishing surface generating mechanisms are analyzed. |
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