Study On Erosion Mechanisms And Polishing Technology Of Hard-Brittle Materials Machined With Precision Abrasive Waterjet

Abrasive waterjet machining has many advantages, such as high machining versatility, small cutting forces, almost no heat generated and low machining cost. However, there is a few of study on erosion mechanisms and precision polishing technology for hard-brittle materials with abrasive waterjet. Therefore, in this thesis, erosion mechanisms and brittle-ductile transition mechanisms of hard-brittle materials machined with abrasive waterjet are studied, material removal models of fracture and ductile erosion by single abrasive particle are established and a precision abrasive waterjet micromachining system is developed.A study on actions of impact and shear caused by normal erosion and low incidence erosion with waterjet and abrasive waterjet is conducted respectively. It is shown that waterjet erosion has a low erosion capability for hard-brittle materials. The erosion for hard-brittle materials with waterjet includes impact caused by waterjet directly and shear by lateral flow of waterjet. The smaller the incidence angle is, the stronger the shear action is. The erosion capability is enhanced greatly by abrasive particles. The material erosion rate will increase with an increase in the jet pressure. The removal of alumina or silicate glass is achieved by the intersection of intergranular cracks in subsurface. Those intergranular cracks in subsurface are parallel or vertical with the material surface. In low incidence erosion, the main characteristics on the erosion surface include the weakened impact action, the decreased fracture area and the increased shear deformation and cutting action. So, the quality of erosion surface is improved. The impact action increases the material removal rate but decreases the integrality of erosion surface. The shear action plays a secondary role in the removal of material but is in favor of improving the quality of erosion surface.Based on quasi static indentation fracture mechanics, an analysis on erosion process with single abrasive particle is conducted. Erosion kinetic energy critical models of hard-brittle material for radial/median and lateral cracks in erosion process are established. Concepts of fracture erosion resistance Rce and fracture erosion machinable number M are proposed in the present work. Brittle-ductile transition mechanisms for machining hard-brittle materials with abrasive waterjet are studied. Effective erosion kinetic energy models of single abrasive particle in fracture erosion and ductile erosion are established, in which the effect of waterjet velocity distribution, relative hardness of the eroded material and abrasive particle, and the incidence angle are considered. An erosion kinetic energy critical condition of brittle-ductile transition on machining hard-brittle materials with abrasive waterjet is derived and verified by experiments. The experimental results i ndicate t hat plastic indentation, radial/median and lateral cracks will be formed in the erosion process. Micro plastic flow of material is generated in the plastic zone of the indentation, the material damage is caused by radial/median cracks and the material fracture is induced by lateral cracks. In the erosion condition, the main erosion mechanism is the fracture erosion, smaller fracture erosion resistance R_CE and larger fracture erosion machinable number M leads to more prominent fracture erosion and higher erosion rate. Contrarily, the erosion rate is lower. It is proved that fracture erosion resistance R_CE and fracture erosion machinable number M can evaluate directly the erosion resistance and machinability by fracture erosion of materials. The erosion mechanisms of machining hard-brittle materials with abrasive waterjet can be classified into three categories such as fracture erosion, ductile erosion and erosion without any cracks. The erosion areas corresponding to the three mechanisms mentioned above are fracture erosion area, ductile erosion area and erosion area without any cracks. In experiments, it can be found that brittle-ductile transition mechanisms of machining hard-brittle materials with abrasive waterjet do exist and the theoretical predictive and experimental critical value of effective erosion kinetic energy of single abrasive particle on brittle-ductile transition have same magnitude. Under higher erosion kinetic energy conditions, the fracture erosion occurs in material, so, the erosion process has a higher material removal rate and a lower specific energy. Under lower erosion kinetic energy conditions, the ductile erosion occurs, so the erosion process has a lower material removal rate and a higher specific energy. In the critical erosion kinetic energy condition, fracture and ductile erosion occurs simultaneously and some partial ductile erosion is formed.An analysis on erosion by single abrasive particle is conducted using indentation fracture mechanics and elastic mechanics. Based on the effective erosion kinetic energy of single abrasive particle, material removal models of fracture erosion and ductile erosion are established respectively. The experimental results indicate that the prediction values of the models are in good agreement with the experimental data. The material removal is very small in the ductile erosion and it is in favor of improving the surface quality. The elastic stress field model on erosion process by single abrasive particle is established. Using the stress field model, the shear deformation and crack nucleation in plastic zone are analyzed. The analysis results indicate that the incidence angle plays an important role in the shear action for material. In normal erosion process, there is not any shear action induced by erosion and some pile-up of material deformed in the plastic zone around the erosion indention. In low incidence erosion, the stress field deflects along with the erosion direction and the shear stress can cause the bigger shear deformation in the front of the erosion zone which results in the pile-up and chips in the material.A precision abrasive waterjet micromachining system with an accumulator pulse abrasive supply system is developed. The micromachining system includes five subsystems such as pressure generation system, jet system, abrasive supply system, motion and control system and assistant system. The pressure generation system can generate a jet with a pressure of 2-15MPa and the accumulator pulse abrasive supply system can steadily supply micro abrasive particle with a diameter of 10-38um by a mass rate of 0.4-0.2mg per pulse. Using the micromachining system, a group of polishing experiments is conducted for three kinds of hard-brittle materials such as silicate glass, aluminum and silicon nitride. In polishing experiments, the surface qualities of all the eroded workpiece obtain a great improvement. The surface roughness of silicate glass achieves Ra93.195nm in the measured area and Ra40-46nm averaged on the two perpendicular sections. The surface roughness of aluminum achieves Ra131.22nm in measured area and Ra40-50nm averaged on two perpendicular sections. The surface roughness of silicon nitride achieves Ra38.616nm in measured area and Ra26-38nm averaged on two perpendicular sections. The experimental results indicate that the precision abrasive waterjet polishing technology is a promising technique with some advantages for polishing complex shape surface, without any effect induced by polishing plate deformation and without scratch damage.