The Coupling Of The Thermal And Mechanical Influence On The Surface Integrity By The High Speed Outer Cylindrical Grinding

The structure ceramics own unique characteristics, such as high temperature re-sistant, anti-corrosion and wear and high strength-weight ratio. However, its hardness and difficult to machining restrict its applications in the area of military defense, aero-space and high-speed rail. The technology of high speed grinding is assumed to be a solution which can be used for the high efficiency and high precision machining of difficult-to-cut materials in various industries. The extensive elevated grinding veloci-ty will lead to a high strain rate in the contact area and make the workpiece material to experience the phase transformation. In the academic field, even the top engineers are still lack of knowledge on the topic of the mechanism of brittle material behavior un-der high strain rate and the optimal process parameters to obtain non-damage machin-ing surface. For rotating components, such as ceramic shaft or bearing rings, the measurement technologies for the determination of the temperature and forces during grinding operation are still left blank, which inversely prevents the understanding of thermal and mechanical coupling mechanism by high speed grinding.In order to solve the aforementioned bottle-necks, this dissertation carried out re-searches on the topic of the coupling of the thermal and mechanical influence on the surface integrity by the high speed grinding. The structure ceramic silicon carbide (SiC) is selected as the workpiece material and the whole grinding operations are fo-cused on rotating component such as ceramic shaft. The research highlights and con-clusions are summarized as below:1.The dissertation proposed an idea on the micro-crack toughening mechanism by high strain rate induced in the high speed grinding. A finite element method (FEM) model is used to reveal the fracture behavior under the coupling of thermal and me-chanical loads, combining the construction law, the equation of state (EOS) and the material damage evolution. The simulation results show that the micro-crack induced by high strain rate and the softening behavior under high temperature is the essence of toughening of SiC. This is the so called thermal and mechanical coupling effect by the high speed grinding. Also the ground subsurface damage observation by scanning electron microscope (SEM) in the physical experiment proved the simulation results quite well. The toughening mechanism raised above overcomes the shortcomings of traditional critical depth of cut theory, which supposes that under a certain static depth of cut, no crack will initiated without concerning the effect of strain rate and thermal softening influence.2. In engineering, it is lack of a quantitative index to evaluate the grinding perfor-mance of structure ceramics from the aspects of material removal ability and surface damage level. Therefore, by the analysis of principal stresses both in the direction of parallel and perpendicular to the grinding velocity, this dissertation proposed a so called brittle grind ability index Gμ. Through high speed grinding experiments and stress calculation, a Gμ-vs diagram is got, and exhibits that the index Gμ is nearly constant when vs=20-60m/s. While it increases dramatically since vs=80m/s and1.2-1.5times when the grinding velocity reached140m/s. This can be concluded that the micro-crack induced by high strain rate and ceramic softening by elevated grinding temperature is the core reason for the toughening behavior of SiC during high speed grinding operation.3. In this dissertation, a new grinding temperature and force apparatus is invented for the outer cylindrical grinding. Because of the rotation movement of the workpiece during the outer grinding process, it is quite difficult to mount the sensor and acquisi-tion system. Also there is no precedent to measure the temperature and force simulta-neously by outer grinding. This invention successfully overcomes the above men-tioned obstacle and gets the thermal and mechanical data during one stroke grinding process, which makes the measurement much more significative for the analysis of the coupling of thermal and mechanical loading. Additionally, the dissection work-piece can also be used for the subsurface damage observation and avoid the second damage, that is to cut the workpiece after grinding.4. By the measurement of surface temperature during grinding process, this dis-sertation proposed a so called RW-WST method for the calculation of heat flux pro- portion penetrated into the workpiece. This method overcomes the shortcoming of the widely used method, which needs the assumptions of heat flux profile and initial pro-portion. Besides, the traditional method requires continuously iteration in order to obtain a relatively accurate RW. RW-WST method can exhibits the real heat flux profile which loads in the contact area. A further analysis on the heat transfer microscope mechanism reveals the fundamental relationship between the grinding speed, the heat transfer coefficient and heat flux proportion RW.5. This paper investigated the effects of grinding process parameters on the sur-face roughness and residual stress. The influence of unconventional transient stress field and strong thermal field on the contact area during high speed grinding was stud-ied. The material upheaval behavior of α-phase (4H,6H hexagonal) from β-phase (3C cubic) transformation of SiC was revealed. The results of these experiments show that the surface roughness is associated with multiple factors, such as the average effective grinding edge cross-sectional area, cooling fluid, abrasive fineness, grinding wheel speed, workpiece speed. The greatest impact among them is grinding wheel speed. With the increments of chip thickness and single grain load on workpiece surface, the residual compressive stress in the grinding contact area increases. Residual stress model has been established.