Study On The Surface Integrity And Subsurface Damage In High Speed Grinding Of Hard Brittle Materials

Hard brittle materials such as optical glass, sapphire and monocrystalline silicon are widely used in defense and civil fields. However, due to its high hardness and low toughness, defects such as crack and residual stress are easy to produce during the conventional processing of hard brittle materials. Among the numerous defects, the most critical issue is the large subsurface damage after machining, which should be removed by the subsequent grinding and polishing. As a result, the production cycle is prolonged and the processing efficiency is reduced.High and ultrahigh speed grinding is the leap of grinding techniques with small grinding force, low grinding temperature and high material removal rate, etc. Furthermore, with the integration of rough and finish machining, high and ultrahigh speed grinding is also a really efficient way to obtain good surface integrity and subsurface quality in hard brittle materials grinding. On the other hand, hi gh speed grinding is complex engineering problem including multidisciplinary field such as physics, mechanics and tribology. As a highly nonlinear and coupled thermomechanical process, high-speed grinding is essentially affected by many factors. Therefore, comparing with the traditional grinding mechanism, there are still lots of challenges need to be figured out.In order to obtain a good machined quality with high efficient and low damage, the interaction between abrasive grain and workpiece in high speed grinding was systematically investigated. Firstly, the effects of grinding parameters on distribution and scatter of residual stress were analyzed in detail using the 3D finite element method. Then, taking into account the wheel spindle vibration in high-speed precision grinding, a predictive model for subsurface crack depth was established to study the correlation between surface roughness and subsurface damage. Besides, based on t he force model in micro-grinding and the definition of specific grinding energy, the brittle-ductile transition mechanism in elliptical ultrasonic assisted grinding of hard brittle materials was revealed. At the end, in order to provide guidance for the development of controlled spalling technology and self-healing mechanism which enabled the reuse of cracking substrate, a thermal assisted grinding model was proposed to discuss the growth of subsurface crack. The main achievements and innovations of this paper are as following:(1) A three-dimensional finite element model considering the grain’s rotational motion and the thermomechanical coupling effect is proposed to analyze the residual stress induced in single grit-workpiece interaction during high speed grinding. Comparing the various stress and temperature histories inside and outside the grinding zone, the complex stress state and temperature distribution are revealed. The effects of grinding speed and grinding depth on residual stress scatter are analy zed in detail using the mathematical statistics method. Both a smaller grinding depth and a higher grinding speed are found to be beneficial for a better consistency of residual stress. The obtained result can give advice to the optimization of grinding parameters and further predict the reliability of a ground component which is of important engineering value.(2) Based on the grinding kinematics characteristic and indentation fracture mechanics, a new predictive model for surface roughness and subsurface damage is established by considering the wheel spindle vibration effect in micro grinding process. By comparison of the theoretical analysis and the experimental data, the variation of surface roughness and subsurface d amage with grinding parameters are obtained. The influence of wheel spindle vibration on surface roughness and surface crack depth are further discussed. Results show that both the surface roughness and subsurface damage increase with the increased workpiece speed and vibration amplitude resulting in bad surface and subsurface quality. On the other hand, the quality of ground surface and subsurface can be improved by increasing the grinding speed and decreasing the vibration frequency. Since the vibration frequency usually equals to the rotation frequency of wheel spindle, a high enough stiffness of wheel spindle is suggested to acquire a high natural frequency in order to avoid the happen of resonance in grinding and finally achieve a small vibration amplitude. From the fitting result, the subsurface damage depth is found to be nonlinear monotone increasing with surface roughness. The obtained results can be used for improving the prediction accuracy of surface and subsurface damage and as guidance for the design and manufacturing of wheel spindle.(3) Based on the force model in micro-grinding and the definition of specific grinding energy, a predictive model for the critical undeforme d chip thickness is proposed to investigate the brittle-ductile transition mechanism in elliptical ultrasonic assisted grinding of hard brittle materials. The effects of grinding speed and ultrasonic vibration parameters on grinding force and specific grin ding energy are analyzed in detail. Results show that t he axial vibration amplitude leads to a slight reduction in grinding force, whereas the vertical vibration amplitude results in a significant reduction of grinding force. The increase of wheel speed and ultrasonic vibration frequency can reduce the grinding force as well. Meanwhile, the grinding force ratio remains relatively steady fluctuating from 1.37 to 1.56. Additionally, the critical undeformed chip thickness increases with the increasing grinding speed, axial vibration amplitude and ultrasonic vibration frequency, while firstly increases and then decreases with the increasing vertical vibration amplitude. Especially, a ductile-mode grinding in micron-level can be achieved when the axial vibration amplitude is above 3μm and a totally brittle fracture occurs when the axial vibration doesn’t exist.(4) Several theoretical models for growth of subsurface crack in grinding are established by considering factors such as plastic deformation and surface point heat source. The impacts of grinding parameters, grain geometry and thermal parameters on stress intensity factors near subsurface crack tip are analyzed in detail. Results show that the main fracture mode for median crack induced in brittle material is opening rather than shear. A small grinding speed, a sharp large tool, a large table speed and grinding depth will lead to strong anti-shielding effect on mode I crack propagation and strong shielding effect on mode II crack propagation. Additionally, whether the median crack propagation is inhibited or promoted by the heat source mainly depends on the relative positions between thermal loading and mechanical loading. On one hand, the stronger heat source in front of the abrasive grain can inhibit the growth of subsurface crack. On the other hand, if the heat source be situated behind the abrasive grain, a proper matching of heat intensity and indenter force should be emphasized in order to make the growth of subsurface crack well controlled. The study on subsurface crack propagation can provide guidance for the development of controlled spalling technology and self-healing mechanism which enabled the reuse of cracking substrate.