Research On Subsurface Damages In Ultrasonic Assisted Grinding Of BK7 Optical Glass

Optical glass materials are widely used in related fields due to the superior physical,chemical and optical properties.However,the typical characteristics of high brittleness and low fracture toughness result in poor machinability on this kind of hard and brittle materials.It would be disadvantage for machining quality and efficiency in conventional grinding(CG)process,which would easily induce damage such as micro cracks and residual stress in the subsurface of optical glass components.In consequence,researching on improving the machining quality in subsurface is helpful to reduce the later ultraprecision machining time.Ultrasonic vibration assisted grinding(UAG)technology has great advantages of reducing the average cutting force and micro cracks in process as well as improving the materials removal rate.However,there are not some definite theories about the ultrasonic vibration influence on propagation process of subsurface micro cracks in optical glass and the formation reasons and prediction of subsurface damage in optical glass under ultrasonic vibration effects are not clarified.Therefore,studying on subsurface damage of optical glass in ultrasonic vibration assisted grinding has theoretical significance and practical value to realize high precision and low damage in process.The foundation in this research is the analysis of the effects of ultrasonic vibration on propagation of subsurface micro cracks in optical glass.This paper stated the ultrasonic vibration effects on the trajectory and kinematics of a single abrasive grain.Through the quasi-static indentation experiment,the model which showed the internal stress fields distribution in BK7 optical glass under quasi-static loads was established and was used to analyze the mechanism of micro cracks nucleation and propagation driven by these stress fields.The results showed that the main reasons for the decline of micro cracks propagation depth were the median cracks shielding effect resulted from the overlap trajectories of abrasive grains and ultrasonic vibration.When analyzing the influences of impact loads on optical glass brittle fracture through rotation scratching experiments with ultrasonic vibration,the results showed that the dynamic mechanical properties of BK7 optical glass had changed due to the inertia force of abrasive grains,which made the micro cracks or inner defect nucleate more easily.In the same time the micro cracks closure which resulted in brittle fracture had become the main reason for the removal of BK7 optical glass.These studies provided the theoretical foundation for researching the optical glass subsurface damage formation laws and depth prediction.In ultrasonic vibration assisted grinding process,the existences of multiple abrasive grains and ultrasonic vibration had great influence on the subsurface damage degree and morphology.This paper carried out ultrasonic vibration assisted grinding experiments,and obtained the morphology of subsurface micro cracks in machined BK7 optical glass by using scanning electron microscope.The analysis of the features and mechanism of subsurface cracks under multiple abrasive grains and loads showed that the abrasive grains loading direction and materials dynamic mechanical properties changes caused the morphology of subsurface micro cracks changing.Based on that result,the grinding parameters influences on the maximum depth of subsurface cracks were found,which would helpful for build the prediction model for the maximum depth of subsurface cracks.The effective prediction on the maximum depth of subsurface cracks would be the key to reduce the time in subsequent polishing process,also improve process efficiency and the quality of grinding.Based on the above research this paper used least square support vector machine(LS-SVM)and Gaussian process regression(GPR)method respectively to establish the prediction models for the maximum depth of subsurface cracks of BK7 optical glass in ultrasonic vibration assisted grinding.The prediction models were both verified through grinding machining experiments,and the prediction accuracy of the models were compared,which provided the basis for the optimization of the ultrasonic vibration grinding process parameters of optical glass materials.