Surface Morphology Generating Mechanism And Optimization Technology Of Si₃n₄ Ceramics For Rotary Ultrasonic Grinding Machining

Si₃N₄ ceramics has excellent physical and chemical properties,which is considered to be the most promising new materials that used widely under the condition of high temperature aerospace.However,when the machined surface morphology is poor,it is easy to occur strong squeeze friction in contact with the high-speed flow of gas-liquid mixing under the condition of high temperature,and generate lots of aerodynamic heat.Inevitably,it results in friction coefficient increases,strength decreases,and even lead to component failures,affecting its performance and reliability.Thus it is necessary to investigate on the surface morphology generating mechanism and optimization technology for improving the surface quality.The results have important significance and engineering value.Rotary ultrasonic grinding machining is the development trend of Si₃N₄ ceramics complex parts manufacturing.The unique material removal mechanism of Si₃N₄ ceramic leads to produce special processing surface morphology,because Si₃N₄ ceramic has high hardness and low fracture toughness.There is no in-depth investigation on surface morphology generating mechanism and optimization technology,which is important foundation to improve surface quality of Si₃N₄ ceramics.Surface morphology generating mechanism theoretical research for rotary ultrasonic grinding machining will be implemented firstly in the paper.Then surface morphology characteristics and the influence rule are studied.The mapping model between surface morphology and machining parameters is proposed.Finally machining parameters optimization can be realized.Surface morphology generating mechanism should be revealed combination abrasive motion macro mechanics mechanism with microstructure mechanical in machining.So elastoplastic dynamics response characteristics of Si₃N₄ ceramic in rotary ultrasonic grinding machining is presented,with grain movement model and material dynamic characteristics of ultrasonic impact loading.Si₃N₄ ceramics properties change also is investigated.Nonlocal theory is introduced to construct material model,and the small parameter variation under ultrasonic vibration is studied.On this basis,the paper analyzes the propagation conditions of medical cracks and lateral cracks in the nonlocal stress field.Surface morphology formation mechanism of Si₃N₄ ceramic in rotary ultrasonic grinding machining has been analyzed based on single particle simulation and scratches experiments.Si₃N₄ ceramic surface morphology fractal characterizations for rotary ultrasonic grinding machining are studied.Firstly,surface morphology characteristic statistical distribution is analyzed.And the self-similar characteristics of surface morphology are proved through qualitative and quantitative analysis.Differential box-counting and double-blanket coverage methods were used for fractal dimension calculation.However,it is found that the calculation accuracy is low.In order to improve fractal dimension calculation accuracy of gray images,the paper proposes a new fractal dimension calculated method based on the relation of perimeter and volume,combined with the morphology structural characteristics.The calculation accuracy has been tested.The surface morphology feature is divided with fractal dimension.What is more,the influence of spindle speed,cutting depth,feed rate and cutting force on fractal dimension has also been investigated.The effect of ultrasonic vibration diamond tools on surface morphology has been investigation.To investigate on the influence of diamond size,concentration and type of bonding on surface morphology from the perspective of cutting force change,the cutting force mathematical model is established on the basis of rotary ultrasonic grinding mechanism studies.And the model has been proved.The relationship between machining tool parameters and cutting force was studied through mathematical models,and the influence on surface morphology could also be studied.In order to further explain the tool parameters effect on the surface morphology,tool wear mechanism and tool life have been studied.Meanwhile,the surface morphology at the different machining stages is analyzed based on above results.For obtaining machining parameters optimization objective function,the paper has established an BP neural network model between surface morphology and spindle speed,cutting depth,feed rate,diamond size,diamond concentration,bonding type.However,the mode cannot converge to target error,using standard BP algorithm and the improved BP algorithm with momentum term.And the discrete of the predicting results is big using LM algorithm,though it can converge to target error.So an improved BP algorithm based on Powell’s method was proposed to solve the problem.The convergence rate and prediction error have been verified.From the objective function and constraints function,machining parameters are optimized based on genetic algorithm.Thus the fractal dimension and root mean square deviation of surface quality have been reduced.