Study On Techniques In Electrical Discharge Diamond Grinding Of RB-SIC Ceramic

Due to its high specific stiffness and thermal stability,reaction-bonded silicon carbide(RB-SiC)ceramics can meet the demand for fabrication of large space optical mirror with complex lightweight structure and negligible dimensional changes.Therefore,RB-SiC has become the best candidate material of large lightweight mirror,which has been reported to successfully apply in fabrication for green body of various space-based mirror and ground-based mirror.According to the requirment of space optics for high image quality,these mirrors are usually needed to machine into spherical or aspheric surfaces.However,the high hardness and brittleness of RB-SiC ceramics make that the resinoid-bonded diamond wheel is needed in grinding of the aspheric surface because of its excellent self-sharpness.As a result,low efficiency and serious wheel wear come into being.To solve these problems,metal-bonded diamond wheel is expected to apply in grinding of the aspheric surface of SiC mirror.Nevertheless,the in-process dressing of metal-bonded diamond wheel is waiting to solve when it is applied to machine large-scale surface.Therefore,a high efficient grinding technology for aspheric surface is develop to improve machining efficiency and sharp diamond wheel.In the paper,a hybrid process termed as electrical discharge diamond grinding(EDDG)was applied in grinding of RB-SiC ceramics for the first time in consideration of its electrical conductivity.Based on the variations of mechanical property influenced by temperature,the material removal mechanism,machined surface quality,and damage were investigated under the interaction between electrical discharge machining(EDM)and diamond grinding.The mechanism and characteristic of diamond wheel wear were also investigated in EDDG of RB-SiC ceramics.At last,multiple-response optimization of EDDG process was conducted using gray relational analysis.The thesis can provide theoretical guidance for application of EDDG technology in aspheric surface grinding of large-aperture SiC mirror.And it is also significant to enrich the basic theory of the hybrid process in precision grinding of RB-SiC ceramics.By a combination of the analysis of EDDG process and the finite element simulation by COMSOL,the paper firstly invsitaged the temperature distribution induced by EDM and its influence caused by discharge energy in RB-SiC matrix.According to the simulated results,the indentation tests heated by laser were conducted to explore the influence of different temperature on hardness,elastic modulus and fracture toughness of RB-SiC ceramics.The results revealed that the high temperature changed the load range of elastic recovery in RB-SiC ceramics.The temperature facilicated plastic deformation of RB-SiC ceramics.As a result,the hardness and elastic modulus decreased with increase of temperature at a same degree.Moreover,the temperature also changed fracture mode from transgranular to intergranular,resulting in enhanced toughness of RB-SiC ceramics.The fracture toughness increased with increase of temperature.Whereas it decreased at excessive temperature because of excessive softening of Si phase and thermal stress.According to the study on mechanical property of RB-SiC ceramics at different temperature,the results of scratching test heated by laser further indicated the influence of temperature on ductile or brittle material removal regime of RB-SiC ceramics,making it ductile remove at a deeper depth.The varied critical depth of ductile-to-brittle transition was mainly influenced by variation of fracture toughness at different temperature.Moreover,the grinding mechanism of RB-SiC ceramics induced by EDDG was studied.It revealed that the EDM melted and evaporated the Si phase while decomposed the SiC phase of the RB-SiC ceramics.The material ductile removal of RB-SiC ceramics induced by diamond grinding was apparently facilicated by the influence of temperature caused by EDM.The material removal induced by the interaction between EDM and diamond grinding led to the surface formation in EDDG of RB-SiC ceramics.The influence factors of surface formation were also discussed in detail,providing guidance for investigation of machined surface quality and processing parameter optimization.The machined surface quality of RB-SiC ceramics was investigated at different polarities and grit size of diamond wheel according to the analysis of surface formation in EDDG process.The results revealed that low surface roughness,surface damage,and subsurface damage were obtained at positive-wheel polarity.The grit size had an influence on the dominant material removal regime induced by EDM or diamond grinding.Surface quality was improved by a suitable diamond girt size.Moreover,the analysis of microstructure of SiC phase revealed the interaction between EDM and diamond grinding on the formation mechanism of surface and subsurface damage in EDDG of RB-SiC ceramics.The results showed that the temperature induced by EDM and the mechanical pressure induced by diamond grinding were responsible for phase transformation in subsurface of RB-SiC ceramics.The experimental investigation was conducted to explore the mechanism and characteristic of metal-bonded wheel wear in EDDG of RB-SiC ceramics.With wheel wear test heated by laser,wear regime of diamond grits and strength metal bond influenced by different temperature were discussed.As a comparsion,the mechanism of wheel wear in EDDG of RB-SiC ceramics revealed that the discharge-induced thermal impact efficiently removed adhension of diamond grit induced by high temperature and enhanced its grinding performance.The main wear mechansim was fracture and pullout for diamond grits and erosion for iron bond.Compared with conventional grinding,there were only initial stage and stable stage for iron-bonded wheel wear in EDDG of RB-SiC ceramics.At the stable stage,the material removal rate(MRR)increased by 13%-23%,whereas tangential and normal forces decreased by60.2% and 61.6%,respectively.Therefore,an in-process dressing technology based on grinding process was developed for metal-bonded wheel.A verification experiment revealed the excellent dressing effect of this technology by calculation of diamond grit density.By orthogonal experimental design and grey relational analysis,the processing parameters were optimized to obtain good grinding performance criteria including surface roughness,MRR,wheel wear rate(WWR),and normal force.The results of orthogonal experiment showed the different influences of parameters on grinding performance criteria.Therefore,the weight of grinding performance criteria was added in gray relational analysis of parameter optimization according to the aim of the paper.The optimal parameter combination was then obtained and verified.The results showed that,compared with orthogonal experiment and conventional grinding,more excellent grinding performance was obtained under the optimal parameter combination in EDDG of RB-SiC ceramics.