The Constitutive Model Of KDP Crystal And Modeling Analysis On The Ultra-precision Flycutting Of The Material

Potassium Dihydrogen Phosphate?KH₂PO₄,KDP?crystal possesses superior nonlinear optical properties,and it is the only artificial crystal which can grow up to 500 mm in dimension.Thus,it plays an irreplaceable role in the laser path of the Inertial Confinement Fusion System.At present,the most general method to produce large aperture K DP crystal which satisfies the requirements of high energy laser system is single point diamond flycutting?SPDF?.However,the flycutting process utilized at home now is of low efficiency,and it is still a challenge to manufacture qualified KDP crystal efficiently and stablely.The deeper reason of the problem is that it is still lack of knowledge about the mechanical property and the cutting mechanism of the crystal.Besides,understandings about the surface forming process and the reason for degrading the surface quality are rather limited.Thus,this project is going to figure out the fundamental issues in processing KDP crystal,such as the anisotropic cutting mechanism and the surface forming process,from the point of view of the mechanical property of the material.Based on that,suggestions for optimizing the flycutting process have been proposed.The results of this research work is of great significance in optimizing the flycutting parameters,improving the surface quality and the increasing the machining efficiency in flycutting of KDP crystal.Firstly,a new constitutive model which is more suitable for describing the mechanical properties of KDP crystal has been proposed.Anisotropic elastic matrix is used for correlating the stress and strain vector within the elastic range,and an ellipsoid is adopted as the yield surface.In addition,maximum tension stress failure criterion is adopted as the fracture criterion of the material.The mathematic expression of the new model was derived and then programmed and integrated into the material library of the commercial finite element?FE?software LS-DYNA.Subsequently,the mechanical properties of the Switch,Doubler and Tripler planes of KDP crystal have been studied by combining nanoindentation experiment,nanoscratch experiment and FE simulation.Finally,elastic,plastic and fracture parameters on these three planes were successfully identified.On the basis of that,the brittle ductile transition?BDT?anisotropy of KDP crystal has been stutied.By utilizing the new constitutive model and the material parameters obtained before,KDP cutting process was simulated,and the influence of cutting direction as well as rake angle on the BDT depth has been analyzed.Meanwhile,groove cutting tests were carried out on the Switch,Doubler and Tripler plane of KDP crystal,and the BDT depths under different cutting direction and rake angle have been determined experimentally.Then,the condition for achieving crack-free surface in processing KDP crystal by circular edge cutting tool has been proposed,and the theoretical model of this condition has been established.Finally,the simulation and the calculation results have been validated by comparing with the experiment results.Then,the cutting force anisotropy in processing KDP crystal has been investigated.A 3D FE model of processing KDP crystal by circular edge cutting tool has been established,and the variation of cutting force with cutting direction as well as feed rate have been calculated.Besides,for calculating the cutting force more efficiently and conveniently,a theoretical model for calculating the cutting force in processing KDP crystal by circular edge cutting tool has also been established.Then,based on the nanoindentation instrument,cutting force in processing the Switch,Doubler and Tripler plane of KDP crystal were measued.Finally,the cutting forces obtained by these three methods were compared,and the accuracy of the simulation as well as the theoretical results has been verified.Finally,surface defects formed in flycutting of KDP crystal has been studied.The forming reason of the surface defects have been clarified by painted-surface cutting experiment,Energy Dispersive Spectrometer?EDS?analysis and Scanning Electron Microscope?SEM?analysis.Based on these knowledges,cutting experiments were carried out on the Doubler plane of KDP crystal,influence of tool edge radius,lubrication conditions and cutting directions on the surface defects have been analyzed.These results can provide guidelines for suppressing the surface defects.Finally,a new diamond cutting tool for eliminating the surface defects has been designed and and it has been proved that the new cutting tool can suppress the surface defects effectively.