Ultra-precision Machining Mechanism And Processing Technologies Of Polycrystalline Magnesium Aluminate Spinel

Polycrystalline magnesium aluminate spinel（MgAl₂O₄）,as an infrared transparent ceramic,has been qualified as an ideal material for making key optical components under extreme working conditions due to its supurb optical transparency,high mechanical strength and high thermal stability.However,the stable chemical constitution and solid lattice structure bring great processing challenges.The high brittleness evokes an easiness of generating surface damages during machining,which makes the low-damage and ductile machined surface very hard to be obtained.On the other hand,the high hardness and elastic modulus also give rise to severe tool wear.These factors make spinel as an representative hard-and-brittle material.Although the sintering techniques of synthesizing spinel has been mature enough to reach the transparency requirement in recent decades,the processing technology of generating optical-grade machined surface still meets many unsolved problems.The deep reason for the situation is that the bridge between the above two domains,which is studying the machinability,removal mechanism,damage mechanism and the fundamental processing techniques for spinel is still in the early stage.Aiming at realizing high surface quality and low surface damage in machining spinel,this thesis conducted a series of research work to fill up the research gap between the synthesizing techniques and the high-quality surface-machining techniques of spinel.The main research content of this thesis includes the following aspects.Nano-indentation and Vickers indentation tests are firstly carried out to investigate the fundamental mechanical properties and key mechanical parameters such as the nano-hardness,micro-hardness,elastic modulus are obtained.Acoustic emission system was applied to monitor the crack behaviors during and after the indentation tests and study the cracks generation and propagation patterns.Then the scratching tests applying cutting tools on spinel were conducted to acquire the critical cutting depths for ductile-to-brittle transition.The DBT mechanism was analyzed and verified through the surface damage topographies.The crystal structure deformation of the machined surface was studied through transmission electron microscopic observation.A crystallographic model was established to uncover the deep relationship between the lattice structure and the ductile/brittle performances.To optimize the key cutting parameters for spinel,the critical cutting depth for realizing entirely ductile machiend-surface was obtained through a group of constant-depth groove-cutting experiments.The machining results of tools with different rake angles were compared.The effects of the stress fields generated by different rake angles on enhancing the machining ductility or damage tendency were analyzed.By means of TEM observation,the lattice structure pre-deformation induced by hydrostatic compression stress of rake faces distributed on the critical cutting zone were characterized.Then FEM cutting models were established to simulate the situations of cutting across two grains and cutting with different rake angles.In the simulation results,the stress distribution and surface damages were comprehensively analyzed.In elliptical vibration cutting process,the determination of tool tip trajectory was analyzed through investigating different cutting and vibration parameters.The material removal mechanism was explored through the elliptical trajectory characteristics and key parameters influencing the machined surface generation were found out.Two prediction models were improved for predicting critical DBT cutting depth in EVC.Then cutting experiments with both conventional cutting and EVC methods were carried out to testify the numerical models and to compare the surface quality.The nominal tool rake angle and the norminal cutting speed for EVC spinel were optimized.On the basis of previous fundamental research work,diamond turning experiments were conducted on spinel surface with applying the optimized cutting parameters and tool parameters.With analyzing the machined surface roughness and the flatness error,it is concluded that ductile-regime and low-damage cutting on（?）10 mm spinel surface can be realized by conventional cutting method.Then elliptical vibration cutting spinel was carried out with the optimized EVC parameters to improve the surface integrity and reduce the sub-surface damage.In addition,the tool wear patterns and tool life for all the used tools were evaluated.The processing chain of“rough cutting with positive-rake-angle tool—fine cutting with tools of negative rake angle and large nose radius—elliptical vibration cutting”was proposed on cutting spinel.