Research On Ultra-precision Machining Mechanism Of Fused Silica Based On SPH Method

Optical glass,because of its excellent physical properties,is widely used in aerospace,information,energy,chemical,microelectronics and other fields.Among many kinds of optical glass,fused silica,owing to its high purity,high temperature resistance,radiation resistance,good stability and other characteristics,as the superior nature of the optical glass,has been widespread concern,which has an irreplaceable role in some harsh conditions.Today,fused silica precision and ultra-precision machining technology has become the focus of advanced manufacturing research direction.However,due to its high brittleness and high hardness,fused silica processing is prone to crack and damage.It is difficult to achieve the development of optical technology to meet the requirements of ultra-smooth surface.On the micro-nano scale,simulation technology has been more and more widely recognized owing to its unique advantages.The method of mesh-less simulation of smooth particle hydrodynamics has solved the problem of grid distortion in finite element method.It is a very promising method to fit the limit precision of current ultra-precision machining.Based on the above problems and the present situation,this paper analyzes and predicts the phenomenon of material separation mode,crack propagation law and residual stress distribution in the process of ultra-precision of fused silica processing with cutting and single abrasive grinding process as the breakthrough point.The experiment verifies the correctness of the simulation.Specific work is as follows:Firstly,built on the constitutive properties of brittle materials,a fused silica SPH model was established,and fused silica ultra-precision cutting process was simulated.With cutting depth being 0.1-1 ?m,material removal mode and strain distribution were analyzed.When different rake angle of tools was chosen,crack formation mechanism and its impact on the ultra-precision machining process under cutting process were analyzed.Simulation results indicate that fused silica can be machined in ductile mode on the micro-and nanoscale.Through the research of relation between micro-cracks and plastic strain,the critical cutting depth of brittle-ductile transition is confirmed,and the value is 0.18 ?m under given simulation conditions,which are 0° rake angle,10 m/s cutting speed and 0.1 ?m blunt radius of diamond tool.Negative rake angle cutting tool can get better surface quality,and it indicates that the negative rake angle cutting is more suitable for ultra-precision machining of fused silica.Secondly,in order to obtain high-quality fused silica optical surface,the SPH method was used and the fused silica grinding process by using a single grain under micro and nano-scale was modeled and simulated.The fused silica separation mode under multiple sets of machining depth,grinding force,the maximum equivalent stress and changes in residual stress and its impact on the sub-surface damage were analyzed.The simulated results indicate that fused silica can be machined in a ductile mode by grinding.Under the simulation conditions,critical depth of the brittle-ductile transition process is 0.36 ?m,grinding force ratio is greater than 1,and crack extension at the depth of about 1.2?m ceases,which provides a basis for prediction of subsurface damage degree.High-speed grinding by single particle experiments validate the simulation result is reasonable.Finally,the fused silica scratch experiment was carried out.After comparing with the simulation results,the correctness of the simulation was proved and the deficiency in the simulation was complemented.When the constant load makes the scribing depth more than 260 nm during the scratch,there is a brittle fracture phenomenon,but not obvious,and the removal is still plastic way.There is a difference in the friction coefficient under steady state,which increases with the increase of the scratch depth,but the increase trend is gradual.