Investigation On The Influence Of Ultraprecision Turning Aluminium Alloy Surface Micro Morphology On The Diffraction Effect

Ultra-precision turning technology is extensively applied in the fabrication of large-size metal reflection mirror.However,when the metal mirror finished by ultra-precision turning process is directly employed in visible wavelengths,severe diffraction effect will occur,which degrades its optical performance and leads to that the mirror cannot be utilized in the practical application.On the current condition,polishing method is commonly recommended as post-treatment process to eliminate the diffraction effect.However,the disadvantages of time-consuming and high-cost cannot fulfill the requirement of large amounts of high-quality metal mirror manufacture.To fabricate the aluminium mirror working on the visible light range without diffraction effect,investigations on the origins of the diffraction effect of ultra-precision turned-surface and corresponding elimination technology are performed in this work.First,research works on surface roughness,surface topography,diffraction effect and corresponding elimination technology are summarized and reviewed.According to the summary of the above works,key influencing factors of the diffraction effect are concluded.Meanwhile,corresponding investigation path for eliminating diffraction effect is also proposed.Investigation on roughness prediction model for ultra-precision turning surface is developed in the light of the technique path.A surface roughness model considering the duplication effect of cutting edge,material plastic side flow,spring back and random factors is established.Cutting experiments are carried out to validate the roughness model and proves high prediction accuracy for the fine-grain aluminium alloy workpiece material.Based on the surface roughness model,origins for the size effect of surface roughness in ultra-precision turning process are analysed.For the fine-grain aluminium alloy,the size effect can be attributed to the competition between tool edge duplication component and plastic side flow component.However,for coarse-grain aluminium alloy,the size effect derives from the impurity defects in material matrix.Second,based on the corresponding calculation models in roughness modelling,ultra-precision turning surface topography model is established in this work.For 2D surface topography,duplication effect of cutting edge,material spring back and plastic side flow etc.are successively integrated.According to the theoretical result of 2D surface topography model,3D surface topography models without and with consideration of the spindle vibration are developed.Surface defects are crucial to the surface topography model.Therefore,the influence of grain boundary protrusion,hard particle protrusion,scratch,stomata and oxide film are synthesized.3D surface topography models with respective and comprehensive consideration of the above surface defects are correspondingly established.Systematic cutting experiments are performed to validate the surface topography model developed in this work.Meanwhile,influences of spindle vibration and surface defects on 3D surface topography for ultra-precision turning process are also discussed.Third,based on the 3D surface topography model,influencing factors of the diffraction effect in ultra-precision turning process are modelled and analysed.For the calculations of diffracted light spatial distribution and diffraction efficiency parameters,light field tracing method based on Fourier fast transformation(FFT)and Fourier modal method(FMM)are employed,respectively.Calculation results show that feed rate,tool edge waviness and peak-valley surface roughness(without consideration of the random factors,e.g.surface defect)only affect the diffracted light distributed on the horizontal direction.The spindle vibration only affects a small area around the center of workpiece surface.For surface defects,they not only affect the diffracted light on horizontal direction,but also act as the critical origin of diffracted light on the vertical direction.Furthermore,for surface defects,the impacts of its parameters in the height/depth direction are obviously larger than those in other directions.Finally,the quantitative relationship model between the surface optical efficiency parameters and surface oughness is established;and the critical condition for elimination of diffraction effect on the visible light band is hence acquired with comprehensive consideration of all the influencing factors in single point diamond turning process.As for the influence of surface defects,a full-scale treatment technology of powder metallurgy,rapid solidification,and surface rolling process is developed in this work.According to the surface topography model proposed in this work,surface roughness control model is established to select the suitable cutting parameters for eliminating the diffraction effect.Cutting experiments and optical performance testing experiments are successively performed under the selected cutting parameters.The results prove the effectiveness of the proposed elimination method for the diffraction effect on the wide infrared-visible wavelength band in this work.The mirror achieved with this technology can fulfill the requirement on the infrared-visible wavelength band without post polishing processing.It supplies the key technology for the fabrication of large-size metal mirror in our country.