Study On The Technology Of Laser Repairing For Cracks And Stress Relieving For The Ground Surface Of Fused Silica

Fused silica is a typical brittle and hard,difficult-to-machine material.The production cycle of large-aperture aspherical optical fused silica lens can be reduced significantly by using the process route "precision creep feed grinding--repairing surface cracks by CO2 laser--ultra-precision ductile grinding".In this study,CO2 laser was introduced to repair surface and subsurface micro-cracks on fused silica.However,a great deal of residual tensile stress was caused on the surface of fused silica.This thesis deals with the technology of laser repairing for cracks and stress relieving for the ground surface of fused silica so as to reduce the surface roughness and residual stress of fused silica after laser repairing.A laser repairing platform was built,which was composed of laser source and optical path,three-dimensional motion platform and control system.Experiments of laser repairing for the ground surface cracks of fused silica were done.It is shown that the micro-cracks were not caused on the subsurface of fused silica under the present laser repairing conditions.And the healing mechanisms of cracks were revealed.A temperature distribution finite element model of fused silica was built for the laser repairing process.The effect of laser repairing parameters on the temperature distribution of fused silica was analyzed.It is shown that the surface temperature of fused silica can be reached up to 2050K?2350K at the selected laser repairing parameters.The effect of the laser repairing parameters on the finished surface roughness was studied.It is revealed that the finished surface roughness was firstly decreased and then increased with an increase in the laser power.Excessive laser power could cause surface burn of fused silica.The finished surface roughness was decreased with an increase in the laser spot diameter,and it was decreased with a decrease in the laser scanning interval.While the laser scanning speed had little effect on the finished surface roughness.The cause of the residual stress and stress relieving technology by laser heating for the fused silica after laser repairing was researched.The visco-elastic constitutive model of fused silica was determined.A finite element model of residual stress on the fused silica surface after laser repairing was built.The residual stress distribution of the repaired fused silica was obtained and the model was experimental verified.It is shown that the cause of residual stress is the thermal stress relaxation resulted from the laser irradiation within the temperature range of fused silica transformation.The effect of the laser repairing parameters on the residual stress was researched.It is shown that the residual stress is decreased with an increase in the laser spot diameter and a decrease in the laser scanning speed respectively.A new processing scheme was put forward that the fused silica was treated with a larger laser spot,a slower scanning speed and a reduced laser power so as to reduce the residual stress.It was proved by theoretical and experimental results that the new stress relieving process can reduce the residual stress effectively.The stress relieving technology of fused silica by a low temperature heating process was studied.The stress relieving process by a low temperature heating was analyzed by finite element method.And the results showed that a heat preservation of 12h at 800 ? and cooling to 740 ? by a cooling rate of 10 ?/h could relieve the residual stress of fused silica.The optical birefringence retardation of the fused silica can be reduced to meet the first grade of GB standard of optical glass after stress relieving treatment.Microwave heating technology was introduced as the heating method for the low temperature stress relieving process.Experiments were done to investigate the chemical elements pollution on the fused silica in the thermal treatment process due to the microwave heating medium.The result showed that the contamination layer was little than 0.3?m,which can be removed by the followed grinding and polishing process.