| Fused silica has been widely used in industry fields,because of its high strength,good low density thermal stability,good abrasion resistance excellent performance,such as in aerospace,National defense equipment,semiconductor,optoelectronics,and nuclear energy industry.Diamond grinding is a common processing technology,but its material removal rate of low,processing cost and high surface/subsurface crack extremely difficult to detect,which seriously hindered its wide range of applications.So far,there are various non-traditional processing technologies have been developed,but it cannot achieve good processing results due to the high hardness and brittleness of fused silica.Therefore,processing technology of fused silica component with efficient,reliable and flexible has been in urgent need.Laser-assisted machining(LAM)is a promising machining method,with the advantages of reducing cost,improving efficiency and reducing surface damage,which has demonstrated tremendous potential in the field of difficult to cut materials.In order to make the LAM of fused silica become a viable industrial technology,it is very necessary to conduct a comprehensive study.Therefore,given the difficult processing characteristics of fused silica,this study was first time attempt to explore the LAM of fused silica,which was beneficial to achieve the efficient processing of fused silica with high efficiency,high precision,and high quality.The main research contents of this study include:(1)The thermal model of LAM was studied.Based on the interaction mechanism between laser and fused silica,the thermal property parameters were measured experimentally,such as absorption rate,emissivity and thermal conductivity etc.The heat source subroutine was developed to simulate the moving laser with the source model in the plane,and the temperature evolution law of the workpiece surface under static and dynamic conditions was revealed.Based on the heat transfer theory,the 3D transient temperature field model on the cylinder surface was developed,and the temperature distribution of the surface and interior of workpiece was analyzed through the cross section and longitudinal section.The accuracy of the thermal model was verified by infrared temperature measurement experiment,and the influence of laser power,spot size,spindle speed and other parameters on the temperature field was obtained.Given the insufficient internal heating of workpiece,the fixed point preheating and scanning preheating were proposed,which effectively improved the temperature distribution of workpiece and was more beneficial to the temperature conduction to the interior of the workpiece.The preheating effect of three heat source models with the circular,oval and square heat source models are discussed from two aspects of temperature distribution in workpiece surface and temperature gradient in three directions.The results show that the temperature distribution in the square spot is wider and more uniform.The analysis of temperature field in the cutting layer shows that the lowest temperature of the cutting layer is located at the tool tip and the average temperature of the cutting zone is the highest when the laser is in the middle of the chamfer.By analyzing the temperature field of the material removal plane,the influence of the angle and distance between the cutting depth laser and the laser spot on the cutting tool temperature and the average temperature in the cutting area is obtained(2)The cutting model of LAM was studied.Mechanical parameters of fused silica at high temperature were obtained by tensile test and Split Hopkinson Pressure Bar(SHPB)test at high temperature.The results show that the tensile strength and elastic modulus of the material decrease sharply under high temperature.There are coupling effects of high temperature softening and strain rate strengthening in high temperature dynamic impact test.Based on Smoothed Particle Hydrodynamics(SPH)method,the SPH/FEM coupled cutting model of uniform temperature field was established,and the maximum equivalent stress,the variation rule of cutting force,surface roughness,surface residual stress and dynamic cutting process were analyzed and discussed.The results show that the maximum equivalent stress,cutting force and surface roughness decrease with the increase of temperature.The influence of the radius of the tool on the minimum cutting thickness is analyzed by using the cutting model,and the scaling effect is revealed in terms of stress distribution,cutting force and specific cutting energy.Based on Finite Element Method(FEM),the cutting model of thermo-mechanical coupling non-uniform temperature field was developed to analyze the chip formation mechanism in the machining process from the aspects of laser power,cutting thickness and preheating time.The results show that the non-uniformity of laser heating leads to the change of the friction coefficient at the chip interface and the non-uniformity of the residual strain in the cutting area,which causes the chip to change from spiral curl to a variety of shapes,including fragment,block,curl and bifurcation.The accuracy of the cutting model is verified by the ring tool device test.The chip morphology,stress distribution and cutting force variation under different tool rake angle are discussed.The results show that with the increase of the cutting tool rake angle,the continuity of chip morphology increases and the cutting force decreases.The mechanism of tool wear during the LAM process is analyzed,and the influence of process parameters on the maximum equivalent stress of the tool is obtained.(3)The machinability of fused silica in LAM was experimentally studied.LAM setup and system was built,and the machinability of fused silica was analyzed from the aspects of surface roughness,tool wear and cutting force.Compared with conventional machining(CM),laser-assisted heating can significantly decrease the surface roughness and improve the surface integrity.According to the large size semi-continuous chip analysis,the material removal mechanism in LAM is a mixture model of plastic deformation and brittle deformation.The multi-objective response optimization was designed based on the Response Surface Method(RSM),and the regression model was optimized and predicted with the comprehensive satisfaction function.The value of comprehensive satisfaction was 95.30%,and the optimal process parameter combination was obtained.The tool wear test showed that under the same cutting conditions,VB value was 370.44?m in CM,while VB value was230.08?m in LAM,which indicated that the tool life increased by 38.79%during LAM.The influence of PCBN,PCD,TiN coated ceramic tool on cutting performance was analyzed,and the wear mechanism of three kinds of tools was analyzed by SEM and EDS.The results showed that PCD tool was most suitable for LAM.The cutting force test shows that LAM technology can significantly reduce the cutting force:the feed force(F_x)decreases by68.64%,the thrust force(F_y)decreases by 66.89%,and the main cutting force(F_z)decreases by 53.61%.Meanwhile,the fluctuation of cutting force and specific cutting force decreases during LAM.Taguchi method(TM),RSM and Artificial Neural Network(ANN)were used to optimize the cutting force.The results show that ANN optimization is more accurate and robust than TM and RSM method.In sum,the high-energy beam laser has changed the cutting mechanism of fused silica from brittle fracture to a mixture model of plastic deformation and brittle deformation.Therefore,LAM technology can effectively improve the cutting performance of fused silica.In this study,the frame system of LAM of fused silica was established preliminarily from the aspect of machinability through modeling,simulation and experiment.However,due to the uneven distribution of Gaussian spot energy,it is easy to cause the difference of brittle-plastic transformation in the cutting area,leading to the complexity of laser-assisted processing.Therefore,it is necessary to further reveal the cutting mechanism of LAM from the microstructure. |
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