Research On The Mechanism And Technology Of Nanosecond Laser Milling Of Al₂O₃ Ceramic Materials

Laser milling,an advanced manufacturing technology developed in recent years,uses laser irradiation on the surface of materials to instantly melt or gasify the material and remove the material.It does not need fixture and is not subject to the limitation of material types.It provides a better way for the processing of ceramic materials.However,due to low efficiency of laser milling and surface quality of formed parts,its application is limited.The thesis analyzes the forming mechanism,numerical simulation,experimentation and processing optimization of laser milling of Al₂O₃ ceramics to improve the quality of laser milling of Al₂O₃ ceramics.It is of great significance both theoretically and practically.The main research contents are as follows:?1?The basic theory of laser-material interaction is described.The process of material melting and gasification induced by pulsed laser is emphatically introduced.The maximum melting depth,relations of laser energy parameters and the computational formula of back stamping force of expanded gas are deduced,which lays a foundation for the establishment of melt injection model in laser milling process.According to the melting and gasification process caused by laser action on material surface,a melt ejection model in pulse laser milling process was established.The dissertation also analyzes the causes of material gasification and expansion in single pulse laser milling,the main power source of melt ejection and the calculation method of punching force which plays a major role in melt ejection.Then,a melt ejection model for laser continuous milling with a certain overlap rate is established,and the influence of scanning direction and melt ejection direction on melt ejection removal efficiency is analyzed.Finally,the driving force formula of melt injection is obtained after the driving force of melt injection is analyzed.The melting power density threshold of nanosecond laser on ceramic material is solved by reasonably choosing the heat conduction equation to remove the material threshold in laser milling and simplifying the heat conduction equation.?2?On the basis of theoretical analysis,the finite element simulation technologyof laser milling forming temperature field is studied.APDL?ANSYS Parametric Design Language?is used to program the dynamic simulation program of laser milling temperature field.Laser milling can not be simulated by means of "life and death unit" technology.At the same time,the dynamic change of temperature field is also engraved.At the same time,by means of ANSYS post-processing program and "life and death unit" code,the process of temperature field changing with time in laser milling at different time and the shape of transverse and longitudinal profile of sample surface after laser milling are studied,and the causes of slag formation at the bottom of concave pit of milling parts are analyzed.In order to further understand the influence of processing parameters on temperature field,different processing parameters?laser power and scanning speed?are adopted to simulate the laser milling process of Al₂O₃ ceramics,and the effects of different laser power parameters on milling temperature field,milling depth and surface quality are analyzed.Under fixed conditions,the temperature field,milling depth and width under different laser power and scanning speed are studied respectively.The simulation of temperature field provides a basis for the selection and optimization of laser milling parameters.?3?Based on the theoretical analysis of laser milling and the results of finite element simulation of temperature field,laser milling experiments of Al₂O₃ ceramics are carried out by using nanosecond laser.The surface microstructures,surface roughness and milling depth of the samples after milling are measured by means of scanning electron microscopy?SEM?and laser confocal microscopy?LSCM?.The influence of laser parameters?laser power,spot overlap rate,laser repetition frequency?and scanning parameters?scanning times and scanning paths?on the surface quality of laser milling are analyzed.The process and mechanism of nanosecond laser milling are analyzed by studying the technological rules.?4?On the basis of laser milling experiments,with the goal of surface roughness and milling depth,the laser milling processing parameters?laser power,laser spot overlap rate,laser repetition frequency and scanning times?are optimized by using orthogonal experimental design and response surface methodology,which determine the contribution rate of laser processing parameters to the depth and roughness ofmilling.Then,the most significant factors affecting the processing parameters are determined.Next,the optimum combination of parameters,with the latest surface roughness and the maximum milling depth as the optimal goal,is found to predict the minimum surface roughness value and the maximum milling depth.Laser milling experiments are carried out with the best parameter combination,and the microstructures,surface roughness and milling depth are measured.The optimal control model of laser milling processing parameters is obtained by comparing them with the predictive value of processing optimization.