Theory And Experimental Investigation On Laser Hot-Assisted Machining Al₂O₃ Engineering Ceramics

Engineering ceramics have many excellent properties, such as higher hardness and melting point, lower density and heat expension coefficient,fine corrosion- resistance and anti-oxidization, excellent heat and abrasion-resistance, outstanding self-lubricating ability and environmental duration, so they are getting more and more applications in machine, automobile, chemical engineering, aerospace aviation and other industrial fields.However, due to their high hardness and brittleness, they are difficult to be machined using traditional machining methods such as turning, milling and drilling.Laser hot-assisted machining (LHAM) is one of the effective methods to solve this problem.Funded on the Hunnan university science reaserch project ,the dissertation reffered to solve the main problem during CO₂ LHAM of Al₂O₃ engineering ceramics. Based on numerical simulation and experimental investigation, the mechanism of LHAM Al₂O₃ engineering ceramics has been studied, emphases are on the invesigation of temperature histories variation and their influences on the forces, tool wear, chip formation and machining quality.The main contents are:1. Based on the optical law, and using GaAs inspector, the polarization of the laser beam through after optic road transportion output from the PHC-1500 laser unit is experimentally verified; A set up for workpiece reflectivity measurement is designed.Both of the Al₂O₃ and Si3N4 engineering ceramics'surface reflectivities to line polarized CO₂ laser beam of both parallel and perpendicular to incidence plane at different incidental angles are measured. The experimentally-obtained data are in good agreement with the ones calculated by means of Fresnel formula. Based on the above data, both of the ceramics'refractive index, Brewster angle and absorption coefficients are obtained.2. The experimental results indicate that both of the Al₂O₃ and Si3N4 engineering ceramics'energy utilization under line polarized CO₂ laser beam heating are better than in circle polarized after the comparison between their measured surface absorption coefficients to line and circle polarized CO₂ laser beam which is parallel to incidence plane at Brewster incidental angle, and Si3N4 engineering ceramics are more difficult to be machined than Al₂O₃ on the same condition.3. A LHAM experimental sysytem incuding laser beam road which consisted of four reflecting mirrors and a focusing mirror and machining sysytem is founded, which not only is in demand of laser beam polarization ,also for the angle adjustment between laser beam centre and cutting tool corner during LHAM.4. After theoretical analysis of heat conduction in LHAM, according a formed elliptical belt on the workpiece under Gauss laser beam heating,a quasi-stable heat conduction model is established combined with the heat conduction control equation.5. Using finite difference method, the quasi-stable heat conduction model of LHAM Al₂O₃ engineeering ceramics is solved. The simulation results indicate that the isotherms are ovoid on the surface, parabolic in cross-section and symmetrical in longitudinal section, and the farther is the distance between the area and the laser spot,the lower is its'temperature. Compared with the results under the laser beam heating the workpiece perpendicularly, the elliptical laser spot heated on the workpiece is prolonged and broadened, where the laser beam heated the workpiece in tangential Brewster incidental angle, and the workpiece is heated uniformly with higher absorbtion, though the power density is lower,and the softened depth is bigger along the ellipse lengthen axis movment beneath the material molten temperature of 2323K. The numerical caculation results indicate that the internal mechanism of LHAM is rapidly raising the temperature of to-be-machined material by the use of laser beam heating,which lower the hardness and strength, inforced the plastics and ductility and make the material softened to be easily plastical deformation to keep continuos and stable machining.The theoretical circular distance between laser beam centre and cutting tool corner and feed are also obtained based on the workpiece temperature distribution under the combination of differential laser and machining parameters in LHAM.6. Using the non-contact infrared thermometer, near the cutting tool corner to be machined workpiece surface temperatures under different parametres such as laser power, beam radius, moving speed and incidence angle are experimental measured, and the changement trend are in good agreement with the numerical model calculated temperatures in the same zone.The heat saving, material emission and distance coefficients are reasons of temperture errors between measuremental and model calculation.7. Using the piezoelectric dynamometer,the variations of machining forces in LHAM Al₂O₃ ceramics are experimentally investigated.The results indicate the three forces decerase obviously.The influences analysis of different laser and machining parameters to machining proccees indicate that raise the laser power would broden the heat affect zone and increase the to-be- machined material's temperature ,which is advantages to material softening and cutting resistence decreasing .This would decrease machining forces ;When using small cutting depth ,the forces can be decreased because of the small deformation and fraction in cutting zone;With the increasing of workepiece rotating,the forces would increase for the reason of relative shortened heating time and the lower temperature and insufficient softening volume in specimen;The forces would decrease and machining quality would be improved by using smaller feed ,which is better for material heating and softening;The forces would also decrease when using certain beam spot heating workpiece which can attain softening volume.The thoeretical analysis and experimental results indicate that the forces under tangantial laser heating are obviously smaller than the forces under perpendical laser heating in LHAM. When properly improve the circular distance between laser beamcenter and cutting tool corner,it can not only decrease the maching forces assuring the LHAM effection,also prevent the cutting tool from life decreasing on the elevated temperature heating.8. The cutting tool wear and chip shape are experimental investigated. There exsist face,flank and boundary wearings on the face,flank and the contacting areas between the major ,minor cutting edge of diamond cutting tool and the work or machined surface of workpiece,where exist the elevated temperature and pressure. The chip shape varies under the condition of different heating which make the to- be- removed material's property vary.The chip would be collapsed-like on lower laser power heating,but unit-like on the higher laser power heating.9. The workpiece machining accuracy, crack distribution and roughness under different machinig parameters of LAHM are measured and inspected. According to the experimental data and theoretical analysis, the proper machining parameters are:laser power 200～325W, beam radus0.75¹.25mm, cutting depth0.2⁰.6mm, feed0.10 0.16mm/r rotation speed147～475r/min circular distance between cutting corner and laser beam center3.93～6.54mm. Higher machining accuracy and surface quality can be obtained through the proper combination these parametersof in LAHM.The above investigation results have certain theory and application references to the LHAM of engineering and other porcelain ceramics.