| Engineering ceramics,cemented carbide,titanium alloy and other difficult-to-machine materials have excellent physical and mechanical properties and are widely used in aerospace,energy,automobile,general machinery manufacturing and other industries.At present,the precision machining of such materials is mainly achieved by super-hard abrasive grinding.In view of the problems that difficult-to-machine materials are prone to surface/subsurface damage and poor surface integrity in conventional negative rake angle grinding,a new concept of super-hard abrasive positive rake angle grinding of difficult-to-machine materials is proposed,and a new type of grinding tool with the inclination angle at the top of the abrasive grains less than 90° was developed by using a femtosecond pulsed laser.Based on the summarizing of the characteristics of the existing grinding methods for difficult-to-machine materials,aiming at improving the grinding surface integrity of difficult-to-machine materials,the present work systematically studies the femtosecond laser fabricating diamond grinding tool with positive rake angle and the positive rake angle grinding performance by means of theoretical research,numerical calculation and experimental exploration.The mechanism of femtosecond laser processing of diamond was revealed,the problem of precise and efficient machining of diamond materials is solved,diamond grinding tools with positive rake angle with good shape accuracy and surface quality were fabricated,and a comparative study on grinding difficult-to-machine materials with positive and negative rake angle grinding tools was carried out to further develop the grinding theory of difficult-to-machine materials.The main contents and results are as follows:(1)In order to reveal the mechanism of femtosecond laser processing diamond materials,the Fokker-Planck kinetic equation including free electron source term was used to study the free electron excitation and evolution law of free electron density in the process of femtosecond Gaussian laser beam processing single crystal diamond(SCD)materials.Considering the change of the optical properties of the material surface caused by plasma excitation,the classic two-temperature model was improved and the energy coupling process between free electrons and the material lattice subsystem was simulated.The results show that during the femtosecond pulse duration,the valence band electrons are first excited to the conduction band by multiphoton ionization,and with the increase of laser intensity,the free electron density increases rapidly due to the co-excitation of multiphoton ionization and avalanche ionization.When the laser energy density reaches the peak,the free electron density exceeds the critical electron density.In the electron-lattice energy coupled heat transfer process,the electrons are rapidly heated to the peak temperature within hundreds of femtoseconds,and then the electron temperature gradually decreases while the lattice temperature increases slowly,reaching a thermal equilibrium state in tens of picoseconds.(2)The ablation threshold of femtosecond laser processing SCD was studied experimentally.The single pulse ablation threshold of SCD under different pulse width was calculated by area extrapolation method,which verifies the theoretical simulation calculation.According to the energy accumulation effect of multi-pulse ablation,the power function law of the multi-pulse ablation threshold of SCD with the number of effective laser pulses was obtained.The experimental research on femtosecond laser processing of SCD material was carried out systematically.The effects of laser average power,laser scanning speed and repeated scanning times on the ablation width,depth and quality of SCD microgrooves were analyzed and summarized.The degree of graphitization on the surface of SCD microgrooves processed by femtosecond laser was detected by a confocal Raman spectrometer,and the results showed that only very weak graphitization occurred on the finished surface.SCD microgrooves with small heat-affected zone and metamorphic layer and good processing quality were obtained via optimal femtosecond laser processing parameters.The optimal combination of process parameters for femtosecond laser processing of polycrystalline diamond(PCD)was also determined referring to the experimental research of femtosecond laser processing of SCD.(3)The grinding process of Ti6Al4 V titanium alloy with single crystal diamond abrasive grains with positive rake angle and negative rake angle was studied by single grain grinding method.The femtosecond laser manufacturing process experiment of positive rake angle SCD abrasive grains was carried out,and single diamond abrasive grains with positive rake angle structure were fabricated.Combined with ABAQUS finite element simulation and single grain grinding experiments,the variation law of grinding force for single grain with positive and negative rake angle with parameters including grinding speed,grinding depth and rake angle of abrasive grain was comparatively analyzed.The effect of abrasive rake angle on the surface morphology and surface roughness of single grain grinding was investigated,and the mechanism of positive rake angle grinding was explored.The results show that when the rake angle of a single abrasive grain increases gradually from negative to positive,both the tangential grinding force and the normal grinding force decrease significantly.For abrasive grains with different rake angles,with the increase of grinding speed,the tangential and normal grinding force both decrease;with the increase of grinding depth,the tangential and normal grinding force increase gradually.During positive rake angle grinding,a single abrasive grain has a cutting-like effect on the workpiece,with less extrusion and plastic deformation.Therefore,the machined surface has shallow wear marks and fewer machining defects,and the machined surface roughness value is 58% ～ 66% lower than that of negative rake angle grinding.(4)By reasonably setting the femtosecond laser processing parameters and scanning path,a PCD end grinding tool with clear contour and good surface quality of micro abrasive cutting edge area,with obvious positive rake angle structure and regular arrangement of abrasive grains was manufactured by a "two-step method".The process experiments of PCD face grinding tool with positive rake angle and electroplated diamond face grinding wheel with negative rake angle for grinding YG8 cemented carbide materials were carried out respectively.The grinding performance(grinding force,surface morphology and roughness,subsurface damage and wear resistance)of positive and negative rake angle face grinding tools for grinding carbide materials was comparatively investigated.The results show that in face grinding,with the increase of workpiece feed rate and grinding depth,the grinding force in all directions increases,the surface quality of workpiece deteriorates,and the grinding surface roughness increases.Under the same processing conditions,the grinding force,surface roughness and subsurface damage layer depth of the positive rake angle face grinding tool are reduced in different degrees compared with the negative rake angle grinding.In the positive rake angle face grinding,there is no obvious abrasive grain fractur and shedding,and abrasive grain wear is dominant. |
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