Self-sharpening Of Polycrystalline CBN Grains Brazed Using High-frequency Induction Heating

Polycrystalline cubic boron nitride(CBN)abrasive grits are particular superabrasives,which are made of CBN microcrystalline particles and AlN binders through sintering under high temperature and high pressure.The polycrystalline CBN grits not only have the same high hardness and high wear resistance as the single crystal CBN grits,but also have the isotropic physical characteristics.Benefited from the self-sharpening behavior,the polycrystalline CBN grits can always maintain the high sharpness during the grinding process.In addition,the application of high frequency induction brazing technology to produce a single-layer superabrasive wheel can effectively reduce the thermal deformation of the metallic wheel matrix compared to the wheel matrix brazed in the vacuum furnace.Meanwhile,the wheel brazed using induction heating has a high holding force to abrasive grits and a higher manufacturing precision.For this reason,it is of great significance for the manufacture and application of the single-layer polycrystalline CBN wheels based on the induction brazing technique.However,owing to the lack of deep understanding of the distribution characteristics of stress in the polycrystalline CBN grains and the performance and mechanism of the self-sharpening behavior,grinding performance of the grinding wheel is difficult to control,which limits the grinding potentials of polycrystalline CBN wheels.Therefore,it has been put forward to investigate the self-sharpening of the polycrystalline CBN grits brazed using the high-frequency induction heating technique.Based on the study on temperature field in the induction brazing process of polycrystalline CBN wheels,the current research clarified the influencing factors and distribution of brazing stress and grinding stress in polycrystalline CBN grits;furthermore,the grinding experiments with a single grit has been carried out to explain the micro fracture behavior of polycrystalline CBN grits,and finally the performance and mechanism of grit self-sharpening behavior was revealed.The research fruits obtained are helpful to deepen the understanding of the fabrication and application mechanism of single-layer polycrystalline CBN superabrasive wheels with high-frequency induction brazing.The main research work and achievements are as follows:(1)The basic characteristics of the temperature distribution during high-frequency induction brazing were analyzed with the method of experiments and simulation.The influences of current magnitude,current frequency,heating gap and scanning speed on the temperature distribution during induction brazing were also investigated.The result shows that,when the current frequency is 1 MHz and the heating gap is 2 mm,the temperature distribution of induction brazing is suitable for the fabrication of the single-layer polycrystalline CBN wheel with the following three groups of brazing parameters(including current and scanning speed): 19 A and 0.5 mm/s,20 A and 1 mm/s and 21 A and 1 mm/s.(2)The microstructure model of polycrystalline CBN abrasive grits was established with Voronoi tessellation.The brazing-induced residual stress distribution and grinding stress distribution were calculated.The influences of internal stresses of brazed polycrystalline CBN grits were analyzed with different embedding depth,particle size and grinding force;accordingly,the embedding depth of grits was optimized.Afterwards,the residual stress between single crystal CBN and polycrystalline CBN grits were comparatively analyzed.The grinding experiments with the brazed polycrystalline CBN wheel were also carried out,through which the stress results in the brazed polycrystalline CBN grits were proved experimentally.(3)Grinding experiments of titanium alloy TC4 with a single grit were conducted.The effects and mechanisms of grinding parameters on grit crushing and self-sharpening behavior were analyzed by morphology observations and fractal dimension of polycrystalline CBN grits.It has been found that,during the grinding process,the polycrystalline CBN grits are more prone to micro fracture,rather than crush and macro-fracture of single crystal CBN grains.Therefore,the grinding performance of polycrystalline CBN grains is always stable.