Fracture Mechanism Of Polycrystalline CBN Grains For Single-layer Brazed Abrasive Wheel

Polycrystalline cubic boron nitride(PCBN for short)abrasive grains are sintered under high temperature and high pressure using the microcrystalline CBN particles and AlN binder materials.In theory,the special microstructure of this kind of abrasive grain makes it possible to achieve self-sharpening of the grinding wheel through grain micro-fracture during grinding process,which can avoid the attrition wear and large fracture of the microcrystalline cubic boron nitride and thus ensure the long-term sharpness of abrasive wheel.Therefore,single-layer brazed PCBN abrasive wheel have become an important tools for high-speed and ultra-high-speed grinding of difficult-to-cut materials.However,at present,the related research still stays on the qualitative level,such as the characterization of fracture morphology of the PCBN abrasive grains and the classification of the fracture patterns.Due to the lack of quantitative research on the influence factors and mechanism of the fracture of the PCBN abrasive grains,grinding parameters are mainly determined empirically in order to control the fracture behavior of the PCBN grains,which greatly limits the high-speed and ultra-high-speed grinding potential of the PCBN abrasive wheels.For the abovementioned reason,an investigatation on the fracture mechanism of PCBN grains for single-layer brazed abrasive wheels is put forword in this work.Finite element simulation models of the fracture process of the PCBN grains is established to study the fracture mechanism of the PCBN grains from two aspects,e.g.,the material characteristics of the PCBN abrasive grains and the grinding parameters.Based on the simulation results,the internal and external factors influencing the fracture of the PCBN grains are analyzed and the control strategies of the fracture are proposed.The main work and results are as follows:(1)The open-source software Neper has been chosen to build the Voronoi diagram of the PCBN geometry model,based on which two-dimensional and three-dimensional geometry model and fracture model of the PCBN grain have been established.The boundary conditions and the load parameters were determined according to the internal and external factors influencing the fracture of the PCBN grains.(2)Based on the two-dimensional fracture model,the energy conversion between strain energy,kinetic energy and surface energy during the fracture process of the PCBN grains was illuminated.The release rate of bulk fracture energy of the PCBN abrasive grain was determined as an index to evaluate the fracture toughness.The effects of grain boundary strength ratio,grain size of microcrystalline CBN particles and effective strain rate on the fracture patterns and fracture toughness of the PCBN abrasive grains were discussed,according to which the optimal grain boundary strength ratio and grain size of microcrystalline CBN particles were selected.The strain rate strengthening effect of the fracture toughness of the PCBN abrasive grains was found,which proved the inherent advantages of PCBN abrasive grains for high-speed and ultra-high-speed grinding.(3)Grinding experiments of GH4169 nickel-based alloy with single PCBN grain have been conducted to elucidate the evolution of the wear morphology of the PCBN grains and the trend of the grinding forces.The optimal grain size of microcrystalline CBN particle and grain boundary strength ratio were adopted into the three-dimensional fracture model to analyze the influence mechanism of the undeformed chip thickness and grinding speed on the fracture of the PCBN grains.According to the analysis,the optimal range of grinding parameters were selected,that is the undeformed chip thickness was around 0.2 ?m to 0.4 ?m and the grinding speed was above 60 m/s.Grinding experiment using the grinding parameters chosen from the optimal range of grinding parameters was conducted to verify the reliability of the selected optimal range of grinding parameters.