Development Of (W,Ti)C-Based Cermet Tool Modified With Core-Shell Structure Nano-Solid Lubricant

In order to meet the high-quality,high-efficiency,and green processing requirements of the manufacturing industry,it is necessary to further improve the mechanical properties and cutting performance of（W,Ti）C-based cermet tools.In this thesis,nano-solid lubricants with core-shell structure（Al₂O₃coated h-BN,h-BN@Al₂O₃）were prepared.Using h-BN@Al₂O₃ as a new additive phase of cermet tool,the relationship between the mechanical properties and lubricating properties of the tool with the addition of solid lubricant is coordinated,the tool processing efficiency is improved,and the environmental pollution of the cutting fluid is reduced.The（W,Ti）C-based cermet tools with nano-h-BN and micro-h-BN were prepared by SPS sintering,respectively,and the relative density and mechanical properties decreased with the increase of h-BN content.Due to the larger sheet diameter,the"splitting"effect of micron h-BN on the tool material is greater than that of nanometer h-BN.Therefore,under the same content,the mechanical properties of the tool with nano-h-BN addition are higher than those of the tool with micro-h-BN addition.The introduction of solid lubricant h-BN has a great influence on the hardness and flexural strength of the tool,but because h-BN has a certain hindering effect on crack propagation,its influence on fracture toughness is relatively small.In order to improve the influence of h-BN on the mechanical properties of（W,Ti）C-based cermet tools,h-BN@Al₂O₃ core-shell particles were successfully prepared by the heterogeneous nucleation method.On the surface of h-BN,dense Al₂O₃ tightly wraps h-BN particles.Through finite element analysis,the introduction of the core-shell structure reduces the stress directly acting on the h-BN particles,replacing the weak interfacial bonding between h-BN and the（W,Ti）C matrix.With the increase of the volume fraction of the core-shell particles and the shell thickness of the core-shell particles,the internal residual stress of the（W,Ti）C matrix material increases gradually,and the range becomes wider.The h-BN@Al₂O₃-added（W,Ti）C-based cermet tool was sintered by SPS,and the microstructure,mechanical properties and toughening mechanism were analyzed.The introduction of h-BN@Al₂O₃ generates residual stress around the matrix,which can improve the bonding strength between grains,and have effects on crack propagation such as bifurcation,pinning,deflection,and crack arrest,which increases the difficulty of crack propagation.This is the main reason for the improved bending strength and fracture toughness of the tool.The（W,Ti）C-based cermet tool with h-BN@Al₂O₃ content of 5vol.%has the best mechanical properties,and its flexural strength,hardness and fracture toughness reach 1465±30 MPa,19.7±0.3 GPa and 9.9±0.3 MPa·m^1/2,which are 38%,3%and 23%higher than those without h-BN@Al₂O₃ addition,respectively.Using single factor test,the cutting test of 40 Cr was carried out with（W,Ti）C cermet tool without additive phase under different cutting parameters.The optimal cutting parameters are determined as v_c=240 m/min,a_p=0.2 mm,f=0.102 mm/r.The effects of nano-h-BN and micro-h-BN on the cutting performance of the tool are studied.The results show that the introduction of solid lubricant h-BN into the tool can reduce the cutting force and cutting temperature.However,due to the inability to take into account the lubricating properties and mechanical properties of the tool,the overall cutting performance of the tool is limited.The effect of h-BN@Al₂O₃ on the cutting performance of the tool was investigated.On the one hand,the introduction of h-BN@Al₂O₃ improves the mechanical properties of the tool,improve the grinding quality of the tool,and effectively increases the life of the tool.On the other hand,the solid lubricating film formed by h-BN@Al₂O₃ in the tool-worker contact area can effectively reduce the friction coefficient,accelerate the discharge of chips,and reduce the thermal wear of the tool.Under the condition of minimal quantity lubrication,the synergistic effect of the core-shell structure nano-solid lubricant and the micro-lubricating oil strengthens the self-lubricating performance of the tool and effectively improves the service life of the tool.The effective cutting distance of the W@5 tool in 40Cr steel exceeds 9000 m.