Study On The Fabrication Of Laser Cladding Cermet Cutting Tools And Its Cutting Performance

To solve the problem of excessive wear and low cutting life of high-speed steel（HSS）tools,in this thesis,GNPs/TiN/Co-based cladding layer is prepared on the surface of W₆Mo₅Cr₁₄V₂（M2）HSS tool material using laser cladding technology,which significantly improves the hardness and wear resistance,thus increases the cutting life of the tool.Therefore,this thesis first explores the laser process parameters based on TiN/Co-based cladding layer,then investigates the mechanism of graphene（GNPs）in the melt pool under laser irradiation,and finally prepares GNPs/TiN/Co-based cladding layer tools and compares them with HSS tools for cutting experiments.TiN/Co-based cladding layer tool material is prepared and the optimum pulsed laser heat input is determined through the study.At a scanning speed of 16 mm/s and a pulse frequency of 4 Hz,the mechanical properties of the cladding layer are optimised,with an average hardness of 590.3 HV,a friction coefficient of 0.426 and a wear volume of 1.3×10⁷μm³.On the one hand,the low frequency pulsed laser（4 Hz）can reduce the grain size and transform the coarse dendrites into fine equiaxed grains,on the other hand,it can make the TiN particles small and uniformly distributed,compared to the high frequency laser（4000 Hz）,the average TiN particle size is reduced by 45.6%（0.37μm）and the mechanical properties of the cladding layer are improved by the strengthening effect of the particles.Secondly,the evolution and mechanism of action of GNPs within the Co-based melt pool under laser irradiation are investigated and GNPs/Co-based cladding layer tool materials are prepared.The high surface energy,high activity and good homogenisation of GNPs are due to their nano-size,sheet-like structure and the presence of many folds on the surface.The melting of GNPs occurs under the action of high-energy laser irradiation,which generates carbides such as Cr₂₃C₇ and Cr₇C₃ in situ with the alloying elements in the melt pool,increasing the hardness of the cladding layer,with a maximum average hardness of 576.8 HV,an increase of approximately43.3%,and the formation of temperature gradients during the formation of carbides,leading to fluctuations in composition and energy fluctuations,increasing the nucleation rate of the melt pool and thus refining the grain.The average grain size of the Co-based cladding layer with 3%GNPs decreased by 47.5%（4.2μm）compared to the cladding layer without GNPs.The unmelted GNPs in the melt pool are retained within the cladding layer as the melt pool cools,providing wear reduction and lubrication.The optimum friction coefficient of the GNPs/Co-based clad layer is 0.358and the wear volume is 4.86×10⁷μm³,a reduction of approximately 26.0%and 49.5%respectively,according to frictional wear tests.GNPs/TiN/Co-based cladding layer tool materials are prepared with an average hardness of up to 777.2 HV,an increase of 33.5%over that without GNPs.The increase in hardness is not only due to the in-situ generation of carbides such as Cr₂₃C₇ and Cr₇C₃ by GNPs during the melting process,but also due to the in-situ generation of Ti（C,N）solid solutions by GNPs and TiN under laser irradiation,which is explained by thermomechanical analysis to make the decomposition of TiN and the generation of Ti（C,N）within the melt pool possible during laser irradiation.The geometry and properties of the cladding layer change significantly when the pulsed laser duty cycle and laser power are simultaneously varied while controlling the same laser heat input.With the decrease of the laser duty cycle,the depth of the melt pool and the thickness of the heat-affected zone of the cladding layer increase,and the TiN aggregation uplift phenomenon gradually disappears.At a laser duty cycle of 65%,the TiN/Co-based cladding layer achieves the finest and most uniform structure,the average TiN particle size decreases to 0.27μm,the average hardness reaches the highest,approximately 779.0 HV,and the wear volume decreases to 0.84×10⁷μm³;the hardness of the GNPs/TiN/Co-based cladding layer reaches 921.0 HV.Finally,45 steel is selected as the cutting workpiece and the cutting performance of GNPs/TiN/Co-based cladding layer tools and HSS tools is investigated.The optimum cutting process is obtained at a cutting speed of 40 m/min,a back draft of 0.2mm and a feed of 0.102 mm/r.Compared to HSS tools,the main cutting force of the cladding layer tool is reduced by approximately 10.0%（119 N）and the cutting temperature is reduced by approximately 16.7%（253.6°C）due to the excellent high temperature hardness of the cladding layer and the excellent wear reduction and lubrication of the GNPs,which effectively improves the cutting performance of the cladding layer tool.The effective cutting distance is approximately 5700 m.Wear is mainly in the form of slight bonded wear on the front face and abrasive and bonded wear on the rear face.