Study on High Speed Milling and Wear Resistance of Particle Reinforced Metal Matrix Cladding Coating
Supervisor:zuo dun wen
Laser cladding particle reinforced metal matrix composite coating(PRMMCC)possess high wear resistance,and has a broad application prospect in the surface strengthening and surface repair of mold.However,the application of this coating is seriously restricted because of the high hardness and poor subsequent machinability of the composite coating.Therefore,high-speed milling technology was proposed for the subsequent processing of PRMMCC.The wear resistance of PRMMCC was systematically studied by combining theoretical analysis with experimental research.The surface formation of high speed milling of PRMMCC was discussed.The influence of high speed milling on the hardness and wear resistance of the composite coating was also studied.On this basis,the relationship between the thickness of cladding coating and the service life of the mold was discussed,and the design principle of the minimum initial thickness of cladding coating was proposed.The main work is as follows.(1)PRMMCC is prepared by different laser techniques,and their microstructure and wear resistance were analyzed.The results show that the solid Nd:YAG laser cladding has the advantages of short heating time,low heat input,small heat affected zone and thermal deformation,and can meet the working requirements of the mold.The PRMMCC composed of different carbide ceramic reinforced particles and different alloy base powders was studied.It was found that the composite coating had compact structure,uniform particle distribution,high hardness and good wear resistance.(2)The surface characteristics of high speed milling of PRMMCC are analyzed.It is found that the machined surface is mainly composed of cutting corrugation produced by periodic feed of milling cutter.The surface features of microcracks,matrix tearing,matrix overlaying,plastic side flow and vibrating grain are also observed.The mode of particle removal has an important influence on the surface formation process of PRMMCC.The mechanism of serrated chip formation in high speed milling of PRMMCC was also discussed.(3)The surface roughness evaluation model of PRMMCC with high speed milling was established.The influence degree and rule of milling parameters and particle on the surface roughness of laser cladding PRMMCC were analyzed.(4)A theoretical model for hardness analysis of laser cladding PRMMCC with high speed milling was established.The influence of high speed milling on the hardness of laser cladding composite coating was studied from two aspects,such as the speed of machine tool,axial cutting depth,feed rate of each tooth and the content and size of reinforced particles.The average microhardness of machined surface increases with the increase of machine speed.When the relative speed of the machine tool is not high,the thermal effect of the milling process is stronger than that of the cold work hardening in the milling process,which softens the coating surface.The microhardness of the heat affected zone increased after high speed milling.With the increase of axial cutting depth,the surface hardening degree of coating becomes larger.The average surface microhardness of the composite coating does not increase monotonically with the increase of the mass fraction of the particles,but increases with the increase of the diameter of the particles.(5)The effect of high speed milling process parameters and particle size on wear resistance of laser cladding composite coating was studied systematically by theoretical analysis and experiment.With the increase of milling speed,the wear resistance was improved.The wear resistance of the coating does not increase monotonously with the increase of feed per tooth.The effect of axial cutting depth on the wear resistance of the coating after milling was smaller.The wear volume of the composite coating does not decrease monotonically with the increase of the particle content,and does not change monotonically with the increase of the particle diameter.The milling speed was the most important factors affecting the wear resistance of laser cladding PRMMCC on high speed milling.(6)The wear resistance of the thin coating is poorer than that of the thick coating.The influence of coating thickness on processing allowance and bearing capacity should be considered comprehensively in coating design.The designed coating should have enough processing allowance,so that the composite coating after milling has enough thickness to support the working load of the mold part.(7)Based on the design life of the mold,the initial thickness design model of PRMMCC on the mold surface was established considering the milling parameters,and the design principle of the minimum initial thickness of the coating was proposed.