Microstructure And Properties Of Surface Electroplating And Electron Beam Remelting Of Titanium And Titanium Alloys

Titanium alloy is widely used in aerospace,chemical industry and biomaterial structure field due to its excellent comprehensive performance,and it has become an indispensable important material.With the increasingly severe working conditions and development requirements,the shortcomings of titanium's lo w hardness and large coefficient of friction seriously affect the service life and increase costs,thus limiting the application of titanium alloys.In order to solve the problems of low hardness and poor wear resistance of titanium alloys,the method of improving hardness and wear resistance through surface modification has attracted more and more attention,which can maximize the advantages of titanium alloy in various applications.Therefore,it is of great significance to study the surface modification of titanium alloys.In the case of surface modification of titanium alloy by single electron beam,the problem of powder repulsion is easy to occur during the interaction between electron beam and material in vacuum chamber.In this paper,the combination of electroplating and electron beam surface remelting modification technology was studied,so the structure and mechanical properties of the coating prepared by composite technology were tested and analyzed.The titanium material is TC4 titanium alloy and TA2 pure titanium.The electroplating method uses nickel plating technology to pre-coat the titanium surface,and performs composite plating with magnetic stirring.The ceramic powder TiC is added to the plating solution and mixed evenly.Based on density functional theory,the intrinsic properties of intermetallic compounds were calculated using first-principles to verify their related properties.Related research contents are as follows:The surface of TC4 titanium alloy substrate was pre-plated with nickel and re-melted by electron beam.After testing and analysis of the coating,the coating has no cracks and pore defects,and the interface exhibits metallurgical bonding characteristics.The composite coating is composed of Ti2Ni,TiNi intermetallic compound and nickel-rich titanium phase.Due to the higher cooling rate,in the non-equilibrium molten pool,the Ti2Ni phase is dendritic or needle-like,and the TiNi phase is dispersed in a point-like manner.The grain size inside the cladding layer is significantly reduced,and the grain boundaries become blurred and interwoven.A large number of dislocations and stacking faults appear inside,and there are needle-like structures with martensite transformation in the structure.The Ti2Ni phase and TiNi phase dominate the composite coating in the form of plasticity and elasticity,respectively.The average nano hardness of the composite coating is 6.5±0.5 GPa.Affected by the residual tensile stress,the highest hardness position can reach 7.03 GPa at about 15 ?m from the surface.Grain refinement,increased dislocation structure and in-situ formed Ti2Ni/TiNi are the main reasons for the increase in coating hardness.The surface of TA2 pure titanium was first electroplated with nickel/titanium carbide,and then re-melted by electron beam to obtain a metallurgical bond between the coating and the substrate without pores or cracks.The conposite coating consists of a composite phase of Ti2Ni,TiNi intermetallic compound and ceramic particles TiC.The TiNi phase is composed of irregular dendrites in the coating,while the Ti2Ni phase is present in the coating as dendrites or fine dendrites/cells.Due to the rapid melting and cooling of the molten pool,a large number of dislocations and twins gather inside the coating grains,and the grain boundaries are no longer clear.During the high-temperature electron beam remelting process of TiC particles,part of the edges of the particles will dissolve and recrystallize into elliptical particles embedded in the Ti2Ni phase.Due to the dispersion of ceramic particles TiC and the in-situ formed Ti2Ni and TiNi phases,the maximum hardness of the coating reached 7.95 GPa.Compared with the original TA2 matrix,the hardness has been increased by more than three times.The value of the friction coefficient(0.33)is the lowest among the substrate and the simple composite plating,and the wear resistance is greatly improved.Compared with 0.61 of the original substrate value and 0.49 of Ni/TiC coating value,the friction coefficient of remelting after composite plating has the lowest value(0.33),and the wear resistance is greatly improved.The spatial structure of Ti2Ni,TiNi and TiNi3 was optimized using the Castep software,and the first-principles method was used to calculate the elastic constant,bulk modulus,shear modulus,elastic modulus,Poisson's ratio and hardness characteristic of the titanium-nick el intermetallic compound.At the same time,the characteristics of Total Density of State and Partial Density of State are analyzed.Related research shows that with the increase of the relative content ofNi,the relative values of bulk modulus,shear modulus,elastic modulus and Poisson's ratio increase in the order of Ti2Ni,TiNi and TiNi3.Among the titanium-nickel intermetallic compounds,using the empirical relationship between hardness and elastic modulus,TiNi3 has the highest hardness in the order of TiNi3>Ti2Ni>TiNi.In the vicinity of the Fermi level(-10?15 eV),the density of states is the transition metal of Ti and Ni.Compared with 3p and 4s electrons,3d electrons have the greatest influence on the density of states.As the relative content of Ni increases,the characteristics of the covalent bond of the alloy increase,while the characteristics of the metal bond decrease.