Study On Boriding Of TA2 By Molten Salt Electrolysis In Na₂B₄O₇-Ca Cl₂ System

As a new type of structural materials,titanium and its alloys have gradually become"all-round metal"and"strategic metal"in industry and civil fields and have been widely applied in all walks of life because the properties of low density,high strength,high and low temperature resistance,good corrosion resistance,non-toxic,non-magnetic and so on.However,titanium and the alloys exhibit the disadvantages of low hardness and poor wear resistance in the application,which seriously affects the service life of titanium parts and makes the preliminary surface treatment of the titanium parts necessary.In this thesis,the technology of molten salt electrolytic boronizing was applied to TA2 to increase its surface hardness.Firstly,the electrolysis mechanism of Na₂B₄O₇-CaCl₂ system was analyzed by cyclic voltammetry and chronological potential method.The experimental results showed that sodium ions in the molten salt were reduced to sodium atoms on the titanium surface,and then sodium atoms reacted with B₂O₃ to produce boron atoms.Finally boron atoms diffused into the titanium matrix to form the borides layer.Secondly,boronizing experiments were carried out with 90%Na₂B₄O₇-10%CaCl₂molten salt to investigate the influences of current density,during time and temperature on the borides layer.The cross sectional morphology and element content of the samples were analyzed by scanning electron microscopy?SEM?and energy dispersive spectroscopy?EDS?;the surface of the samples was analyzed by X-ray diffraction?XRD?;and the surface hardness of the specimen was measured by Vickers hardness tester.The results showed that the thickness of the boriding layer changes parabolicly with the increase of current density.The thickness of the TiB₂ layer is about 4.5?m,and the surface hardness of the boriding sample reaches to 1621HV at the current density of500A/m²,with the temperature of 1193K and duration of 60min;the thickness of the boriding layer increases with the prolongation of duration.The thickness of the TiB₂layer is about 7.9?m and the maximum depth of TiB penetration into the substrate reaches to 26.6?m,with the surface hardness of the boriding sample being 2696HV when boriding at 1193K for 240mins;the boriding layers form more rapidly at higher temperature.The thickness of the TiB₂ layer is about 6.4?m and the average length of the TiB whiskers reaches to 16.4?m,with the surface hardness being 2899HV at 1293K with duration of 30mins.Finally,the diffusion constant K₀ and the diffusion activation energy Q were calculated based on the experiment data,and then growth kinetics equation of the boriding layer was established as the following:d_TiB2=4781.5?exp?-26849/T?t?^1/2,1153K?T?1293K,500A/m²m²...