Study On The Boriding Of TC4 Titanium Alloy By Molten Salt Electrolysis

Having the advantages of low density, high strength, good corrosion resistance and biocompatibility, TC4 titanium alloy has been one of the most widely used titanium alloys. However, the surface hardness of TC4 is low and the wear resistance is not good, hence it is necessary to carry out the resrarch work on surface hardening technology for TC4 to improve the surface hardness. In this thesis the method of boronizing by molten salt electrolysis, which has the advantages of low cost, easy operation and obvious effect, was applied to obtain borides coating on the surface of TC4.In the experiments, the boriding of TC4 substrates was carried out in an electrochemical cell in which a mixture of sodium tetraborate and sodium carbonate was used as the base ingredients of molten electrolyte, the TC4 test pieces were attached to the cathode holder of the electrochemical cell while a graphite crucible served as the anode. DSC-TG analysis was conducted on the mixture of sodium tetraborate and sodium carbonate with different compositon to obtain the proper one. The boriding of TC4 was carried out with different current denstity, processing time and temperature. X-ray diffraction method was used for chemical phses analysis of the molten salt and the borides coating, Scanning electron microscopy(SEM) and EDS were applied to observe microstructure and anylyze the element content of the borides coating of the TC4 substrate respectively. The microhardness tester was applied to measure the surface hardness of the boriding TC4. The effects of current density, process duration and temperature on boride layer thickness, chemistry and morphology was investigated and the mechanism of boriding was discussed.According to DSC-TG analysis, the physical and chemical changes happened when heating the mixture of sodium tetraborate and sodium carbonate were discussed,and it shows that the mixture of salt with 20%Na2CO3 was suitable for electrolytic boriding. By analyzing the microstructure and chemical compositing of the boriding sample, it was found that a double layer boring coating can be obtained on the TC4 titanium alloy when TC4 titanium alloy is 3500 A/m2 with boriding temperature of 900? and process duration of 110 min; the outer layer of the boring sample was a continuous dense layer, with the main components being TiB2, and the inner layer was mainly comprised of TiB whisker. The thickness of dense layer is about 14.8 ?m, and the surface micro hardness of the boriding sample reaches to HV712, which is 2.5 times of that for TC4 titanium alloy. In addition, the influences current density, process duration and temperature on the thickness and microstructure of the boriding layers were analyzed; as some pore and small cracks existed in the junction of diffusion layer and substrate, the reason for this phenomenon was discussed.According to the chemical analysis of the solid salts and tests results from DSC-TG for the mixed salts, combining thermodynamics calculation of the related reactions, the mechanism of boriding was discussed and the main reactions during boriding were summarized.