Study On Microstructure And Properties Of Ti6Al4V Alloy Fabricated By Additive Manufacturing Before And After Solid Boronizing

In this paper,the microstructure and mechanical properties of Ti6A14V alloy fabricated by SLM were studied at first,and then the solid powder boronizing preparing boron-titanium compounds layer was studied to improve the surface properties of Ti6A14V alloy fabricated by SLM.The morphology,structure and performance of boride layers were analyzed by Scanning Electron Microscopy(SEM),Optical Microscopy,X-ray Diffraction,Microhardness Tester,Friction and Wear Testing Machines and other instruments.The main conclusions are as follow:Ti6A14V alloy fabricated by SLM consists of a martensite phase.In addition,there exists anisotropy between the horizontal and vertical section microstructure due to the occurrence of epitaxial growth,and the former microstructure seems equal-axis and the latter is column.There are some pores and non fusion defects in the range of 200 μm near the surface,and the inner parts of the formed parts are almost dense and have no obvious defects.The main mechanical properties of Ti6Al4V fabricated by SLM(tensile strength of 1084MPa,yield strength of 997MPa,elongation of 13.2%)are higher than that of the traditional cast and forged Ti6Al4V.The tensile fracture mechanism is mainly the ductile fracture at room temperature.The primary selection of activator experiments showed that the KBF4 could not as titanium alloy boronizing activator,La2O3 and ZrO2 had some catalytic effect during the process of boronizing were found,while CeO2 and HfO2 limited the growth of the boride layer.RE(rare earth)-boronizing experiments were carried out on Ti6A14V alloy fabricated by SLM.Single factor tests and orthogonal experiments were carried out to determine the best proportion of solid boronizing agent and the best boronizing process,namely the boronizing heating temperature is 1050℃,boronizing holding time is 15h,boronizing agent formulated as a proportion:20%B4C+4%La2O3+76%SiC.The thickness of the boride layer was 43pm after optimization process.Analysising phase composition of boride layer after the optimization process showed that boride layer is composed of two phases,that are,TiB2 and TiB.The effect of La2O3 on hardness、friction properties、wear resistance of the boride layer on Ti6A14V alloy fabricated by SLM were investigated.The results showed that permeation boronizing layer is composed of two kinds of boron-titanium ceramic phases,that are,the outer layer of TiB2 and internal TiB.The microhardness of permeation boron layer is 1910HV0.2,this value is about 6 times higher than that of the substrate surface.The friction coefficient of boride layer is stable between 0.2 and 0.25,compared with 0.45,which of the substrate,this value increased nearly two times.According to the principle of thermodynamics and diffusion principle,by discussing the formation of dual phase boride(TiB and TiB2)and catalytic mechanism of RE,it was found that the addition of La2O3(RE)can reduce the energy requirement for the system reaction,and adsorb on the surface of substrate to form an active center,to further promote the formation of TiB and TiB2.