Study On Microstructure And Mechanicai Properties Of Additive Manufactured Titanium Alloy Based On Electron Beam Melting

The macroscopic microscopic and mechanical properties of Titanium Alloy(Ti-6Al-4V)processed by electron beam melting were investigated by micro-CT,optical microscopy(OM),scanning electron microscopy(SEM),and electron backscatter diffraction(EBSD),nanohardness test system and tensile machine techniques.The results show that,first,the density for all the examined mate rials is up to 99.5%.Internal defects are mainly spherical pores,elongated pores,and unmelting powder.As the building direction changes,the elongated pores exhibit a strong directivity,that is,the long axis direction of the pores is perpendicular to the building direction.The pore distribution has a size effect,and in the large-geometrical sized sample,the spherical pore is dominant,conversely,the elongated pore is dominant.The rich microstructural features at multiscale level contain the columnar dendrites which are parallel to the building direction and basket weave,macroscopically,and there are sub-micro columnar phase of the transition phase and nano-sized ? spherical phase between ? phase.High angle grain boundaries,especially twin boundaries(?13b and ?11b)are the major part of the total GB in acicular ? phase,the distribution of the ? phase texture is random,the Burgers OR is satisfied between ? and ? phase.Second,the single heat treatment method does not significantly reduce internal defects.The microstructure after heat treatment is still composed of columnar crystals consisting of ?(?')clusters approximately parallel to the building direction.However,the width of the columnar crystals decreases slightly with the increase of temperature,and the width of the ? laths increases with the increase of temperature.After the heat treatment,the ? phase is still dominant,but some orientations between ? and ? phase deviate from Burgers OR.The samples in the three states have similar grain boundary misorientation distributions,and the grain boundaries with misorientation angle of around 60°,especially ?13b,are the major part of the total GB in ? phase.Third,the nanohardness of the EBM Ti6Al4 V alloy fluctuates greatly near the substrate,but gradually approaches the periodic fluctuation with the increase of the building height,and the overall hardness is between 2-5 GPa.Tensile test results show that the samples in the horizontal building direction has the best comprehensive mechanical properties,followed by the vertical building direction,and the mechanical properties are the worst at diagonal building direction.The tensile fracture morphology shows that the samples in the horizontal building direction are intergranular ductile fractures,and the samples in diagonal building and vertical building directions are ductile-brittle mixed fractures,and the plasticity is improved after annealing at 850 ?.