The Microstructure And Tensile Behavior Of TC4 Titanium Alloy Produced Via Electron Beam Rapid Manufacturing

Electron beam rapid manufacturing(EBRM),as one of 3D printing technologies,employs an electron beam as a heat source and alloy wire as feedstock.The main attractions of EBRM technology are its high efficiency and economy in fabricating large,complex near net shape components dielessly and only needing limited machining.In this study,the microstructure,texture and mechanical properties of as-built,HIP-ed and annealed TC4 alloy fabricated using EBRM have been investigated by using the techniques of scanning electron microscopy,electron backscatter diffraction,transmission electron microscopy and X-ray diffractometry.Tensile deformation and fracture behavior of TC4 alloy under unidirectional loading conditions are studied using in situ/quasi-situtensile.To improve consistency of mechanical propertiesof TC4 alloy,the main factors affecting the mechanical anisotropy and deformation inhomogeneity of TC4 alloy are analyzed by theoretical calculation method.The microstructure of the TC4 alloy contains columnar prior ? grains growing along the building direction(Z direction).With the increase of the deposited height,the mean width of columnar grains increases rapidly in the initial stage,and then the increasing rate of the columnar grain slows down.Inside columnar prior ? grains,fine a lamellae microstructure with typical a texture transformed from strong<100>p fiber texture can be observed.The intensity of a texture increases with the deposited height.Due to different thermal history,the a lamellae at the top layer are relatively fine with a certain amount of dislocations inside them,whereas the relatively coarse a lamellae are at the bottom layer.The a lamellae also exhibit typical spatial orientation distribution.The calculated result shows that the angles between the X direction(scanning direction of electron beam)and the trace direction of the growth direction of a lamellae on the X-Z plane are mainly distributed around?50° or?130°.The morphology of the columnar prior ? grains and the a texture does not be changed obviously after HIP-ed or annealing process,but the intensity of the a texture increases with the annealing temperature.The deformation behavior of alloy is highly affected by its microstructure.For the as-built alloy,some planar slip lines along the a lamellae can be observed.Prismatic slips are the main mode of deformation,as well as a small number of basal and pyramidal slips.After being HIP,prismatic and basal slips are the main mode of deformation,accompanied by abundant cross slips and coexistent slips,and most of these slip lines are blocked within an a lamellae.For the annealing in the ?+? phase region,the slip line is preferentially formed along the primary a lamellae.The fracture behavior of the alloy is also highly influenced by the crystal and spatial orientation of the a lamellae.When the a lamellae have a great Schmid factor for the prismatic slip,and the maximum shear stress is consistent with the broad face of a lamellae,crack will be easily nucleated and grown along the a lamellae.The prior(3 grain boundary can effectively hinder the crack propagation.Some pores and unfused defects can be observed within the as-built alloy.Due to the orientation and distribution characteristics of unfused defects in the as-built alloy,the strength,plasticity and impact toughness of tensile sample in Z direction significantly decreases,but these defects play a relative weak influence on other oriented samples.The tensile strength and ductility of the as-built samples slightly increase with the deposited height.HIP-ed process can effectively eliminate pores and unfused defects of the as-built alloy.The strength of the HIP-ed sample slightly decreases with the increase of the deposition height.The annealing temperatures also have a significant effect on the mechanical properties of the alloy.In the ?+? phase region,with the increase of annealing temperature,the refined a lamellae can lead to an increase of the strength and ductility.In the ? phase region,the coarsening prior ?grains can result in a significant reduction of the ductility.For the HIP-ed or annealing alloys(in the ?+? phase region),obvious anisotropy in tensile properties can be observed on the X/Y-Z plane.The X/Y-Z-22.5/45 samples exhibit higher strength but lower ductility,whereas the sample in Z direction has the best ductility but the lowest strength.The samples on the X-Y plane exhibit approximately tensile isotropy.The calculated result shows that the crystal and spatial orientation of the a lamellae are two main factors that lead to the anisotropic tensile strength and ductility.Obvious inhomogeneous deformation can be found in the samples when the loading direction is along the X-Y plane or 45° to the X-Y plane.This inhomogeneous deformation is highly associated with crystal orientation of prior? grain.For the samples along the X-Y plane,a relatively low deformation resistance is found when the minimum angle,0,between the<001>p direction and the projection of the loading direction onto X-Y plane is close to 0°,but when the ?is close to 45°,the prior P grain shows higherdeformation resistance.In a contrast,for the 45°samples,the prior ? grain has relatively low deformation resistance when the 0 is close to 45°,but higher deformation resistance when the 0 is close to 0°.