Research On Microstructure Evolution Of Ti-6Al-4V Alloy Additively Manufactured By Selective Electron Beam Melting

Additive manufacturing(AM),which is a near-net-shapping process of joining materials to make objects from 3D model data,usually layer upon layer,as opposed to subtractive manufacturing methodologies,offers great flexibilities and freedom in designing and fabricating cutomerized,geometrically complex components with a low buy-to-fly ratio and shorter lead time,therefore,being referred to as a "renaissance in advanced manufacturing industry".Selective electreon beam melting(SEBM),a branch of AM,has higher energy efficiency,higher power density,faster manufacturing capability and lower running cost as well as can protect the material due to the vacuum building environment,comparing to selective laser melting(SLM).SEBM is suitable to produce components of Ti alloys and other mateials that have a high affinity to oxygen and nitrogen,thus has attracted a tremendous attention in aerospace,biomedical implant and transportation industries,etc.However,the microstructures of the SEBM-fabricatd Ti alloys are quite inhomogeneous resulting into non-consistant mechanical peroperties.So far,literatures available all focus on small SEBM-fabricated samples,the results of which can not be fully applied to guide the manufacturing process of practical component.This present work is trying to address this issue.Firstly,we characterized the microstructure in the as-built Ti-6Al-4V sample using OM,SEM,XRD,EBSD,TEM and FIB and studied the phase transformations during the building process by Thermal-Calc.Secondly,we characterized the surface textures,building defects and microstructures in the various samples built in different diameters(01.5mm,02.5mm and ?6.0mm)and at different building directions(0°-90°)and studied their influences on mechanical peroperties.Finally,we tried to homogenize the microstructures and heal the defects by hot isostatic pressing(HIP)in order to acquire consistant mechanical peroperties.The following conclusions are made:(1)During AM of Ti-6Al-4V alloy by SEBM,martensitic transformation(???'),long-range diffusional transformation(???+?)and massive transformation(???m)all took place.The as-built microstructure formed as a mixture of bascketweave,widmannstatten and patchy-shapped ultra-fine ?+? microstructures(the product of massive phase in-situ decomposition)due to the cyclic heat treatment.(2)The occurrence of the P to am massive transformation in Ti-6A1-4V during SEBM has been identified.Massive grains mostly associated with the prior-? grain boundaries(GBs)and part of the massvie grains could even grow across the prior-?GBs.Massive grains took irregular shapes and covered wide misorientations.Massive phase transformation occurred between 800?-893? with a growing speed of about 152?gm/s.Due to the high temperature of the powder-bed and cyclic heat treatment,massive phase in-situ decomposed into ultra-fine a+p.(3)Staircase effect due to slicing the 3D CAD model,ridges formed as a result of the random melting flow and the un-melted powders attached to the surface all contributed to the as-built rough surface finish.The sample diameters have little effect on the surface roughness.The surface roughness first increased with increasing building direction at lower angles and then decreased a little at higher building direction.Both the building diameters and directions significantly influced the microstructures.The higher the building direction is and the smaller the sample diameter is,the finer the microstructure is and more martensite there is in the sample.The mechanical peroperties showed certain non-consistence resulting from a combined effect of building defect and microstructures.(4)The resulting microstructures and mechanical properties of 3 00mm long Ti-6A1-4V rods built vertically and perpendicular to the powder bed by SEBM were characterised along their building direction by dividing each rod into three segments(top,middle and bottom),both before and after HIP.The as-built microstructure of each rod was inhomogeneous;it was coarsest in the top segment,which showed a near equilibrium-lamellar structure,and finest in the bottom segment,which featured a non-equilibrium mixed structure.The tensile properties varied along the rod axis,especially the ductility,but all tensile properties met the requirements specified by ASTM F3001-14.HIP homogenised the microstructure thereby leading to highly consistent tensile properties along the rod axis.The as-built inhomogeneous microstructure is attributed to the temperature gradient in the deep powder bed.Post heat treatment is thus necessary for Ti-6Al-4V samples or parts manufactured from a deep powder bed by SEBM.(5)The resulting microstructure and microtexture of the 220mm high,5mm thick Ti-6A1-4V plates built vertically by SEBM were characterized.HIP lead to a significant increase in maximum pole density.HIP homogenized as well as coarsened the microstructure.Before HIP,the soft a plates(with higher Schimid factors)were surrounded by hard plates(with lower Schimid factors)indicating a high deformation constraint that kept the crack changing the direction during propagation when meeting the hard a plates.The fractured surface was much rougher.After HIP,most of the colonies were reoriented into a direction facilitating the prismatic slip system activation.Because slips would easily cross over the colonies formed after HIP,which lessened the deformation constraint,the plate became more sensitive to the shearing stress at about 45° relative to the tensile loading direction.The fractured surface represented an edge shape.Fortunately,both the mechanical properties before and after HIP met the requirements specified by ASTM F3001-14.