Investigation On Microstructure And Properties Of Ti-6Al-4V Parts Fabricated By Electron Beam Selective Manufacturing

Ti-6Al-4V（TC4）is the most prevalent titanium alloy in contemporary manufacturing industry.However,TC4 is difficult to be machined because its high hardness and low thermal conductivity.The rapid manufacturing as a new technology named electron beam selective melting（EBSM）can fabricate the parts with complex and porous structure by melting powder melting and solidification layer by layer,which provides a new method for the formation of TC4 titanium alloy and other materials.Consequently,it has broad application prospects and of great significance in the field of aerospace,biomedical and chemical industry.In this paper,TC4 specimens at four angles of 0°、30°、60°and 90°were fabricated by EBSM using Ti-6Al-4V powder with Arcam Q10 equipment.The influence of different heights and orientations on microstructure,solidification and mechanical properties of TC4specimens were investigated.The main conclusions are as follows:（1）EBSM-TC4 specimens mainly consist of a large amount ofαwith a structure of hcp and a little ofβwith a structure of bcc.Theβcolumnar crystals and theβgrains are grown through several bands in the direction（the maximum temperature gradient direction）of the alloy powder deposition.With the decrease of the distance and intersection angle between the specimens and base plate,theβcolumnar crystals’width increases,continuity and tendency to columnar crystals decreases,which is gradually shift to coarseβgrain.In the 90°specimen,with the decrease of between the specimens and base plate,the length of acicular martensiteα’decreases while its quantity increases and the hardness decreases and then increases.The maximum hardness at the top of the 90°specimen is 406.7HV_0.1.The average microhardness on the direction of powder deposition is 353.35HV_0.1.Microstructure at the top of 0°specimen mainly consists of acicular martensiteα’.The maximum hardness is 355.3 HV_0.1.With the decrease of deposition height,the hardness decreases.Becauseα’and its strengthening effect disappear as being heated.And bandedαis formed at the middle and lower of 0°specimen.The average microhardness is333.92HV_0.1.There is a thin layer of ultrafine dense near equal axial grain at the side of every specimen.Refined microstructure enables microhardness to be enhanced compared with that at the center of specimen.（2）Microstructure of EBSM-TC4 specimens at angles of 90°、60°、30°consist of colony Widmanstatten structure formed by parallel distributed acicular martensiteα’atβcolumnar crystals boundary with the same orientation and basketweave Widmanstatten structure formed by crossed acicular martensiteα’inβcolumnar crystals.The transverse yield strength（YS）,ultimate tensile strength（UTS）and elongation（EL）at different heights and angles are similar.The maximum value in 30°specimen is 920MPa and 984MPa.EL is increased to 7%.The direction of longitudinal tensile load is parallel to the axis of columnar crystals.YS and UTS is lower compared with transverse value as the decrease of grain boundary’s strengthening effect.And EL is increased to 12%.With the decrease of the intersection angle between the specimen and base plate,the microstructure is gradually transformed from basketweave Widmanstatten structure formed by dense,fine and crossed acicular martensiteα’to colony Widmanstatten structure formed by parallel distributed bandedαinβgrains in the 0°specimen.YS and UTS are decreased to 758MPa and854MPa with the weakened effect of refined microstructure and the shape transformation ofβfrom columnar crystals to grains.Furthermore,the shape of grains makes the compatibility of strain and the increase of plasticity,which enables EL increase to 14.5%.（3）The fracture toughness(K_IC)is affected by the columnar crystal in the 90°EBSM-TC4 specimen,which is intergranular fracture when the crack is extended in the direction of the powder deposition.When the crack is extended in the direction of the powder deposition,K_ICC is 94.94MPa·m¹/2/2 because of the decrease of grain boundary’s strengthening effect,K_ICC is decreased to 85.33MPa·m^1/2when the crack is extended in the electron beam scanning direction,which is ductile transgranular fracture.The size and distribution morphology ofαphase have effect on K_IC.K_ICC valued 101.45 MPa·m^1/2is generated when the crack is extended along the boundary of the colony formed by parallel distributed bandedαin eachβgrain is greater than that when the crack is extended in basketweave Widmanstatten structure formed by fine and dispersive acicular martensiteα’crossed with each other inβcolumnar crystals.