The Microstructure And Properties Of TA7/TC4 Dual Titanium Alloy Produced Via Electron Beam Rapid Manufacturing

Titanium alloys are widely used in engine blades,wheel discs,aircraft landing gear,fuselages and other structural parts because of their high specific strength and other performance advantages.Due to the increasingly complex working conditions on aircraft,the performance of single alloy parts cannot meet the requirements,and double titanium alloys can better meet the performance requirements of different parts of the same part.At the same time,additive manufacturing technology has obvious advantages in forming double titanium alloy and gradient materials.The electron beam additive manufacturing technology is less studied in terms of the forming process and the influence law on the tissue properties.In this study,the forming process and tissue properties of TA7/TC4 double titanium alloy by electron beam additive manufacturing are mainly investigated.The effects of different process parameters and different interpass lap rates on the macroscopic morphology of the deposited layers of TA7 alloy were investigated.The results show that the layer height increases and then decreases with the increase of electron beam flow,while the layer width continues to increase;the deposited layer width and height both tend to increase and then decrease with the increase of wire feeding speed;the layer height continues to increase and the layer width continues to decrease with the increase of moving speed;too high or too low lap rate will result in poor forming quality,so a lap rate of 35% is chosen for deposition.The differences in the organization and properties of single titanium alloy along the deposited layer direction were investigated.The results show that the TA7 alloy has a lamellar and serrated α organization at the bottom of the deposited layer,while the lamellar αorganization at the top of the deposited layer.With the increase of height,the tensile strength increases,the elongation decreases,and the fractures all show ductile fracture;TC4 alloy shows a net basket organization with high tensile strength and plasticity in the deposition layer direction,in addition,the interlaced organization of strip α phase and lamellar α′ phase is formed at the top of the deposition layer.The TA7 side of the double titanium alloy shows mainly massive α-phase,which is close to the top of the deposited layer of the TA7 alloy,while the TC4 side shows an acicular martensitic transformation phase and vertical transition layer growth.Due to the anisotropy of the tissue structure,the microhardness increases significantly from the TA7 side to the TC4 side.River-like patterns and quasi-dissociation surfaces are observed in the fracture,which is shear fracture and indicates good plasticity.After heat treatment,a little β organization appears inside the TA7 alloy,and the middle of the deposited layer has a Weiss structure,and the comprehensive mechanical properties are slightly enhanced,and the fracture still shows a typical ductile fracture;while the TC4 alloy shows a certain degree of coarsening of intracrystalline α after heat treatment,and the continuous α phase on the original β grain boundary is more clearly defined,and the secondary α phase content increases.The elongation of the specimens decreased compared with the deposited state,and the fracture mode was all ductile fracture.After the stress relief of the double titanium alloy,the TC4 side shows a mesh basket organization and the TA7 side is equiaxed.The coarse α structure is generated along the initialβ grain boundary,which makes a little lamellar α appear on the TA7 side,indicating a certain fusion of the transition layer;during the recrystallization process,the grains on both sides of the grain boundary grow again,the columnar α on the TA7 side extends along the parallel deposition direction with a certain directionality,and the striped α+β phase on the TC4 side grows in the perpendicular grain boundary direction in a staggered manner,which constitutes a typical net basket organization and makes the transition boundary more blurred.There is no significant change in hardness and tensile strength of the double titanium alloy after heat treatment,but the elongation increases significantly,and the fracture mode is shear fracture.