Effect Of Microstructure Heterogeneity On Mechanical Properties Of Electron Beam Welding Joints Of Titanium Alloy

TA15 titanium alloy electron beam welding joint is composed of weld zone(WZ),heat affected zone(HAZ)and base material(BM).As the different microstructures in different zones have different mechanical properties,the microstructures of welding joint are heterogeneous.The heterogeneity of microstructure can significantly influence the mechanical properties of different zones in welding joint,and further influence the mechanical properties of whole joint.In this paper,observation methods were performed to study the different microstructures in different zones of welding joint.Nano-indentation tests were performed to determine the mechanical properties of different microstructures.Afterwards,the mechanical properties of different zones in welding joint were performed.In the end,scanning electron microscope observation on tensile specimen and the microstructure-based finite element simulation were adopted to investigate the microscopic strain distribution.The main results were listed as follow:(1)BM is mainly composed of many equiaxed primary ? phase and structure of ? transformation.The structure of ? transformation is composed of secondary ? plate and retained ? with interval distribution.HAZ is mainly composed of equiaxed primary ? phase and acicular '? phase,and WZ is mainly composed of acicular '? phase with irregular orientation and distribution.A few retained ? exist in HAZ and WZ.BM has strong texture features,while the structures of weld zone and heat affected zone occur dynamic recovery and recrystallization,which lead to disappearance of texture features;(2)The hardness,yield strength and work hardening exponent increase in the order of primary ? phase,the structure of ? transformation,'? phase.The main reason is that the size of secondary ? cluster in the structure of ? transformation is small,which lead to the limited length of slip,while the martensite cluster boundary is a strong barrier to the expansion of slip band;The fracture toughness increase in the order of '? phase,the structure of ? transformation,primary ? phase.The main reason is that the primary ? is pure and uniform.As a result,the crack initiation is difficult.The structure of ? transformation is composed of secondary ? plate and retained ? with more crack initiation,while '? phase exist with hard brittle phase,such as orthorhombic ''? phase,transient ? phase,which lead to the worst resistance to crack initiation;(3)The hardness and yield strength increase in the order of BM,HAZ,WZ,while the impact toughness and resistance for brittle failure increase in the order of WZ,HAZ,BM.Crack blunting occurs in the ? phase and '? phase,which lead to the circuitous crack expansion path,while the crack expands along straight lines in primary ? phase;(4)For tensile specimens from BM and HAZ,a large number of slip bands with 45 degree from tensile direction,gather in primary ? during tensile process.The stress and strain localize in phase boundary.Stress localize in the side of the structure of ? transformation or '? phase,while strain localize in the side of primary ?.With the increase of macro-stress,more and more strains localize in phase boundary,the strain localization bands form in primary ? phase.