Microstructures And Mechanical Properties Of Electron Beam Welded(?+?)Titanium Alloy

Electron beam welding technology is an important joint method to titanium alloy,which has a significant influence on promoting the application of titanium alloy.In this paper,TC4 and Ti62A titanium alloy were used as experimental materials.The micro structure and mechanical properties of base material(BM),heat-affected zone(HAZ)and fusion zone(FZ)of welded joints were investigated by scanning electron microscopy,electron probe micro-analyzer and electron backscatter diffraction.The micro structure evolution of the welded joints and the characteristics of mechanical behavior were analyzed.For the above two electron beam welded joints,the micro structure variation from BM to FZ included the reduction of primary a grains,the increase of ? transformed structure and secondary a phase.The rim-a,dendritic a,abnormal secondary a(as)colonies and the "ghost" structures formed in the HAZ.The phase transformation and the redistribution behavior of the elements occurred simultaneously due to the welding thermal cycle process.The atoms diffusion will reduce the concentration gradients at the ?/? interface and may even cause the abnormal distribution of elements in some secondary a phases.The crystallographic orientations of Rim-a and the primary a grain is consistent,and the formation of Rim-a is related to the large concentration gradient of elements at the ?/? interface.The dendritic a may have evolved from tiny dendrites remaining in the coarsening behavior of primary a grains.The abnormal as colony and the "ghost" structures are both originated from ???s transformation.The high(or low)concentration of A1(or V)in the two a structures facilitates the growth of as,resulting in the large size of as lamella.The nanoindentation experiments of TC4 titanium alloy show that the hardness of the "ghost" structure and the adjacent transformed ? structure regions is mainly dominated by the anisotropy of the hexagonal close packed structure of the a phase.The hardness of "ghost" structures is higher than that of the nearby ?t regions possibly due to the low volume fraction of ? phase and high Al concentration.The in-situ tensile test of TC4 titanium alloy shows that the maximum tensile force of BM,HAZ and FZ are different.The maximum tensile force of FZ is the highest,the parent metal is lower,and the heat affected zone is the lowest.The plastic deformation characteristics of the notched in-situ stretching process are observed and the result indicates that slip zones were produced in the primary a grain of BM,slip zones were produced along 45° to the normal direction of the arc in FZ,slip zones were firstly produced in the primary a grain near transformed ? structure and then rapidly expanded into the primary a grain.The hardness test of Ti62A titanium alloy shows that the hardness of BM is lower than that of FZ due to the thinner a lamellas in the FZ.The hardness distribution of the HAZ is close to that of FZ,but the hardness distribution of this zone is very uneven,which is related to the transition state of microstructure in this zone.The tensile test of Ti62A titanium alloy shows that the strength of the welded joint is high while the plasticity is slightly low.Compared with the single aging treatment after welding,the a layers in the FZ and the HAZ of the Ti62A titanium alloy welded joint are coarsened after the solution aging heat treatment.The thicker a layers result in the decrease of hardness and tensile strength and the increase of plasticity and fracture toughness.