Temperature-gradient induced microstructure evolution in heat-affected zone of electron beam welded Ti-6Al-4V titanium alloy

The heat-affected zone（HAZ） of electron beam welded（EBW） joint normally undergoes a unique heat-treating process consisting of rapid temperature rising and dropping stages, resulting in temperature-gradient in HAZ as a function of the distance to fusion zone（FZ）. In the current work,microstructure, elements distribution and crystallographic orientation of three parts(near base material（BM） zone, mid-HAZ and near-FZ) in the HAZ of Ti-6Al-4V alloy were systematically investigated. The microstructure observation revealed that the microstructural variation from near-BM to near-FZ included the reduction of primary α（αp） grains, the increase of transformed β structure（β;） and the formation of various α structures. The rim-α, dendritic α and abnormal secondary α（αs） colonies formed in the mid-HAZ, while the "ghost" structures grew in the near-FZ respectively. The electron probe microanalyzer（EPMA） and electron back-scattered diffraction（EBSD） technologies were employed to evaluate the elements diffusion and texture evolution during the unique thermal process of welding. The formation of the various α structures in the HAZ were discussed based on the EPMA and EBSD results. Finally, the nanoindentation hardness of "ghost" structures was presented and compared with nearby β;regions.