The non-uniform temperature field and cooling rates in the molten pool during solidification process fabricate microstructural and mechanical heterogeneities in fusion zone (FZ) and heat-affected zone (HAZ) of thick titanium alloy weldment. In this study, electron beam welding was used to assemble thick Ti-5Al-2Sn-2Zr-4Mo-4Cr (TC17) alloy plates. Microstructural heterogeneity concerning recrystallized martensitic structures with different morphology and distribution features in FZ and retained microstructures after experiencing thermal exposure in HAZ were analyzed to unravel their effects on multiscale mechanical behavior. It was found that needle-shaped martensite (α') exhibited a comparative high nano hardness and the precipitation of needle-shaped martensite cluster contributed to a more severe tensile strain localization in FZ and premature brittle fracture of the joint. The bottom layer of weldment, where the FZ was verified to be martensitic free, exhibited a better combination of strength and ductility. Mo and Al exhibited a better thermostability than Cr, which held the responsibility for the microstructural stability in HAZ during severe thermal exposure after welding.
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