| Summary: | Electron beam welding (EBW) is suitable for joining titanium alloys. However, the utilization of EBW leads to an uneven microstructure within the weldment due to the rapid solidification and non-uniform temperature gradients, which results in the formation of coarse columnar β grains, ghost α phases, and metastable martensite α/α′ phases, decreasing the mechanical properties. Post-welded heat treatment (PWHT) is a practical method of enhancing mechanical properties by changing the phase composition and grain size. PWHT can promote decomposition and recrystallization of the metastable martensite α/α′ phase in the fusion zone (FZ) and heat-affected zone (HAZ), creating dense nanoscale acicular α phases to build α+β colonies. As the holding time and temperature increase, the thickness and spacing of the acicular α phases increase. The increase in acicular α phases thickness and spacing has a more significant impact on the nanohardness but a lesser effect on the microhardness. Optimal PWHT is 630 °C for 2 h, followed by air cooling, which promotes the precipitation of dense nanoscale acicular α phases in the columnar β grains and ghost α phases, resulting in fine-grain strengthening effect, thereby greatly enhancing the mechanical properties of the welded joint with a tensile strength of 1053.8 MPa, an elongation of 14.33%, and a fracture at the base metal (BM). Moreover, due to the formation of dense nanoscale acicular α phases, more grain boundaries are generated, which hinder the dislocation movement, so that the localized strain in the FZ and HAZ is much lower than that in the BM.
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