Optimizing microstructure and mechanical properties of Ti-5Al-2Sn-2Zr-4Mo-4Cr alloy electron beam welded joint through post-weld heat treatment

Abstract

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.