Influences of multi-scale stress concentration on the low-cycle fatigue behaviors of electron beam welding joints of TC4-DT titanium alloy with the thickness of 130 mm

Damage-tolerant titanium alloy TC4-DT has been widely applied in the manufacturing of large equipment used in aerospace due to its excellent mechanical properties. The microstructures and low-cycle fatigue (LCF) behaviors across various layers of electron beam welding (EBW) joints of TC4-DT titanium alloy with a thickness of 130 mm were investigated. Experimental results show that the microstructures in the weld are not uniform along the thickness direction. Compared with other weld metal (WM) zones, the weld root exhibits more α′ phases with a dispersed distribution and smaller size, resulting in the highest microhardness in this zone. The weld root is a region with relatively poor LCF behaviors in super-thick EBW joints, where the significant microhardness gradient is a key factor contributing to the poor fatigue performance. Fatigue cracks propagate through the connection of micro-pores in series, and stress concentration in micro-areas is a crucial factor in the formation of micro-pores that can lead to fatigue damage.