In the application of Ti-6Al-4V to aerospace structural components, when welding thick plates similar of the thickness of the components, microstructure and hardness gradients emerge between the base material (BM) and the joint. This leads to the issue of significant stress concentration in the BM under tensile stress. To address this problem through post-welding heat treatment, this study conducted heat treatments at temperatures both below (mill annealing, MA) and above the beta-transus temperature (beta annealing, BA) on electron-beam weldments of 18 mm thickness Ti-6Al-4V plates. Subsequently, microstructures and hardness were analyzed at different depths from the upper surface and areas (fusion zone (FZ), heat-affected zone (HAZ), and BM), and tensile properties were measured at various depths. The results indicated that α′ observed in FZ and HAZ was resolved through both MA and BA. Particularly after BA, the microstructural gradient that persisted even after MA completely disappeared, resulting in the homogenization of widmanstätten α + β. Consequently, after BA, the hardness gradient in each zone also disappeared, and the tensile strength was higher than in just-welded and MA heat-treated plates.

For the application of Ti-6Al-4V alloys in urban air mobility, safety is very important, so achieving excellent strength and toughness is essential to prevent fractures. Regarding toughness, which is a combination of strength and ductility, it is necessary to derive the optimal heat treatment conditions for this combination of Ti-6Al-4V alloy and further understand its microstructure and fracture characteristics. For this purpose, this study investigated the microstructure in terms of grain size, plate thickness, and element distribution, as well as mechanical properties, including phase hardness and tensile properties, of Ti-6Al-4V alloy subjected to solution treatment and aging (STA) heat treatment under various aging conditions. As a result, this study suggests that solution treatment followed by aging at 630 °C for 480 min can achieve approximately 26% higher toughness than the just-solution treatment process. This is because there is little difference in hardness between the equiaxed α and basketweave structures, and β plates, which contain an excessive V between α plates, function like fibers and delay fracture.