Tailoring microstructures and mechanical properties of laser welded joints in laser powder bed fusion Ti6Al4V alloys

Ti6Al4V alloys fabricated via Laser Powder Bed Fusion (LPBF) exhibit excellent mechanical properties, rendering them highly desirable for advanced engineering applications. However, the relatively limited build size of LPBF components constrains their use in large-scale parts. To address this limitation, the joining of LPBF-fabricated Ti6Al4V alloys using welding technique has emerged as a viable strategy for manufacturing large-scale components. However, the ductility of the welded LPBF Ti6Al4V is significantly reduced after welding as reported in the literature. In this study, a Laser Metal Deposition (LMD) process was employed to weld of LPBF Ti6Al4V alloys, with particular focus on improving the ductility in the welded joints. The microstructure of the WM in the as-welded joint contained continuous grain boundaries α (α_GB) and coarse Widmanstätten grain boundary α (α_WGB) with inhomogeneously sized ά martensite, which resulted in a deficiency of ductility. To further enhance ductility, a post-weld annealing heat treatment was conducted at a temperature slightly below the β-transus temperature. This treatment facilitated a transformation of the WM microstructure into a mixture of lamellar and globular α phases with an intergranularly dispersed β phase. During heat treatment, the nucleation mechanism of α_WGB shifted from induced nucleation to interface instability nucleation. Consequently, the fracture location transitioned from weak interfaces between α_GB and α_WGB in the as-welded condition to the α + β basket-weave structures within the β-Ti columnar grains of the WM. Compared to the as-welded joints, the heat-treated joints exhibited a 10 % reduction in Ultimate Tensile Strength (UTS) but demonstrated a remarkable 110 % increase in Elongation Index (EI), achieving a better strength-ductility balance.