Microstructural and mechanical heterogeneity of Ti-6Al-4V electron beam welded joints: insights into deformation behavior and damage mechanisms

Titanium alloys, especially Ti-6Al-4V, are widely used in aerospace and marine applications due to low density, high specific strength, and excellent corrosion resistance. Electron beam welding (EBW), with its vacuum environment and high energy density, provides an efficient method for joining Ti-6Al-4V. However, during EBW of titanium alloys, the combination of a high fusion temperature and low thermal conductivity generates steep thermal gradients near the fusion zone (FZ), leading to heterogeneous microstructures from the base material (BM) to the FZ. This heterogeneity in EBW joints limits their strength-ductility balance. This study systematically investigates the microstructural evolution and mechanical behavior of Ti-6Al-4V EBW joints at room temperature quasi-static tensile loading. The BM exhibits a bimodal α + β microstructure, while the heat-affected zone (HAZ) progressively transforms from α + β to martensite α′ toward the FZ, which is fully converted into lath martensite α′. Higher beam currents promote coarsening of the α′ laths. Plastic deformation is dominated by dislocation slip, with basal and prismatic <a> slips preferentially activated, while pyramidal <a> and limited pyramidal <c + a> slips progressively contribute at higher strains. The cooperative activation of multiple α (α′) slip systems, efficient slip transfer across grain and colony boundaries, and coordinated deformation of residual β lamellae maintain the joints’ plasticity and mechanical integrity under large strains. At the microscale, nanoscale slip bands (SLBs) within α grains dominate BM deformation, whereas strain localizes at α/β interfaces, facilitating crack initiation. In the FZ, microscale shear bands (MSBs) accommodate higher plastic strain. Their propagation is constrained by high-angle α′ grain boundaries, α′ colony boundaries, and prior-β grain boundaries, preventing catastrophic shear band formation. These results reveal how microstructural heterogeneity and slip system activity govern strain distribution, plastic accommodation, and damage evolution, providing critical guidance for optimizing their strength-ductility balance.