Temperature-dependent tensile anisotropic behavior of WAAM-TC4 dual-phase titanium alloy: Microstructural evolution, deformation mechanisms, and fracture characteristics

Understanding the anisotropic behavior of Wire Arc Additively Manufactured (WAAM) TC4 titanium alloy is essential for optimizing the mechanical performance. This study examines the anisotropic characteristics of WAAM-TC4 by analyzing microstructure, mechanical properties, and deformation mechanisms across multiple length scales and temperatures. The alloy features a hierarchical microstructure with coarse prior-β columnar grains aligned along the build direction, significantly influencing anisotropic response. different α phase structures, with α laths enclosed by thin β interlayers, further contribute to this behavior. Tensile tests at 0°, 45°, and 90° orientations reveal pronounced anisotropy, with the highest ultimate tensile strength at 45° and the greatest elongation at 90°. At lower temperatures (20 °C and 400 °C), fine α laths and dense phase boundaries restrict dislocation motion, increasing strength but reducing ductility. As the temperature increases, α laths coarsen, phase boundaries decrease, and ductility improves. These variations arise from the growth direction of columnar grains, phase transformation orientation, and the brittle-to-ductile transition with increasing temperature. Variant selection within prior-β grains plays a critical role in anisotropic deformation, where favorable orientations facilitate slip, while others act as barriers, affecting the overall mechanical response. Deformation mechanisms, including slip and phase boundary slip (PBS), are influenced by α/β interfaces, resulting in strain localization and anisotropic fracture modes. Multiple PBS events lead to shear strain localization bands (SSLBs), which initially enhance ductility but contribute to strain localization and cracking in later stages. The hierarchical microstructure induces anisotropy through multi-scale deformation mechanisms, where grain boundaries, α variant distributions, and slip system orientations generate localized strain heterogeneity. These findings offer valuable insights for optimizing the mechanical properties of WAAM-TC4 in advanced structural applications.