Diffusion effects of β-Stabilizing element to interface microstructure evolution and nano-mechanical characteristics in dissimilar TC4/TC17 LFW joints

This study used Linear Friction Welding (LFW) to join dissimilar titanium alloys joints of TC4/TC17, to investigate diffusion effects of interface elements and their relationship to phase transformations and nano-mechanical properties. Results showed that short-range diffusion of β-stabilizing elements like Mo and Cr occurred across the interface: with these elements migrating from the TC17 side toward the TC4 side. This movement induced the development of a narrow acicular band dominated by orthorhombic martensite (α′′). The transmission electron microscopy analyses confirmed that the α′′ phase exhibits a Burgers orientation relationship with both α and β phases on either side of the interface, demonstrating good crystallographic orientation inheritance following welding. Combined high-angle annular dark-field and energy dispersive spectroscopy analyses identified inside the recrystallized β grains on the TC17 side of depletion locally of β-stabilizing elements. In these grains nanoscale superlattice β precipitates were identified; which were consistent with β matrix and appeared as one-third order superlattice reflections in selected area electron diffraction patterns. However, in the TC4 side and further away from the interface, the lack of β stabilizers and cooling down below the β-transus temperature, produced solely equiaxed α instead of α′ grains. Nanoindentation measurements showed a graded range of mechanical properties across the interface, across α′′ > α > β, with α′′ showing the largest hardness value (4.96 GPa). These findings show the important role of β-stabilizer diffusion to interface phase transformations and its effects on mechanical properties locally, and provide essential insights into microstructural changes and allow to design for performance the TC4/TC17 linear friction welded joints.