Effect of Ta microalloying on joint performance by tailoring the microstructure during laser beam welding of Ti-22Al-27Nb

The development of Ti₂AlNb based alloys has set a strong competition between conventional Ni-based superalloys and titanium aluminides for aerospace industry due to their compatible/superior mechanical properties. With advent of time, progressive manufacturing technologies are being worked out to transform this novel alloy system into tangible engineering products. Laser beam welding (LBW) of Ti₂AlNb based alloys is a provocative task due to evolution of metastable B2 phase in the fusion zone (FZ). In the present study, a comparative analysis on LBW of Ti-22Al-27Nb was carried out by preparing the joints with/without an interlayer of Tantalum (Ta) microfilm. The evolution of microstructure in the weld zone (WZ) was analyzed through Electron probe microanalysis (EPMA), Electron backscattered diffraction (EBSD) analysis, High-angle annular dark-field (HAADF), Bright field (BF) and Selected area electron diffraction (SAED) imaging. The effect of Ta on laser beam welded joint (LBWJ) of Ti-22Al-27Nb was established by correlating the room temperature mechanical properties (tensile properties and hardness) with the evolution of phases, grains and grain size distribution and inter/intra grain misorientations in the weld zone (WZ). The addition of Ta into FZ triggered heterogeneous nucleation, causing epitaxial as well as non-epitaxial mode of solidification in the weld pool. A substantial increase in tensile strength of Ta adulterated LBWJ of Ti-22Al-27Nb was the provenance of grain refinement and greater fraction of high angle grain boundaries (64.89%). The tensile strength and percentage elongation of LBWJ prepared with 0.01 mm thin interlayer of Ta was measured up to 1049 MPa (i.e. 99.15% of that of the base material) and 9.53% respectively. Further dilution of the LBWJ with Ta contributed no significant increase in tensile strength due to nucleation of Intermetallics along the periphery of FZ and caused an early failure.