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

Abstract

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.