Effect of Ti-Hf-Zr-Cu-Ni high entropy alloy addition on laser beam welded joint of Ti₂AlNb based intermetallic alloy

Ti₂AlNb based intermetallic alloys, a potential competitor for next-generation super alloys, are susceptible to high-temperature embrittlement due to nucleation of a metastable single B2 phase in the fusion zone (FZ) during laser beam welding (LBW). In this study, a high entropy alloy (HEA), Ti-Hf-Zr-Cu-Ni, was self-developed and introduced as an interlayer into laser beam welded joint (LBWJ) of Ti-22Al-27Nb to analyze its impact on the evolution of microstructure in the weld zone (WZ) and subsequently on joint performance. Microstructural examination was carried out through electron probe micro analysis (EPMA), electron backscattered diffraction (EBSD) analysis, high-resolution scanning transmission electron microscopy (HRSTEM) comprising bright field (BF), selective area electron diffraction (SAED) and high angle annular dark-field (HAADF) imaging. Addition of the HEA into FZ of LBWJ triggered heterogenous nucleation during solidification, resultantly, fine-grained B2 with a greater proportion of high angle grain boundaries (HAGBs) was developed. FZ of Ti-22Al-27Nb LBWJ, prepared with an interlayer of HEA, was composed of planar, cellular, columnar and equiaxed dendritic grains; a solidification mode which was different from that observed in LBWJ prepared without adulteration of the HEA. The impact of heterogenous nucleation during epitaxial solidification on mechanical properties was established through micro vickers hardness mapping and tensile test, conducted at room temperature. The average hardness, 343.5 HV, in the FZ of LBWJ prepared with an interlayer of HEA, was compatible with that of base material (BM), 345 HV. The ultimate tensile strength (UTS), 1062 MPa, and percentage elongation, 11.2%, of the HEA tempered LBWJ were found in close approximation with that of BM, 1060 MPa and 13.4%, respectively. A ductile mode of failure was observed during tensile test of the Ti-Hf-Zr-Cu-Ni supplemented LBWJ of Ti-22Al-27Nb, while quasi-cleavage mode of fracture was apparent in the joint of Ti-22Al-27Nb welded without addition of the HEA.