Role of Pd interlayer on NiTi to Ti6Al4V laser welded joints: Microstructural evolution and strengthening mechanisms

[Display omitted] •Dissimilar laser welds between NiTi and Ti6Al4V exhibiting the NiTi superelasticity and sustaining stress and elongation up to 520 MPa and 5.6 % were achieved using Pd interlayer.•The Pd interlayer significantly lowered the volume fraction of the brittle Ti2Ni intermetallic compound from 83 to 10 %.•The higher thermodynamic formation potential of Ti-Pd compounds than the Ti-Ni compounds facilitated the formation of TiPd and Ti2Pd compounds at the expense of Ti2Ni.•The accumulated irrecoverable strain on the dissimilar joint was below 1% after 30 load/unload cycles. Obtaining a reliable NiTi to Ti6Al4V dissimilar joint exhibiting NiTi's superelasticity can provide design flexibility in aerospace and biomedical fields via integrating distinct material benefits. However, this materials couple is vulnerable to severe embrittlement due to the development of excessive intermetallic compounds (IMCs), namely Ti2Ni. Pd-free and Pd-interlayered NiTi-Ti6Al4V laser joints were evaluated for their microstructure, compositional changes, thermodynamic mechanism, and mechanical properties. The presence of Pd constrained the formation of Ti2Ni IMC, and NiTi-Ti6Al4V joints with excellent mechanical properties demonstrating superelastic behavior were achieved for the first time. The tensile strength and rupture strain of the Pd-added NiTi-Ti6Al4V joint improved more than twofold, reaching 520 MPa and 5.6%, respectively. During cyclic tensile testing, the Pd-added joint demonstrated superelasticity and a comparable irrecoverable strain to NiTi (1 versus 0.65%). Multiscale characterization revealed that the fraction of Ti2Ni decreased from 83 to 10% near the NiTi boundary and 24 to 6% at the weld center compared to the Pd-free joint. The superior thermodynamic formation tendency of Ti-Pd compounds over Ti2Ni IMC favored their development, and thus Ti-Pd and NiTi compounds dominated the fusion zone (FZ) at the expense of Ti2Ni IMC, explaining the improved mechanical performance of the Pd-added joint.