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

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.