Strengthening synergies of various interlayers in NiTi toTi6Al4V dissimilar laser joints

Abstract

A sound NiTi-Ti6Al4V dissimilar joint featuring NiTi's superelasticity is crucial for merging both material benefits in aerospace and biomedical fields. Yet, the solidification of excessive intermetallic compounds (IMCs) for dissimilar par, notably Ti2Ni, poses a considerable risk of severe brittleness. Altering the chemistry of the fusion zone (FZ) could prevent embrittlement and enhance the mechanical performance of the joints. In this regard, understanding the impact of filler materials on the solidification behavior is vital. This study unveils the synergistic impact of employing these diverse interlayers, characterized by varying melting points, metallurgical compatibilities, and reaction spontaneity, on solidification behavior, microstructure evolution, and mechanical properties and presents a viable approach to enhance the microstructure and performance of NiTi and Ti6Al4V joints. A notable improvement in properties was observed in the Pd-interlayered joint, where the strong reaction spontaneity in the Ti-Pd system facilitated the preferential solidification of the TiPd phase in the FZ. This significantly reduced the amount of detrimental Ti2Ni IMC, ultimately decreasing joint embrittlement. Mechanical testing data demonstrated a progression in performance in the following sequence: interlayer-free, Co-interlayered, Zr-interlayered, and Pd-interlayered. Using interlayer materials that form lower-energy compounds with Ti and offer good mechanical properties, such as TiPd in this study, is presented as an optimal approach to enhance joint mechanical properties. The present work advances the metallurgical knowledge associated with non-equilibrium processing of NiTi and Ti6Al4V alloys.