Interlayer enabled crack-free joining of NiTi shape memory alloy to AlCoCrFeNi_2.1 eutectic high entropy alloy by arc-offset GTAW

Dissimilar fusion welding of NiTi shape memory alloy (SMA) to an AlCoCrFeNi_2.1 eutectic high-entropy alloy (EHEA) offers a route to multifunctional hybrid structures for smart and extreme-service applications. Both materials exhibit vibration-damping capability, making the combination relevant for automotive shock absorbers and seismic damping components in civil infrastructure. Their corrosion resistance and favorable thermophysical response further motivate integration for demanding environments. In such hybrids, NiTi can act as a sensor/actuator through shape memory and superelasticity, while AlCoCrFeNi_2.1 provides high-strength structural support, enabling cost-effective designs for components such as engine housings and other elevated-temperature assemblies.

However, direct joining is severely constrained by the formation of brittle reaction products and cracking [1,2]. In this work, gas tungsten arc welding (GTAW) with an arc-offset strategy was employed using (i) a Nb interlayer and (ii) a combined Nb–Cu interlayer approach to tailor heat input and interfacial metallurgy. The arc-offset configuration enabled a controlled weld-braze condition at the NiTi–interlayer interface while promoting managed mixing in the fusion zone.

Microstructural and phase evolution were investigated by optical and electron microscopy, SEM backscattered electron imaging with EDS, synchrotron X-ray diffraction (SXRD), and CALPHAD-based thermodynamic simulations. Dissolution of Nb (and Cu) altered solidification pathways, promoting the formation and intensification of topologically close-packed (TCP) phases (notably C14 Laves and σ) together with Ti-rich intermetallics such as Ti₂Ni. Multiple interfacial reactions on the NiTi side introduced significant local strain, increasing hardness and creating preferential sites for crack initiation under mechanical loading. Overall, the interlayer-assisted strategies enabled stable, crack-free joints and confined severe interfacial reactions compared with catastrophic cracking observed in direct joining. While the resulting joint properties remain lower than those of the base materials, the demonstrated metallurgical feasibility motivates further development of interlayer architectures and process strategies to enhance mechanical performance while retaining defect-free joining of these alloys.