Mechanical Properties, Microstructure, and Actuation Behavior of Wire Arc Additive Manufactured Nitinol: Titanium Bimetallic Structures.

Nitinol (NiTi) is well known for its corrosion resistance, shape memory effect, superelasticity, and biocompatibility, whereas Titanium (Ti) is well known for its high specific strength, corrosion resistance, and biocompatibility. The bimetallic joint of NiTi and Ti is required for applications that require tailored properties at different locations within the same component, as well as to increase design flexibility while reducing material costs. However, because of the formation of brittle intermetallic phases, connecting NiTi and Ti is difficult. In the present study, a systematic experimental investigation is carried out to develop NiTi-Ti bimetallic joint using wire arc additive manufacturing (WAAM) for the first time and to evaluate its microstructure, mechanical properties, martensitic transformation, and actuation behavior in the as-built condition. The defect-free joint is obtained through WAAM and microstructural studies indicate the formation of intermetallics at the NiTi-Ti interface leading to higher microhardness values (600 HV). Shape recovery behavior and phase transformation temperature were also enhanced in comparison to NiTi. An improved actuation and bending angle recovery is observed in comparison with NiTi. The present study lays the way for the use of WAAM in the construction of NiTi and Ti bimetallic structures for engineering and medicinal applications.