Searching New Solutions for NiTi Sensors through Indirect Additive Manufacturing

Shape Memory Alloys (SMAs) can play an essential role in developing novel active sensors for self-healing, including aeronautical systems. However, the NiTi SMAs available in the market are almost limited to wires, small sheets, and coatings. This restriction is mainly due to the difficulty in processing NiTi through conventional processes. Thus, the objective of this study is to evaluate the potential of one of the most promising routes for NiTi additive manufacturing-material extrusion (MEX). Optimizing the different steps during processing is mandatory to avoid brittle secondary phases formation, such as Ni₃Ti. The prime NiTi powder is prealloyed, but it also contains NiTi₂ and Ni as secondary phases. The present study highlights the role of Ni and NiTi₂, with the later having a melting temperature (Tm = 984 °C) lower than the NiTi sintering temperature, thus allowing a welcome liquid phase sintering (LPS). Nevertheless, the reaction of the liquid phase with the Ni phase could contribute to the formation of brittle intermetallic compounds, particularly around NiTi and NiTi₂ phases, affecting the final structural properties of the 3D object. The addition of TiH₂ to the virgin prealloyed NiTi powder was also studied and revealed the non-formation of Ni₃Ti for a specific composition. The balancing addition of extra Ni revealed priority in the Ni₃Ti appearance, emphasizing the role of Ni. Feedstocks extruded (filaments) and green strands (layers), before and after debinding & sintering, were used as homothetic of 3D objects for evaluation of defects (microtomography), microstructures, and mechanical properties. The composition of prealloyed powder with 5 wt.% TiH₂ addition after sintering showed a homogeneous matrix with the NiTi₂ second phase uniformly dispersed.