Martensitic transformation, microstructure and functional behavior of thin-walled Nitinol produced by micro laser metal wire deposition

Additive manufacturing (AM) of Nitinol could enable realizing smart 3D metallic structures that combine the functional properties of shape memory alloys with higher geometrical flexibility. However, powder feedstocks suffer from issues related to availability, chemical composition stability, and long-term stocking. Therefore, wire-based AM processes are considered promising for reducing the time required for the process development stage and for maintaining quality over time. In this study, micro laser metal wire deposition (μLMWD) was used for manufacturing thin walls with heights of up to 40 mm by using commercial superelastic Nitinol wires of 0.4 mm diameter. The microstructure, martensitic transformation (MT), and mechanical behavior were analyzed. The results showed that μLMWD could be used to produce Nitinol walls with high aspect ratio and sub-millimeter thickness, that were free of cracks and produced with relative density as high as 99%. The as-deposited wall section was characterized by an almost homogenous composition, which was associated with a solubilized condition, owing to the multiple heating cycles during the deposition of advancing layers. Limited amount of Ni loss induced an increase in the characteristic temperatures of the MT, and shape memory behavior was detected with a recoverable strain of 4.4% upon heating up to 120 °C