Fabrication of NiTi through additive manufacturing: A review

Nickel-titanium (NiTi) is an attractive alloy due to its unique functional properties (i.e., shape memory effect and superelasticity behaviors), low stiffness, biocompatibility, damping characteristics, and corrosion behavior. It is however a hard task to fabricate NiTi parts because of the high reactivity and high ductility of the alloy which results in difficulties in the processing and machining. These challenges altogether have limited the starting form of NiTi devices to simple geometries including rod, wire, bar, tube, sheet, and strip. In recent years, additive manufacturing (AM) techniques have been implemented for the direct production of complex NiTi such as lattice-based and hollow structures with the potential use in aerospace and medical applications. It worth noting that due to the relatively higher cost, AM is considered a supplement technique for the existing. This paper provides a comprehensive review of the publications related to the AM techniques of NiTi while highlighting current challenges and methods of solving them. To this end, the properties of conventionally fabricated NiTi are compared with those of AM fabricated alloys. The critical steps toward a successful manufacturing such as powder preparation, optimum laser parameters, and fabrication chamber conditions are explained. The microstructural characteristics and structural defects, the influencing factors on the transformation temperatures, and functional properties of NiTi are highlighted to provide and overview of the influencing factors and possible controlling methods. The mechanical properties such as hardness and wear resistance, compressive behaviors, fatigue characteristics, damping and shock absorption properties are also reported. A case study in the form of using AM as a promising technique to fabricate engineered porous NiTi for the purpose of creating a building block for medical applications is introduced. The paper concludes with a section that summarizes the main findings from the literature and outlines the trend for future research in the AM processing of NiTi.