Torsional behavior and microstructure characterization of additively manufactured NiTi shape memory alloy tubes

• Published in: Engineering structures 2021-01, Vol.226, p.111383, Article 111383
• Main Authors: Safaei, Keyvan, Nematollahi, Mohammadreza, Bayati, Parisa, Dabbaghi, Hediyeh, Benafan, Othmane, Elahinia, Mohammad
• Format: Article
• Language: English
• Subjects: Actuators; Additive manufacturing; Diffraction patterns; Digital image correlation; Grain boundaries; Intermetallic compounds; Laser beam melting; Manufacturing industry; Martensite; Martensitic transformations; Microstructure; Nickel base alloys; Nickel compounds; Nickel titanides; NiTi; Powder; Precipitates; Process parameters; Properties (attributes); Room temperature; Scanning electron microscopy; Selective laser melting; Shape effects; Shape memory alloys; Shear strain; Superelasticity; Thermomechanical properties; Titanium compounds; Torsion; Transformation temperature; Tubes; Unloading; X-ray diffraction
• ISSN: 0141-0296; 1873-7323
• DOI: 10.1016/j.engstruct.2020.111383

[Display omitted] •Torsional behavior of selective laser melted NiTi tubes was investigated.•Ti-rich precipitates formation tailored the transformation temperatures of NiTi tubes.•Local shear-strain distributions were observed via in-situ strain monitoring.•2.3% of stable recovery strain was achieved under torsional loading. The large reversible strain upon heating (shape memory effect) or unloading (superelasticity), high power-to-weight ratio, good functional stability, compact size, and lightweight make the rotary NiTi shape memory alloys actuators an interesting candidate for various engineering applications. Additive manufacturing (AM) provides a single step freeform manufacturing process that not only fabricates the complex geometries but also tailors the properties of the printed parts profoundly. In the selective laser melting (SLM) technique, the process parameters (PPs) and scanning strategy show the remarkable effect on the microstructure, the properties, and the size accuracy of as-built parts. Impurity pick-up during the AM process is an unintended incident altering the microstructure and thermomechanical properties of fabricated parts significantly. In this paper, slightly Ni-rich NiTi powder is utilized to fabricate the NiTi tubes with three different thicknesses via the SLM method. It is shown that the bidirectional scanning strategy results in the size deviation of thin-wall tubes. Transformation temperatures (TTs) of the as-fabricated samples are assessed and compared with those of the starting powder. A large shift in TTs is found between the powder and the SLM tubes. The x-ray diffraction pattern shows the martensite phase at room temperature for the starting powder, while the as-built tubes are in the austenite phase coexisting with a secondary phase of Ti-rich oxide. Scanning electron microscopy (SEM) confirms Ti-rich Ti4Ni2Ox precipitates form along the grain boundaries. The characterization of tubes under pure torsional loading shows the localized shear strain on the tube surface. The thermomechanical behavior of the as-fabricated tubes is investigated and shown to exhibit superelastic response with a stable transformation strain of 2.3% after 10 cycles.