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Sintering of Extrusion-based 3D-Printed Ni-based Shape Memory Alloy Wires and Micro-trusses

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Porous metal structures exhibit numerous advantages over dense materials due to their high specific stiffness, strength, damping, energy absorption, and surface areas, making them suitable for applications ranging from actuators to medical implants. However, traditional foam manufacturing methods do not provide sufficient control of the foam micro-architectures, and the creation of parts with custom or complex shapes can be difficult. Additive manufacturing technologies such as extrusion-based 3D-printing of liquid inks provide micro-architectural control as well as the ability to fabricate complex geometries. This thesis investigates the use of a novel additive manufacturing technique for the fabrication of Fe and Ni, NiTi-Nb, and Ni-Mn-Ga shape memory micro-trusses via (i) extrusion-based 3D-printing of liquid inks containing a polymer binder, solvents, and metallic powders and (ii) subsequent heat-treatments to remove the polymer binder and to sinter the powders. Inks comprised of metallic Fe or Ni powders, an elastomeric binder, and graded volatility solvents are 3D-printed via syringe extrusion and then sintered to form metallic cellular structures. Similar structures are created from Fe2O3 and NiO particle-based inks with an additional hydrogen reduction step before sintering. All sintered structures exhibited 92-98% relative density within their struts, with neither cracking nor warping visible despite extensive volumetric shrinkage (~70-80%) associated with reduction (for oxide powders) and sintering (for both metal and oxide powders). The cellular architectures, with overall relative densities of 32-49%, exhibited low stiffness (1-6 GPa, due to the particular architecture used), high strength (4-31 MPa), and high ductility when subjected to uniaxial compression, leading to excellent elastic and plastic energy absorption. In NiTi bone implants, a porous structure is necessary to reduce the stiffness of the bulk, prevent the stress shielding effect, and improve biointegration. Transient liquid phase sintering via the formation of a Ni-Ti-Nb eutectic phase is utilized to bond the NiTi powders in the 3DP micro-trusses. NiTi-Nb micro-trusses with 400-500 µm 3DP channels suitable for osteogenesis and smaller 20-50 µm pores within the sintered struts have overall porosities of ~75% and compressive stiffnesses of 1-1.6 GPa. Preliminary studies showed good cell adhesion and viability of human mesenchymal stem cells on the sintered NiTi-6.7Nb micro-trusses after 14 days in culture. In Ni-Mn-Ga alloys, porosity is essential for the creation of polycrystalline parts, as it significantly increases the magnetic shape memory effect. A method for the fabrication of ferromagnetic Ni-Mn-Ga micro-trusses by sintering and interdiffusion of 3DP inks containing elemental Ni, Mn, and Ga powders with hierarchical porosity from 3DP channels, residual porosity within the sintered struts from incomplete sintering, and voids originally occupied by the largest Ga droplets is presented. The micro-trusses, sintered at 1000 °C for 12 h and chemically ordered for 10 h at 700 °C and with overall porosities of 73-76%, have uniform compositions near Ni-32Mn-18Ga (at.%) and are comprised of a non-modulated martensite phase. Reversible martensite/austenite transformations between 45 and 90 °C, Curie temperatures of 85-90 °C, and saturation magnetizations of up to 56 Am2/kg are achieved. Additionally, the evolution of microstructure, porosity and grain size in the Ni-Mn-Ga system during sintering and interdiffusion heat-treatments is examined using ex situ annealing studies and in situ X-Ray tomography during sintering. After debinding, Ga-rich regions melt and induce liquid phase sintering of the surrounding Ni and Mn powders, resulting in localized swelling of the wires and an increase in the wire porosity. After solidification of the melt and diffusion of the Ga into Ni and Mn, solid state sintering occurs. At the end of the 4 h sintering period, chemically homogenized, oligocrystalline wires with bamboo-like grains are observed with porosities ranging from 30 to 57%. Furthermore, significant grain growth occurs in wires sintered at 1000 and 1050 °C (11-35 µm vs. 1-10 µm initial powder size). The results from this study will be instrumental in tailoring the porosity and grain size of 3D-printed Ni-Mn-Ga wires and 3D micro-architectures to enhance their magnetic shape-memory and magnetocaloric effects. Finally, the use of pre-alloyed Ni-Ga powders rather than elemental Ga powders in the Ni-Mn-Ga inks improves the porosity uniformity and decreases the strut porosity and grain size of the Ni-Mn-Ga micro-trusses. Recommendations for areas of future study are made for all materials systems studies.

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  • 02/08/2019
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