Advanced Processing Techniques: Additive, Porous, and Others: Session 2
Program Organizers: Aaron Stebner, Colorado School of Mines

Thursday 8:00 AM
July 13, 2017
Room: Comiskey
Location: Hyatt Regency Chicago

Session Chair: David Dunand, Northwestern University

8:00 AM  Invited
Microstructure and Shape Memory Behavior of NiTi SMAs Fabricated Using Laser Based Additive Manufacturing Techniques: Reginald Hamilton1; Beth Bimber-Last1; Todd Palmer1; Mohsen Taheri Andani1; Mohammad Elahinia1; 1The Pennsylvania State University
    In the past decade, Additive Manufacturing (AM) has gained significant attention for processing NiTi because they have circumvented many of the challenges associated with the conventional methods. The AM techniques for NiTi are either powder-bed based technologies such as Selective Laser Melting (SLM), or flow-based methods such as Laser Directed Energy Deposition (LDED). The powder-bed SLM based techniques deal with deposition of the powder through a roller, blade, or knife, while the flow-based LDED technologies deposit the powder through one or more nozzles that directly feed the powder into the laser focus. The laser-based layer-by-layer AM techniques can result in microstructural anisotropy, which is characterized in terms of the grain and microconstituent morphologies. The purpose of this work is to correlate the microstructures to the shape memory behavior of additive manufactured Ni-rich as-built SLM and LDED AM NiTi alloys.

8:40 AM  
Location-dependent Control of Thermal History and Microstructure of NiTi Shape Memory Alloys through Selective Laser Melting: Ji Ma1; Brian Franco1; Gustavo Tapia1; Kubra Karayagiz1; Luke Johnson1; Raymundo Arroyave1; Ibrahim Karaman1; Alaa Elwany1; 1Texas A&M University
    We demonstrate a method to achieve local control of 3-dimensional thermal history in a NiTi shape memory alloy, which resulted in a designed spatial variations in its functional response. The NiTi part was created with multiple shape recovery stages activated at different temperatures using the selective laser melting technique. The multi-stage transformation is caused by differences in thermal history, and thus the precipitate structure, at various locations created from controlled variations in the processing parameters within the same part, achieving effectively micrometer-scale heat treatments. This is an example of precision location-dependent control of thermal history in alloys beyond the surface, and utilizes additive manufacturing techniques as a tool to create materials with novel functional response that is difficult to achieve through conventional methods.

9:20 AM  
Fabricating NiTi Microtubes via Interdiffusion and Kirkendall Pore Formation in Ti-Coated Ni Wires: Ashley Paz y Puente1; Sarah Plain2; Dinc Erdeniz2; David Dunand2; 1University of Cincinnati; 2Northwestern University
    While NiTi wires are used for a variety of applications based on their shape memory, superelasticity, and biocompatibility, additional benefits can be gained from making these wires hollow including more rapid cooling, higher specific strength and stiffness (in bending) and new functionality as tubes. Traditional microtube manufacturing via extrusion is very difficult because of the limited formability of NiTi. In this study, we demonstrate an in-situ approach to microtube fabrication where (i) a Ti-rich coating is deposited on ductile, pure Ni wires with 50 μm diameter and (ii) interdiffusion homogenizes the wires to near equiatomic NiTi composition, with Kirkendall voids developing near the center of the wire forming a continuous longitudinal cavity, thus creating a microtube. The results of ex-situ annealing studies, in-situ X-ray tomography experiments, and mechanical testing are discussed with consideration of extending this technique beyond individual NiTi wires to 2-D and 3-D NiTi wire constructs.

10:00 AM  
Manufacturability of Superelastic Titanium-based Nickel-free Porous Structures for Load-bearing Medical Implants using the Space Holder and Laser Powder-bed Fusion Technologies: Vladimir Brailovski1; Alena Kreitcberg1; Anton Konopatskiy2; Sergey Dubinskiy2; Sergey Prokoshkin2; 1Ecole de Technologie Superieure; 2National University of Science and Technology MISIS
    Ti-22Nb-6Zr and Ti-18Zr-14Nb powders were produced using induction skull melting and gas atomization techniques. The obtained TNZ and TZN powders were used to manufacture open-cell structures of two types: random-oriented structures using the TNZ powder and space holder technology (SH), and regular structures, using the TZN powder and laser powder-bed fusion technology (L-PBF). The obtained structures were characterized from both the pore morphology and the mechanical behavior points of view. For the pore size ranging from 100 to 600 Ám, porosity of the SH-produced TNZ structures varies from 0.35 to 0.45, the Young’s modulus decreases from 13 to 8 GPa and the yield stress, from 260 to 130 MPa. The cell size of the TZN structures corresponds to the minimum resolution of the L-PBF technology, i.e. to 250 mcm. In both cases, the selected alloys offer an extra capacity to mimic plateau-like bone behavior due to their superelasticity.

10:20 AM Break