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Reginald Hamilton1 Beth Last1 Emily Jenkins1 Todd Palmer1

1, Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania, United States

Additive Manufacturing (AM) has gained significant attention for processing shape memory alloys because they have circumvented many of the challenges associated with the conventional methods. Shape memory alloys are a class of multifunctional materials that undergo large shape changes, and upon heating or removing external stimuli “remember” their original shape and form. Underlying reversible solid-state atomic and microstructure length scale phase transitions beget the bulk scale memory. Consequently the shape memory alloy behavior can be tailored using manufacturing techniques that provide freedom to design the microstructure. The AM techniques for NiTi are either powder-bed based technologies such as Selective Laser Melting, or flow-based methods such as Laser Directed Energy Deposition (LDED). LDED deposits powder through nozzles directly into the laser focus melt pool. LDED AM is a potential tool for in-situ, i.e. during fabrication, microstructure design. The laser-based layer-by-layer AM techniques can result in microstructural anisotropy, which is characterized in terms of the grain and microconstituent morphologies. During the additive manufacturing (AM) process, individual passes and layers are deposited. The deposition of passes and layers creates overlapping regions between adjacent passes and interfacial zones between successive layers. Within the overlapping and interfacial regions, previously deposited material is remelted is also reheated as heat is conducted away from the solidifying material. The remelting and reheating in these local regions will bring about microstructure anisotropy. Multi-scale deformation measurements correlate microstructure to underlying physical mechanisms in order to establish the interrelationships between novel fabrication technologies and shape memory functionality. The purpose of this work is to correlate the layerwise built-up microstructures to the shape memory behavior of LDED AM NiTi shape memory alloys.

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