In-situ Synthesis of NiTi Alloys by Laser-directed Energy Deposition
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NiTi alloys are the most widely shape memory alloys in industries such as aerospace, automobile, medical devices, and consumer products due to their unique functional properties (shape memory effect and pseudoelasticity), high shape recovery percentage, the potentiality for amorphization, and high ductility. With the functional properties, the deformed shapes of the shape memory alloys can be recovered through temperature-induced or stress-induced phase transformations. Laser-directed energy deposition, as one of the laser additive manufacturing processes, has been used to in-situ synthesis NiTi alloys. The problems such as high energy and time cost, high impurity-pick up, and limited parts’ geometry induced by conventional manufacturing methods (casting and powder metallurgy) can be reduced or fully solved. Besides, with the track-by-track and layer-by-layer deposition mode, the microstructures and mechanical properties of the NiTi alloys can be very different from the conventionally manufactured NiTi alloys. The objective of this dissertation is to solve some of the problems, to utilize the advantages, and to explore new possibilities of the laser-directed energy deposition to in-situ synthesize NiTi alloys. A comprehensive review is firstly given to provide fundamental knowledge on the microstructural features and mechanical properties of the NiTi alloys that are in-situ synthesized by laser-directed energy deposition. Then, some of the problems are investigated and solved through optimization of the processing parameters, the adjustment of the powder morphologies, the implementation of the ultrasonic vibration, and the addition of TiC reinforcement materials. Last, the formation mechanisms the Ni4Ti3 precipitates, their influences on the mechanical properties, and prediction of the mechanical properties are investigated. The investigations in this dissertation will not only fill the research gaps in this area but also help to establish a high efficiency process to fabricate NiTi alloys with superior pseudoelastic behaviors.Embargo status: Restricted until 09/2172. To request the author grant access, click on the PDF link to the left.