Nickel-Titanium (NiTi) is a shape memory alloy that, depending upon composition, can exhibit shape memory or superelastic properties, recovering up to 8% deformation. Utilizing the shape memory effect it is possible to use NiTi as an actuator replacing traditional mechanical systems with a light-weight system using a fewer number of moving parts. In addition to strain recovery, NiTi undergoes significant changes in its material properties, including elastic modulus and electrical resistivity. With these changes in material properties, it is possible to create NiTi based transducers. Currently, NiTi is limited to niche applications due primarily to difficulty in machining and joining NiTi to traditional structural materials.
The goal of this thesis is to develop and characterize consistent methods of creating adaptive structures using NiTi. The research presented consists of two parts; the first deals with the development and characterization of cost-effective methods of joining NiTi and common aluminum and steel alloys. Laser welding, tungsten inert gas welding, and ultrasonic soldering were used to create joints between NiTi and itself, aluminum 2024, O1 tool steel, and 304 stainless steel. Where applicable, joints were subject to mechanical testing and analysis using optical microscopy.
The second part explores the development and characterization of NiTi/Al metal matrix composite transducers constructed using Ultrasonic Additive Manufacturing (UAM), a low temperature solid-state process also referred to as ultrasonic consolidation. An aluminum UAM matrix was first characterized through mechanical testing and analysis using optical microscopy. Using UAM, aluminum matrix composites with embedded NiTi wires were created with up to a 13.4% NiTi cross sectional area ratio. The composites were tested to characterize their stiffness as a function of temperature. A model was also developed using the Brinson constitutive model in order to predict the stiffness and strain sensing properties of current and future UAM NiTi/Al composites.