1 Fabrication and Analysis of Thermally Invariant Smart Composites via Ultrasonic Additive Manufacturing

Ultrasonic additive manufacturing (UAM), a form of 3D printing based on ultrasonic metal welding, is a fabrication technique that is rapidly altering the development of new components within the research and commercial industries. Through the use of piezoelectric boosters, vibrating at 20 kHz, and the application of normal forces in excess of 5000 Newtons, thin metal foils can be welded in a fusionless, low-temperature process to produce bulk structures. Because of its low-temperature, UAM provides the opportunity to embed thermally sensitive materials, such as nickel-titanium (NiTi), a shape memory alloy. NiTi exhibits a shape change as it undergoes thermally-induced crystallographic phase transformation between martensite, the low-temperature phase, and austenite, the high-temperature phase. During phase transformations, NiTi can recover up to 8 % elastic strain and have a change in elastic modulus of 100%. When embedded, the strain recovery of NiTi can be used to counteract the thermal expansion of the matrix material—specifically aluminum in this study—for the purpose of producing components with low coefficients of thermal expansion (CTE) while keeping the weight at a minimal level. The work herein covers the design, fabrication, and characterization of Al-NiTi composites to aid in the development of a composite that has a coefficient of thermal expansion at, or below, 5 µє/°C. A composite is produced that has a CTE of 13.83 µє/°C; a 40.4 % decrease as compared to Al alone. In addition, electrical resistivity measurements in the longitudinal direction and thermal diffusivity measurements in the out-of-plane directions are presented.