Effects of Plastic Deformation From Ultrasonic Additive Manufacturing

Nuclear energy technology can be exponentially advanced using advanced manufacturing, which can drastically transform how materials, structures, and designs can be built. Ultrasonic Additive Manufacturing (UAM) represents one of the four main additive manufacturing methods, although it is also the newest. As UAM technology and applications develop, a fundamental understanding of the bonding mechanism is crucial to fully realize its potential. Currently UAM bonding is considered to occur through breaking down surface asperities and removing surface oxides. Plastic deformation occurs although its role is currently unclear. This research analyzes material configurations in a variety of geometries, with similar and dissimilar material interfaces, and with pure metals and complex engineered materials. A variety of characterization techniques were used to develop a general description that UAM bonding requires plastic deformation.

First, we analyzed various dissimilar material interfaces created between UAM foils and the coating of embedded optical fibers. Enhanced interdiffusion of elements was found beyond that expected from the thermal profile experienced during bonding. This interdiffusion was rationalized based on enhanced point defect vacancies creating additional diffusion pathways. Following on this study, we analyzed the local strengthening at one of these interfaces. These interfaces strengthened through a complex interaction dominated by dislocation forest hardening, reduced grain sizes, and vacancy clusters created by the agglomeration of vacancies. UAM bonding of pre-treated Al 6061 was also performed and analyzed using multi-length scale characterization. Macroscale strengthening was observed as well as foil-foil interface strengthening. This was a result of dynamic recrystallization, dynamic recovery, adiabatic heating, and precipitate dissolution (as the vacancies allowed enhanced diffusion of elements). Finally, UAM bonding of titanium was analyzed. The HCP phase of titanium significantly resisted plastic deformation, which resulted in a phase transformation to the BCC phase, which was stabilized by the introduction of certain stabilizing elements. The strain induced phase transformation and enhanced vacancy driven interdiffusion were utilized to demonstrate a viable method of improving UAM bonding by focusing on the plastic deformation requirement. The phenomena outlined in this research demonstrates an improvement in our understanding of the fundamental bonding requirements of UAM, and deformation induced vacancy formation.