Doctoral Dissertations

Date of Award

8-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Anming Hu

Committee Members

Seungha Shin, Peter K. Liaw, Sudarsanam Suresh Babu, Raymond Xu

Abstract

For high temperature brazing applications, several design considerations must be evaluated including metallurgical compatibility, mechanical stability, wettability, and many others. One of the obstacles to high temperature brazing is minimizing the melting temperature of the brazing material without sacrificing high operating temperature. Traditionally, this is accomplished by adding melting point depressants such as boron and silicon. However, the addition of boron and silicon exposes the base material to the formation of brittle boride and silicide intermetallic phases and/or low melting eutectic phases. Nanomaterials experience size-dependent melting point depression without the use of melting point depressants via the Gibbs-Thomson effect. Until the studies discussed in this dissertation, nanomaterials as the sole brazing material are seldom investigated. In this dissertation, we discuss nanomaterial brazing (nanobrazing) material performance when joining Inconel 718 and Ti-6Al-4V.In the dissertation studies, we compare the mechanical strength of Cu-Ag and Ag nanomaterials and high entropy alloy nanoparticles for laser brazing Inconel 718 to their bulk counterparts. It was found that the Cu-Ag nanomaterials outperformed a commercially available bulk Cu-Ag brazing alloy in terms of strength due to subgrain Hall-Petch strengthening. The bulk high entropy alloy produced a stronger brazing joint than the nanoparticles; however, the nanoparticles had a much lower brazing temperature. We also investigated Ni/Al reactive multilayer films as a self-powered brazing material for joining Ti-6Al-4V. Due to a lack of interdiffusion between the BAlSi-4 and the reactive multilayer film prevents the joint from achieving high strength. In our study of Ni nanomaterials, we established that Ni can outperform BNi-2 commercial brazing material under the same vacuum brazing conditions. High heating rate, high maximum temperature, and high grain boundary diffusivity were identified as critical factors in the nanobrazing process of Ni nanomaterials.Through these studies, we provide evidence for two hypotheses: (1) Surface melting and diffusion are critical processes for successful nanojoining (2) (Sub)Grain boundary strengthening is an important mechanism for high bonding strength of nanojoints. Furthermore, the research in this dissertation provides a firm foundation for future nanobrazing studies and provides valuable insight to fundamental investigations and technical improvements for optimizing nanobrazing procedures.

Comments

Portions of this document were previously published in the following journals: Journal of Materials Processing Technology, Welding in the World, Materials Letters, Applied Sciences, and Materials & Design. In addition, portions of this document were published in a chapter of the book "Semiconductor Nanocrystals and Metal Nanoparticles: Physical Properties and Device Applications" as well as the conference proceedings for the 46th and 47th North American Manufacturing Research Conference.

Orcid ID

http://orcid.org/0000-0002-0606-8845

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