Author ORCID Identifier

https://orcid.org/0000-0003-1577-890X

Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Mechanical Engineering

First Advisor

Dr. Dusan P. Sekulic

Abstract

In this dissertation, the capillary flow of liquid aluminum alloy in both wetting and wetting/non-wetting systems is investigated.

The impact of gravity and surface topography on the capillary flow has been studied in a wetting/non-wetting assembly (an AA3003/Al2O3 wedge-tee configuration). The research includes (i) kinetics of liquid Al-Si-KxAlyFz alloy triple line movements and dynamic macro advancing and receding contact angles and (ii) free surface profiles of the molten alloy driven by surface tension and impacted by gravity. A trade-off between the surface tension and gravity force has been found by calculating the capillary length. Roughness effect has been also studied for the wetting of the liquid metal on the vertical AA3003 substrate. The capillary flow process is simulated using phase-field model through a collaboration with Washington State University. Good agreements have been achieved. The microstructure has been studied and phase segregation has been found for the re-solidified liquid alloy on the samples.

For aluminum braze capillary flow over a cylindrical wetting/non-wetting assembly (PIN and HOLLOW PIN experiments), the heating rate and amount of the molten alloy change on the wetting distance have been investigated under terrestrial condition. The key experimental finding has been uncovering a phase segregation of re-solidified microstructures under an impact of surface tension and gravity driving forces in the short HOLLOW PIN experiment, in which the liquid metal flows into the inside cavity after reaching the top over the outside wall. The team from Washington State University has predicted the wetting distance and surface profile under earth gravity and zero gravity by using our empirical findings and Surface Evolver. Excellent agreement has been achieved between the experimental and the numerically predicted data under terrestrial condition.

Regarding the molten Al-Si alloy capillary flow in a wetting system, diffusion fields of different elements (Si, K, Cu, Ti, Fe, Mn) involved in an aluminum brazing sheet have been studied under different heating rates. It is demonstrated that Si diffuses more from the clad to the core of the substrate when heating rate decreases. Hence, less liquid metal would be available to form the joint at low heating rates. Silicon diffusion process prediction is essential for an assessment of the volume of clad metal of a composite sheet available to flow into the joint. Hence, a study of both solid and liquid state of Si diffusion across the clad-core interface during the brazing process, including 1) solid-state Si diffusion before clad melting, and 2) liquid state Si diffusion after clad melting, is investigated. Using the joint size obtained under different heating conditions and modeling the joint formation, the Si liquid diffusion coefficient is calculated.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.273

Funding Information

NASA’s Physical Sciences Research Program, Grant #NNX17AB52G (2016-2020)

Available for download on Saturday, December 26, 2020

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