Dissertation
Benchmark aluminum alloy solidification experiments in microgravity and on Earth
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2022
DOI: 10.25820/etd.006674
Abstract
Modeling the physical phenomena that occur during the solidification of metal alloys is a key interest for industry and an ongoing challenge. Gravity driven processes such as melt convection and the transport of buoyant solid strongly affect macrosegregation within a casting as well as the final grain structure, including the columnar to equiaxed transition (CET). Benchmark solidification data is necessary to validate numerical models that predict transport within the molten metal. Cylindrical samples of Al-4, 10, and 18 wt.% Cu as well as Al-7 wt.% Si, all without grain refiner, are solidified in a bottom-up manner in terrestrial and microgravity environments. The Al-Cu alloys have primary solids that are negatively, neutrally, and positively buoyant in their melt. The different relative densities in each alloy results in differing gravity driven transport of free primary solid. The Al-Si alloy is used in an attempt to initiate CET without the addition of grain refiner and to relate the present experiments to other similar experiments on a similar subject. However, the grain structures of the Al-Si specimens were entirely columnar, indicating either grain refiner or different solidification parameters are necessary to observe CET in this alloy. Significant differences are observed between the grain structures of the microgravity and terrestrial Al-Cu samples. All of the samples solidified in microgravity exhibit solely equiaxed grains while the corresponding terrestrial specimens all exhibit a CET. The presence of columnar grains in the terrestrial samples contrasted with their absence in the microgravity samples suggests the presence of melt convection in all terrestrial samples. The Al-4 and 18% Cu samples show evidence of grain sedimentation and floatation respectively. Both eutectic distribution and solute concentration indicate the presence of inverse segregation within all Al-Cu samples. The eutectic and solute measurements generally correspond well with each other. All terrestrially solidified samples exhibit areas of high eutectic fraction and solute concentration that further suggest the presence of melt convection during solidification. The results from these experiments, including temperatures, grain structure, eutectic fraction, and macrosegregation, are presented as benchmark data for validation of future numerical modeling efforts. Heat flux boundary conditions on the samples are determined and provided. Thermal characterization is accomplished by calculating and presenting process parameters on each sample.
Details
- Title: Subtitle
- Benchmark aluminum alloy solidification experiments in microgravity and on Earth
- Creators
- Thomas J Williams
- Contributors
- Christoph Beckermann (Advisor)Hongtao Ding (Committee Member)Albert Ratner (Committee Member)David G Rethwisch (Committee Member)H S Udaykumar (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Mechanical Engineering
- Date degree season
- Autumn 2022
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.006674
- Number of pages
- xiv, 81 pages
- Copyright
- Copyright 2022 Thomas J Williams
- Language
- English
- Date submitted
- 11/11/2022
- Date approved
- 11/21/2022
- Description illustrations
- illustrations, tables, graphs
- Description bibliographic
- Includes bibliographical references (pages 77-81).
- Public Abstract (ETD)
Simulations and modeling are critical to predicting the solidification of metals which is a key interest of industrial metalcasting. Benchmark experimental data is needed to improve and validate numerical models that predict solidification while accounting for various physical processes. Gravity drives both the motion of molten metal and unattached solid during solidification. These processes are difficult to predict. To create proper benchmark data for these models, solidification experiments are performed on Earth and in space aboard the International Space Station. A series of metal alloys are used that have solid densities greater than, equal to, and less than their liquid densities. Each alloy behaves differently during solidification due to the different relative densities. After the experiments, the alloys samples are analyzed. The samples solidified in space have a significantly different structure than those on the ground. The samples solidified on the ground also each display a different structure from one another due to their different solid densities. The analysis of sample structure, the temperatures during the experiments, and other sample characteristics are presented as benchmark data for future modeling efforts.
- Academic Unit
- Mechanical Engineering
- Record Identifier
- 9984422559902771
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