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Characterization and Mitigation of Solidification Defects in Aluminum Casting and Welding

Trometer, Nicole
http://rave.ohiolink.edu/etdc/view?acc_num=osu1721137732665968

Abstract Details

2024, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Aluminum is increasingly favored in the automotive sector for its favorable mechanical properties and good strength-to-weight ratio. Laser welding finds utility in electric vehicle battery assemblies, while high pressure die castings (HPDC) are used in vehicle body construction. Nonetheless, defects pose a risk to the mechanical robustness of aluminum welds and cast parts. These defects include hydrogen porosity, entrapped air, and externally solidified crystals (ESCs). In this study, defects in die-cast and welded aluminum were investigated to understand their formation mechanisms and to explore methods for their prevention. Laser welding of aluminum and copper is commonly used in the battery assemblies of electric vehicles. However, aluminum is prone to forming hydrogen porosity when welded. The investigation of hydrogen porosity in aluminum welds involved the utilization of scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), microcomputed tomography (\textmu-CT), and LECO\textsuperscript{\textregistered} analysis. It was observed that the oxide layer on anodized aluminum served as heterogeneous nucleation sites for hydrogen porosity. Anodized aluminum can contribute to increased hydrogen content in the liquid phase. As liquid aluminum solidifies, hydrogen is forced into the melt, leading to a state of supersaturation in the liquid. This supersaturation prompts the nucleation and growth of hydrogen porosity. Additionally, a cellular automaton (CA) model was expanded to predict hydrogen porosity under different hydrogen concentrations, laser speeds, and powers. To mitigate hydrogen porosity, it is necessary to either improve the cleaning process of the anodized aluminum or utilize an alternative corrosion-resistant material. Die-cast aluminum is frequently employed to reduce vehicle weight. However, the turbulent flow inherent in the die casting process can easily entrap air, leading to gas porosity. The impact of vacuum on entrapped air during die casting was investigated using a combination of MAGMASOFT\textsuperscript{\textregistered} and ProCAST flow simulations, water analog experiments, and die casting trials. The water analog experiments demonstrated consistent repeatability and concurred with the flow simulations. It was observed that the introduction of vacuum resulted in a reduction in the quantity of entrapped air and porosity within the samples. Samples cast with a strong vacuum, below 100 mbar, exhibited greater density and enhanced mechanical properties compared to those cast with a weaker vacuum or none at all. Externally solidified crystals (ESCs) are defects characterized by a distinct microstructure compared to the matrix. Two types of ESCs are commonly found in die-cast samples. Type I ESCs are very large dendrites, approximately 150 \textmu m in size. Type II ESCs have a finer microstructure within the matrix and exhibit a clear boundary with it. This boundary can cause a signification reduction in elongation. The formation mechanism of ESCs was investigated using computer simulations, water analog experiments, and die casting trials, revealing that ESCs primarily form in the shot sleeve after pouring. Type II ESCs form on the walls of the shot sleeve, benefiting from a rapid cooling rate that encourages a finer microstructure. Conversely, Type I ESCs form further away from the shot sleeve wall, where a slower cooling rate facilitates their growth into larger dendrites. Turbulence during the filling process breaks up both the large Type II and Type I ESCs, pushing them into the die cavity. Increasing the melt temperature decreases the occurrence of ESCs in die-cast samples. In conclusion, this research offers valuable insights into optimizing the performance of aluminum in automotive applications.
Alan Luo (Advisor)
Ahmet Selamet (Committee Member)
Xun Liu (Committee Member)
Glenn Daehn (Committee Member)
196 p.
Materials Science
Casting; Welding; Porosity; Externally Solidified Defects

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Citations

  • Trometer, N. (2024). Characterization and Mitigation of Solidification Defects in Aluminum Casting and Welding [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1721137732665968

    APA Style (7th edition)

  • Trometer, Nicole. Characterization and Mitigation of Solidification Defects in Aluminum Casting and Welding. 2024. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1721137732665968.

    MLA Style (8th edition)

  • Trometer, Nicole. "Characterization and Mitigation of Solidification Defects in Aluminum Casting and Welding." Doctoral dissertation, Ohio State University, 2024. http://rave.ohiolink.edu/etdc/view?acc_num=osu1721137732665968

    Chicago Manual of Style (17th edition)

osu1721137732665968
47
© 2024, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.