ABSTRACT

Solidification (liquid to solid transition) occurring during the casting of metallic alloys plays an important role in determining the structures of the resulting materials at the nanometer scale, as well as their mechanical properties. During solidification in a casting process, the liquid metal surface reacts with the environment, resulting in generation of metallic oxides, which are called oxide bifilms. These bifilms can lead to different types of defects in materials, especially in aluminum alloys because of their high rate of oxidation. Oxide bifilms play a major role in reducing the quality and reliability of aluminum castings and can account for as much as 80% of the total effective problems in castings. This award supports fundamental research to study oxide bifilm formation and evolution during solidification of aluminum alloys. The outcomes of this research will enable practical recommendations for controlling and reducing defects in aluminum casting. This research has applications in the automobile, aerospace, and other industries that demand lightweight, high strength and fatigue-resistant metallic alloys. This project will also contribute to the manufacture of lighter and more energy-efficient vehicles.

Oxide bifilms are now believed to be the main cause of micro-cracks, microporosity, and other ailments that greatly weaken the mechanical properties of cast parts. While there is experimental evidence that is consistent with this hypothesis, the role of bifilms in producing defects and the mechanisms of defect creation are mostly conjecture, because no direct observation has been possible, the mechanical properties of bifilms are unknown, and the physics of bifilm evolution and interaction with crystalline dendrites during solidification is not understood. The research team will perform atomistic simulations to determine the high temperature mechanical properties of oxide bifilms. These results will be transferred to a new multi-phase-field model to simulate, for the first time, the interactions between bifilms and solidifying dendrites and to track the evolution and deformation of bifilms during solidification. Macro-scale thermal-fluid analysis will be performed to determine fluid flow and thermal boundary conditions for the micro-scale regions of the phase-field model. Casting and characterization experiments will be performed to validate the model predictions.

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