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    Simulation of freckles during vertical solidification of binary alloys.

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    Author
    Felicelli, Sergio Daniel.
    Issue Date
    1991
    Keywords
    Dissertations, Academic.
    Mechanics, Applied.
    Metallurgy.
    Advisor
    Heinrich, Juan C.
    
    Metadata
    Show full item record
    Publisher
    The University of Arizona.
    Rights
    Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
    Abstract
    The formation of channel segregates or freckles during vertical solidification of Pb-Sn alloys is simulated. The simulation is based on a mathematical model of dendritic solidification, in which the solid plus liquid zone is modeled as an anisotropic porous medium of variable porosity. The alloy solidifies from an initial melt and proceeds until the alloy is completely solidified, and the final macrosegregation can be predicted. The growth process is calculated by solving the fully coupled equations of momentum, energy and solute transport, as well as by maintaining the thermodynamic constraints dictated by the equilibrium phase diagram of the alloy. It is found that the thermosolutal convection that occurs during solidification, under certain growth conditions, can produce localized segregation zones, varying in shape from short streaks or pockets to long and narrow channels or freckles rich in solute. Several numerical examples show how these defects originate and why they subsist or die, depending on whether they face a favorable or adverse convection pattern ahead of the solidification front. The response, in both appearance and location, of channels to the variation of the cooling conditions is in good agreement with experiments, although the preference of channels to form on the outer surface of the casting, rather than in the interior, is greater than what is normally observed in Pb-Sn alloys. A mechanism that favors the formation of freckles within the body of the casting, because of defects in the dendritic mushy zone, is proposed and illustrated numerically.
    Type
    text
    Dissertation-Reproduction (electronic)
    Degree Name
    Ph.D.
    Degree Level
    doctoral
    Degree Program
    Aerospace and Mechanical Engineering
    Graduate College
    Degree Grantor
    University of Arizona
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    Dissertations

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