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Determination of the influence of growth front angle on freckle formation in superalloys

University of British Columbia

Freckles are macrosegregation defects which are usually found in nickel-base superalloys or specialty steels and which occur during solidification. They are presently one of the major defects encountered in advanced casting technology of superalloys. The evaluation of a numerical criterion able to provide quantitative insight on the conditions of freckle formation is now recognized as a major key toward the successful manufacture of large diameter VAR/ESR ingots and large DS/SX castings. This thesis summarizes the main features of freckle formation and gathers most of the criteria proposed in the literature to date, along with a brief critical description. Emphasis is put on the Rayleigh number which seems best adapted to freckling prediction. The various parameters of the Rayleigh criterion are numerically evaluated. Some of the hmitations in the ability of the Rayleigh criterion to predict freckling are also described. It is suggested that the angle of the solidification front with respect to the direction of gravity plays a critical role on freckle formation. An experimental vacuum induction furnace was built in order to directionally solidify freckle-prone superalloys (Waspaloy, Mar-M247) at various angles to the vertical under typical industrial conditions (thermal gradients ranging from 500 to 4000°C/m (5 to 40°C/cm) and growth rates ranging from 1.6x10⁻⁵ to 10x10⁻⁵m/s (1 to 6mm/min)). A comprehensive thermal modeling of this furnace was carried out with the finite element package ProCAST in order to accurately evaluate the solidification conditions (G and R) for each sample. Based on the obtained experimental results, two possible "modified" Rayleigh criteria are suggested, incorporating the anisotropy in the mushy zone permeability and the directionality of gravity, and are shown to accurately describe the effect of the casting conditions and growth front angle on the presence or absence of freckles. Finally, the direct application of these new criteria to actual industrial processes is illustrated.

Thesis/Dissertation

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2009-06-03

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http://hdl.handle.net/2429/8676

Materials Engineering

University of British Columbia

UBCV

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