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UBC Theses and Dissertations

Determination of the influence of interdendritic segregation during the solidification of freckle-prone alloys Auburtin, Philippe Bernard Lucien


Freckles are presently one of the major defects encountered in advanced casting technology. The current state-of-the-art knowledge about freckles is discussed in an extensive critical literature review, including the main physical characteristics of freckles, the current theory of density inversion leading to channel segregation and the various experiments and mathematical models developed so far. This review emphasizes the fact that no quantitative research on freckles has been carried out on actual industrial alloys yet. The goal of this thesis is to start this quantitative research by measuring interdendritic segregation profiles and evaluating the extent of density inversion in the mushy zone and its possible effects on freckle formation. Chemical compositions and segregation were measured by electron microprobe on directionally solidified and quenched samples of the following alloys : IN718, MAR-M002, MAR-M247, C-276 and T l . Freckled samples were also chemically analyzed. Densities, function of temperature and composition, were estimated by a numerical model. It was found that freckles initiated from density inversion of the order of 0.01 (g/cm3)/°C under typical superalloy casting conditions. In superalloys, it was estimated that freckles tend to initiate relatively close to the top of the mushy zone (20°C below the liquidus temperature) where fraction liquid is still high (about 40-60%). In high carbon alloys, especially tool steels, precipitation of heavy alloying elements into carbides may act as a trigger for freckling. Finally, the potential importance in the analysis of freckles of the segregation of minor alloying light elements such as carbon, silicon, zirconium, manganese, phosphorus, etc. was outlined.

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