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Directional solidification (DS) is the process of solidifying a metal alloy from one end to another resulting in aligned primary dendrites which are branched tree like features. Alignment of primary dendrites along  direction and their uniformity and distribution along the DS length determines the mechanical properties. These properties are especially important for single crystal turbine blade applications in modern gas turbine engines. Convection during solidification plays an important role in formation of detrimental defects, such as, misaligned grains, non-uniformity of dendrites and composition inhomogeneity. In this study the microstructural evolution during “Gradient Freeze DS process”, involving an abrupt cross-section decrease has been examined on cylindrical Pb- 5.8% Sb alloy samples. Pb-5.8Sb was selected for this study because of its ease of processing and availability of its well-characterized physical property data. Two furnace cooling rates, 0.5 and 4 oC/min, have been examined during gradient-freeze DS. Morphology of primary dendrites has been observed to change from being branch-less (cellular), to onset of side-branching, to being well-branched having tertiary and higher level branching as a function of increasing solidification distance. “Freckles” caused by “severe plume-type” convection are seen in the slower cooling rate sample, but not in the faster cooling rate sample. Positive macrosegregation of Sb occurs along the DS length at both cooling rates, but is more severe at the slower cooling rate. A solute buildup, just before the section decrease, and solute depletion, just after the area decrease is seen, especially at the higher cooling rate. Trunk diameter of primary dendrites decreases, and their spacing (mean minimum spanning tree branch length) increases as the liquid-solid interface advances from the cold end of the sample to its hot end. This ground based research is in support of a future microgravity experiment in the convection free environment of Space Station. The research was supported by grants from NASA and from the Undergraduate Summer Research Program at Cleveland State University.
Washkewicz College of Engineering
Chemical & Biomedical
Shannon, Michael, "Dendrite Morphology of Pb-5.8 Sb Alloy: Gradient Freeze DS with Cross-Section Area Change" (2018). Undergraduate Research Posters 2018. 69.