Untersuchung, Modellierung und Simulation der Freckle-Bildung an Superlegierungsbauteilen

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Freckles are microstructural defects which are observed in superalloy castings that are directionally solidified or solidified as single crystals. According to a widely accepted assumption, freckles arise as a result of thermo-solutal convection. During the upward directional solidification, the melt located between the dendrites can have a density inversion as a result of the redistribution of alloying elements which leads to instability of the melt flow, as the flow forces become stronger than the resistance forces. The currently discussed hypothesis assumes that the resulting smelting and fragmentation of dendrite arms lead to the freckle formation. Once formed freckles cannot be removed by heat treatment and lead to rejected parts. The mechanisms of freckle formation are, in spite of the long-term investigation, ultimately not entirely clear, and new findings reveal freckles in areas that would not be expected on the basis of the above hypothesis. The phenomenology of freckle formation shows that both process and component parameters as well as processes on the microstructure scale are responsible for freckle formation. For the physically based numerical simulation of the freckle formation, a new, a holistic coupled approach is to be developed in this project. Here, the models used can be classified as follows: micromodels representing crystal growth and macromodels used for the process and component scale, as well as mesoscopic models covering the range on the grain scale between them. In addition, selected fundamental and mechanism-oriented experimental work will be carried out in this project, by varying the influencing variables of the process and the material, in order to provide data and correlations for the further development of simulation models on different scales. A new step in the planned work is the implementation in a multi-stage approach as well as the thorough consideration of the multicomponent and multi-phase material constitution approach. As a result, the developed overall model for freckle formation can also be used in the future for further fundamental questions of the interactions between solidification and flow processes, and can also be applied to other material groups, such as, for example, steel materials, or solidification phenomena, e.g. hot cracking during solidification.