Untersuchung, Modellierung und Simulation der Freckle-Bildung an Superlegierungsbauteilen

斑点是在定向凝固或凝固为单晶的高温合金铸件中观察到的显微组织缺陷。根据一个被广泛接受的假设，雀斑是由于热溶质对流而产生的。在向上定向凝固过程中，由于合金元素的重新分布，位于枝晶之间的熔体可能具有密度反转，这导致熔体流动的不稳定性，因为流动力变得比阻力更强。目前讨论的假说假设，所产生的熔化和枝晶臂的碎片导致雀斑的形成。一旦形成的雀斑不能通过热处理去除，并导致不合格的零件。雀斑形成的机制，尽管长期的研究，最终并不完全清楚，新的发现揭示了雀斑的领域，不会被预期的基础上上述假设。雀斑形成的现象学表明，过程和组件参数以及微观结构尺度上的过程是雀斑形成的原因。对于雀斑形成的物理基础的数值模拟，一个新的，整体耦合的方法是在这个项目中开发。在这里，所使用的模型可以分类如下：微观模型表示晶体生长和宏观模型用于过程和组件规模，以及介观模型覆盖的范围之间的晶粒尺度。此外，本项目还将通过改变工艺和材料的影响变量，开展选定的基础和面向机制的实验工作，为进一步开发不同尺度的模拟模型提供数据和相关性。计划工作的一个新步骤是多阶段方法的实施以及多组分和多相材料构成方法的全面考虑。因此，所开发的雀斑形成的总体模型也可以在未来用于凝固和流动过程之间的相互作用的进一步基本问题，并且也可以应用于其他材料组，例如钢材料或凝固现象，例如凝固期间的热裂纹。

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.