Untersuchung, Modellierung und Simulation der Freckle-Bildung an Superlegierungsbauteilen

雀斑是在定向凝固或单晶凝固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，雀斑是由于热溶解对流而产生的。在向上定向凝固过程中，由于合金元素的重新分布，当流动力大于阻力时，位于枝晶之间的熔体会发生密度反转，从而导致熔体流动不稳定。目前讨论的假说认为，由此产生的树枝状臂的熔化和碎裂导致了雀斑的形成。雀斑一旦形成，就不能通过热处理去除，从而导致零件报废。尽管进行了长期的调查，雀斑的形成机制最终仍不完全清楚，新的发现揭示了在上述假设的基础上不可能预料到的区域的雀斑。斑点形成的现象学研究表明，过程和成分参数以及微结构尺度上的过程都是导致斑点形成的原因。对于基于物理的雀斑形成的数值模拟，本项目将开发一种新的、整体耦合的方法。这里使用的模型可分为：代表晶体生长的微观模型和用于工艺和成分尺度的宏观模型，以及涵盖它们之间的颗粒尺度范围的介观模型。此外，本项目还将通过改变工艺和材料的影响变量，进行有选择的基础性和面向机理的实验工作，以便为进一步开发不同尺度的模拟模型提供数据和关联。计划工作中的新步骤是以多阶段办法执行，以及彻底审议多组分和多阶段材料构成办法。因此，所开发的整体斑点形成模型未来还可以用于进一步研究凝固和流动过程之间的相互作用的基本问题，也可以应用于其他材料组，例如钢材料，或凝固现象，例如凝固过程中的热裂纹。

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.