Fundamental mechanisms and modeling of microstructure evolution during beam and powder bed-based additive manufacturing

在粉末床中进行基于光束的金属增材制造（AM）不仅可以制造复杂的高性能材料定制组件，还可以通过熟练的加工来调整局部材料特性。凝固条件的变化使得微观结构长度尺度的修改成为可能。此外，最新的研究结果表明，部件的纹理在制造过程中也是可调节的。因此，关于轻质部件的优化打开了全新的视角，因为不仅拓扑结构，而且材料的纹理也可根据部件上的局部负载进行调整。为了更好地理解和控制织构的演化，需要从根本上理解流体动力学非平衡凝固过程（晶粒生长、选择和形核）。实验研究表明，特别是在极端条件下的形核AM机制是不够的解决，不能再现经典模型，本建议的目的是识别，从根本上理解和物理模型的微观结构演变，特别是在特殊的凝固形核。这个模型应该在我们主持开发的现有软件中实现。基于IN 718的增材制造样品进行了建模和验证。在项目的最后，该模型应预测凝固组织，晶粒结构和织构的演变，在梁和粉末床为基础的AM。该项目借鉴了我们的软件，在梁和粉末床为基础的AM的固结过程模拟。该软件包含一个格子玻尔兹曼方法来描述熔化和凝固过程中的水和热力学。该方法与元胞自动机耦合，模拟了凝固过程中晶粒结构的演变，忽略了当前的晶粒形核。我们新的理论分析除了包含温度梯度和凝固前沿速度的第一次额外的信息有关的纹理的前层（取向，间距的细胞/枝晶，偏析）和当地组成的熔体在界面处的当前处理层。应研究凝固前沿的取向变化与快速熔化材料中的偏析（熔体记忆）如何通过局部过冷诱导晶粒成核。这些发现是数学利用的晶粒形核模型。

Beam-based additive manufacturing (AM) of metals in a powder bed not only offers the opportunity to build complex, custom-made components of high-performance materials, but also to adjust the local material properties by proficient processing. The variation of solidification conditions enables the modification of microstructure length scales. Additionally, latest research results indicate, that also the texture of the components is adjustable during manufacturing. Therefore, entirely new perspectives are opened regarding optimization of light weight components, because not only the topology, but also the texture of the material is adjustable to the local loads on the component. In order to comprehend and control the texture evolution, the hydrodynamic non-equilibrium solidification process (grain growth, selection and nucleation) needs to be fundamentally understood. Experimental investigations show that especially the mechanisms of nucleation under the extreme conditions of AM are insufficiently resolved and are not reproduced by classical models.The aim of this proposal is to identify, to fundamentally understand and to physically model the microstructure evolution, especially the nucleation under the special solidification. This model should be implemented in existing software, which is developed at our chair. Modeling and verification are experimentally substantiated basing on additively manufactured samples of IN718. At the end of the project the model should predict the solidification structure, grain structure and texture evolution during beam and powder bed-based AM.The project draws on our software for simulating the consolidation process during beam and powder bed-based AM. The software contains a lattice Boltzmann method to describe the hydro- and thermodynamics during melting and solidification. This method s coupled to a cellular automaton modeling the grain structure evolution during solidification neglecting currently grain nucleation. Our new theoretical ansatz contains besides the temperature gradient and the solidification front velocity for the first time additional information about the texture of the previous layers (orientation, spacing of cells/dendrites, segregation) and the local composition of the melt at the interface to the currently processed layer. It should be investigated, how orientation changes at the solidification front in combination with the present segregations in the rapidly melted material (memory of melt) induce grain nucleation by local undercooling. These findings are mathematically utilized for a grain nucleation model.