Rausch Alexander M, Gotterbarm Martin R, Pistor Julian, Markl Matthias, Körner Carolin
Chair of Materials Science and Engineering for Metals, Friedrich-Alexander-Universität Erlangen-Nürnberg, Martens Str. 5, 91058 Erlangen, Germany.
Joint Institute of Advanced Materials and Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, Dr.-Mack-Str. 81, 90762 Fürth, Germany.
Materials (Basel). 2020 Dec 3;13(23):5517. doi: 10.3390/ma13235517.
A microstructure has significant influence on the mechanical properties of parts. For isotropic properties, the formation of equiaxed microstructures by the nucleation of new grains during solidification is necessary. For conventional solidification processes, nucleation is well-understood. Regarding powder bed fusion, the repeated remelting of previous layers can cause nucleation under some conditions that are not explainable with classical theories. Here, we investigate this nucleation mechanism with an unprecedented level of detail. In the first step, we built samples with single crystalline microstructures from Ni-base superalloy IN718 by selective electron beam melting. In the second step, single lines with different parameters were molten on top of these samples. We observed a huge number of new grains by nucleation at the melt-pool border of these single lines. However, new grains can only prevail if the alignment of their crystallographic orientation with respect to the local temperature gradient is superior to that of the base material. The current hypothesis is that nucleation at the melt-pool border happens due to remelting microsegregations from former solidification processes leading to constitutional undercooling directly at the onset of solidification. This study offers the opportunity to understand and exploit this mechanism for different manufacturing processes.
微观结构对零件的力学性能有重大影响。对于各向同性性能而言，凝固过程中通过新晶粒形核形成等轴微观结构是必要的。对于传统凝固过程，形核已得到充分理解。对于粉末床熔合，先前层的反复重熔在某些经典理论无法解释的条件下会导致形核。在此，我们以前所未有的详细程度研究这种形核机制。第一步，我们通过选择性电子束熔化由镍基高温合金IN718构建具有单晶微观结构的样品。第二步，在这些样品顶部熔化具有不同参数的单线。我们在这些单线的熔池边界处通过形核观察到大量新晶粒。然而，只有当它们的晶体取向相对于局部温度梯度的排列优于母材时，新晶粒才能占主导。目前的假设是，熔池边界处的形核是由于先前凝固过程中的重熔微观偏析导致在凝固开始时直接产生成分过冷。这项研究为理解和利用这种机制用于不同制造工艺提供了机会。
