A study of grain selection in two-dimensional (2D) grain selectors during investment casting of single-crystal superalloy

Zhu, Xintao; Bührig-Polaczek, Andreas (Thesis advisor); Wang, Fu (Thesis advisor)

Aachen : Gießerei-Institut der RWTH Aachen (2023)
Book, Dissertation / PhD Thesis

In: Ergebnisse aus Forschung und Entwicklung 35
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2023

Abstract

Ni-based single-crystal (SX) superalloys are widely used to produce turbine blades due to their excellent high-temperature performance. Generally, the SX blades are cast using the directional solidification process through seeding or selector method. Although the seeding method can control the orientation of SX structure, stray grain also forms, and the corresponding process is complex. Due to those reasons, the selector method is preferred in practice. The selector is broadly composed of two parts: the starter block and the selector part. The starter block controls the orientation and the grow dynamics of grain, whilst the selector part favors the growth of individual grains and ultimately determines the formation of SX structure. To obtain high-efficiency grain selectors, several kinds of grain selectors, such as restrictor, angled and spiral selectors, were designed in the past few years. However, quite a large length is required in the restrictor grain selector resulting in low efficiency. While around the corners of the angled grain selector, new grains often nucleate due to the sudden change in the growing direction. In the wax injection stage, the spiral selector cannot be fabricated integrally with the blade due to the complexity of the three-dimensional (3D) shape. It needs to be welded with the blade later, which may bring about some inevitable position errors and sudden failures in grain selection. Moreover, the high stability cannot be guaranteed in spiral grain selectors, although it is employed extensively in casting foundries. To overcome these shortcomings, the new 2D grain selectors, C-form and Z-form, were designed and systematically studied in this work. A series of Bridgman casting experiments were conducted at the Foundry Institute of RWTH Aachen University to study the physical processes occurring in the novel 2D grain selectors with different geometry parameters. The corresponding effect of the geometry on grain selection was quantified by optical microscopy (OM) and scanning electronic microscopy (SEM) equipped with electron backscatter diffraction (EBSD) function. Based on the results, the influences of the dimension in grain selector are discussed, and the underlying mechanism accounting for the experimental results is put forward.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair for Foundry Science and Foundry Institute [526110]