Optimizing Grain Selection Design in the Single‐Crystal Solidification of Ni‐Based Superalloys

Abstract

An attempt is made to model the design of grain selection during single‐crystal solidification of an Ni‐based superalloy by the Bridgman method. Various geometrical designs of the starter block and spiral grain selector are chosen and their effects on crystal orientation of the single‐crystal part are studied. The competitive grain selection is simulated utilizing the cellular automaton finite element module of the ProCAST software. The simulated results are validated against the experimental observation of grain structure along with the crystal orientation measured by the electron backscatter diffraction (EBSD) and rotational orientation X‐ray diffraction (RO‐XRD) techniques. The results show that in order to obtain the single crystal with a minimum deviation from longitudinal axis, the height and diameter of the starter block as well as the height and the take‐off angle of the spiral should be optimized. It is found that the diameter/height ratio of the starter block should be less than 0.6 to obtain a well‐oriented single crystal prior to entrance of solidification front into the spiral. The height of starter block is found to be more influential than its diameter in selecting the suitable single grain.

Effect of Initial Position and Crystallographic Orientation on Grain Selection Procedure in Z-Form Selector for Ni-Based Single-Crystal Superalloy

“…Because of the high anisotropy of the Ni-based single crystal superalloy, accurate control of the crystallographic orientation of the blades has become one of the most essential ways for improving service performance. As a result, a significant effort has been expended on the grain selection process and optimization in order to increase the control of precision on the crystallographic orientation achieved by the grain selection method [6,10,18,[29][30][31][32]. Researchers have clarified the essence of grain selection behavior through in-depth research on competitive grain growth and grain selection behavior, and the main role of the starter block is to obtain the <001> oriented texture through the competitive growth of a large number of grains [13,16,19].…”

Orientation Control for Nickel-Based Single Crystal Superalloys: Grain Selection Method Assisted by Directional Columnar Grains

“…The spiral selector can generally be divided into two parts: the starter block and the spiral part [6][7][8]. The functions of these two parts have received extensive attention and a consensus has been reached [5,7,9]. The main function of the starter block is obtaining well-oriented grains with the <001> texture [10].…”

“…Rezaei et al reported that the withdrawal rate, height of the starter block, and temperature gradient during directional solidification were inter-dependent and careful optimization of these process parameters was indeed needed to achieve a minimum misorientation of the final grain [17]. Some studies have also shown that by adjusting the structure and size of the spiral part carefully, the orientation of the final grain can also be optimized within a small scale [5,9,12]. At present, the misorientation of the final grain obtained by the grain selection method can be controlled within 15 • for turbine blade applications in industrial production.…”

A Geometry Optimization of Spiral Grain Selector During Directional Solidification of Nickel‐Based Superalloy

“…High press turbine blades are the most important hot-end component of the aircraft engines and the industry gas turbines, its microstructure experienced the evolution from equiaxed to columnar and then to SX. The outstanding features of SX blade are the significant improvement of high-temperature properties, which benefit from the eliminated defects of orientation deviation, [1,2] stray grain, [3,4] freckles [5] et al Moreover, it is restricted that the deviation angle of <001> orientation to be within 12°in the aircraft engines to further improve the creep resistance and thermal fatigue strength of the SX blade, which brings great challenge for the manufacture process. Spiral grain selector which includes a columnar starter block and a spiral passage is an effective device that can acquire a qualified SX.…”