Advancing Alloy Design: A Study of Novel and Conventional Cast Ni-based Superalloys in Additive Manufacturing
Year: 2025
Type: dissertation
Abstract: Additive manufacturing, especially powder bed fusion by laser beam of metals (PBF-LB/M), offers significant advantages in the production of complex components by providing a higher design freedom. Ni-based superalloys are the most dominant material class for high temperature applications, but their processing via PBF-LB/M is challenging. They are susceptible to hot- and solid-state cracks and the processing conditions lead to an unfavorable microstructure for high temperature applications. A deeper understanding of those mechanisms is needed to develop new alloys that can overcome these challenges. Seven variants of a new Ni-based superalloy with the baseline composition Ni-19Co-14Cr-3Al-9Ta-2.5Mo-2.5W were developed in this work. The variants differ primarily in their minor element and hafnium content. Additionally, four reference alloys were studied. The goal of this work is to better understand the composition-property relationships as well as the impact factors on the various cracking mechanisms. Therefore, various sample geometries were processed with different processing parameters. Different heat treatments were performed on these samples and the material behavior characterized by in- and ex-situ techniques. It was found that there is a correlation between the strength in the as printed condition and the content of γ’-forming elements that goes beyond the strengthening by solid solutioning even if no γ’-particles are observable. This is explained by the formation of the γ’-phase by spinodal decomposition rather than classic nucleation during PBF-LB/M. The early stages of spinodal decomposition are already capable of strengthening without visible γ’-particles. An additional impact factor on the strength is the carbon content. Due to the fast solidification rates carbides are extremely fine and also precipitate inside grains, which significantly increases their strengthening contribution compared to carbides that form during slower casting routes. By composing the new alloys with a slow precipitation kinetic of the strengthening γ’-phase and a low carbon content, a high plasticity during the PBF-LB/M process could be achieved that increases the crack resistance to solid-state cracks. Besides the buildup of residual stress during the PBF-LB/M process and the volume contraction by the formation of the γ’-phase, the grain boundary strength plays an important role on the SAC susceptibility as well. Boron increases the grain boundary strength and thereby decreases the SAC susceptibility. The alloys that are the most and least susceptible to SAC in this study are variants of the new developed alloy, that do not differ in their γ’-forming elements. This shows that the commonly used design rules that only take the γ’-forming elements into consideration are over simplified. An excellent combination of stress-rupture life and ductility was achieved by the addition of hafnium, which leads to coarse γ’-particles and γ-seams at grain boundaries. Additionally, an abnormal grain growth (AGG) mechanism was found that is triggered by the formation of an intermetallic phase at high temperatures and further increases the stress-rupture life. An even more potent AGG mechanism was discovered in STAL-15DE, but its understanding requires further investigation. By combining alloy development and the study of fundamental mechanisms, this work contributes to a deeper understanding of additively manufactured Ni-based superalloys and advances their alloy design.
Author: Dörries, Kai
Institution: Technische Universität Braunschweig
Language: English
Subfield: Mechanical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7083682555
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