Laser powder bed fusion processing and heat treatment of Ni-base superalloys: microstructure and properties

Licentiate thesis, 2022

Nickel-base superalloys are indispensable materials for the energy and aerospace industries. The additive manufacturing (AM) of these materials by powder bed fusion – laser beam (PBF-LB) presents a valuable opportunity to improve component performance and ease manufacturing and supply chain complexity in these industries. However, only a limited number of Ni-base superalloys are currently available for PBF-LB. This is due to several challenges encountered during PBF-LB processing, including microcracking, post-process cracking, development of an AM-specific microstructure, and lack of heat treatment optimization.

The aim of this thesis study is to develop better understanding of the extent of these issues in different superalloys, their causes, and potential remedies. To understand aspects of processability, the alloy Haynes® 282® was studied to assess its feasibility for manufacture by means of PBF-LB, including susceptibility to cracking. Results showed excellent processibility of Haynes® 282® by PBF-LB, allowing to reach full-density crack-free components over the wide range of energy input, while also being resistant to post-process cracking.

Conventionally manufactured superalloys – cast or wrought – are currently considered as the benchmark in terms of mechanical performance. The microstructure and mechanical performance of PBF-LB processed Haynes® 282® after standard heat treatment was evaluated and compared to its wrought counterpart from the literature. PBF-LB processed Haynes® 282® showed finer grain sizes and discontinuous grain boundary carbides compared to wrought microstructure. Despite excellent room temperature tensile properties, clear anisotropy in high temperature mechanical performance of PBF-LB processed Haynes® 282® was observed, which is proposed to be addressed by heat treatment optimization.

Heat treatment is a critical post processing step for any precipitation strengthened alloy, and this is especially true for PBF-LB processed superalloys. Heat treatments developed for cast or wrought alloys may not be optimal for the same alloys in PBF-LB processed form because PBF-LB processed superalloys have a starting microstructure that is very different from equivalent cast or wrought microstructures. This aspect was studied in detail by evaluation of the as-built microstructure of Inconel 939, a high γ’-fraction superalloy. No γ’ precipitates were found in the as-built microstructure, however, η phase was found at inter-dendritic regions. This secondary phase was observed to grow upon ageing, lowering the ductility of the material. This demonstrates the importance of a solution treatment for Inconel 939, regardless of γ’ in the as-built condition. Further study also aimed to optimize the ageing heat treatment steps for PBF-LB manufactured Inconel 939. This resulted in a proposed ageing heat treatment which is shorter than the one used for conventional cast Inconel 939, which also produces improved and more isotropic tensile performance. Another aspect of heat treatment in PBF-LB processing is potential contamination of an alloy from the stress relief heat treatment carried out while a part is fused to a dissimilar building platform material. This was addressed in a study on Haynes® 282® built onto a carbon steel building platform. The study showed that no large-scale change in chemical composition occurred, suggesting that steel platforms are suitable for use with Ni-base superalloys.