Additive manufacturing of Ni-base superalloys: Processing, heat treatment, productivity, and properties

Doktorsavhandling, 2025

Nickel-base superalloys are essential materials for the energy and aerospace industries. The additive manufacturing (AM) of these materials using powder bed fusion – laser beam (PBF-LB) offers a valuable opportunity to improve component performance and simplify manufacturing and supply chain complexities in these industries. However, several challenges are encountered during PBF-LB processing of superalloys, including microcracking, deficient mechanical performance, and low productivity. The aim of the research in this thesis is to develop a better understanding of the extent of these issues in different superalloys, uncover their causes, and propose potential remedies.

The challenge of microcracking susceptibility was investigated for a γ´ strengthened superalloy IN738LC, where the alloying of minor grain boundary strengthening elements B and Zr was systematically varied. PBF-LB processing tests indicated that alloying with B and Zr together promoted severe microcracking, while omitting either or both elements from the composition reduced the susceptibility to microcracking. Importantly, the creep resistance of the alloy was maximized only with alloying of both B and Zr. This presented a conundrum where microcracking could only be prevented at the expense of the most desirable properties of the alloy. Based on the fine grain size and poor creep resistance in PBF-LB processed IN738LC, an alternative approach was proposed in the form of a modified IN738LC with heavily increased B alloying. Processability tests showed that microcracking was mitigated in the modified alloy, and CALPHAD solidification simulations suggested this was potentially due to a liquid backfilling effect in the last stages of solidification. The modified alloy also showed enhanced creep resistance relative to the standard composition, thus showcasing a novel method of overcoming microcracking susceptibility while simultaneously improving high temperature capability.

Investigations into heat treatment optimization were made for the alloy Haynes® 282®, since the application of the standard wrought heat treatment to the PBF-LB material was shown to produce severe anisotropy in tensile ductility and deficient creep rupture life. The use of solution heat treatments at 1250°C could radically alter the microstructure by coarsening grains, increasing twin boundary fractions, and forming a characteristic carbide structure at grain boundaries. The anisotropy in tensile properties was resultantly reduced, and creep rupture life of the alloy could be enhanced even above the level of its wrought counterpart.

The design of heat treatment for PBF-LB superalloys necessitates a detailed understanding of the as-built microstructure, which is often different from the cast or wrought counterpart of the same alloy. This aspect was investigated in the case of IN939, a high γ´-fraction superalloy. No γ´ precipitates were observed in the as-built microstructure, however the η phase was found at inter-dendritic regions. This secondary phase was observed to grow when aged without solution treatment, lowering the alloy’s ductility. The study demonstrates the importance of a solution treatment for IN939, regardless of γ´ precipitates in the as-built condition.

Investigations into high productivity PBF-LB processing of IN625 and Haynes® 282® were performed to understand the effect of high layer thickness parameters on porosity and mechanical performance. X-ray Computed Tomography analysis revealed that high density parts with porosity volume fractions < 0.05% could be produced with high productivity processing. However, the large size and sharp morphology of the porosity resulted in degraded mechanical properties compared to conventional processing. Despite this, high productivity PBF-LB processing presents an attractive route towards efficient manufacturing, especially if the trade-off in properties can be accepted.