Abstract
Additive manufacturing (AM) of non-weldable high-γ’ Ni base superalloys is challenging due to various issues, but notably because of their inherent cracking propensity. Typically, the segregation of melting point-depressant elements to grain boundaries (GB) drastically increases the solidification interval, allowing the high processing-induced stresses in the parts to pull apart the liquid film at GBs. To achieve a better understanding of the consolidation process of nickel superalloys as well as the origin of defects and cracks, a simplified model γ/γ’-strengthened Ni-Cr-Al-Ti alloy with reduced solidification interval, related to the commercial CM247LC alloy, is investigated under a large parameter survey. The consolidation behavior is typical of nickel superalloys produced by AM, with the optimal condition being a compromise between cracking and porosity. The cracking mechanism is, however, changed to solid-state cracking, localized at high-angle GBs, and likely due to the lack of GB strengthening phases and the inherently low strength of this simplified alloy. Transmission electron microscopy and atom probe tomography reveal elemental segregation of Ti, and to a lower extent Cr and Al, to the solidification cell boundaries, in agreement with Calphad calculations. No γ’ precipitates are observed in the as-processed condition, indicating that all elements remain in solid solution. No chemical differences are observed between cracked and non-cracked boundaries. Trace amounts of oxygen contained in the powder lead to Al2O3 slag formation, as well as nano oxide dispersoid incorporation. Sulfur, a critical contaminant in superalloys, is detected but rendered harmless by the formation of TiS nanoprecipitates.
Highlights
Recent atom probe studies reveal that Si and Zr segregation is only observed at high-angle grain boundaries, while C and B segregate to every grain boundary [9]
Atom probe data on modified IN738LC with lowered Zr and Si suggest that the actual grain boundary segregation level of Zr and Si is unchanged by the alloy modification, indicating that the actual cracking mechanism is more complex [9]
The Calphad calculations are compared to differential scanning calorimetry (DSC) measurements on laser powder bed fusion (L-PBF) specimens (Table 2)
Summary
Research on AM of γ/γ’ Ni-base superalloys has predominantly focused on the commercial alloys Inconel 738LC [4,5,6,7,8,9,10,11,12,13,14,15,16,17], due to strong commercial interest for industrial gas turbines, as well as CM247LC, originally developed for directional solidification casting, and its parent MarM247 [1,18,19,20,21,22,23,24,25]. As we do in the present study, has the potential to shed light on the validity of alloying concepts that have been established decades ago on complex commercial alloys for processes quite different from the capabilities of modern AM machines Such concepts include: i) the critical influence of the Al + Ti content on processability, ii) the importance of grainboundary strengthening elements, iii) the solidification challenges associated with heavily segregating elements, often acting as grain boundary strengtheners, iv) the detrimental nature of large solidification intervals, v) the importance of controlling powder contamination, vi) the formation of dispersoids during L-PBF from strong oxide forming elements present in the alloy. The observed behavior is compared to commercial Ni-base alloys and conclusions for future alloy and process development are drawn
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