Microstructure and defects in a Ni-Cr-Al-Ti γ/γ’ model superalloy processed by laser powder bed fusion

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.

Keywords

• Additive manufacturing
• Laser powder bed fusion
• Nickel superalloy
• Cracking mechanism