Cobalt-based superalloys strengthened by a γ’ precipitation are relatively new alloys compared to the nickel-based ones. However, the optimisation of their chemical composition takes profit of the former studies made on their predecessors, especially for the minor element contents. These minor elements as carbon, boron or zirconium are initially added to achieve high temperature mechanical resistance. Even if these additions are beneficial when the alloys are elaborated using traditional processes, the additive manufacturing of superalloys designed for conventional metallurgy leads to numerous issues [1]. Indeed, the fast cooling rate and the high thermal gradients induce non-equilibrium state in the sample during the 3D manufacturing. Chemical segregations often lead to a large enrichment of elements and particularly minor elements during the last stages of solidification, in interdendritic areas, and even cause the precipitation of low melting point phases. These segregations and undesirable phases could then be re-melted due to successive layer depositions inherent to the additive manufacturing process. The particular thermal path undergoes by the 3D parts can thus generate large and numerous cracks at the liquated areas under thermal stresses (figure 1). In this regard, chemical optimisation of the compositions is needed to elaborate crack free cobalt-based superalloys by additive manufacturing without diminishing their remarkable high temperature strength.
In this study, the role of the minor elements in the crack sensitivity is highlighted in the case of cobalt-based superalloys. It was observed that low melting point phases are detrimental to the material health and can be avoided by chemical modifications. Thus, based on a fine characterization of the alloy solidification path supported by Thermo-Calc simulations, chemical contents were appropriately modified [2].
The new chemical contents permit to avoid liquid phase cracking mechanisms while keeping the two-phase γ/γ’ microstructure. These chemical optimisations also lead to an enlargement of the crack free sample manufacturing range by directed energy deposition. These improvements are beneficial for the development of new manufacturing processes to elaborate near net shape parts made of these recent alloys.

References
[1] A., Després et al., Materialia, On the role of boron, carbon and zirconium on hot cracking and creep resistance of an additively manufactured polycrystalline superalloy, 2021, 19, 101193.
[2] T., Froeliger et al., Manuscript submitted for publication, Interplay between solidification segregation and complex precipitation in a γ/γ’ cobalt-based superalloy elaborated by directed energy deposition, 2022.