Ni20Cr/IN738LC functionally graded materials manufactured by laser directed energy deposition

초록

Despite the great potential of directed energy deposition (DED) for repairing high-grade Ni-based superalloy components, understanding of the continuous interfaces between dissimilar materials remains limited. To address this issue, this study fabricated a functionally graded structure transitioning from an IN738LC substrate to a Ni20Cr bond coat using DED, and its microstructural evolution and mechanical behavior were investigated under various post-heat treatment conditions. The Ni20Cr/IN738LC functionally graded material (FGM) exhibits a continuous grain structure and gradual chemical composition transitions due to varying fractions of Ni20Cr and IN738LC. The reduced C content suppressed the formation of primary MC and Cr-rich M23C6 carbides as the Ni20Cr fraction increased. Additionally, the reduction of Al and Ti with increasing Ni20Cr fraction delays gamma ' phase initiation and growth, thereby altering both the gamma ' phase fraction and morphology in the transition zone of Ni20Cr/IN738LC FGM. These complex microstructural changes influence the mechanical properties of the Ni20Cr/IN738LC FGM, indicating that the hardness in each region generally increases with increasing gamma ' phase fraction, with additional strengthening provided by networks of Cr-rich M23C6 carbides. The evolved gamma ' phase and Cr-rich M23C6 carbides in the Ni20Cr/IN738LC FGM cause its stress-strain behavior to resemble that of IN738LC rather than Ni20Cr. These results suggest that chemical compositional changes during DED critically affect microstructural evolution, including grain structure, phase distribution, and secondary phase development after post-heat treatment.

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