The effect of heat treatments on the elevated temperature mechanical performance of directed energy deposition repaired Ni-based superalloys

Laser-blown powder directed energy deposition (LBP-DED) is being explored for repairing forged Ni-based superalloy components. However, the high cooling rates associated with these processes develop an as-deposited microstructure that is distinct from that of the forged substrate. As such, elemental segregation, brittle interdendritic phases, and local mechanical mismatches can occur within these deposits. The current knowledge gap relates to how repair-compatible ageing heat treatments impact Alloy 718Plus deposits made on both forged Alloy 718 and forged Alloy 718Plus substrates, and if they can be utilised to enhance metallurgical compatibility while avoiding homogenisation heat treatments. For this reason, both forged substrates were repaired using Alloy 718Plus powder by LBP-DED and exposed to either Alloy 718 or Alloy 718Plus compatible ageing heat treatments. Microscopy, compositional analysis, electron backscatter diffraction, Vickers microhardness, tensile testing at 650 °C, and fractographic analyses were used to characterise the repairs. Both heat treatment conditions displayed distinct substrate, bond-line, heat affected, and LBP-DED repair zones. The repair deposit exhibited columnar dendritic grains with chain-like and granular interdendritic Laves phase remaining, indicating that the inherited as-deposited microstructure dominates the final microstructure. Heat treatment resulted in the precipitation of γ′, as well as the local formation of η phase near Laves phase regions, yet did not significantly alter grain size or dendrite arm spacing. Additionally, the prior forged microstructure controlled epitaxial grain growth behaviours with larger forged Alloy 718 grains promoting larger LBP-DED grains. The 718-718Plus condition exhibited improved ductility at 650 °C compared to the 718Plus-718Plus condition. Fractographic analysis confirmed that failure occurred within the LBP-DED repair zone and was attributed primarily to strain incompatibility between the brittle Laves phase and the γ matrix. In summary, ageing heat treatment resulted in limited changes to the as-deposited Alloy 718Plus microstructure, whereas the substrate and as-deposited microstructures had more pronounced effects on the final metallurgical and mechanical behaviour.