Crystallographic Structure of Single-Crystalline René N4 Ni-based Superalloy Deposited on a Seeded Polycrystalline Substrate by Laser Powder Deposition

The outstanding high-temperature properties of Ni-based superalloys presently used in aerospace gas turbine engines are ensured by a single-crystalline structure and a microstructure composed of cuboidal γ’-phase precipitates densely distributed in a γ-phase matrix. This microstructure must also be present when single-crystalline components are manufactured or repaired by additive manufacturing methods, such as Laser Powder Deposition (LPD).

Since single-crystalline parts are extremely costly to manufacture by casting, they are good candidates for manufacturing by LPD. This method, due to the freedom of movement of the laser head, allows building-up complex shaped parts in relatively short times and at moderate cost. It is well known that single crystal Ni-based superalloys can be deposited by LPD, provided that a properly oriented single-crystal substrate and an adequate deposition strategy are used. Single crystal deposition on polycrystalline substrates would allow, for example, building-up turbine blades directly on top of a polycrystalline disk, but no process exists to do so. The aim of the present work was to investigate the possibility of depositing single crystalline superalloys on a polycrystalline substrate by seeding it with a properly oriented single-crystal. The structure of the deposited material was analysed by SEM and SEM-EBSD in order to establish relationships between the deposition method and the deposited material crystallographic structure. Strategies to improve this structure while minimizing cracking at the single/poly-crystal interface and heat treatments leading to a suitable γ/γ’ microstructure in the deposited materials were also devised.