Microstructure of a coated single crystalline René N5 part repaired by epitaxial laser deposition

Abstract

Laser Metal Deposition has been applied to the repair and manufacture of single crystalline Ni-based superalloys components. The extensive research carried out so far showed that single crystalline depositions of the same alloy of the substrate, or similar, can be produced in appropriate conditions. However, repair is often performed on previously refurbished parts, combining materials with different compositions and structures. In this work we investigated the deposition of René N4 Ni superalloy on a René N5 single crystalline part previously coated by TIG welding with René 195. Deposition was performed on the (100) surface of the single crystalline substrate. The results show that the multilayer deposits inherit the single crystalline structure and orientation of the original crystal when the material is laid down on the single crystalline regions of the substrate. Above the polycrystalline René 195 a heavily textured columnar dendritic solidification structure develops, similar to the structure of directionally solidified cast alloys. Above the dilution zone of the first deposited layer, both the chemical composition and the microstructure of the deposited René N4 alloy remain constant across all the deposited layers showing that repairing is feasible even on heterogeneous substrates. Several heat treatments aimed at developing a suitable γ/γ’ microstructure within the deposited material were tested and an optimized heat treatment method consisting of a solution step, followed by two ageing steps at specific temperatures was devised. • Over the single crystal, the deposit maintains the orientation of the substrate. • Above the coating, solidification structure is similar to a directionally cast alloy. • The coating does not interfere with the deposit orientation over the single crystal. • A three-stage heat treatment is required to obtain the γ/γ’ microstructure.