Repairing weld for directly solidified Ni-based superalloy substrates using directed energy deposition of Inconel 625

Abstract

Although the maintenance, repair, and operation of gas turbine systems are important issues in enhancing the operation of power units, advanced welding repair processes have limitations in depositing directionally solidified Ni-based superalloy components. To overcome this problem, in this study, a directed energy deposition method was applied to repair directionally solidified components using an Inconel 625 alloy. The rapid cooling rate and small laser beam diameter during directed energy deposition minimized the heat-affected zone and duration in the mushy zone, resulting in crack inhibition at the interface. The first laser scan partially melted the substrate and formed a mixing zone with a chemical composition gradient. By combining the chemical compositional gradient and dislocation accumulation due to repetitive solidification, that is, with the melting cycles of directed energy deposition, the thermal stability decreased, resulting in recrystallization and grain growth in the mixing zone. Consequently, the crack-free continuous interface of the partially repaired components did not induce tensile fracture near the interface. Although the mechanical properties of the IN625 deposit layer were degraded by severe heat treatment of the substrate, the results demonstrated that energy deposition with post-heat treatment is an effective approach in repairing Ni-based superalloy components without cracks and inclusion initiation.