Control of the microstructure formation in the near-net-shape laser additive tip-remanufacturing process of single-crystal superalloy

Abstract

A successful near-net-shape tip-remanufacturing process of single-crystal (SX) turbine blades should sustain a monocrystalline nature in the restored zone. Laser additive manufacturing (LAM) technique shows the huge potential to faultlessly restore the damaged tip of SX turbine blades. In this study, the microstructure formation in the near-net-shape tip-remanufacturing process of SX superalloy is studied through LAM experiments and simulation. The results indicate that due to the asymmetric heat diffusion condition at the top surface edge of SX substrate, the increase of laser power destroys the symmetry of morphologic size and microstructure formation in the single-track bead with a rapidly increased melting depth at the left side. Disoriented grains at the right side of the sing-track bead multiply layer by layer, gradually irrupting the epitaxial-growth continuity of columnar dendrites and resultantly destroying the monocrystalline nature of remanufactured tip structure. The continuous forced cooling of substrate not only shrinks the bead morphologic size, but also inhibits the disoriented grains formation at the right side of the single-track bead by directionally enhancing the epitaxial-growth ability of columnar dendrites. With the optimized laser power 550 W and continuously cooling substrate to −20 °C, a near-net-shape remanufactured tip structure with completely SX microstructure is achieved. The results provide an effective method to control the microstructure formation in the laser additive tip-remanufacturing application of SX turbine blades via coupling adjusting the laser power and continuous substrate forced cooling.