Microstructure evolution and laves phase control in IN718 superalloy repairing using direct energy deposition for enhanced performance

Abstract

IN718 nickel-based superalloy is extensively utilized in high-temperature applications, particularly in aerospace and energy sectors, where its repair is crucial for extending component lifespan and reducing costs. Direct energy deposition (DED) emerges as a promising repair technique, yet it introduces microstructural complexities, notably the formation of detrimental Laves phases. This study investigates microstructure evolution and Laves phase control during DED repair of IN718, focusing on temperature field manipulation via laser power and scanning speed. Experiments using electron backscatter diffraction (EBSD), microscopy, and mechanical testing, combined with numerical simulation, revealed that higher laser energy increases Laves phase volume (6.5% to 16.8%), depletes Nb, and suppresses γ″ precipitation. Lower laser energy generally improves mechanical properties (tensile strength 1049 MPa, hardness 377 HV), but excessively low energy can cause porosity. Simulations showed thermal accumulation in multi-layer deposition leads to grain size variations and temperature gradients influencing Laves phase. Simulation results demonstrated that thermal accumulation during multi-layer deposition increased grain sizes by 20∼30% and reduced temperature gradients to 72% of their initial values. Through optimization of the temperature gradient (G) to solidification rate (V) ratio (G/V) via regulation of laser power and scanning speed, this study successfully reduced the Laves phase volume fraction from 16.8% to 6.5%. Low temperature gradients and high cooling rates were found to suppress Nb segregation, thereby minimizing the formation of Laves phases(Ni₂Nb). And achieve uniform microstructure and superior mechanical performance in repaired IN718. This research provides a foundation for high-quality DED repair of IN718 for critical components.