Effect of thermal cycles on laser direct energy deposition repair performance of nickel-based superalloy: Microstructure and tensile properties

Solidification conditions and cooling rate can significantly affect the microstructure and the mechanical properties of Nickel-based superalloy GH4169 components repaired by laser direct energy deposition (L-DED). In this work, we employ inter-channel dwell time in the L-DED repairment of GH4169 superalloy groove in order to control the thermal cycles during deposition, thus regulating the microstructure and tensile properties of repaired GH4169 groove. The tensile properties of materials repaired by using different inter-channel dwell times were compared by tensile tests with digital image correlation (DIC) technique, and the microstructure and fracture mechanism were also investigated by scanning electron microscope (SEM). Results show that the dendrite structure, grain size of recrystallization, primary dendrite arm spacing, distribution of Laves and γ'' phases of the repaired material are significantly different under different inter-channel dwell times of 0 s, 4 s, 8 s and the associated different thermal cycles. The content of the Laves phases is lower, the grain size is smaller and distribution of γ'' phases are relatively uniform with the additional dwell time of 8 s. Owing to the difference in microstructure, the tensile strengths of both the repaired material and the substrate material increase with the increase of inter-channel dwell time. Moreover, the repaired GH4169 materials exhibit similar or even higher tensile strength than that of the substrate GH4169 material. Besides, due to the uneven precipitation of Nb element in repaired material and the associated change of Laves phases and carbides, the measured elongations of the substrate are larger than those of the repaired materials. The changes in microstructure under varying dwell times also lead to differences in strain localization and strain hardening exponent of the repaired materials. The present study paved a novel way to achieve high-quality repair of superalloys via laser direct energy deposition.