Microstructural evolution and precipitated phase characteristics in the fusion zone for the as-repaired Inconel 718 alloy by directed energy deposition additive manufacturing

In this study, five laser powers were employed to repair Inconel 718 alloy by directed energy deposition additive manufacturing technology. Under various solidification conditions, microstructural evolution and precipitated phase characteristics in the fusion zone (FZ) of the as-repaired Inconel 718 specimens were investigated. The results showed that the main dendritic morphology in the FZ changes from equiaxed dendrites to columnar dendrites with increasing laser power, accompanied with the morphological evolution of numerous Laves phases from a long-chained shape to a blocky shape. However, these morphological changes for the Laves phase are not only relevant to the solidification structure characteristics but also affected by the repetitive vertical thermal cycles induced by layer-by-layer deposition because multiple thermal cycles could be regarded as conducting mild heat treatment to the FZ microstructure. On the one hand, the gradually increasing heat accumulation effects are conducive to partially dissolving Laves phase with increasing laser power; on the other hand, there are a number of γ′′ phase, γ′ phase and δ phases precipitated in the Nb-rich zone. When laser power is 1400 W, the volume fraction of the Laves + δ phases is the highest but the volume fraction of the γ′ + γ′′ phases is the lowest among the five as-repaired specimens, resulting in the lowest micro-hardness.