Effect of energy input on microstructural evolution of direct laser fabricated IN718 alloy

In this study IN718 samples were deposited by direct laser fabrication (DLF) technology in argon atmosphere from pre-alloyed powders. The microstructural evolution of the samples and the effects of energy input (Ev) on microstructural architectures, dendritic morphology, precipitated phases and thus microhardness were investigated in detail. For microstructure of the as-DLFed samples, at a lower Ev, the columnar grains were very continuous and uniform, while at a higher Ev, the columnar grains were no longer continuous and a layer banded structure was present. With increasing Ev, there was a dendrite to cell transition (DCT) in dendritic morphology evolution of the as-DLFed IN718 samples. Also, two critical points of Ev to determine whether the dendritic morphology of the as-DLFed IN718 sample is dendrites, cells or both were found to be about 220 J/mm3 and 550 J/mm3 respectively in this study. For precipitated phases, the size and the amount of the Laves phase within interdendritic boundaries were increased with increasing Ev, while the volume fraction of precipitated γ″ and γ′ phases in matrix γ of the as-deposited IN718 samples was getting small. As a consequence, the microhardness of the sample increases by decreasing Ev for a constant overlap rate between two neighbor cladding tracks, similarly, the microhardness also increases by decreasing Ev for a constant laser scanning velocity.