Effects of thermal cycles on the microstructure evolution of Inconel 718 during selective laser melting process

A staircase Inconel 718 block was fabricated to investigate the effects of the thermal cycles on the microstructure evolution in the selective laser melting (SLM) part using optical scope (OM), scanning electron microscope (SEM), and electron backscatter diffraction (EBSD). The laser beam scanning strategy was clearly shown in the part under OM, including laser scanning pattern and hatch spacing. The Y-plane (side surface) was characterized by elongated colonies of cellular dendrites with an average cell spacing of 0.511 ∼ 0.845 μm. In addition, Laves phase was observed in the inter-layers and inter-cellular regions. Under the continuing effects of the thermal cycles, the fraction of the Laves-phase showed a significant drop with their morphology changing from coarse and interconnected particles to discrete Laves phase. This is attributed to the reheating process as Laves phase can be dissolved at a proper heat treatment. In terms of the width of the cellular dendrites, the longer the thermal cycle period is, the coarser the elongated grains are. Due to the preferred orientation of the crystal, the Y-plane and Z-plane had a strong texture of < 1 0 1> and < 0 0 1>, respectively. With the repeating thermal cycle period elongating, the maximum intensity of the texture, together with the fraction of larger grains and the high misorientation angles, increased. At the same height, there was no significant changes with the grains size, but the fraction of the high misorientation angles (>15°) increased with the subsequent building of the part. Moreover, the area fraction of the porosity was below 0.2%, with no remarkable effects found from the thermal cycles and the build height.