The crystal growth, intercellular spacing and microsegregation of selective laser melted Inconel 718 superalloy

The solidified microstructure of SLM-fabricated IN718 was investigated from macroscopic to microscopic perspective in this study using optical microscope, scanning electron microscope, transmission electron microscopy. Based on the experimental results and theoretical analysis, the solidification behavior, the characteristics of the primary dendrite spacing lambda(1) and microsegregation were elucidated. In the present SLM-produced IN718, the solidification starts at the molten pool boundaries by the epitaxial growth. The solidification rate is estimated in the range of 0.1-1 m/s. The newly-formed crystals grow into cellular dendrites, the direction of which is either parallel to the original direction or changed by 90 degrees. The primary dendrite spacing lambda(1) varies over a wide range of 27.6 x 10(-2) similar to 147.3 x 10(-2) mu m due to dramatically changeable average cooling rate from 1.1 x 10(5)-1.2 x 10(7) degrees C/s. There is a significant microsegregation of Nb in the inter-dendritic region, leading to the formation of Laves phases, the amount of which is about 9.0 +/- 1.3%. Meanwhile, a solute trapping of Nb in the dendritic trunk occurs during rapid solidification.