Direct metal laser melting of Inconel 718: Process impact on grain formation and orientation

Line scan specimens of Direct Metal Laser Sintering (DMLS) processed Inconel 718 were fabricated under a wide range of process parameters in order to correlate the laser energy density (LED) input with the resulting microstructure. High quality specimens at LED values between 0.14 and 1.50 J/mm were prepared for Electron Backscatter Diffraction (EBSD) microscopy. Cross sections of samples perpendicular to the build direction showed a highly oriented crystallographic texture in the 〈001〉 direction. LED value directly affects the heat flow direction which was found to have a pronounced effect on the preferred orientation of the grains. Also, the results show that crystallographic texture along the build direction not only depends on manufacturing parameters, but most importantly is a function of heat flow direction and former layer's grain orientation. Furthermore, morphology of grains in the bead area depends on the geometry of bead. Narrow beads with low LED value have columnar grains along the build direction. However, wide beads with high LED values have grains converging to the top middle of the beads. The higher LED experiments show characteristics similar to high energy processes, such as electron beam welding as excessive voids and porosity similar to key-hole effect observed in electron beam welding was found in substrate and melt pool region.