Effect of process parameters on melt pool geometry and microstructure development for electron beam melting of IN718: A systematic single bead analysis study

To understand the fundamentals of microstructure formation in an electron beam melting (EBM) additive-manufacturing process, which is classified as a type of electron beam powder bed fusion (EB-PBF) in ISO562910/ASTM-F42, single bead experiments were conducted by using an electron beam to scan an IN718 plate, using various combinations of power and scan speed, focusing on the relationship between (i) the beam irradiation level, (ii) the melt pool geometry, and (iii) the solidification microstructure. The width and depth of the melt pool increases almost linearly with the line energy. Elongated grains, which are generally called “columnar grains” were observed in almost the entire cross-section of the beads regardless of the process parameters. Temporal evolution of the temperature distribution for the single bead experiments was simulated by finite element analysis (FEA) with thermal conduction and recoalescence taken into account. The surface heat source model used in the simulation was modified to cause the geometry of the simulated melt pool to align with that which was observed experimentally. The distributions of the temperature gradient (G) and solidification rate (R) on the solidification interface were evaluated from the simulation results. The distributions of the microstructures were constructed from the distributions of G and R, as obtained from a solidification map in the literature. Contrary to the experimental observations, the constructed microstructure consisted mostly of equiaxed and mixed grains. The reasons for this contradiction are discussed.