Differences in microstructure and nano-hardness of selective laser melted Inconel 718 single tracks under various melting modes of molten pool

Single tracks of Inconel 718 were manufactured by selective laser melting (SLM) under the same volumetric energy density (VED), but via various scanning speed and laser power, with the aim of discerning the influence of the melting mode of molten pools on their solidification behavior, microstructure and nano-hardness. For this purpose, the geometric appearance of each Inconel 718 single track was firstly investigated. The results indicate that the molten pool geometries are significantly different despite melting under the same VED condition; the typical melting modes of molten pools, including the keyhole and conduction modes, can be obtained. The solidified microstructures of both modes evolve from cellular at the bottom to cellular dendrites at the top; but the dendrites are more developed under the keyhole mode. In addition, the grains of both modes follow the epitaxial columnar crystal growth pattern along the boundary of molten pool. The preferential growth direction of grains under the conduction mode is closer to the <001> direction, but the grains orientation of the keyhole mode tends to be <001> and <101> direction. Moreover, in comparison with the conduction mode, the average nano-hardness for different regions of molten pools corresponding to the keyhole mode is slightly higher.