Microstructure and Mechanical Behavior of Bright Crescent Areas in Inconel 718 Sample Fabricated by Selective Laser Melting

Bright crescent areas were obviously apparent at the bottom of melt pools of Inconel 718 samples prepared by selective laser melting (SLM). Their microstructure, composition, spatial morphology, and mechanical behaviors were systematically investigated. These areas consisted of finer columnar dendrites with underdeveloped secondary arms, which resulted in a bright color under optical field and dark color under SEM. Both growth and crystal orientations were consistent with those of the grains in the melt pool of the former layer. Enrichment of elements Nb and Ti occurred among the dendrites in these areas, leading to the formation of Laves phases. The crescent areas underwent great deformation together with the melt pools during tensile testing, which showed that they were of great plasticity and no gaps or cracks were found around these areas. In addition, the bright crescent areas possessed higher hardness than that of the surrounding regions. Furthermore, the simulated temperature field results showed that the incomplete re-melting of the formed dendrites and a large number of nucleated dendrites among them are the main reasons for the formation of the bright crescent areas.