Effects of Heat Treatment and Carbon Element Invasion on Microstructure and Mechanical Properties of Inconel 718 Alloys Fabricated by Selective Laser Melting

Nickel-based superalloy Inconel 718 was prepared by selective laser melting and carbon element invasion was unexpectedly generated. In this work, the microstructural evolution, phase transformation and mechanical properties variations in the As-built state and after four conventional heat treatments (SHT + Age, SHT, Age, SR), as well as the effect of carbon intrusion, were studied. The results showed that the brittle phase Laves increased due to poor printing, and all the Laves phases were reduced and eliminated after heat treatment. The Laves phases generated the 8 phase in the near areas and the y'=y'' phase in the relatively distant areas due to diffusion effect. However, the NbC phase could not be eliminated after heat treatment and remains at the grain boundaries. Age heat treatment produces a special Laves phases, ductile network Laves phase. The intrusion of carbon element was observed by XRD and EDS. The diffused C element captures the Nb element, robbing the element of the main strengthening phase y'' and causing the reduction of ultimate tensile strength, yield strength and microhardness of SHT (538 Mpa, 307 Mpa, 286 HV), SR (739 Mpa, 454 Mpa, 241 HV) and SHT + Age (1051 Mpa, 903 Mpa, 485 HV) under high temperature treatment than As-built (786 Mpa, 510 Mpa, 322 HV). Under lower temperatures on Age (1097 Mpa, 863 Mpa, 479 HV), the effect of carbon intrusion is not obvious. However, the solid solution strengthening and interstitial strengthening of carbon element greatly increase the ductility of SHT (8.54%) and SR (12.12%).