The effects of the solution heat treatment temperature on the precipitates, grain boundary evolution and response of the microhardness of Inconel 718 (IN718) superalloy fabricated by selective laser melting (SLM) were investigated. It was found that: (1) The long-chained Laves phases formed in the as-deposited condition dissolved into the matrix when the solution temperature rises above 980 °C. (2) The width-to-length ratio was maintained at approximately 1.6 when the solution was heated from 980 °C to 1080 °C, and dropped down to 1.03 when heated to 1130 °C. (3) Low-angle grain boundaries kept the same number fraction of 65% from 980 to 1080 °C as the as-deposited condition, and decreased dramatically from 1090 to 1130 °C to 4%. (4) Annealing twin boundaries occurred at 1090 °C with a number fraction of 3%, and quickly increased to 65% when heated to 1130 °C. It is concluded that the static recrystallization of IN718 fabricated by selective laser melting (SLM) occurred at 1090 °C and fast proceeded to full recrystallization at 1130 °C. The forming of annealing twins accompanies the recrystallization process and is an effective way to refine the recrystallized grain size.

The microstructure and precipitates of INCONEL 718 alloy (IN718) fabricated by selective laser melting (SLM) are investigated. The matrix is comprised of the elongated γ grains along Z direction, tilting a few degrees. The microstructure mainly consists of a cellular–columnar structure with fine first dendrite arm space of 1–1.5 µm originated from the rapid solidification rate locally. The precipitates of the SLMed IN718 are confirmed to be a significant amount of Laves in the form of irregular bulk distribution, small amount of carbonitride and spherical Al₂O₃ particles in nanoscale. Some of the carbonitrides grow separately and some of the carbonitrides enwrap the Al₂O₃ fully or partly, forming the shell structure of the Al₂O₃–carbonitride. The reheating effect from the following layers to the previous layers have not reached the solid-state transformation temperature of the γ′ and γ″.