  Homogenization is one of the critical stages in the post-heat treatment of
additive manufacturing (AM) component to achieve uniform microstructure. During
homogenization, grain coarsening could be an issue to reserve strength, which
requires careful design of both time and temperature. Therefore, a proper
design of homogenization becomes particularly important for AM design, for
which work hardening is usually no longer an option. In this work, we
discovered an intriguing phenomenon during homogenization of suction-cast and
AM Inconel 718 superalloys. Through both short and long-term isothermal heat
treatments at 1180{\deg}C, we observed an abnormal grain growth in the
suction-cast alloy but continuous recrystallization in the alloy made by laser
powder bed fusion (LPBF). The grain size of AM samples keeps as small as 130
{\mu}m and is even slightly reduced after homogenization for 12 hours. The
homogeneity of Nb in the AM alloys is identified as the critical factor for NbC
formation, which further influences the recrystallization kinetics at
1180{\deg}C. Multi-type dislocation behaviors are studied to elucidate the
grain refinement observed in homogenized alloys after LPBF. This work provides
a new pathway on microstructure engineering of AM alloys for improved
mechanical performance superior to traditionally manufactured ones.
