A Comparative Analysis of Inconel 718 Made by Additive Manufacturing and Suction Casting: Microstructure Evolution in Homogenization

TL;DR

This study compares the microstructure evolution during homogenization of Inconel 718 produced by additive manufacturing and suction casting, revealing distinct grain growth behaviors and the influence of Nb homogeneity on recrystallization.

Contribution

It uncovers the different grain growth mechanisms in AM and cast Inconel 718 during homogenization, highlighting the role of Nb homogeneity in microstructure control.

Findings

AM Inconel 718 maintains small grain size after homogenization.

Suction-cast alloy exhibits abnormal grain growth during homogenization.

Nb homogeneity influences NbC formation and recrystallization kinetics.

Abstract

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.