Microstructure Engineering During Heat Treatment of Inconel 718 Superalloy Processed by Laser Powder Bed Fusion

Post-heat treatment of additively manufactured (AM) Inconel 718 components is essential for optimizing microstructure and property since deformation processing is no longer applicable. Although several commercial heat treatments have been developed for wrought and cast alloys, a systematic study on the microstructure engineering during heat treatment of AM Inconel 718 is needed because its different initial microstructures from the traditional alloys can cause varied microstructure change during heat treatment and can thus influence material properties. This dissertation comprehensively studied the microstructure engineering during the heat treatment of Inconel 718 alloy fabricated by laser powder bed fusion (LPBF). Experiments along with modeling were conducted to understand the phase transformations. The microstructure evolutions in the heat treatment stages, i.e., homogenization, continuous cooling, and aging, were investigated and the process-structure-property relationships were inferred. The microstructure change of LPBF Inconel 718 during homogenization was studied by both experiments and kinetics simulation. Homogenization at a higher temperature (1180oC) than what is used traditionally could effectively introduce recrystallization, remove columnar grain texture, and reduce residual stress. Additionally, grain refinement was achieved through recrystallization and the Zener pinning effect in LPBF Inconel 718, confirming that the mechanical properties of AM components could be potentially improved by heat treatment. During a continuous cooling process, the phase transformations of LPBF alloys were found to be different from those in suction-cast alloys and were impacted by homogenization durations. The different phase transformation behaviors were attributed to the varied Nb homogeneities caused by diverse homogenization conditions. Microstructure evolution during isothermal aging process at intermediate temperature could be affected by homogenization conditions; more isotropic mechanical properties could be obtained with aging after high-temperature homogenization. A novel optimized heat treatment strategy was developed based on the inferred phase transformation behaviors, which achieved a combination of good strength and ductility while maintaining the properties isotropic. Moreover, a high-throughput approach leveraging the printing flexibility of LPBF was developed and the potential of accelerating heat treatment design for AM alloys was demonstrated