Evolution of Microstructure and Its Effect on Mechanical Properties of Additively Manufactured Ni-based Superalloy During Heat Treatment

Abstract

Additive Manufacturing (AM) is a powerful tool to manufacture parts with unique benefits over traditional manufacturing, such as design freedom, reduction in manufacturing time and time, enabling use of broad range of materials etc. Laser Powder Bed Fusion is an AM technique that provides better part density and high dimensional accuracy. But there are some challenges associated with AM/LPBF process such as porosity, inclusion, complex residual field etc. which affect microstructure and mechanical properties of the material. Post-processing can tailor the microstructure to wrought standard, hence designing suitable heat treatment for LPBF parts is crucial. This study investigates microstructural evolution through various heat treatment schedules at different temperatures for different times that provides better understanding of tailoring desired microstructure and mechanical properties. LPBF Inconel 625 parts were heat treated at 700oC, 900oC and 1050oC for various lengths of time from 1 hour to 40 hours depending on objectives of the study. Then microstructure and hardness are compared between as-printed LPBF and wrought material. Fine dendritic microstructure with visible melt pool boundaries was detected in as-printed LPBF IN625. Columnar growth of grains along the build direction was also observed that is caused by downward heat transfer towards the substrate. Strengthening γʺ precipitate was detected in as-printed condition using transmission electron microscopy (TEM) that not only proves its presence, but also reveals its type. Very few researchers mentioned this phenomenon previously, but did not present with convincing proof. Hardness of as-printed IN625 is much higher than traditionally wrought IN625 due to its fine dendritic microstructure with the presence of γʺ precipitates. LPBF IN625 that were heat treated at 700℃ for 1h showed presence of γʺ precipitates as well. The 3rd variant is also visible after this heat treatment which indicates presence of tensile residual stress in as-printed condition that promoted the growth of this variant. The dendritic microstructure as well as the melt pool boundaries are still visible after heat treatment at 700℃ for 1, 2, 5 and 10h, but the columnar growth seems to be remodeled with increasing heating time. The hardness initially increases after 1h from as-printed, then decreases with increasing time until it increases again after 10h. Heat treatment at 900℃ caused precipitation of δ phase that appears as plate or needle in the microstructure. With increasing time from 1 to 40h, the precipitates grow in size and amount. The precipitates on grain boundaries grew larger than the ones inside the grains. Hardness values for this temperature are lower than those for 700℃ due to the dissolution of strengthening γʺ phase and formation of δ phase. Further analysis on this precipitate revealed that precipitation occurs much faster on the surface exposed to the furnace ambience during heat treatment compared to that in the bulk of the sample. Volume fraction analysis revealed that 1% precipitation of δ phase occurs in between 2 to 5 h in LPBF IN625 when heat treated at 900℃. The anisotropic and heterogenous microstructure becomes fully recrystallized after heat treating at 1050℃. Melt pool boundaries, dendritic structure and precipitates are completely dissolved, however, carbides are detected at this temperature. These carbides get redistributed and remodeled with increasing heating time. Hardness values are lowest at this temperature due to the dissolution of precipitates as well as stress relief. Metal oxide inclusions in the form of Al2O3 has been found in all heat treatment conditions. This study is a useful resource to understand the evolution of microstructure and precipitates in LPBF IN625 on its way to complete recrystallization. We have also been able to achieve a range of hardness that can be tailored using suitable heat treatment. Volume fraction analysis of δ phase can contribute to the establishment of new TTT diagram for LPBF IN625 in future.