Achieving simultaneous control of second phase and grain structure in directed energy deposited Inconel 718 alloy: insights from heat treatment

During the directed energy deposition (DED) of superalloys, rapid cooling rates and nonuniform temperature gradients usually lead to the formation of coarse columnar grains and complex precipitates. The appropriate post-processing techniques are necessary, and the heat treatment shows the promising in optimizing microstructure of the DEDed superalloys. To simultaneously control second phase and grain structure in the DEDed Inconel 718 alloy, two heat treatment schemes are designed and evaluated in this study: H schemes (involving only homogenization treatment) and AH schemes (combining pre-aging and homogenization treatments). For H schemes, as the homogenization temperature rises or homogenization time extends, Laves phase dissolves progressively. Moreover, at lower homogenization temperatures (1000 ℃ and 1050 ℃), the growth of columnar grain takes place. As the homogenization temperature rises (1100 ℃ and 1150 ℃), the initial columnar grains transform into equiaxed grains due to the progress of recrystallization, which weakens and randomizes the initial strong {001} < 100 > rotated cube texture. For AH schemes, the pre-aging treatment facilitates the dissolution and morphology transformation of Laves phase. This allows that Laves phase dissolves completely under a lower temperature and shorter time in the following homogenization treatment. Furthermore, the pinning effects of δ phase on grain boundaries retard grain boundary migration during the following homogenization treatment, and the finer grain structure is obtained under AH schemes. The optimal heat treatment for simultaneous control of second phases and grain structure is identified as pre-aging-treated at 900 ℃ for 720 min and following homogenization-treated at 1100 ℃ for 60 min.