1. Conclusions

In this work, we performed a detailed analysis of the precipitation kinetics of an AM IN625 alloy fabricated using L-PBF during heat treatment at 700 ◦C. Whereas previously reported residual stress heat treatments at 800 ◦C and 870 ◦C can effectively reduce the residual stress, they lead to the formation of large δ phase precipitates at a significant volume fraction and create unfavorable conditions for applications requiring good ductility, fracture toughness, and corrosion resistance. Our ex situ SEM data show that heat treatment at 700 ◦C leads to significantly slower precipitation of the δ phase precipitation when compared with 800 ◦C. The in situ synchrotron XRD data show that the δ phase is the only observable precipitating phase at 700 ◦C. The time-dependent lattice parameters of the FCC matrix and the δ phase show a continuous contraction of the FCC unit cell and a continuous expansion of the δ phase unit cell, which is consistent with a slow diffusion of Nb and Mo from the matrix phase to the δ phase. The in situ SAXS results reveal that the morphological evolution of the δ phase precipitates behaves differently at 700 ◦C compared to 800 ◦C and 870 ◦C. The major dimension of the platelet δ phase precipitates reached a stable value of 154 ± 7 nm after 10.5 h at 700 ◦C, which is in contrast to a continuously increasing major dimension that reached 961 ± 94 nm after 10 h at 870 ◦C. In the context of a residual stress heat treatment, a stress relief heat treatment at 700 ◦C for as long as 10 h results in δ phase precipitates (major dimension ≈150 nm) significantly smaller than those developed during typical residual stress heat treatment of AM 625 (major dimension ≈500 nm after one hour at 870 ◦C or two hours at 800 ◦C). We also compared the experimental findings with a TC-PRISMA-based precipitation simulation. The simulation captured the general trend in precipitation kinetics with good agreement between the observed and simulated precipitate size. The simulation overestimates the volume fraction of the precipitates, which is possibly due to factors such as the assumed spherical geometry of the precipitates, the effects of the dislocation density, and any temperature dependence of the interfacial energy. In general, this work unequivocally established significantly slower precipitation kinetics of the δ phase for AM IN625 at 700 ◦C than at 800 ◦C or 870 ◦C, which are temperatures commonly used for residual stress relief, and this work also provides the rigorous microstructural kinetics data required to explore the feasibility of a lower-temperature stress-relief heat treatment for AM IN625.