Effect of stress-relief heat treatments on the microstructure and mechanical response of additively manufactured IN625 thin-walled elements

Metallic structures fabricated with laser powder-bed fusion (LPBF) often have elemental segregations that are far from equilibrium and high levels of residual stresses due to the localized melting and rapid solidification that occur during printing. These unique characteristics result in mechanical property debits that make it difficult to employ such printed parts in critical applications. However, post-processing heat treatments can minimize the negative effects of residual stresses and reduce segregation. In this study, we measure the effect of two stress-relief heat treatments, 870°C for 1 h (industry-recommended) and 800°C for 1 h (potential alternative), on the microstructure and room temperature, monotonic quasi-static mechanical response of LPBF-processed thin-walled Inconel 625 T-elements. Electron backscatter diffraction analysis revealed an overall columnar microstructure with a strong texture that is closely aligned with the build direction. The well-developed dislocation cell walls in both stress-relieved cases were found to be decorated with two types of blocky precipitates, namely, (Nb, N)-rich MX and (Nb, Mo, Si, N)-rich -M6X with scant evidence of the deleterious δ phase. The slightly larger size and higher volume fraction of precipitates in the 870°C case was found to have a small effect on the mechanical response of the T-elements under the current loading conditions and the difference in mechanical response of the two heat treatments was only significant at the very later stages of deformation. These findings make 800°C for 1 h a viable stress-relief alternative for our IN625 samples under the loading conditions studied here.