On the thermomechanical aging of LPBF alloy 718

Heat treatment of products post additive manufacture are considered hugely important since the metallurgical condition post process is suboptimal. In the case of nickel-based superalloys, grain size, precipitate distribution and precipitate size are distinct from wrought equivalents. Appropriate heat treatment is required to ensure material performance. In this study, LPBF alloy 718, post-processed using a standard heat treatment, is explored under thermal and thermomechanical exposure conditions (with and without applied stress) to illustrate textural and microstructural evolution.The results show the instability of the LPBF microstructure in terms of grain size, precipitate density, and crystallographic orientation, illustrating the need for an appropriate heat treatment in relation to future service conditions. During thermal exposure only, the instability of the LPBF alloy microstructure was evident as the texture increased with time before decreasing and almost disappearing at the time of fracture. This contrasts with wrought alloy whose texture increases throughout creep testing and reaches a maximum at the time of fracture. An ideal microstructure for improved creep performance was identified and includes large equiaxed grains, elimination of texture, dissolution of Laves and delta phase and the precipitation of small carbides and gamma'' pre-cipitates. Recommendations on how to heat treat LPBF alloy 718 to reach this microstructure are given. Overall, this work showed that LPBF components may become more performant than wrought and conventional equivalents.

Automated summary

Heat treatment is crucial for post additive manufactured products to optimize their metallurgical condition. This study focuses on LPBF alloy 718 and reveals the instability of its microstructure under thermal and thermomechanical exposure conditions. The study emphasizes the importance of appropriate heat treatment in relation to future service conditions, and provides recommendations for achieving an ideal microstructure for improved performance.