# Additive Manufacturing of PH 13-8 Mo Family: A Review
## 2 PBF-LB of PH 13-8 Mo Family
### Post-Processing Heat Treatments: Microstructure  
Post-processing heat treatments are very important to improve the microstructure and properties of the PH 13-8 Mo family. Therefore, several authors have investigated the relationship between different heat treatments and the resulting microstructure and tried to find the optimum post-processing heat treatment. Post-processing heat treatment procedures after the PBF-LB process from the literature can be found in Table 3.  
The influence of three different heat treatments on the microstructure of PBF-LB/CX was investigated by [PERSON] et al. [47]. These heat treatments were solution treatment at 900 \({}^{\circ}\)C for 1 h, aging at 530 \({}^{\circ}\)C for 3 h, and solution treatment at 900 \({}^{\circ}\)C for 1 h + aging at 530 \({}^{\circ}\)C for 3 h. The as-built sample had only 0.3% austenite, and after solution treatment, the austenite was not detected in the sample, which suggested that the solution heat treatment successfully eliminated the austenite. However, the solution annealed microstructure was  
Figure 2: Microstructure of as-built PBF/CX for (**a**,**b**) vertical, (**c**,**d**) horizontal (Reprinted from Ref. [53], Copyright (2022) with permission from Elsevier). coarser. Aged and solution-aged samples had higher austenite content (4.5% and 3.5%, respectively). It was considered that during aging treatment, a reverse transformation from martensite to austenite was promoted, which increased the austenite content. NiAl precipitates (3-25 nm) were observed via TEM in the as-built microstructure. On the other hand, larger NiAl precipitates (7-30 nm) at a higher intensity were found in the solution-aged specimen. Those precipitates were rod-like. [PERSON] et al. [49] used the same heat treatment procedures and achieved similar results regarding the effectiveness of solution-aging heat treatment. In addition, [PERSON] et al. [48] conducted experiments with the same heat treatment procedure for PBF-LB/CX. It was confirmed that solution annealing increased the average size of martensite, and reverted austenite was present after the aging treatment. It was added that the aging treatment refined the grains due to the recovery process compared to solution-annealed and as-built conditions. However, solution-aging heat treatment suppressed the formation of NiAl precipitates. Direct aging heat treatment was more successful in promoting the formation of precipitates. [PERSON] et al. [30] studied the microstructure of Corrax in three different conditions: heat-treated wrought, heat-treated after PBF-LB, and heat-treated in the HIP (hot isostatic pressing) after PBF-LB. In that study, the recommended heat treatment procedure (solution treatment at 850 \({}^{\circ}\)C for 30 min + aging at 525 \({}^{\circ}\)C for 4 h) was used. Mostly spherical B2 NiAl precipitates (1-9 nm) were observed using Atom Probe Tomography (APT) in all conditions, as shown in Figure 3. The size distribution of precipitates can be found in Figure 3d. The precipitates of heat-treated PBF-LB/Corrax (d50 = 4.3 nm) were slightly higher than the other conditions. The HIPed microstructure was coarser and presented the highest austenite amount compared to the other conditions investigated. Figure 3: APT results—NiAl precipitates in (**a**) heat-treated wrought Corrax (C/SA), (**b**) heat-treated PBF-LB/Corrax (AM/SA), (**c**) heat-treated PBF-LB/Corrax in the HIP (AM/HIP), (**d**) precipitates size distribution (Reprinted from Ref. [30], Copyright (2020), with permission from Elsevier). [PERSON] et al. [57] heat-treated the PBF-LB/Corrax with solution heat treatment at 900 \({}^{\circ}\)C for 1 h and aging at 530 \({}^{\circ}\)C for 3 h. However, NiAl precipitates were detected in the same range (1-10 nm) and had a similar average size (4.27 \(\pm\) 1.20 nm) when comparing the microstructure in both conditions. Similarly, [PERSON] et al. [58] investigated the influence of different heat treatments on the microstructure of PBF-LB/Corrax.