# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 5 Effect of Heat Treatments in AM Processed Maraging Steels
### Strengthening Mechanisms' Models
#### 5.1.3 Precipitation Strengthening
Since Ni and Ti have the lowest interaction energy, they tend to nucleate in the form of Ni\({}_{3}\)Ti with an association of Co and Fe atoms with formation energies similar to the Ni and Ti sites. The formation energy can be represented by Equation (22). \[\sigma_{p}=\frac{1}{b}\frac{2S}{\sqrt{\pi}w_{q}}\Big{(}\frac{\gamma\ w_{r}r_{ p}}{S}\Big{)}\frac{\sqrt[3]{f}}{r_{p}} \tag{6}\]9  
where \(nE_{Ni_{3}Ti-Fe/Co}\) is the energy of the supercell with one Co or Fe atom, \(3/4n\) or \((3/4n-1)\) is the bulk energy of Ni atoms, and \((1/4n-1)\) or \(1/4n\) is the bulk energy of one Co or Fe atom. Figure 16 shows that, regardless of Co's and Fe's occupied Ni and Ti sites, the formation energy of Ni-Ti clusters is less endothermic, and as aging progresses, Co and Fe will be rejected from Ni\({}_{3}\)Ti. For example, in the early work by [PERSON] et al. [113], high-Ni-content-based maraging steels with Ti led to the precipitation of Ni\({}_{3}\)Ti phases. Partial substitution of the Ni by Fe and Co in some sublattices was also reported, together with the formation of Fe\({}_{2}\)Mo phases at extended aging treatments [110; 113]. This is consistent with the results of [PERSON] et al. [93] and [PERSON] et al. [114] on the Heatar alloy and 18 Ni-300, respectively. In the latter study [114], it was observed that heat treatment led to variations in both mechanical and microstructural properties. Surprisingly, Directly Aged (DAG) samples and Solution treatment followed by Aging samples (SAG) displayed a comparable hardness (654 HVN) and tensile strength (2126 MPa), despite their distinct microstructures. Austenitization above 900 \({}^{\circ}\)C resulted in the growth of austenite grains, producing large lathe martensite, and solution treatment  
Figure 13:0 Clustering of (**a**) Co, Ni, and Ti, (**b**) Ni, and Ti, and (**c**) Co distributions relative to their dispersion in BCC Fe. Reprinted with permission from [95]. Copyright 2019 Acta Materialia Inc.  
facilitated the migration of segregated Mo and Ti into the grain, causing the Mo- and Ti-rich white grain boundaries to disappear. Previous studies, by [PERSON] et al. [115], also observed the disappearance of alloy-rich grain boundaries after solution treatment. However, white borders with high Mo and Ti content were seen in the grain boundaries compared to the DAG samples. This suggests that the maximum temperature used for aging (560 \({}^{\circ}\)C for 12 h) did not provide sufficient energy to diffuse Mo and Ti from the grain boundaries into the grain interiors. For maraging steels generated by LPBF, solution treatment may be unnecessary, and a direct aging strategy can be utilized due to the rapid cooling (103 to 108 K/s [16; 116]) during processing, which produces lathe martensite. While employing DAG may be a cost-effective option for post-heat treatment of maraging steels generated by additive manufacturing, this may not always be true [114; 117]. Solution treatment is known to improve ductility and reduce micro-segregation, and achieving a good balance of ductility and strength is crucial for practical applications. However, at extended periods, it can cause the reversion of martensite into austenite [85]. Additionally, residual stress from additive manufacturing processes can adversely affect mechanical performance [12]; therefore, solution treatment is deemed a necessary step. It is worth noting that this study failed to clearly explain the response of martensite lathes' size to different heat treatments, which is a crucial factor in establishing the effectiveness of each heat treatment procedure. In a previous study by [PERSON] et al. [110] on 18 Ni-300 steel, it was found that localized heating during the AM process did not result in phase transformations due to the rapid heating/cooling. However, after aging at 480 \({}^{\circ}\)C for 5 h, three main phase precipitates were observed, namely (Fe,Ni,Co)\({}_{3}\)(Ti,Mo), (Fe,Ni,Co)\({}_{3}\)(Mo,Ti), and (Fe,Ni,Co)\({}_{7}\)Mo\({}_{6}\) (\(\mu\)-phase).