# 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
Atom probe tomography results identified the existence of two Ni\({}_{3}\)X phases, with one phase having a lean Ti concentration compared to Mo in a ratio of 1:3 and the other phase having a lean concentration of Mo compared to Ti in a ratio of 1:3. Reverted austenite was suggested to be present in the Ni-rich patches detected in the precipitate-free zone. The equilibrium concentration of austenite at 480 \({}^{\circ}\)C was calculated using thermodynamic calculations and was confirmed through experimental validation. The study observed theoretical and empirical deviations in Ni concentrations for the martensite, with 4 at. % and 10 at. %, respectively. The cause of this discrepancy was not clear, as it could have been due to the precipitation of the new intermetallic or slow diffusion of Ni [110]. [PERSON] et al. [118] focused on increasing the number density of intermetallic precipitates to improve the homogeneity of 300 maraging steel produced using LPBF. The absence of nano-precipitates in the as-printed state was confirmed by the earlier findings by [PERSON] et al. [110]. The authors suggested that Ni segregation under direct aging at 510 \({}^{\circ}\)C for 6 h possibly reduced the potential active sites for nucleation, favoring growth over nucleation. On the other hand, solution aging (900 \({}^{\circ}\)C for 1 h + 510 \({}^{\circ}\)C for 6 h) promoted fine precipitates with significant density, limiting austenite reversion [118]. These findings can expand the ability to control the different mechanical properties of materials by adjusting the volume density of precipitates. In contrast to the findings of [PERSON] et al. [114] and [PERSON] et al. [117], the results obtained in this study demonstrated that solution treatment is an essential step for effectively managing and controlling precipitation, as well as achieving the desired mechanical properties. [PERSON] et al. [93] conducted a study on the Heatar alloy, comparing the effects of solution and DAG on the precipitates' evolution in the as-printed state. The study found that the alloy exhibited a Laves phase during solidification along the melt pool boundaries where Mo was segregated. At ambient conditions, this phase was found to possess brittleness, making it prone to act as crack nucleation sites [119]. A theoretical equilibrium partition coefficient of 0.34 and a solid front velocity solidification averaging 0.5 m/s confirmed this observation together with early reports on Fe-Mo systems [109]. To address this issue, solution treatment was conducted, which simultaneously dissolved the eutectic phase and improved the alloy's elongation from approximately 4% to 15%. The solution treatment was carried out at an elevated temperature of 1233 K for 30 min, which ensured the redistribution of alloying elements inside the grain interior. M\({}_{6}\)C and MC carbides were identified, consistent with [PERSON] et al. 's work [120], who confirmed the observations. These carbides provide sufficient strength and ductility to the alloy. A comparison of the as-printed and solution-treated samples showed a good balance of the strength and ductility in the solution-treated samples, making the heat treatment suitable for low-strength, high-ductility applications. Direct aging at 903 K for 240 min resulted in the highest strength value, averaging 1980 MPa. During the aging of managing steels, Fe\({}_{2}\)Mo precipitates with a size of approximately 36 nm were formed [120], which were confirmed by TEM analysis. The disappearance of previously observed melt pool boundaries was also observed, similar to the solution-treated samples. The most-complex material was produced with a distribution of around 20% precipitates and a hardness of over 2000 GPa.