# 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
However, there was a considerable loss of ductility of 13% EL compared to the ST samples due to the production of hard precipitates, as shown in Figure 13:1  
To gain a better understanding of the effect of aging temperature on strength, aging at temperatures of 883 K, 903 K, and 923 K, based on the Lifshitz-Slyozov-Wagner (LSW) model, was simulated. Unlike other alloying systems, aging was found to cause severe negative drops in the mechanical properties. However, the Heatvar alloy showed a minimal reduction of only 2 HRC after being aged, indicating excellent high-temperature properties. In contrast to other LPBF alloy systems previously studied, such as MS1, which produces a soft austenite phase at elevated aging temperatures due to over-aging, Heatvar exhibited superior mechanical stability at a temperature of 893 K for 100 h (Figure 17). Beyond this optimal temperature, Orowan looping became more pronounced as the precipitates coarsened. Previous studies on wrought H13 and MS1 showed significant drops in the HRC values [93], demonstrating the Heatvar alloy's superior mechanical properties. However, this study did not address the combined method of applying solution treatment and DAG to optimize the alloy's ductility-versus-strength ratio. Further research is needed to analyze the effect of solution treatment before aging and the results of aging time at different temperatures, as in the previous work by [PERSON] et al. [114]. To maximize the mechanical and functional qualities of alloys, the development of an optimal HT method is crucial. Different heat treatment procedures with the same chemical composition may produce different reactions under similar chemical compositions. An LPBF-EOS SS CX alloy and a wrought alloy belonging to the PH13-8 Mo was reported [98] to possess a martensitic structure with a distribution of austenite as a result of austenite stabilizing elements (Mo, Ni, and Mn). Moreover, these elements decreased the martensite finish temperature. The fraction of retained austenite in the LPBF and the wrought alloy was 6% and 3%, respectively. Due to limited energy and time for the austenite-stabilizing elements to diffuse out of the austenite phase during the cooling process, significant elemental segregation was observed [118]. A similar solution treatment protocol was applied to the hybrid materials [98], consisting of heating at 850 \({}^{\circ}\)C for 30 min (Figure 18). A peak hardness of 50 HRC was achieved for both alloys after 2 h of aging. In this study, the LPBF alloy precipitated a larger (100 nm) NiAl phase and less reverted austenite (1%) compared to the wrought counterpart, which had a smaller (<40 nm) NiAl phase and approximately 5% retained austenite. This difference in reverted austenite could be attributed to the influence of nitrogen gas during powder atomization, as reported by [PERSON] et al. [121] in their work on PH17-4. The \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Treatment** & **Yield Strength (MPa)** & **Tensile Strength (MPa)** & **Elongation (\%)** \\ \hline As-printed & \(1165\pm 12\) & \(1190\pm 16\) & \(4\pm 1\) \\ Aged & \(1660\pm 18\) & \(1980\pm 20\) & \(2\pm 1\) \\ Solution-treated & \(1156\pm 12\) & \(1369\pm 18\) & \(15\pm 1\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Change of mechanical properties with heat treatment [93]. LPBF alloy exhibited a superior yield strength of 1700 GPa compared to the wrought alloy's yield strength of 1600 GPa. In contrast, the wrought alloy exhibited reduced ductility of approximately 4% compared to 10% for the LPBF alloy. This highlights the significance of investigating the substantial retainment of austenite, as suggested by [PERSON] et al. [122], in order to better understand the underlying factors contributing to the mechanical properties of these alloys. LPBF metals produce fine grain structures that strengthen metals due to a high grain boundary following the Hall-Petch mechanism, hence a higher yield strength [16].