## 4 Discussion
### Natural Aging, Nanoprecipitation, and the Impact on Tensile Behavior  
Much of the existing literature focuses on the impact that artificial aging has on the microstructural and mechanical behaviors of AM aluminum, but other than in small-scale \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Sample Location** & **Figure 9 ID** & \
\begin{tabular}{c} **Area Weighted Mean** \\ **Grain Size (\(\upmu\)m)** \\ \end{tabular} &
\begin{tabular}{c} **Maximum Grain** \\ **Size (\(\upmu\)m)** \\ \end{tabular} \\ \hline \multirow{4}{*}{6 NA} & 1 & B & 4.6 \(\pm\) 1.7 & 9.0 \\  & 2 & 4.8 \(\pm\) 1.7 & 10 \\  & 3 & & 6.4 \(\pm\) 2.3 & 13 \\  & 1 & C & 9.4 \(\pm\) 3.7 & 18 \\
12 NA & 2 & & 8.9 \(\pm\) 3.4 & 20 \\  & 3 & & 11 \(\pm\) 4.3 & 22 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._laboratory cases, natural aging is a reality that cannot be ignored. In industry, structures can remain at room temperature for a period of time prior to artificial aging, as it is not always realistic to move from fabrication to heat treatment immediately. This study's results clearly indicate a negative impact on tensile strength as the direct result of natural aging behavior. A theory has been developed to explain the diminishment in properties and the microstructural evolution influenced by natural aging. From the literature, Al\({}_{3}\)(Sc,Zr) precipitates are seen to form upon subsequent artificial aging; however, these temperatures are far from room temperature at which natural aging occurs [31; 33; 35]. Several studies in the literature for conventionally processed aluminum alloys have indicated that natural aging can promote the formation of solute clusters and Guinier Preston zones [39; 43]. Further treatment via artificial aging can significantly influence the final state of the precipitates and produce coarser precipitates than those seen without natural aging [51]. The precipitation sequence for Al\({}_{3}\)Sc has been noted not to contain Guinier-Preston zones or metastable phases [52; 53], so it could be theorized that, during natural aging, the SSSS promoted the formation of solute clusters which act as nucleation sites to form fully actualized Al\({}_{3}\)(Sc,Zr) nanoprecipitates upon artificial aging. However, several factors complicate this, such as the slow diffusion of scandium and zirconium in aluminum and the large free energy requirements for phase formation. These factors make precipitation at natural aging seem impossible, but the primary consideration is temperature, and without elevated temperatures, there is not enough energy to form precipitates. Considering the situation from a purely energy standpoint, it can be seen that temperature is just one energy source. A study by [PERSON] et al. explored the application of a cyclic force to generate compressive and tensile stresses to eliminate the high-temperature barrier for precipitation of age-hardened aluminum alloys, Al-Cu-based Al2024, Al-Mg-Si-based Al6061, and Al-Zn-based Al7075 [54]. The authors demonstrate that precipitation is possible as long as the energy is provided [54]. Another study by [PERSON] et al. [55] explored the impact of quench-induced residual stresses on the precipitation stresses of aluminum 7085 and determined that the elastic lattice distortion caused by the presence of residual stresses had a positive impact on precipitation during artificial aging. While processing occurred at elevated temperatures, the authors noted the importance of residual stresses for promoting nucleation [55]. [PERSON] et al. [56] noted the positive influence of deformation during quenching on precipitation. The authors found that the combined effect of deformation through stressing an aluminum 2024 part and the rapid cooling inherent in quenching lead to favorable, in situ precipitation behavior [56]. In conjunction with the literature and the evidence presented in this study, a working theory has been developed. The rapid cooling rate inherent to AM processing has been shown to produce extensive residual stresses [31; 57].