## 4 Discussion
### Natural Aging, Nanoprecipitation, and the Impact on Tensile Behavior
[PERSON] et al. [58; 59] theorize the potential of large residual stresses due to AM processing being a catalyst for precipitation processes by altering diffusion kinetics for aluminum. Additionally, studies have noted that AM processes can produce a microstructure that contains many vacancies and dislocations [60; 61], which can also influence diffusion kinetics and precipitation [61]. The magnitude of the residual stresses caused by DED processing and the presence of vacancies and dislocations noted in AM microstructures are theorized to destabilize the supersaturated solid solution, leading to the formation of solute clusters over an extended period of time at room temperature and paving the way for the growth of nanoprecipitates during artificial aging. Subsequent artificial aging at 325 \({}^{\circ}\)C for 4 h produces nanoprecipitates designed to promote strengthening and hinder grain growth. Examination of the evolution of nanoprecipitate size presented in Figure 8 shows an increase in nanoprecipitate size and a decrease in the number of precipitates with longer natural aging times. Two schools of thought are presented by [PERSON] et al. concerning the precipitation behavior of solute clusters formed at room temperature: (1) All clusters formed at room temperature are too small and revert back to SSSS upon artificial aging and, in some cases, resulting in an overall coarser precipitate phase for naturally aged conditions relative to cases not naturally aged, (2) At room temperature solute clusters formed can be small or large, but only the large clusters form precipitates and the small clusters revert upon artificial aging [62]. From the study, the authors noted that, while the solute clusters did not increase in size during natural aging, there was an increase in the number density during the first few thousand kiloseconds at room temperature, whereafter the number density held relatively constant [62]. [PERSON] et al. noted that, as a result, the longest natural aged case exhibited the most dense clusters and the greatest drop in hardness after artificial aging [62]. The theories presented by [PERSON] et al. could explain the phenomenon noted in this study: precipitates are coarser with natural aging prior to artificial aging [62]. Furthermore, as the precipitates evolve, their ability to pin the grain boundaries diminishes, resulting in greater coarsening and decreased tensile strength. The impact of natural aging time prior to artificial aging on YS and grain size is depicted in Figure 10, indicating the connection between tensile properties and microstructural evolution. It is worth noting that, while grain size plays a considerable role in the mechanical behavior due to Hall-Petch strengthening, grain size is only one factor for this alloy as there are other strengthening mechanisms. The precipitate phase, defect status, and processing method also impact strength. While the grain size was seen to increase due to the evolution of the precipitates, the strength change is not a directly proportional change. This is a result of the other factors involved and the combinatory effect of these factors.