## 5 Conclusions  
The effects of natural aging prior to artificial aging at 325 \({}^{\circ}\)C for 4 h on the microstructural evolution and tensile behavior have been investigated. From this study, the following conclusions can be drawn:  
1. Application of natural aging time is seen to result in decreased tensile strength. After 6 days of natural aging prior to artificial aging (6 NA), the average YS and TS are diminished by 10 and 8.6%, respectively. As natural aging time increases to 12 days (12 NA), average YS decreases by 23%, and TS decreases by 18% relative to the 0 NA case. Additionally, an overall diminishing trend was noted for elongation after fracture with the application of natural aging. Figure 10: Correlation between tensile properties and microstructural evolution across natural aging time. 2. Accompanying the decreases in YS and TS as natural aging time increases is the increase in median nanoprecipitate size (Feret diameter) by 26% after 6 days (6 NA) and 73% at 12 days (12 NA) relative to 0 days of natural aging (0 NA). With the increase in precipitate size, it was also noted that the number of precipitates was decreased, with precipitate area fractions for 0 NA, 6 NA, and 12 NA being 1.2, 1.1, and 0.74%, respectively. The increased size and decreased number of precipitates negatively impact the ability to precipitate strengthen and can be linked to the decrease in YS and TS. 3. Like the nanoprecipitates, the Al\({}_{3}\)(Sc,Zr) micron and sub-micron phase particles experienced a size increase with natural aging. The particle size increased by 58% after 12 days at room temperature (12 NA) relative to 0 days of natural aging (0 NA). With the increase in size, it was noted that the number of particles decreased, likely due to growth at the expense of others. The increased size and decreased number of secondary Al\({}_{3}\)(Sc,Zr) phase particles limit strengthening capabilities. 4. Along with the evolution of precipitates, the microstructure demonstrated increased grain sizes with increasing natural aging time. Of note is the dramatic increase in grain size for 12 NA compared to 0 NA and 6 NA. The increased grain size is theorized to be a key piece of the explanation behind the initial decrease in YS and TS seen on days 2-10, and the further decrease occurs on 12 NA. The increasing grain sizes mean that the number of grains and grain boundaries is decreased. As a result, the Hall-Petch or grain boundary strengthening is negatively impacted and can be linked to the decreased YS and TS. 5. From the results of this study, cases where natural aging can be eliminated, and artificial aging can proceed immediately following fabrication are vastly preferable. However, if natural aging is unavoidable, the time frame for DED Scalmalloy(r) components should be constrained to time frames less than 12 days (12 NA). 6. Future Work: The impact of natural aging on tensile strength has been well documented for conventional processing of select common aluminum alloys; however, the lack of literature surrounding the impact on AM aluminum leaves a considerable gap in the knowledge base. The knowledge gap stems from the fact that, in most cases, it is impossible to eliminate any natural aging time between fabrication and artificial aging. As a result, it is necessary to understand how delaying artificial aging treatments impacts behavior so that a solution can be found to address and balance the issue of property diminishment while allowing for a waiting period between fabrication and artificial aging. These studies should also be expanded to document additional causal behavior observed as an effect of natural aging with an emphasis on minimizing any adverse mechanical performance due to natural aging. Future studies are recommended to document this behavior in other aluminum alloys used in AM. Conceptualization, R.B.-N. and S.P.I. ; methodology, R.B.-N. and S.P.I.