## 2 Materials and Methods
### Sample Preparation  
For this investigation, a 6061 structural aluminum alloy was used, and its composition is shown in Table 2 as it was determined using a spark spectrometer. The aluminum alloy used was a commercial 6061-T6 alloy (as-received material). The designation of the T6 condition corresponds to a standardized classification system used to denote the heat treatment of an alloy (typically referring to a sheet or plate) aimed at altering its mechanical properties and hardness. In the case of alloy 6061, a range of designations from T1 to T38 exists. Specifically, T6 indicates that the alloy initially underwent a T4 treatment (a solutionizing heat treatment), followed by a subsequent precipitation hardening thermal treatment involving heating at 530 \({}^{\circ}\)C, succeeded by aging at 160 \({}^{\circ}\)C for 18 h, resulting in the designation T6 [24; 25]. It is worth noting that we performed all tests with the alloy in the T6 condition from the supplier. The initial state characteristics are specified in the subsequent sections, presented in the results sections of the experimentation conducted in this research. For the heat treatment processes, samples with dimensions of 6.35 mm in thickness, 25 mm in width, and 70 mm in length were cut. To conduct the research, we proposed a design of experiment (DOE) using a Taguchi L27 (3\({}^{\circ}\)4) array [26] consisting of four factors: (A) solutionizing temperature, (B) aging temperature,(C) solutionizing time, and (D) aging time, and three levels: (1) low, (2) medium, and (3) high. The combination of levels and factors is presented in Table 3; in this table, each row represents a unique combination of factors and levels to be evaluated. For example, row 1 corresponds to testing factor A at level 1, factor B at level 1, factor C at level 1, and factor D at level 1. Similarly, row 2 corresponds to testing factor A at level 1, factor B at level 2, factor C at level 2, factor D at level 2, and so on. The selection of solutionizing and aging temperatures and duration was based on existing literature, mainly the references analyzed in the Introduction [8; 9; 10], where similar values are reported, but also relevant international standards and guidelines [24; 27]. Also, we selected the temperature and time intervals that we found to be useful in heat treatments that we have used in welding processes [14; 28; 29]. The three levels for the solutionizing temperature were 520, 530, and 540 \({}^{\circ}\)C; the levels for the solutionizing time were 3, 4, and 5 h; for aging temperature, we proposed 150, 160, and 170 \({}^{\circ}\)C; and for the aging time, we used 12, 18, and 24 h. A muffle furnace was utilized to perform solutionizing and aging heat-treatments based on the combinations described in Table 3. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & & \multicolumn{3}{c}{**Element**} & \multicolumn{3}{c}{**Other Not Specified**} \\  & & & **(wt. \%)** & & & & **(wt. \%)** \\ \hline
**Si** & **Fe** & **Cu** & **Mn** & **Mg** & **Cr** & **Zn** & **Ti** & **Individual** & **Total** \\ \hline
0.75 & 0.62 & 0.38 & 0.14 & 1.10 & 0.33 & 0.25 & 0.15 & \textless{}0.05 & \textless{}0.15 \\ \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of as-received 6061-T6 aluminum alloy. In the context of experimental design, selecting factors and levels is crucial in determining the efficiency and effectiveness of the study. When designing our experiment, we carefully identified known or hypothesized variables to influence the response variable, which, in our case, is the ultimate tensile strength (UTS) of the alloy. These factors were chosen based on a literature review, as mentioned in the precedent paragraph, and our understanding of the material properties and heat-treatment processes involved. Each selected factor has a potentially significant impact on the response. Furthermore, the decision to include four factors and three levels each was made carefully considering the capabilities of the Taguchi DOE methodology [18].