## 2 Methodology and Materials
### Equipment, Tools, and Materials  
The following equipment, tools, and materials were used to conduct the study:  
* Tongtai TMV510 Machining center (Tongtai Machine and Tool Co, Luzhu Dist, Kaohsiung City, Taiwan) equipped with Fanuc series OI-MC numerical control (FANUC Iberia, Castelldefels, Barcelona, Spain) (Figure 2a).
* Cold Compressed Air system from Cold Air Gun Vortex (Vortec, Cincinnati, Ohio, U.S.). This equipment uses vortex tube technology and filtered compressed air to produce a compressed air of a working temperature of 8 \({}^{\circ}\)C. The equipment employs no moving parts, does not require electricity, just a compressed air source [61]. (Figure 2b).
* Mitutoyo Surftest SJ 401 Roughness tester (Mitutoyo America Corporation, Aurora, IL, USA) (Figure 2c).
* A 9 mm diameter high performance twist drill with supplier reference HSS-E-PM-A1 1257. Supplier Garant (Hoffmann Iberia Quality Tools S.L., San Fernando de Henares, Madrid, Spain) (Figure 2d).
* Multimaterial or hybrid compound specimens for re-drilling. These specimens consist of three intercalated plates of magnesium/aluminum/magnesium alloys of 15 mm thickness. The combination of the different material layers is designed to allow the drill bit passing from one material to another during the machining of the hole. The operations performed aim to simulate the repair operations on hybrid components. For this reason, specimens used already have 8 mm pre-drilled holes, and the procedure consists of re-drilling them to a slightly larger diameter of 9 mm. The shape of each plate is a parallelepiped whose dimensions are \(50\times 50\times 15\) mm. The three parallelepipeds are mechanically fixed so that the surface roughness inside the machined holes can be dismantled and measured relatively easily (Figure 2e). The chemical compositions of the magnesium and aluminum alloys used are available in Table 3.  
Figure 2: _Cont._A schematic diagram of the experimental configuration for re-drilling tests on Mg-Al-Mg multimaterial specimens is shown in Figure 3. The figure schematically shows the different stages of Taguchi-based research and the main materials employed. Predrilled specimens were tested using a machining center and cutting tool. Tests were carried out according to a predefined test plan in a Taguchi-based experimental design. Tests allow data collection of the response variable at each position predefined as potentially significant. Finally, this data is statistically analyzed using descriptive statistics and analysis of variance.  
\begin{table}
\begin{tabular}{c c} \hline
**UNS M11917 (AZ91D)** & **UNS A92024 (AA2024 T351)** \\ \hline Al 8.30–9.70\% & Al 90.7–94.7\% \\ Cu \(\leq\) 0.03\% & Cr \(\leq\) 0.1\% \\ Fe \(\leq\) 0.005\% & Cu 3.8–4.9\% \\ Mg 90\% & Fe \(\leq\) 0.5\% \\ Mn \(\geq\) 0.13\% & Mg 1.2–1.8\% \\ Ni \(\leq\) 0.002\% & Mn 0.3–0.9\% \\ Si \(\leq\) 0.1\% & Si \(\leq\) 0.5\% \\  & Ti \(\leq\) 0.15\% \\ Zn 0.35–1\% & Zn \(\leq\) 0.25\% \\ \hline \end{tabular}
\end{table}
Table 3: Chemical composition of the materials used for the manufacturing specimens of UNS M11917 (AZ91D) and UNS A92024 (AA2024 T351).  
Figure 2: Main equipment, tools, and materials used during the trials. (**a**) Tongtai TMV510 machining center; (**b**) Vortec Cold Air Gun; (**c**) Mitutoyo Surftest SJ 401 roughness tester; (**d**) helical drill bits HSS-E-PM A1 1257 manufactured by Garant; (**e**) overall dimensions of the re-drilling specimen; (**f**) specimen Mg-Al-Mg.