## 3 Results, Analysis, and Discussion
### Analysis and Discussion
Besides, in Figure 4b, where the effects of the different levels chosen on the factors are shown separately, it can be generally seen that lower values of _Ra_ are obtained for \(f\) = 0.10 mm/rev, \(N\) = 500 rpm, under dry machining conditions, in the second magnesium plate, _LRI3_(Mg3), and in the measurement made at the hole entrance, _LRS1_. Figure 5 shows a higher surface roughness at the exit of the re-drilling of the central aluminum plate, _LRS2_, than at the entrance, _LRS1_, in the case of CCA cooling, with a directional effect on the process. The effect is however the opposite, although less pronounced, in the case of dry machining. A significant interaction between the type of cooling system, \(C\), and the Location Regarding the Specimen, _LRS_, was detected as well by the ANOVA analysis and summarized in Table 5. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Source** & **Squares Sum** & **Percentage of Variability** \\ \hline \(C\) & 1.882 & 20.7\% \\ _LRI_ & 4.983 & 54.7\% \\ \(C\) *_LRS_ & 2.243 & 24.6\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Mg-Al-Mg test. Percentage of variability of each factor on Ln_Ra_ according to ANOVA. Figure 4: Mg-Al-Mg test. (**a**) Normality of the distribution Ln_Ra_. (**b**) Main effects over Ln_Ra_. Based on the results of the trial, the following preliminary conclusions are drawn:  
* It is possible to obtain all \(Ra\) values below the maximum limits of the strict tolerances required in the aeronautical sector when repairing hybrid Mg-Al-Mg components by re-drilling using sustainable lubrication systems. Tests 5 and 7. * The factors with a significant effect on the response variable \(Ra\) are \(C\) (20.7%), \(LRI\) (54.7%), and the interaction \(C\)* \(LRS\) (24.6%). * Tests 2 and 3 would allow the re-drilling of 15 mm magnesium plates and 7.5 mm aluminum plates. For thicker aluminum plates, the data obtained give roughness values outside the optimum range for the aeronautical sector. * The factor of the type of cooling system, \(C\), has a clear significant influence on the roughness obtained in the aluminum plate and, in particular, on the output values of the re-drilling (\(LRI\)2-\(LRS\)2). In this area, by dry machining, the roughness obtained at the exit of the drill is within the range typical of the aeronautical industry (1.30 \(\upmu\)m in test 5) or higher than those accepted, but very close (1.70 \(\upmu\)m in test 8) while, with cold compressed air cooling, all the values obtained are out of tolerance (3.00, 3.95, 2.11, and 2.91 \(\upmu\)m in tests 1, 2, 3, and 4, respectively). * Based on the results, there seems to be a dependent directional effect in addition to the cooling/lubrication used, and consequently, the conduction of new tests is proposed to verify the influence of the drilling direction on the \(Ra\), that is, if there is a significant difference in making the re-drilling of the hybrid compound starting from the magnesium plate or the aluminum plate. These conclusions confirm and complement the factors of influence found in previous studies on Mg-Al-Mg multimaterial compounds, confirm the possibility of using sustainable machining processes in the repair of magnesium-based multimaterial compounds, provide new knowledge of the process such as the dependence of the final quality of the machining on the thickness of the plates of the multimaterial compound. This new knowledge allows to establish the basis for designing new configurations of hybrid multimaterial specimens to continue advancing in the development of sustainable machining techniques aimed at reuse after repair of magnesium-based multimaterial components, and with application in the aeronautical and/or automotive sector. Another novelty contributed by this study is to carry out a comparative analysis regarding previous studies, giving an enriching global perspective applicable to magnesium-based multimaterials. Figure 5: Mg-Al-Mg test.