## 4 Conclusions  
This study is included in a larger project that focuses on the development of sustainable machining techniques applicable to hybrid multimaterial components made of magnesium-based light alloys. As a novelty, it presents an analysis of magnesium-based multimaterial compounds focusing on the relationship between the thickness of the plates that form the compound and the multimaterial combination used, and the directionality of the process. This analysis was carried out from a joint perspective and applying the previous knowledge acquired, so that in addition to analyzing the data obtained in the individual experiment carried out for this study on a Mg-Al-Mg component, a comparative analysis was also carried out with the data obtained in a previous trial framed in the same project, and carried out on a Mg-Ti-Mg multimaterial combination. The data from both studies are comparable as they follow the same methodological structure based on Taguchi, and designed for this purpose. The data from the previous study were reanalyzed following the perspective, objectives, and structure of the present study. Specifically, the design selected was a product of a complete factorial \(2^{3}\) and a block of two factors (\(3\times 2\)) that includes eight re-drilling tests. The following conclusions were drawn from this experiment:  
This study aimed to establish a basis for adapting or redesigning the current project specimens to allow them to be used in the study of applications involving the riveting of multimaterial plates made of light alloys in the aerospace industry, especially those made of magnesium-based alloys. To this end, the relationship between the surface quality obtained after re-drilling different combinations of magnesium-based multimaterials, the thicknesses of the plates that form them, and the cutting parameters were analyzed, all oriented towards repair and maintenance processes of parts by re-drilling in the aeronautical industry, and with the essential need to guarantee the design requirements throughout their useful life. The following conclusions were drawn from this experiment:  
Figure 6: Mg-Ti-Mg test. (**a**) Multiple interval plot with groups: tool type, \(T\), Location Regarding Insert, _LRI_, and Location Regarding Specim, _LRS_; (**b**) main effects over Ln\(R\)a. Based on data from [18]. * It is possible to obtain all \(Ra\) (\(\upmu\)m) values below the maximum limits of the strict tolerances required in the aeronautical sector for the repair by re-drilling of hybrid Mg-Al-Mg components through sustainable lubrication systems. Tests 5 and 7. * The factors with a significant effect on the response variable \(Ra\) (\(\upmu\)m) are the type of cooling system, C (20.7%), the Location Regarding Insert, \(LRI\) (54.7%), and the interaction between the type of cooling system
* Location Regarding Specimen, 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 sheets, however, the data obtained give values of roughness outside the optimal range for the aeronautical sector. * Factor type of cooling system, C, has a clear effect on the roughness obtained on the aluminum plate, since, whereas with dry machining, the values obtained are within the tolerance for the plate of 15 mm (1.24 and 1.30 in test 4) or relatively close to the standard values (1.66 and 1.7 \(\upmu\)m in test 8), with Cold Compressed Air cooling all the exit values of the aluminum plate are out of tolerance (2.11, 2.91, 3.00, and 3.95 \(\upmu\)m); therefore, using this cooling system, the intermediate plate could not be thicker than 7.5 mm. * Based on the results, there seems to be a dependent directional effect in addition to the cooling/lubrication used, and consequently, it is proposed the conduction of new tests 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.