**2. Materials and Methods**

We followed the guidelines provided by Montgomery [87]. First, in the planning stage, factors, levels, ranges, and response variables were determined (Table 1). Then, the design of experiments (DOE) was performed. The design was elected according to the fixed resources and objectives. In this case, the goal was to analyze the variability in the surface roughness inside the holes obtained by drilling of hybrid stacks composed of UNS M11917 magnesium alloy and UNS R56400 titanium alloy (Table 2). The design selected was the product of a full factorial 23 and a block of two factors (3 × 2; *LRI* × *LRS*) with a total of 48 experimental runs, as can be observed in Table 2. In this table, each line represents two experimental runs, one for each level of the *LRI* factor.

Next, the machining trials were completed. To execute the drilling trials, it was first necessary to establish the protocols both for the cutting conditions to be used and for registering the obtained data. Then, it was necessary to prepare the workpieces with the specimens of the hybrid stacks, the tools, and the machine tool, introducing the cutting conditions selected. Next, the machining operations were performed, and finally, we photographed and recorded videos of all the trials for further analysis once the process was finished.

Then, we measured the response variable. Surface roughness was selected as the response variable, measured as the roughness average (*Ra*) [88]. Once the data were obtained, we performed a statistical analysis. The variability of the average roughness values, *Ra*, was modelled using ANOVA, identifying both the influential factors and interactions among them on surface roughness. In addition, an exploratory data analysis was performed to obtain a clear graphical view of the key aspects in terms of the distribution of the influential factors on the surface finish of hybrid magnesium–titanium–magnesium stacks. The relationships between pairs of influential factors were illustrated and analyzed. After the statistical analysis of the results, some conclusions were established.



**Table 2.** Experimental design: product of a full factorial 23 and a block of two factors 3 <sup>×</sup> 2.

#### **3. Trials**

This work focused on the manufacturing process of drilling and on repair operations of holes used to join parts of different materials by means of rivets. Materials, cutting tools, cutting conditions, and measurement locations were established as influential factors on the variability in surface roughness.

#### *3.1. Specimens: Materials and Geometries*

The materials used in the manufacture of the workpieces for hole repair drilling operations included magnesium alloy UNS M11917 and titanium alloy UNS R56400, whose compositions are outlined in Table 3. In the absence of standards, national or international, in relation to the design and manufacture of test pieces, we decided to use 50 × 50 × 15 mm parallelepipeds (Figure 2) [57]. The hybrid component stacks were composed of a total of three pieces or parallelepipeds called specimens: two of magnesium alloy and one of titanium alloy. The two UNS M11917 magnesium alloy specimens were called the base and the UNS R56400 titanium alloy specimen, located between the two magnesium pieces, was called the insert; this is similar to previous works involving other processes and other materials analyzed. The specimens were positioned one above the other to differentiate the measurements when collecting data. The three parallelepipeds or specimens were mechanically fixed together so that it was possible to disassemble and measure the surface roughness inside the machined holes with relative ease. In order to simulate repair and maintenance operations, a test piece was predrilled with a number of holes in accordance with the requirements for the planned experiments.


**Table 3.** Chemical compositions of materials used for manufacturing specimens.

**Figure 2.** (**a**) Design of the geometry of each of the three parts of the workpiece; (**b**) Predrilled workpiece used during the trials to simulate hole repair operations on hybrid stacks [56,57].

#### *3.2. Tools*

Two types of tools were selected for drilling operations with the same geometry (Figure 3a) but with different coatings (Figure 3b) (GARANT, Hoffmann Iberia Quality Tools S.L., San Fernando de Henares, Madrid, Spain). We selected tools made of High-Speed Steel (HSS CO): two-flute twist drills with 130◦ point angle and in two different qualities, A11240 and A11253. A11240 is recommended for titanium, steel, and stainless steel, whereas A11253 is coated with titanium nitride (TiN) and recommended for steel, stainless steel, titanium, aluminium alloys, and copper alloys. This selection allows us to use the tools in the largest number of trials involving different material combinations, considering that this work is inside a wider project that involves different machining processes (turning, milling, drilling), materials (steel, aluminium, titanium, and magnesium), tools, and workpieces of several types, shapes, and sizes.

**Figure 3.** (**a**) Drill tool geometry and dimensions; (**b**) Materials and uses of the tools: A11240 is composed of High-Speed Steel and recommended for titanium, steel, and stainless steel and A11253 is composed of High-Speed Steel coated with titanium nitride and recommended for steel, stainless steel, titanium, aluminium alloys, and copper alloys [64].
