**1. Introduction**

Reduction in energy consumption is a constant demand in various industries, such as aeronautic, aerospace, and automotive, due to economic and environmental reasons. However, as energy consumption is closely linked to weight, one method to reduce energy consumption is to use lighter components composed of light alloys, such as titanium (Ti), aluminium (Al), and/or magnesium (Mg), all of which have an excellent weight/mechanical properties ratio.

The parts used in the aforementioned industries have to be rigid, strong, and light. In addition, they have to meet high precision and quality standards. As such, the production of these light alloys has been extensively studied [1–33]. Their repair and maintenance has also been thoroughly studied. Given their uniqueness and high cost, replacement parts are often not available when they have to be maintained or repaired [34–49].

The development of new alloys or polymers that increase the number of applications may be expensive but not always achievable. Sometimes, even when a material exists, it struggles to simultaneously satisfy all the necessary properties for concrete application using the material alone. Then, the combination of two or more materials is an alternative for producing hybrid components

whose properties satisfy the requirements in areas not covered by the individual components separately [50–58]. Different combinations of materials and technologies have been studied in order to improve their properties [50–70], so the use of hybrid structures is becoming more common.

The complexity of aeronautic and aerospace parts means that most have to be mechanized until achieving their final geometry. Then, using hybrid components, machining together several materials simultaneously is necessary. Drilling is one of the most usual machining operations in these industrial sectors, since it is used to create a large number of holes for assembling the parts. As these holes are subjected to fatigue cycles, every so often they must be checked for their maintenance since small cracks or damage in its contour can quickly degenerate into the breakage of the piece. Given the nature of these environments, breakages can have serious consequences. Due to the cost and complexity of these types of pieces, replacement parts are not maintained in stock to be used for repair or maintenance [3,36,49].

Performing machining operations in complex geometries of hybrid components or stacks is a significant challenge in terms of accuracy and quality, especially when the combined materials are magnesium and titanium. Magnesium is the lighter metal and has good mechanical properties that make magnesium alloys attractive for applications in industries where weight is crucial. Titanium is in demand due to its physical, chemical, and thermal properties. However, both materials present some problems. Magnesium is easily ignited by the heat generated during the machining process. The low thermal conductivity of titanium increases risk of tool wear, its low modulus means that parts move away from the cutting tool during machining, and its high chemical reactivity and low hardness tend to produce cracks in the cutting tools [71–73].

For solving these problems, different compatible and environmentally sustainable lubricants and coolants have been developed [26,38–40,45,74]. However, more research is needed because machining hybrid components causes an increase in the instability of the process due to the different cutting characteristics of the different materials. Many investigations have been reported for the simultaneous machining of materials in the search for optimal combinations of cutting conditions, tools, and cooling systems [56,57,75–89].

This paper focuses on drilling processes, and especially on finding the best combinations of cutting conditions and type of tools to perform hole repair and maintenance operations with dry machining on hybrid stacks of Mg–Ti–Mg in the shortest time and most sustainable method possible.

The magnesium–titanium–magnesium stack (Figure 1) was predrilled to simulate repair operations in hybrid components that are assembled with rivets. The two parts of magnesium were considered the base of the stack and the titanium was considered the insert.

**Figure 1.** Hybrid magnesium–titanium–magnesium stack for (**a**) drilling and (**b**) hole repair trials [56,57].

When a rivet hole is damaged, it has to be drilled to a larger diameter and an oversized rivet has to be assembled. This repair process is widely used in aeronautics and has to be performed with care as cracks in the structure can produce catastrophic consequences.

In this work, the surface roughness on the inside the holes was obtained by drilling the hybrid stacks. The stacks were composed of a magnesium alloy and titanium alloy to determine if it is possible to efficiently and sustainably repair or maintain aeronautic and aerospace industry parts with very strict surface finish requirements. To achieve this goal, a design of experiments (DOE) was defined for the drilling trials and the surface roughness measurements. The design selected was the product of a full factorial 2<sup>3</sup> and a block of two factors (3 × 2), whose possible influential cutting parameter factors were feed rate, *f*, cutting speed, *V*, and tool coating type, *T*. The location factors in terms of measuring the surface roughness included location on the specimen, *LRS*, and location on the insert, *LRI*. The obtained results were analyzed via analysis of variance (ANOVA), which helped establish a ranking of the parameter combinations based on the surface roughness achieved in the machining trials and helped determine the optimum combination of factors for performing these types of operations.
