## 1 Introduction
There are different working lines to optimize consumption and reduce the emission of pollutants: the development of new engine technologies [9], the use of alternative technologies [12], and the reduction of the aircraft's weight, either by reducing their size or by replacing conventional heavy materials with new lightweight structural materials, or combinations of them while maintaining the same performance or improving it [14]. The present work is framed in this last working line. The consensus regarding the benefit of mass reduction is clear and has been outlined in many studies. For each 100 kg of vehicle mass reduction, there is a reduction in fuel consumption of 0.38 L/100 km, and a reduction of 8.7 g of CO\({}_{2}\) per km [15]. Each kilogram of mass reduction produces a saving of 150,000 US dollars in fuel over the life of the engine [16]. A 10% reduction in the mass of a vehicle results in a reduction of fuel consumption of 5-7% [17]. One of the research and development lines currently open is based on the use of lightweight structural materials such as lightweight titanium, aluminum, and magnesium alloys, all with an excellent density/mechanical properties ratio [18; 19; 20; 21; 22; 23], or plastic and polymer compounds [21; 22]. For example, in the case of automotive vehicles, 40% of the total mass is in the car body; therefore, this is a key area for potential mass reduction [24]. On the other hand, it is fundamental to bear in mind that the recyclability of the materials selected for the design is a must from the very beginning. Over the next 20 years, more than 12,000 aircraft are expected to reach the end of their lifetime. Airbus has established a method for dismantling and recycling its aircraft parts, achieving 90% recycling or reuse and safe and sustainable disposal when recycling is not possible. Since 2007, Airbus has recycled 117 aircraft worldwide with 92% reuse of the remaining parts and 100% recycled engines [2; 25]. Aluminum and magnesium are two of the most abundant materials on earth, and they have been widely studied separately, but the number of studies in which a multimaterial combination of them is machined is very limited. From a structural point of view, their main characteristic is an excellent density/mechanical resistance ratio [26]. This feature makes them very attractive when seeking to reduce mass to reduce fuel consumption and emissions while maintaining mechanical performance. In particular, aluminum alloys are the most commonly used lightweight structural materials and have been the subject of many scientific articles in recent years [21; 22]. Moreover, Al and Mg alloys offer additional advantages such as their great capacity to be recycled [27; 28] and their good machinability. However, they also present some common disadvantages such as lower mechanical resistance than other structural materials such as titanium or steel; lower working temperature; corrosion problems and worse weldability, among others [29]. On the other hand, titanium has very good mechanical, chemical, and thermal properties that complement those of magnesium and aluminum. It has a relatively low density, 4500 kg/m\({}^{3}\) so that the overall properties of the multimaterials in which titanium is combined maintain a good ratio between mechanical properties and density [29]. All this makes the study of possible combinations among these three light alloys in multimaterials, such as Mg-Al-Mg or Mg-Ti-Mg, very interesting for their potential applications in the reduction of the mass of vehicles and aircraft in a sustainable way. Additionally, the current demands on high-performance parts make it difficult to design them using a single material. Therefore, it is often necessary to benefit from the properties of several materials to create a multimaterial or hybrid component with superior properties [30]. It is also feasible to develop new alloys with the required properties, but the process is long, and the target is not always achieved.