**1. Introduction**

Mg-alloys are being considered as one of the most versatile material choices among the structural materials that exhibit both energy efficiency and environmental benefits. Mg-based materials (alloys and composites) have enormous and unlimited potential to replace aluminum, steel, and structural plastics in diverse industrial and commercial sectors such as automotive, aviation, defense, biomedical, sporting equipment, consumer electronics, etc. These applications result from the need to use magnesium and its alloys as a material with favorable physical properties, especially high relative strength (Rm/ρ). Due to the good casting properties of magnesium, it was used primarily in cast structural elements. Wrought alloys have been used on a smaller scale so far, but material research and plastic formability processes to produce semi-finished products from magnesium alloys are currently under intensive development. The development is caused by the possibility of using various types of light structures in the construction, including vehicles made of magnesium-based materials, for which, for example, thin sheets are the

basic initial material. This is mainly due to the attempt to reduce the weight of the structure and ensure adequate strength.

The interest in magnesium alloys for various structural elements, e.g., for the aviation industry, dates back to the 1940s. Examples of applications concerned, for example, the Northrop XP-56 plane, in which virtually all parts exposed to elevated temperature were made of magnesium and its alloys [1], B-36 bomber or [2], the S55 helicopter by Westland Aircraft Ltd. (1950 r.) [1, 3].

In later years, the use of magnesium alloys for aircraft structural elements was significantly limited, which was mainly due to the rapid corrosion of alloys (the main disadvantage), opinions about the flammability of magnesium, low metallurgical purity, low strength, high processing costs through plastic formability and poor machinability [4–7].

Since the beginning of the twenty-first century, there has been a renewed, significant increase in interest in magnesium alloys for applications in the aerospace industry. It results from the development of new coatings that can protect alloys against corrosion, new alloys, new technologies of obtaining semi-finished products by casting methods, and the improvement of various plastic forming technologies, which significantly improves the properties of the product and allows the use of these alloys wherever engineers look for very light construction elements, relatively durable and with anticorrosion protection at the same time [8–10].

The main problem in the development of techniques for processing magnesium alloys with metal forming methods is low formability [4, 11]. Low deformability at room temperature as well as temperature increased to 200°C of magnesium alloys result from a limited number of hexagonal lattice slip systems. The test results show that the microstructure of Al-Mg-Zn alloys deformed at temperatures up to 200°C shows bands slip and deformation twins [11]. In the range from 200 to 300°C, limited dynamic recovery and the formation of nuclei of dynamic recrystallization are observed. Continuous dynamic recrystallization takes place above 300°C, which results in an almost twofold increase in formability.

Metal forming of magnesium and its alloys is carried out, depending on the content of alloying elements, only in a narrow temperature range. Grains with an average diameter of up to 10 μm were obtained in magnesium alloys by thermoplastic treatment. The fragmentation of the grains below 10 μm is only achieved by introducing large deformations. The use of unconventional methods of deformation allows for obtaining the grinding of magnesium alloys to sub-micrometric or nanometric sizes. Therefore, these deformation methods constitute a very significant support for conventional forming methods [12–14].

In magnesium alloys, deformation processes are carried out at an elevated temperature. Therefore, it is practically impossible to obtain nanometric grain sizes obtained by the development of shear bands. The most common methods of large deformation, leading to the grinding of the grains of magnesium alloys, are equal channel angular pressing (ECAP) [15, 16] or hydrostatic extrusion [14, 17].

Basic magnesium alloys for metal forming contain up to 8% Al and the addition of Mn (up to 2%), Zn (usually up to 1.5%), Si (about 0.1%), and traces of Cu, Ni, Fe. There are three basic groups of magnesium alloys for metal forming. The first group includes mainly alloys with the addition of aluminum, zinc, and manganese. The second group includes alloys containing mainly the elements Zn, RE, Y, Zr, Th, and the third group, which is in the phase of intensive research, consists of new ultralight alloys containing lithium.

Magnesium alloys for metal forming are still used to a relatively small extent, which results from technological difficulties in metal forming and high production costs [13, 18, 19]. The main disadvantage of magnesium and its alloys is low deformability at ambient temperature, which results from the type of crystal lattice. At ambient temperature, only one in-plane glide system (0001) is active. In addition to low temperature slip, a significant amount of twinning is observed in magnesium alloys.

Hot forming of magnesium alloys, depending on the chemical composition and deformability, is performed using the following methods [20, 21]:


The formability effect of magnesium alloys has been recently also used in a relatively new solid-state joining process, under the influence of frictional heat, i.e., Friction Stir Welding (FSW). There is no melting involved in the process unlike conventional Fusion welding processes. This method gives very good results in the creation of strong joints, competitive to other joining techniques that require additional joining materials (e.g., riveting, bolted connections, welding, and others). Joining elements (e.g., sheets) made of magnesium alloys with this technique are very effective and require careful selection of process parameters, taking into account the special features of the plasticization of magnesium alloys and the stirring effect in the joint area [4, 22–24].
