**2. Evaluation of plastic formability of Mg alloys**

Various magnesium alloys for plastic deformation have difficulties in carrying out metal forming processes. The evaluation of the plastic formability of magnesium alloys can be conducted by determining the mechanical behavior of samples of tested materials in compression, torsion, and tensile tests. These tests reflect relatively well essential features of the state of stress or deformation in technological processes of metal forming, including extrusion, forging, and rolling, respectively.

To evaluate mechanical behavior of the material in extrusion process, the upsetting test was used to realize plastic deformation under various conditions and to look for adequate their choice for real deformation process. The grades AZ31, AZ61, AZ80, WE43, and magnesium alloys with lithium, as casted ingots and extruded preforms, were used in the research work. In order to study feasibility of these magnesium alloys in extrusion process, the upsetting test of cylindrical specimens was carried out and let to determine flow stress–strain relationships [25–30]. Before upsetting, the specimens were heated in a furnace to established temperature (**Figure 1**).

On the example of AZ31, AZ61, AZ80, WE43 alloys, and magnesium and lithium alloys, the determined flow stress–strain relationships between flow stress and strain for different values of temperature and strain rate allow for adequate adjustment of plastic deformation parameters based on specific relationships between the structure and deformation under conditions of hot compression test. Documented occurrence of two deformation mechanisms: slip and twinning in the presented relationships between plasticity characteristics such as: maximum yield

**Figure 1.**

*a) Setup of the upsetting/test at high-temperature mounted on 1000 kN hydraulic press, b) flow stress–strain relationships for AZ 31 [28].*

stress and strain corresponding to the maximum, and the Zener-Hollomon parameter allows for an appropriate interpretation of the effects of microstructure transformation [31]. In order to prepare technological process, it is necessary to define precisely the plastic properties and microstructure changes of those alloys. Comparison of the plasticity and microstructure of magnesium alloys with from 3 to 8% aluminum content from group Mg-Al-Zn-Mn let to choose proper parameters of the process. On the basis of tensile tests, the plasticity changes were determined at temperature from 150 to 450°C. Conducted compression test at temperature from 250 to 450°C and deformation speed from 0.01 to 10 s–<sup>1</sup> provided important data concerning the influence of process parameters on flow stress and microstructure changes connected with recrystallization process.

The characteristics of the relationships obtained in the compression test: flow stress σp- strain **ε** for the tested alloys AZ31, AZ61, AZ80, WE43 (**Figure 2**), and magnesium alloys with lithium (**Figure 3**) show the influence of temperature on their course, which allows for an adequate choice of technological parameters for plastic deformation methods with the dominant state of compressive stress (e.g., forging, extrusion).

Tests of magnesium and lithium alloys [32] with lithium content of 2.5, 4.5, 7.5, and 15% of the mass showed very different flow stress–strain relationship characteristics plasticizing from deformation, which results from the fact that alloys with 2.5 and 4.5% lithium form a lithium solid solution in magnesium and have a hexagonal structure. The 7.5% lithium alloy has an.

α + two-phase structure β., where the α-phase with the hexagonal structure is a solid solution of lithium in magnesium and the β phase has a wall structure centered is a solution of magnesium in lithium. The 15% lithium alloy is a solution of magnesium in lithium and has a wall-centered structure. An example of the flow stress– strain relationship for *Mg-7,5 Li* alloy is shown in **Figure 3**.

Alloys that contain more lithium, which is 7.5%, have good formability at temperature of 150°C. The alloy content of 15% lithium demonstrates very good deformability. The shape of flow curves and microstructure of alloys after deformation at elevated temperatures show significant influence of dynamic recrystallization process.

Determination of the plastic formability can be made on the basis of the results of torsion test too [30].

Flow curves for the magnesium alloy AZ31, most commonly used so far, were determined using torsion test at 300, 400, and 450°C at the speed 1 s<sup>1</sup> (**Figure 4**), while in the compression test at the temperature of 200, 300, 400, and 450°C at a strain rate of 0.01 and 1 s<sup>1</sup> , respectively (**Figure 5**). AZ31 magnesium alloy exhibits

**Figure 2.**

*Flow stress–strain relationships for magnesium alloys AZ31, AZ61, AZ80, and WE43 obtained in compression test [28].*

**Figure 3.**

*Flow stress–strain relationships for magnesium alloy Mg-7,5 Li obtained in compression test [28].*

an increase in deformability with an increase in the torsional temperature from 1.2 at 300°C to 5 at 450°C (**Figure 4**).

Axial-symmetric compression tests carried out on the Gleeble 3800 simulator with simultaneous "freezing" of the microstructure after deformation by rapid cooling with water, in the temperature range from 200 to 450° C with the strain rateέ = 0.1 s<sup>1</sup> and 1,0 s<sup>1</sup> , until the deformation ε = 1, 2 (**Figure 5**) allows the assessment of the influence of these factors on the course of the characteristic and its use in the design of plastic forming processes.

The flow curves obtained in the compression test indicate a similar level of the value of flow stress of the alloy for comparable conditions of its deformation in relation to the torsion curves.

#### **Figure 4.**

*Flow curve of AZ31 alloy determined in torsion test at temperature of 300, 400, and 450°C with a strain rate 1 s*�*<sup>1</sup> [30].*

**Figure 5.**

*Flow curve of AZ31 alloy determined in compression test at temperature of 200, 300, 400, and 450°C with a strain rate: a) 0.01 s*�*<sup>1</sup> , b) 1 s*�*<sup>1</sup> [30].*

Performing plastometric tests for a magnesium alloy allows the identification of two types of flow curves. Classical curve—where the dominant mechanism in the microstructure is slip (e.g., **Figure 6a**) and the characteristic curve, where the dominant mechanism in the microstructure is twinning (**Figure 6b**).

The relationship of the maximum yield stress σpp and strain ε<sup>p</sup> as a function of ln Z, where: Z – Zener-Hollomon parameter, *<sup>Z</sup>* <sup>¼</sup> *<sup>ε</sup>*\_ *exp <sup>Q</sup> RT* is shown in **Figures 7** and **8**.

Currently, plastic forming of magnesium alloys is limited to a few basic grades from the group of Mg Al-Zn (AZ21, AZ31, AZ61) and Mg-Zn-Mn (ZM21) alloys. AZ31 magnesium alloy as the most widely used for rolling metal sheets shows good formability under hot-forming conditions. The obtained flow curves depending on the deformation parameters show two different types of the deformation process. For higher temperatures and lower strain rates, the curve follows the classical course of changes in the yield stress. At lower temperatures and higher strain rates, the course of stress changes is different and characteristic for the process based on the twinning mechanism, which was confirmed in structural studies. It has been shown that there is a relationship between the maximum yield stress σpp and the

#### **Figure 6.**

*Microstructure of magnesium alloy AZ31: a) after deformation at temperature of 350°C—Slip domination, b) after deformation at temperature of 250°C—Twinning domination, strain rate 0.1 s<sup>1</sup> [30].*

**Figure 7.** *Maximum yield stress σpp as a function lnZ [31].*

**Figure 8.**

*Deformation ε<sup>p</sup> corresponding to the maximum yield stress on the flow curve σ<sup>p</sup> as a function lnZ [31].*

corresponding strain εp, and the Zener-Hollomon parameter (**Figures 7** and **8**). A worse fit occurs for curves where twinning dominates, which changes the shape of the curve [30]. Flow stress–strain curves of alloy AZ31 are characteristic for alloy in which during deformation a mechanism of plastic strain called twinning occurs [33].

The microstructure of AZ31 alloy after deformation by hot compression at the temperature of 200, 300, 400, and 450°C with strain rate of 0.01 s<sup>1</sup> and 1 s<sup>1</sup> , respectively, was examined, and an example is shown in **Figure 9**. It was observed after the compression test at 200°C for strain =1.2, for both the strain rates used, the microstructure of the primary elongated grains and the ultrafine grains dynamically recrystallized (**Figure 9**). Samples deformed at a lower strain rate are characterized by a greater advancement of the recrystallization process. Recrystallized grains are observed both at the boundaries and within the primary grains.

The microstructure of AZ31 alloy, after deformation at the temperature of 300°C with the speed of 0.01 s<sup>1</sup> , consists of fine grains that are dynamically recrystallized. For a higher strain rate, chains of recrystallized grains at the boundaries of the deformed primary grains are observed. Increasing the temperature to 400 and 450°C intensifies the recrystallization processes and grain growth. Few deformation twins are also observed (**Figure 10**).

The presented results of plasticity tests of AZ31 magnesium alloy indicate its good formability during hot deformation. The fine-grained recrystallized microstructure was obtained at the temperature of 300°C for a low strain rate. Increasing the temperature leads to the growth of recrystallized grains. Consequently, the average grain diameter after deformation at 450°C is much higher than before deformation.

Traditionally, the compression test specimens are circular in cross section. Taking into account the geometrical profiles of plastically formed products, e.g., in the

*Microstructure of the AZ31 alloy after compression at temperature 200°C: a) with a strain rate 0.01 s<sup>1</sup> , b) with a strain rate 1 s<sup>1</sup> [31].*

#### **Figure 10.**

*Microstructure of the AZ31 alloy after compression at temperature 450°C: a) with strain rate 0.01 s<sup>1</sup> , b) with strain rate 1 s<sup>1</sup> [25].*
