*3.1.1 Rolling*

Rolling of magnesium alloys is currently limited to a few basic grades from the group of Mg Al-Zn and Mg-Zn-Mn alloys. The new alloys Mg-Th- (Mn or Zr) and Mg-Li-Al are also susceptible to rolling [30, 32, 35, 38, 39].

The process of rolling magnesium alloy products is very expensive and timeconsuming. This is due to the necessity to carry out annealing between successive operations. As a result of the current growing interest in sheets of magnesium alloys, the so-called "twin rolls casting" technology has been developed, which reduces the number of rolling and heating operations by casting between rolls and subsequent rolling [32].

Rolling AZ31 alloy with heating the billet in a chamber furnace to the temperature of 470°C for 30 minutes and cooling it in the air to the rolling temperature, in the range from 200 to 450°C, allows for obtaining a product with the required mechanical properties at the level of Rm = 220–265 MPa and A50 = 10–12%.

Examples of the microstructure of AZ31 alloy bands after the hot rolling process are shown in **Figures 14** and **15**. In the microstructure of the specimens rolled with a total draft of 44%, it shows a partially recrystallized structure (**Figure 14a**). The presence of primary grains and recrystallized grains was found (**Figure 14b**) fully recrystallized structure with fine grains. The analysis of the microstructure of the

*Metal Forming of Magnesium Alloys for Various Applications DOI: http://dx.doi.org/10.5772/intechopen.101034*

**Figure 15.**

*Microstructure of the AZ31 alloy rolled plate: a) in the surface of the plate, b) in the longitudinal section of the plate, c) in the cross section of the plate [30].*

AZ31 alloy bands shows that it is precisely the application of large creases (here 82%) that allows obtaining a fine-grained structure without visible areas of primary grains.

The existing industrial application of magnesium alloys is currently focused on the utilization of semi-finished products such as sheets. The effects of processing parameters and special aspects of the rolling process on the mechanical properties and sheet formability is examined, and recent developments is presented in [37, 40].

#### *3.1.2 Forging*

Among the most common forming processes, forging is a promising candidate for the industrial production of magnesium wrought products. The basics of magnesium forging practice are described, and possible problems as well as material properties are presented and discussed in many papers. Several alloy systems containing aluminum, zinc, or rare earth elements as well as biodegradable alloys are evaluated to focus on the process control and processing parameters, from stock material to finished parts including mechanical properties and analysis of microstructure [37, 40, 41].

The final properties of the forgings made of Mg alloys depend on the type of alloy and the string technological process leading to the final product. Most often feet cast magnesium is plastically deformed into semi-finished products, mainly by extrusion or rolling. Usually, forgings are made of semi-finished products in the form of extruded bars or rolled plates, which can be supplied in various conditions depending on the type of heat treatment applied. Forgings after being forged are subjected to precipitation hardening, recrystallization annealing, or stress relief annealing or leaving heat untreated. Each stage of the production line affects the structure and final properties of the product.

Forging as a process of forming the material through multistage deformation for magnesium alloys is a typical process based on hot forming at a narrow temperature range. An example of multistage forging of the airplane wheel hub and the AZ31 magnesium alloy control system lever of the helicopter is shown in **Figure 16** [42–44].

Forging should be carried out on hydraulic or low-speed hydraulic presses. In specialist literature, it is not recommended to use die hammers and high-speed presses due to the cracking of the material during forging. When designing the transverse flow tendency of the magnesium alloys, the rod axis as well as difficult flow in the longitudinal direction should be taken into consideration. Die blanks should be polished to facilitate material flow and avoid surface defects. Free removal of the forging from the blank is possible thanks to forging inclinations equal to 3°, and in some cases even smaller [45, 46].

#### **Figure 16.**

*View of forgings from AZ31 alloy: a) semi-hub of wheel, b) lever after process at beginning temperature of 410°C [28].*

A very important process parameter is the temperature of the charge and tools. Magnesium alloys are good heat conductors and quickly cool down in contact with the tools, the more so that during forging, the material deformed over a relatively long time is in contact with a large surface with the die shape. For this reason, the temperature of the tools should be kept slightly below the load. Too much cooling of the billet leads to the formation of cracks. Conversely, too high a temperature also causes cracking due to the occurrence of hot brittleness. Lubrication is applied during the forging operation. Greases based on graphite or molybdenum disulfide are recommended for the forging temperature range of magnesium alloys.

Magnesium alloys are highly strain rate sensitive and exhibit good workability at a narrow forging temperature range. Consequently, parts made of these materials are usually forged with low-speed hydraulic presses, using specially designed tool heating systems in order to ensure near isothermal conditions. This study investigates whether popular magnesium alloys such as Mg-Al-Zn can be forged in forging machines equipped with high-speed forming tools.

Results presented in work [47] have demonstrated that AZ80A is not suitable for forging with either the screw press or the die forging hammer, that AZ61A can be press- and hammer-forged but to a limited extent, and that AZ31B can be subjected to forging in both forging machines analyzed in the study.

Examples of the application of magnesium alloys in aviation structures obtained by forging indicate the effective use of the possibilities of this technology (Examples shown in **Figures 17**–**22**).

*Metal Forming of Magnesium Alloys for Various Applications DOI: http://dx.doi.org/10.5772/intechopen.101034*

**Figure 18.**

*Stages of forging the helicopter control system lever: a) forging, b) forging AZ31 alloy.*

**Figure 19.**

*Drop forgings made in industrial conditions: (a) AZ31B, screw press; (b) AZ31B, forging hammer; (c) AZ61A, screw press; (d) AZ61A, forging hammer [47].*

The forging process was carried out on a drop forging hammer. The initial material was ingots heated to the initial forging temperature equal to 350°C and 410°C, followed by upsetting, forging, forging in a die blank. The correct forging was obtained for the alloy annealed at 410°C [41–44].

The results of the research on the forging process in industrial conditions of two selected parts of aircraft structures: i.e., the aircraft wheel hub and the helicopter control system levers showed that the appropriate geometric parameters of these magnesium alloy elements and the determination of the conditions of the hammer forging process allowed for obtaining final products without defects with the required final properties (geometric and mechanical properties).

#### *3.1.3 Extrusion*

The conventional process of extrusion of magnesium alloys is carried out at a temperature range from 320 to 450°C, at a speed of 1–25 m/min. The developing method of hydrostatic extrusion allows for plastic deformation at lower temperatures and to obtain greater grain grinding of magnesium alloys [28, 43, 48, 49]. The extrusion process was carried out on a counter-press with a heated container. AZ31, AZ61, AZ80, and WE43 magnesium alloy ingots with a diameter of 100 mm, heated to a temperature of 400°C, were extruded at various speeds from 0.04 to 0.16 m/s and with a different extrusion ratio of 6,25–25. The most favorable effect on the microstructure was observed after billet extrusion with an extrusion ratio of λ = 25 (**Figure 20a**–**d**). As a result of plastic deformation and recrystallization, fine recrystallized grains were obtained, although in the case of AZ61 and AZ80 alloys, a banded microstructure was observed (**Figure 20b** and **c**).


**Figure 20.**

*Microstructure and mechanical feature of magnesium alloys after indirect extrusion [28].*

Different shapes of cross section of profiles for aviation application were obtained in the way of backward hot extrusion process. Some results of final products are presented in **Figure 21**. It is possible to obtain profiles of complex shape with elements of varied wall thickness and with thin walls. Microstructure of tested alloys in initial condition after extrusion is shown in **Figure 20**. Before deformation the tested alloys AZ31 and AZ61 were characterized by single-phase microstructure of solution α-Mg, whereas in microstructure of alloy AZ80 the presence of intermetallic phase γ-(Mg17Al12) was found and which was confirmed by prior X-ray tests. The extrusion of profiles with a complex cross-sectional shape and large differences in wall thicknesses (**Figures 21** and **22**) allowed the assessment of high plastic deformation possibilities of these alloys, favorable microstructure, and obtaining very good mechanical properties [28, 49–51].

**Figure 21.** *Complex shape profiles of Mg alloys obtained during indirect extrusion [28].*

The measurements of the grain size (acc. to ASTM E 112) and the microhardness HV0.1 of the magnesium alloy on the cross section of the extruded profile in the characteristic areas A, B, C (**Figure 22**) show different values of microhardness and grain size (**Table 1**), which proves the influence of the cross-sectional geometry on the microstructure and properties of the magnesium alloy.

Parameters of the direct and hydrostatic extrusions are presented in **Table 2**. In the field of extrusion of magnesium alloys, the method of hydrostatic extrusion has been developed quite intensively in recent years. Due to favorable thermomechanical conditions, the hydrostatic extrusion process can be carried out at lower-temperatures and a greater grain grinding of magnesium alloys [52, 53].

The comparison of microstructures of various magnesium alloys after plastic forming during extrusion with the same extrusion ratio λ = 25 for alloys: AZ31, AZ61, AZ80 WE43 (**Figure 23**) indicates diametrical differences in the final plastic
