*3.1.4 KOBO extrusion*

Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultrafine-grained metal [54]. Possibility to influence on the microstructure and the mechanical properties by heat treatment is also important to plan an SPD process [52, 55]. The KOBO extrusion process as SPD process is an unconventional extrusion method based on the idea of cyclical deformation path change during the process and localized plastic flow [53, 56]. The deformation path change is carried out by setting the die into an oscillating rotary motion around its axis. The method combines the extrusion of the material with an additional plastic deformation caused by reversible

torsion of die. Being induced this way super plastic mode of deformation makes possible to deform metals with very high deformation (SPD process) at low temperature (room temperature). Hence, despite very low load capacity of the press, the methods allow for the variety of metallic products to be extruded at room temperature from the billet, with size and dimensions with value of extrusion ratio λ up to several hundreds. The example for KOBO extrusion of MgLi4 is presented in **Figure 24**.

Superplastic behavior of a metals under such deformation conditions is proven also in exact filling of a die opening, regardless how complicated it is and mode of extrusion forward or sideway. The most important advantage of the KOBO method is the ability to deform metals with a very high λ coefficient (extrusion ratio) [53, 57–59] in the "cold" process (without preheating the billet or tooling), regardless of the hardening state of the initial material. The use of the KOBO method allows for reducing the work of deformation, so the extrusion forces are incomparably lower than in conventional extrusion processes. The examples of measured parameters of KOBO process for magnesium alloys AZ31 and WE43 are shown in **Figure 25**.

KOBO extrusion is a process carried out in a system of concurrent extrusion with an oscillating matrix. In turn, the possibility of forming the products of a specific crosssectional shape, as in conventional extrusion processes, makes it possible to obtain products, not only specimens, with very fine grains and favorable mechanical properties, which qualify the KOBO method for potential industrial implementations. The significant reduction of the extrusion load and the lack of the need for preheating make the method very economically attractive. Another, extremely attractive feature of the KOBO method is the possibility of low-temperature consolidation of postprocessing chips [28, 53, 57–64] and obtaining a solid product as a result of plastic deformation in the process. The deformation effects of magnesium alloys, both in the form of a solid billet and in the form of consolidated postprocessing chips, are shown in **Figure 26**.

#### **Figure 24.**

*a) Scheme of KOBO extrusion process, b) extruded wire of Mg Li4 alloy, extrusion ratio λ = 10,000.*

#### **Figure 25.**

*a) Extrusion of magnesium alloy AZ31 round bar with stable extrusion speed. b) Extrusion of magnesium alloy WE43 round bar with stable extrusion speed. c) Extruded round bar ϕ3 mm.*

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

#### **Figure 26.**

*Extrusion of metal chips in the KOBO process: a) a chip before and after compaction of chips into billet, b) an extruded monolithic profile, c) extrusion of solid profile of complex shape of cross section.*

Low temperature—above 200°C in KOBO extrusion technology of magnesium alloys, possibility to control the final properties of the extruded profiles let to maintain a fine grain structure of the alloys [65]. It comes from the number of KOBO process parameters, such as amplitude and frequency of die oscillations. It is possible to use the backward extrusion and KOBO processes for bulk metals and chips. To determine proper parameters of extrusion process, it is necessary to know exact information on initial features of given magnesium alloy—its macro and microstructure, mechanical properties, and final results of transformation of the internal structure under conditions of plastic deformation.

#### *3.1.5 Joining. Friction Stir Welding*

Joining. Friction Stir Welding FSW is solid-state joining process under the influence of frictional heat. There is no melting involved in the process unlike conventional fusion welding process. The FSW process was originally invented in institute TWI in the United Kingdom in 1991 [65–67]. The principle of utilizing the frictional heat between a rotating tool and the two joining metal interfaces is shown in **Figure 27**.

The rotating tool serves two basic purposes: heating of the workpiece materials due to friction and plastic deformation and stir movement and containment of materials to produce joint. Rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of sheets or plates to be joined and subsequently traversed along the joint line. Advancing and retreating side orientations require knowledge of the tool rotation and travel directions. It is especially important when FSW process is designed for given metallic material. Magnesium alloys require special attention from the point of view of adequate parameters choice, especially for joining very thin sheets, because FSW process is very sensitive to the technological parameters of welding. From the point of view of plasticization of metals, the FSW process is very complex. The rotating tool in the first phase of the process supplies more and more heat, then the welded material is strongly deformed plastically. The material, in contact with the pin of the moving tool, experiences a state of stress and deformation comparable to that in extrusion and forging. The last stage is cooling of the weld [67]. It is important to know the influence of the selected technological and geometrical parameters of the process on the plasticization of the materials to be joined.

**Figure 27.** *Scheme of linear Friction stir welding process.*

Assessment of joint quality on the basis of tests of the mechanical properties of the obtained joints (static strength, fatigue strength, microhardness), microstructure tests in the area of weld, and parent material as well as the measurement of forces acting on the tool as a material response to the load resulting from the adopted process conditions. As a result of the conducted research on linear, FSW stir-mixed butt welding of thin sheets with a thickness of 0.5 mm made of AZ31B magnesium alloy, the basic goal was achieved, which was to obtain a solid, durable, free from defects connection of elements. When welding 0.5 mm thick sheets, the variable parameters were the rotational speed and the tool feed. The friction stir welding process was carried out with the use of magnesium alloy sheets, including AZ31B, 0.5 mm thick. The FSW joining process was carried out as a butt joint. An example of a connection is shown in **Figure 28**.

#### **Figure 28.**

*FSW joints: a) view of joint with advancing and retreating sides, b) FSW joint of the AZ31B magnesium alloys, a) face and ridge of joint FSW joint of AZ31 B sheets of 0.5 mm of thickness, l = 180 mm.*

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

Due to the difficult conditions for the implementation of the process, an important issue is the selection of an appropriate tool with the appropriate geometry and kind of material of the tool. During FSW welding of magnesium sheets a cemented carbide tool was used. Taking into account the interdependence of mechanical and structural effects in the FSW process, special attention should also be paid to the analysis of the microstructure in the joint area, identification of characteristic zones as a result of the transformation of the microstructure resulting from the plasticization of the joined materials and the generated heat due to friction and plastic deformation, heat dissipation, among others in contact with the tool and tooling and the overall heat balance resulting in the final state of the joint. These are issues for thin sheets, especially for light metals (aluminum alloys, magnesium alloys) [68, 69]. **Figure 29** shows the FSW weld microstructure of a sheet with a thickness of 0.5 mm of AZ31B alloy. The presented sample was made with the welding speed of 80 mm/ min and the tool rotation speed of 2000 rpm. It is difficult to identify the zones characteristic for the FSW joint in the case of welding very thin sheets. In this case, the locations of areas such as the heat affected zone, the thermomechanical interaction zone, and the weld core were estimated based on the dimensions of the tool. The area of direct contact of the rim of the tool resistance is characterized by fragmented grain due to high plastic deformation. Equal-axis grain was also observed in the HAZ (heat-affected zone) and TMAZ (thermomechanical-affected zone) as shown in **Figure 28**.

The presence of equiaxed grains in the HAZ and TMAZ zone indicates that the material has fully recrystallized during the FSW process. The dynamics of recrystallization affects the grain size. Magnesium alloys are more prone to dynamic recrystallization than aluminum alloys because the Mg recrystallization temperature is approximately 523 K—lower than that for aluminum alloys. The factor that inhibits grain growth as a result of high temperature is also the cooling rate. In ultrathin materials (sheet thickness equal to or less than 0.5 mm), it is relatively high—the material cools down quickly, which is noticeable already during the process. The joint shown in **Figure 30** does not contain any crater defects or discontinuities.

The obtained results of the FSW joining process of magnesium alloys showed that the process of linear friction stir welding (FSW) is a favorable method of joining difficult-to-weld magnesium alloys [70–73]. Appropriate selection of parameters allows for obtaining joints free from defects. The assessment of the quality of the joints made on the basis of: the results of the joint microstructure analysis, measurements of the microhardness of the joint and the base material as well as the static tensile test of the weld material allowed for an unambiguous determination of the joint effectiveness. Proper selection of technological and geometric parameters allows for obtaining a connection with approximately 90%

#### **Figure 29.**

*Typical scheme of zones of butt FSW joint (SZ – Stir zone/nugget zone,TMAZ – Thermomechanically affected zone/HAZ – Heat-affected zone/BM – Base material/parent material, RS – Retreating side, AS – Advancing side).*

**Figure 30.**

*Typical microstructures of an FSW joint made at different rotational speed and welding speed. a) Microscopic structure of FSW welded AZ31B of 0.5 mm in thickness. View of base material BM. b) Microscopic structure of FSW welded AZ31B of 0.5 mm in thickness. View of HAZ and TMAZ zone from the retreating side. c) Microscopic structure of FSW welded AZ31B of 0.5 mm in thickness. View of stir zone SZ and TMAZ of advancing side.*

efficiency in relation to the parent material, determined on the basis of a static tensile test. During the tensile test, all joints broke in the thermomechanical impact zone on the retreating side. This is due to the resulting structural notch at the boundary of the weld nucleus and the thermomechanical influence zone. When welding 0.5 mm thick sheets of Mg AZ31B alloy, the maximum force acting on the tool in the Z axis was 4.5 kN. The values of the forces acting on the tool in the radial direction range from 40 to 100 N. After making a total of approximately 20 m of linear FSW weld, the tungsten carbide tool was not worn according to visual assessment. The FSW process is an excellent alternative to riveting or conventional welding. It does not require the use of an additional connector, which contributes to reduce the weight of the structure. Friction stir welding of the FSW joint allows for the creation of a high-quality, durable, defect-free joint with high mechanical properties (static and dynamic tests) and favorable internal structure of the AZ31B alloy sheet material.

The average obtained connection efficiency with appropriately selected input parameters of the process ranges from 85 to 95% compared with the parent material. These values are fully acceptable to the aviation (min. 80%) and automotive (min. 70%) industries. The analysis of the variability of forces generated during the FSW process requires knowledge of material properties and process conditions. It is necessary to adequately select the process parameters to the type of alloy and thickness of the joined elements (**Figure 31**).

The estimated value of the FSW process temperature should be approximately 0.8 solidus temperature. In this case, for alloy AZ31B, solidus temperature is 605°C, so temperature for FSW process is 430°C. AZ31 alloy with aluminum as the main alloy additive is very brittle at room temperature (only one slip plane (0001)). Only when the temperature of 200°C is exceeded, other slip planes are activated, which facilitates further plastic deformation. Reaching the temperature of 200°C is associated with the phenomenon of recrystallization. Achieving these conditions during welding is possible while providing the rotating and plunging tool with a sufficiently long time stoppage. This treatment is necessary to heat the material due to the pressure and friction of the rotating tool (dwell time). The recorded force courses may indicate the most favorable time to achieve the plasticizing effect and achieve the appropriate plasticization conditions of the material over 450 MPa, which exceeds the value of the yield stress for AZ31B alloy. A sudden increase in radial forces acting on the tool was noticed. Then the welding stage begins, where the stabilization of both axial and radial forces was observed. Very

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

**Figure 31.** *Matrix of technological parameters for joining AZ31B alloy sheets of 0.5 mm thickness [68].*

good results of joining thin sheets of the AZ31 B alloy [64–69, 74, 75] prove the purposeful use of friction stir welding to join sheets of various thicknesses. Butt joints with smooth surface, without voids and flash, can be obtained by cylindrical flat shoulder and pin tool made from tungsten carbide. Tensile tests revealed a durability increase up to 90% compared with BM, which is thought to be mainly attributed to the preferred basal orientation and the activation of the extension twins. The SZ and TMAZ experienced full dynamic recrystallization and thus consisted predominantly of equiaxed grains. The grain size in SZ increased with increasing heat input.

During the FSW, the process must be carried out at the temperature preferably higher than the recrystallization temperature of the base material BM to be joined for the dynamic recrystallization to take place in the SZ. With increasing tool rotational speed or decreasing welding speed supplied more heat energy and generated a higher temperature in stir zone SZ. This led to a weaker or more random texture stemming from the occurrence of more complete dynamic recrystallization. After the FSW of AZ31B alloy of 0.5 mm in thickness, the lowest hardness occurred at the center of SZ through the HAZ and TMAZ of the welded joints; however, the differences are minor.

The welding speed and rotational speed had a strong effect on the UTS (ultimate tensile strength). When choosing the technological parameters for the process, tool feed rate played an important role compared with a tool rotation speed. The plastic flow in the welded regions is also observed with uniform grain orientation.
