**3. Friction stir welding**

*Magnesium - The Wonder Element for Engineering/Biomedical Applications*

automotive industry because it is inexpensive, easily automated, and capable of producing high-quality welds [8]. The process is based on the contact resistance between the metals to be joined and the electrodes [9]. During the process, a weld nugget is formed by the melting of the materials as the temperature increases due to the materials' resistance. The heat generated during resistance spot welding (RSW) is based on Joule's law. However, the presence of surface oxides on magnesium alloys necessitates the use of high welding current because of the high contact resistance which also leads to rapid erosion of electrode tips used in the welding process. The scientific literature, however, shows that the heat input by RSW dissimilar joining of Al-Mg alloys leads to the formation of Al12Mg17 and Al3Mg2 intermetallic compounds within the weld nugget [10–12]. A recent study using Ni/TiO2 interlayer showed that the intermetallic formation during dissimilar resistance spot welding of magnesium to aluminum can be controlled with the inclusion of nanoparticles at the interface (see **Figure 3**) however, the addition of a coating step in the process extends the process time [13]. Despite the advancements that have been made regarding the impact of interlayers such as Zn as an interlayer between Al/Mg alloys, gold coated Ni interlayer [14, 15] and

adhesive interlayers [16], to enhance the mechanical properties of the joints.

been shown to limit the mechanical strength of the weld.

Laser welding, on the other hand, permits dissimilar joining of magnesium alloys while limiting the width of the HAZ, due to a smaller diameter beam and greater control of power input. However, the equipment used for laser welding is expensive and requires skilled operators. Furthermore, laser welding is usually applied to delicate and thin sheets due to the low penetration of laser beam into the materials. Ultrasonic spot welding (USW) have also been shown to eliminate some of the challenges encountered during RSW since the amount energy input into the material during welding is significantly lower, typically in the range of 0.5–1.2 kJ and requires a shorter welding time [17]. **Figure 1(B)** shows the microstructure of a bond made by ultrasonic welding. The formation of Mg2Sn layer at the interface has

*SEM micrograph showing: (A) Al/Ni–TiO2/Mg spot weld, (B) microstructure of point-8, (C) Al/weld nugget* 

**86**

**Figure 3.**

*interface and (D) microstructure of point-9 [13].*

Friction stir welding (FSW) is a solid-state welding process that was developed at The Welding Institute (TWI) [22]. As explained by Mishra and Ma, the process is carried out by the use of a non-consumable rotating pin which travels along the joint to be welded [23]. The friction between the tool and the materials generates heat which softens the material [24]. **Figure 4** shows a schematic of the FSW

**Figure 4.** *Schematic of the friction stir welding process.*

#### **Figure 5.**

*Weld produced by friction stir welding and the microstructure generated [26].*

**Figure 6.** *Hardness profiles across Mg-alloys welded using friction stir welding and TIG welding.*

#### **Figure 7.**

*EPMA results of the FSW region of the sample welded by using a weld-pitch ratio of 1400/40 r/mm in the cross-section perpendicular to the tool transverse direction. Element distribution: (A) Al and (B) Mg [36].*

**89**

*Dissimilar Welding and Joining of Magnesium Alloys: Principles and Application*

dissimilar welding, some IMCs will still form as shown in **Figure 7**.

below. A schematic of the diffusion bonding process is shown in **Figure 8**.

Solid state diffusion bonding occurs due to the migration of atoms at the interface of the materials to be joined. The faying surfaces do not undergo any metallurgical discontinuity and as a result, the mechanical and microstructural property

**4.1 Solid-state diffusion bonding of magnesium alloys**

process. The combination of tool rotation and localized softening of the material causes movement of the material leading to a weld. The process was first developed to weld aluminum alloys [22] previously described as non-weldable because of the adverse solidification microstructural formed in the fusion zone during fusion welding. Since its development, however, the technology has been applied to various metal and alloys. Zhang [25] demonstrated that friction stir welding is a viable

Mg joints produced by FSW are characterized by high joint strength, high fatigue resistance, less distortion, since consumables are not required hence it is cheaper and no loss of alloying elements occurs during the process [27, 28].

The literature shows that weld quality is dependent on the optimization of the welding parameters; tool shape, jig rotational speed, tool speed, and joint configuration. In comparison to the traditional welding zones, the weld produced by FSW was characterized by four distinct zones as shown in **Figure 5** [26, 29]. These regions include; stirring zone (SZ), thermo-mechanical affected zone (TMAZ), heat affected zone (HAZ), and base metal (BM) [26, 30]. Singh et al. [23] demonstrated that FSW is capable of Mg weld with shear strength ranging from 66 to 410 MPa and the hardness from 50 to 110 HVN which represents 60–195% joint efficiency compared to the base metal alloy. **Figure 6** shows the hardness variation across the weld and confirms the hardness variations reported by Singh et al. The difference between the hardness recorded at the center of the weld and the base metal was attributed to the formation of IMCs with the weld during the FSW process. Similar findings were recorded by other researchers who studied different Mg alloys were welded using FSW such as; MgAZ31 [31, 32] and MgAZ31B [33], dissimilar joint of Al6061/MgAZ31B [34, 35]. Mohammadi et al. [36] demonstrated that while FSW can reduce the volume of intermetallic compounds formed during

Diffusion bonding is a non-conventional solid-state welding process that can join a variety of materials in the solid state below the melting point of the base materials. The core mechanism involves the interdiffusion of atoms between the faying surfaces at the interface [37]. The process is frequently completed by one of two methods; firstly, solid-state diffusion bonding in which the base metals to be joined are heated to approximately 60% of the melting temperature of the metals under the influence of static load. Melting at the interface of the faying surfaces is prevented, however, interdiffusion of the diffusing species leads to the formation of solid-state bond. The second method is referred to as a transient liquid phase (TLP) diffusion bonding in which an interlayer is placed between the metals to be joined. Interdiffusion between the interlayer and the base metals facilitates the formation of a eutectic reaction which transitions from liquid to solid by isothermal solidification as the composition of the eutectic liquid changes due to diffusion. Each method will be further discussed

*DOI: http://dx.doi.org/10.5772/intechopen.85111*

technique for joining magnesium alloys.

**4. Diffusion bonding**

#### *Dissimilar Welding and Joining of Magnesium Alloys: Principles and Application DOI: http://dx.doi.org/10.5772/intechopen.85111*

process. The combination of tool rotation and localized softening of the material causes movement of the material leading to a weld. The process was first developed to weld aluminum alloys [22] previously described as non-weldable because of the adverse solidification microstructural formed in the fusion zone during fusion welding. Since its development, however, the technology has been applied to various metal and alloys. Zhang [25] demonstrated that friction stir welding is a viable technique for joining magnesium alloys.

Mg joints produced by FSW are characterized by high joint strength, high fatigue resistance, less distortion, since consumables are not required hence it is cheaper and no loss of alloying elements occurs during the process [27, 28].

The literature shows that weld quality is dependent on the optimization of the welding parameters; tool shape, jig rotational speed, tool speed, and joint configuration. In comparison to the traditional welding zones, the weld produced by FSW was characterized by four distinct zones as shown in **Figure 5** [26, 29]. These regions include; stirring zone (SZ), thermo-mechanical affected zone (TMAZ), heat affected zone (HAZ), and base metal (BM) [26, 30]. Singh et al. [23] demonstrated that FSW is capable of Mg weld with shear strength ranging from 66 to 410 MPa and the hardness from 50 to 110 HVN which represents 60–195% joint efficiency compared to the base metal alloy. **Figure 6** shows the hardness variation across the weld and confirms the hardness variations reported by Singh et al. The difference between the hardness recorded at the center of the weld and the base metal was attributed to the formation of IMCs with the weld during the FSW process. Similar findings were recorded by other researchers who studied different Mg alloys were welded using FSW such as; MgAZ31 [31, 32] and MgAZ31B [33], dissimilar joint of Al6061/MgAZ31B [34, 35]. Mohammadi et al. [36] demonstrated that while FSW can reduce the volume of intermetallic compounds formed during dissimilar welding, some IMCs will still form as shown in **Figure 7**.
