**1. Introduction**

The interest in the use of lightweight materials and alloys has significantly increased in recent years due to excellent mechanical and chemical characteristics; such as high specific strength, low weight, and corrosion resistance. Magnesium is a popular industrial metal and it is the lightest of all, estimated to be 33% lighter than aluminum and ~75% lighter than steel, with a density of 1.74 g/m3 [1]. Magnesium has a hexagonal closed pack (HCP) crystalline structure that resists the slip to parallel basal planes and therefore, magnesium cannot be plastically deformed at room temperature, because the work hardening rate is high, and ductility is low. Therefore, magnesium alloys are formed above 226°C with a range of 343–510°C as

the slip process becomes easier at elevated temperatures. Anisotropy during deformation is the other consequence of the HCP structure in cold-formed sheets.

The industrial and biomedical demand for components made from light-weight materials have only increased over the last decade [1]. In its pure state, magnesium is ductile and possesses low tensile strength in the range of 20 MPa. Engineering applications, however, require greater strength as such magnesium if frequently alloyed with elements such as; Al, Mn, Zn, Li, Ag, Ca and Cu along with various minor alloying elements. The utilization of magnesium alloys has seen an increase at the rate of 15% and is predicted to go even higher over the next decade. Currently, magnesium is used widely as a structural material due to low density and favorable damping characteristics, low casting costs and machinability [2]. However, new magnesium alloys are being developed with enhanced corrosion resistance and enable these materials to operate satisfactorily inside the human body. The corrosion resistance of these materials has been shown to be inversely proportional to the volume of impurity (Fe, Ni, and Cu) present in the alloy. Currently, the high purity ZX50 Mg alloy and high purity Mg-Al alloys have shown the greatest degree of success.

A major challenge in using these materials is the unavailability of suitable welding technique capable of both similar and dissimilar welding of magnesium alloys [3]. Currently, magnesium alloys used in dissimilar welding processes are joined by resistance spot welding, laser welding, friction stir welding, and diffusion bonding. Extensive research has been devoted to the development of technologies capable of joining these alloys while preventing significant microstructural changes commonly observed during fusion welding of advanced composites. This chapter will investigate welding and joining technologies which are currently used to join magnesium alloys with emphasis on diffusion bonding as a technique for the development of multi-material structures for applications in the biomedical industries. Multimaterial structures often provide the most efficient design solution to engineering challenges. The use of several metals in the construction of multi-material structures is limited by the ability of the available welding/joining technologies to join dissimilar materials together.

### **2. Welding metallurgy of magnesium alloys**

Fusion welding of magnesium alloys has been studied using several technologies which include; gas tungsten arc welding (TIG), gas metal arc welding (MIG), laser welding and ultrasonic welding. During TIG welding, an arc is produced between a non-consumable electrode and the workpiece, which melts the base metal to form a weld/joint as shown in **Figure 1**. The joint and surrounding areas can be divided into regions consisting of the weld pool (with a cast structure) and a heat affected zone (HAZ). The microstructural changes within these regions occur due to the temperature gradient from the weld pool to the base metal alloy as shown in **Figure 2**. Shielding flux either in solid or gaseous form is required to protect the weld pool from exposure to oxygen.

TIG welding is considered as the most preferred industrial welding method for reactive materials such as magnesium; this is attributed to better economy and applicability, which makes it an excellent choice for joining magnesium and its alloys. Previous work on joining Mg alloys showed that TIG welding may result in a joint strength of up to 94% of the shear strength of the Mg base metal alloy. However, grain size variation from 6 to 23 μm occurred, across the weld pool, heat affected zone (HAZ) region, and Mg base metal alloy, when TIG welding is applied without a filler wire [4]. On the other hand, Peng et al. [5] showed that dissimilar joining of magnesium alloys AZ31 and AZ61 using TIG welding can

**85**

similar in both alloys [4, 6].

the mechanical performance of the weld [7].

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

achieve a bond strength of 84%. In their recent work, Song et al. [4] also showed that laser TIG welding produced good weldability and joint strength in Mg alloys when compared with hybrid welding. The hybrid welding approach resulted in a more flexible and reliable methodology for industrial application for magnesium alloys welding. The increased joint strength of Mg-AZ31 joints produced by hybrid TIG welding was attributed to the formation of a partially melted zone (PMZ) [4]. Several researchers successfully achieved dissimilar joining of magnesium alloys. The results of these studies demonstrated that while it is possible to join magnesium using TIG welding the heat input during welding results in significant changes in the HAZ that limits the strength of the joints produced. **Figure 2** shows an optical micrograph of fusion zone and the heat affected zone (HAZ) for two types of magnesium alloys joined using TIG welding, while the chemistry of the materials is different, the microstructural changes within the FZ and HAZ are

*Microstructures of the welding joint produced by TIG welding (a) AZ31 side of the weld; (b) fusion zone; (c)* 

*Micrographs showing the transverse section of joint welded by (a) TIG welding (b) ultrasonic welding [4].*

Similarly, MIG welding has been shown by several researchers to be capable of joining magnesium alloys with greater speed when compared to TIG welding. Unique fast rigging proportions are normally required in the wire feeders since the magnesium terminal wire has a high melt-off rate. The typical wire feeder and power supply utilized for aluminum welding will be appropriate for welding magnesium. The heat input during MIG welding of magnesium alloys, however, results in the formation of a large HAZ containing a coarse grain structure which lowers

Recent studies in resistance spot welding (RSW) of magnesium alloys have demonstrated that dissimilar RSW of magnesium AZ31 to aluminum Al5754 coated with nanostructured electrodeposited coating prevented intermetallic formation while enhancing joint strength. RSW is the most common welding technique used in the

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

**Figure 1.**

**Figure 2.**

*AZ80 side of the weld [4].*

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

**Figure 1.**

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

high purity Mg-Al alloys have shown the greatest degree of success.

dissimilar materials together.

exposure to oxygen.

**2. Welding metallurgy of magnesium alloys**

the slip process becomes easier at elevated temperatures. Anisotropy during deformation is the other consequence of the HCP structure in cold-formed sheets.

The industrial and biomedical demand for components made from light-weight materials have only increased over the last decade [1]. In its pure state, magnesium is ductile and possesses low tensile strength in the range of 20 MPa. Engineering applications, however, require greater strength as such magnesium if frequently alloyed with elements such as; Al, Mn, Zn, Li, Ag, Ca and Cu along with various minor alloying elements. The utilization of magnesium alloys has seen an increase at the rate of 15% and is predicted to go even higher over the next decade. Currently, magnesium is used widely as a structural material due to low density and favorable damping characteristics, low casting costs and machinability [2]. However, new magnesium alloys are being developed with enhanced corrosion resistance and enable these materials to operate satisfactorily inside the human body. The corrosion resistance of these materials has been shown to be inversely proportional to the volume of impurity (Fe, Ni, and Cu) present in the alloy. Currently, the high purity ZX50 Mg alloy and

A major challenge in using these materials is the unavailability of suitable welding technique capable of both similar and dissimilar welding of magnesium alloys [3]. Currently, magnesium alloys used in dissimilar welding processes are joined by resistance spot welding, laser welding, friction stir welding, and diffusion bonding. Extensive research has been devoted to the development of technologies capable of joining these alloys while preventing significant microstructural changes commonly observed during fusion welding of advanced composites. This chapter will investigate welding and joining technologies which are currently used to join magnesium alloys with emphasis on diffusion bonding as a technique for the development of multi-material structures for applications in the biomedical industries. Multimaterial structures often provide the most efficient design solution to engineering challenges. The use of several metals in the construction of multi-material structures is limited by the ability of the available welding/joining technologies to join

Fusion welding of magnesium alloys has been studied using several technologies which include; gas tungsten arc welding (TIG), gas metal arc welding (MIG), laser welding and ultrasonic welding. During TIG welding, an arc is produced between a non-consumable electrode and the workpiece, which melts the base metal to form a weld/joint as shown in **Figure 1**. The joint and surrounding areas can be divided into regions consisting of the weld pool (with a cast structure) and a heat affected zone (HAZ). The microstructural changes within these regions occur due to the temperature gradient from the weld pool to the base metal alloy as shown in **Figure 2**. Shielding flux either in solid or gaseous form is required to protect the weld pool from

TIG welding is considered as the most preferred industrial welding method for reactive materials such as magnesium; this is attributed to better economy and applicability, which makes it an excellent choice for joining magnesium and its alloys. Previous work on joining Mg alloys showed that TIG welding may result in a joint strength of up to 94% of the shear strength of the Mg base metal alloy. However, grain size variation from 6 to 23 μm occurred, across the weld pool, heat affected zone (HAZ) region, and Mg base metal alloy, when TIG welding is applied without a filler wire [4]. On the other hand, Peng et al. [5] showed that dissimilar joining of magnesium alloys AZ31 and AZ61 using TIG welding can

**84**

*Micrographs showing the transverse section of joint welded by (a) TIG welding (b) ultrasonic welding [4].*

**Figure 2.**

*Microstructures of the welding joint produced by TIG welding (a) AZ31 side of the weld; (b) fusion zone; (c) AZ80 side of the weld [4].*

achieve a bond strength of 84%. In their recent work, Song et al. [4] also showed that laser TIG welding produced good weldability and joint strength in Mg alloys when compared with hybrid welding. The hybrid welding approach resulted in a more flexible and reliable methodology for industrial application for magnesium alloys welding. The increased joint strength of Mg-AZ31 joints produced by hybrid TIG welding was attributed to the formation of a partially melted zone (PMZ) [4]. Several researchers successfully achieved dissimilar joining of magnesium alloys. The results of these studies demonstrated that while it is possible to join magnesium using TIG welding the heat input during welding results in significant changes in the HAZ that limits the strength of the joints produced. **Figure 2** shows an optical micrograph of fusion zone and the heat affected zone (HAZ) for two types of magnesium alloys joined using TIG welding, while the chemistry of the materials is different, the microstructural changes within the FZ and HAZ are similar in both alloys [4, 6].

Similarly, MIG welding has been shown by several researchers to be capable of joining magnesium alloys with greater speed when compared to TIG welding. Unique fast rigging proportions are normally required in the wire feeders since the magnesium terminal wire has a high melt-off rate. The typical wire feeder and power supply utilized for aluminum welding will be appropriate for welding magnesium. The heat input during MIG welding of magnesium alloys, however, results in the formation of a large HAZ containing a coarse grain structure which lowers the mechanical performance of the weld [7].

Recent studies in resistance spot welding (RSW) of magnesium alloys have demonstrated that dissimilar RSW of magnesium AZ31 to aluminum Al5754 coated with nanostructured electrodeposited coating prevented intermetallic formation while enhancing joint strength. RSW is the most common welding technique used in the

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.

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 been shown to limit the mechanical strength of the weld.

#### **Figure 3.**

*SEM micrograph showing: (A) Al/Ni–TiO2/Mg spot weld, (B) microstructure of point-8, (C) Al/weld nugget interface and (D) microstructure of point-9 [13].*

**87**

**Figure 4.**

*Schematic of the friction stir welding process.*

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

A major challenge in welding magnesium alloys, however, is the presence of a surface oxide. Magnesium has a relatively low melting point, however, the surface oxide has a high melting point, as such removal of the surface oxide is required before welding of the metal. Additionally, several quality issues have been shown to arise during fusion welding of magnesium alloys. For example; large shrinkage during solidification and high chemical reactivity of Mg makes it very difficult to weld using traditional welding techniques [18], hence, Mg is usually alloyed with aluminum, zinc, and manganese for commercial usage [18]. A critical consideration is necessary for welding Mg alloys that may be affected by the properties of the alloying elements [19]. Any oxygen in contact with the weld pool, regardless of whether from the climate or the protecting gas, causes dross. Therefore, an adequate stream of idle protecting gases is essential, and welding in moving air ought to be avoided. While fusion welding has been successfully used for similar metal joining of magnesium alloys, dissimilar welding of magnesium alloys to metals such as: steel [20], aluminum [21], and titanium has significant limitations due to the formation of intermetallic compounds within the joint region that minimize the joint strength and limits potential applications. The formation of various Mg-Al intermetallic compounds is detrimental to the mechanical performance of the materials. The formation of these compounds can be minimized with the use of interlayers between the materials to be joined. Given the potential for intermetallic formation, various joining techniques have been extensively studied with the objective being to minimize the formation of intermetallic compounds during joining while improving joint performance. Some of these techniques include friction stir welding, resistance spot welding, laser welding, ultrasonic welding, and diffusion bonding. These limitations have led to a preference for solid-state joining techniques to weld/join Mg and its alloys to other metals.

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

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

**3. Friction stir welding**

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

A major challenge in welding magnesium alloys, however, is the presence of a surface oxide. Magnesium has a relatively low melting point, however, the surface oxide has a high melting point, as such removal of the surface oxide is required before welding of the metal. Additionally, several quality issues have been shown to arise during fusion welding of magnesium alloys. For example; large shrinkage during solidification and high chemical reactivity of Mg makes it very difficult to weld using traditional welding techniques [18], hence, Mg is usually alloyed with aluminum, zinc, and manganese for commercial usage [18]. A critical consideration is necessary for welding Mg alloys that may be affected by the properties of the alloying elements [19]. Any oxygen in contact with the weld pool, regardless of whether from the climate or the protecting gas, causes dross. Therefore, an adequate stream of idle protecting gases is essential, and welding in moving air ought to be avoided.

While fusion welding has been successfully used for similar metal joining of magnesium alloys, dissimilar welding of magnesium alloys to metals such as: steel [20], aluminum [21], and titanium has significant limitations due to the formation of intermetallic compounds within the joint region that minimize the joint strength and limits potential applications. The formation of various Mg-Al intermetallic compounds is detrimental to the mechanical performance of the materials. The formation of these compounds can be minimized with the use of interlayers between the materials to be joined. Given the potential for intermetallic formation, various joining techniques have been extensively studied with the objective being to minimize the formation of intermetallic compounds during joining while improving joint performance. Some of these techniques include friction stir welding, resistance spot welding, laser welding, ultrasonic welding, and diffusion bonding. These limitations have led to a preference for solid-state joining techniques to weld/join Mg and its alloys to other metals.
