R.R. Ambriz and V. Mayagoitia

*Instituto Politécnico Nacional CIITEC-IPN, Cerrada de Cecati S/N Col. Sta. Catarina C.P. 02250, Azcapotzalco, DF, México* 

### **1. Introduction**

Welding processes are essential for the manufacture of a wide variety of products, such as: frames, pressure vessels, automotive components and any product which have to be produced by welding. However, welding operations are generally expensive, require a considerable investment of time and they have to establish the appropriate welding conditions, in order to obtain an appropriate performance of the welded joint. There are a lot of welding processes, which are employed as a function of the material, the geometric characteristics of the materials, the grade of sanity desired and the application type (manual, semi-automatic or automatic). The following describes some of the most widely used welding process for aluminum alloys.

### **1.1 Shielded metal arc welding (SMAW)**

This is a welding process that melts and joins metals by means of heat. The heat is produced by an electric arc generated by the electrode and the materials. The stability of the arc is obtained by means of a distance between the electrode and the material, named *stick welding*. Figure 1 shows a schematic representation of the process. The electrode-holder is connected to one terminal of the power source by a welding cable. A second cable is connected to the other terminal, as is presented in Figure 1a. Depending on the connection, is possible to obtain a direct polarity (Direct Current Electrode Negative, DCEN) or reverse polarity (Direct Current Electrode Positive, DCEP). The core wire of the coated electrode conducts the electric current and it provides filler metal to perform the weld.

The heat of the arc melts the wire core and the coating (flux) at the tip of the electrode. The melt material is transferring to the base metal in a drop shape, as is showed in Figure 1b. The molten metal is stored in a weld pool and it solidifies in the base metal. The flux due to its low density floats to the surface of the weld pool and solidifies as a layer of slag in the surface of the weld metal.

The electrode covering contents some chemical compounds, which are intended to protect, deoxidize, stabilize the arc and add alloy elements. There are basically four types of electrode coating types: (i) *Cellulosic* (20-60% rutile, 10-20% cellulose, 15-30% quartz, 0-15% carbonates, 5-10% ferromanganese), which promotes gas shielding protection in the arc region, a deep penetration and fast cooling weld. (ii) *Rutile* ( 40-60% rutile, 15-20% quartz, 0- 15% carbonates, 10-14% ferromanganese, 0-5% organics), this is employed to form slags mainly for slag shielding, it presents high inclusion content in weld deposit. (iii) *Acid* (iron

Welding of Aluminum Alloys 65

Weld metal Arc

Weld pool

Current conductor

Wire electrode

Welding direction

**b)**

Base metal

Gas inlet

Shielding gas

Metal droplet

Contact tube Shielding gas nozzle

Power source

Wire electrode

Flow meter Regulator

Shielding gas

When using an Ar gas arc, the arc energy has a smaller spread than an arc of He, due to the low thermal conductivity of Ar. This aspect helps to obtain a metal transfer more stable and an axial Ar plasma arc. The shielding gas effect on aluminum welding is presented schematically in Figure 3. The penetration pattern is similar to a bottle nipple when using


Ar, whereas when using He, the cross-sectional area has a parabolic penetration.

Ar 75% He+25%Ar

Fig. 3. Schematic representation in aluminum welds using different shielding gases

when using CO2 and He, globular transfer can be obtained at all current levels.

diameter of the electrode and the type of shielding gas.

4, shows the typical range of current for some wire diameters.

There are three basics metal transfer in GMAW process: *globular transfer*, *spray transfer* and

In the *globular transfer*, metal drops are larger than the diameter of the electrode, they travel through the plasma gas and are highly influenced by the gravity force. One characteristic of the globular transfer is that this tends to present, spatter and an erratic arc. This type of metal transfer is present at low level currents, independently of the shielding gas. However,

On the other hand, *spray transfer* occurs at higher current levels, the metal droplets travel through the arc under the influence of an electromagnetic force at a higher frequency than in the globular transfer mode. In this transfer mode, the metal is fed in stable manner and the spatter tends to be eliminated. The critical current level depends of the material, the

In *short-circuiting transfer*, the molten metal at the electrode tip is transferred from the electrode to the weld pool when it touches the pool surface, that is, when short-circuiting occurs. The short-circuiting is associated with lower levels of current and small electrode diameters. This transfer mode produces a small and fast-freezing weld pool that is desirable for welding thin sections, out-of-position welding and bridging large root openings. Figure

Wire drive and control

Wire reel

Cable 1 Cable 2

Fig. 2. Gas metal arc welding process

*short-circuiting transfer* (Kou, 2003).

**a)**

Workpiece

Gun

ore-manganese ore, quartz, complex silicates, carbonates ferromanganese), which provides fairly high hydrogen content and high slag content in the weld. (iv) *Basic* (20-50% calcium carbonate, 20-40% fluorspar, 0-5% quartz, 0-10% rutile, 5-10% ferro-alloys), this coating brings low high hydrogen levels (≤ 10 ppm) and electrodes can be kept dry (Easterling, 1992). Because the temperature is high, the covering of the electrode produces a shielding gas for the molten metal. During the welding process, the covering of the electrode reacts to eliminate the oxides produced in the fusion process and it cleans the weld metal. Also, the slag formed in the solidification process protects the weld metal, especially when the temperature is too high. The electric arc is produced by the ionization of the gases (plasma), which conduct the electric current. Arc stabilizers are compounds that decompose into ions arc in the form of oxalates of potassium and lithium carbonate. They increase the electrical conductivity and improve to conduct the current in softer form. Additionally, the electrode covering also provides alloying elements and/or metallic powders to the weld metal. Alloying elements tend to control the chemical composition of the weld metal; metallic powders tend to increase the deposit rate.

Fig. 1. Shielded metal arc welding process

Some advantages of the SMAW welding process is that it is portable and not expensive compared with others. These features allow that the SMAW process can be employed in maintenance, repair operations, production of structures or pressure vessels. However, in welding of aluminum alloys and titanium, the welding process does not provides a sufficient degree of cleaning because the gas produced by the coating is not enough to obtained welds free of defects and discontinuities. On the other hand, the deposit rate is limited because the electrodes must be changed continuously due to its length and the operator must be stop.

### **1.2 Gas metal arc welding (GMAW)**

GMAW is a welding process that melts and joins metals by heating employing an electric arc established between a continuous wire (electrode) and metals to be welded, as is shown in Figure 2. Shielding protection of the arc and molten metal is carried out by means of a gas, which can be active or inert. In the case of aluminum alloys, non ferrous alloys and stainless steel Ar gas or mixtures of Ar and He are employed, whereas for steels the base of the shielding gases is CO2. When using an inert gas, it is kwon as MIG process (Metal Inert Gas) and MAG when Metal Active Gas is used. GMAW process is one of the most employed to weld aluminum alloys.

ore-manganese ore, quartz, complex silicates, carbonates ferromanganese), which provides fairly high hydrogen content and high slag content in the weld. (iv) *Basic* (20-50% calcium carbonate, 20-40% fluorspar, 0-5% quartz, 0-10% rutile, 5-10% ferro-alloys), this coating brings low high hydrogen levels (≤ 10 ppm) and electrodes can be kept dry (Easterling, 1992). Because the temperature is high, the covering of the electrode produces a shielding gas for the molten metal. During the welding process, the covering of the electrode reacts to eliminate the oxides produced in the fusion process and it cleans the weld metal. Also, the slag formed in the solidification process protects the weld metal, especially when the temperature is too high. The electric arc is produced by the ionization of the gases (plasma), which conduct the electric current. Arc stabilizers are compounds that decompose into ions arc in the form of oxalates of potassium and lithium carbonate. They increase the electrical conductivity and improve to conduct the current in softer form. Additionally, the electrode covering also provides alloying elements and/or metallic powders to the weld metal. Alloying elements tend to control the chemical composition of the weld metal; metallic

Electrode holder

Base metal

Flux covering

Core wire

Shielding gases

Arc

Electrode

Weld metal

Metal droplet Flux droplet

Weld pool

Slag

Welding direction

**b)**

Some advantages of the SMAW welding process is that it is portable and not expensive compared with others. These features allow that the SMAW process can be employed in maintenance, repair operations, production of structures or pressure vessels. However, in welding of aluminum alloys and titanium, the welding process does not provides a sufficient degree of cleaning because the gas produced by the coating is not enough to obtained welds free of defects and discontinuities. On the other hand, the deposit rate is limited because the electrodes must be changed continuously due to its length and the

GMAW is a welding process that melts and joins metals by heating employing an electric arc established between a continuous wire (electrode) and metals to be welded, as is shown in Figure 2. Shielding protection of the arc and molten metal is carried out by means of a gas, which can be active or inert. In the case of aluminum alloys, non ferrous alloys and stainless steel Ar gas or mixtures of Ar and He are employed, whereas for steels the base of the shielding gases is CO2. When using an inert gas, it is kwon as MIG process (Metal Inert Gas) and MAG when Metal Active Gas is used. GMAW process is one of the most employed

powders tend to increase the deposit rate.

Electrode Arc Workpiece

Cable to electrode holder

Fig. 1. Shielded metal arc welding process

**1.2 Gas metal arc welding (GMAW)** 

Power source

operator must be stop.

to weld aluminum alloys.

**a)**

Cable to workpiece

Fig. 2. Gas metal arc welding process

When using an Ar gas arc, the arc energy has a smaller spread than an arc of He, due to the low thermal conductivity of Ar. This aspect helps to obtain a metal transfer more stable and an axial Ar plasma arc. The shielding gas effect on aluminum welding is presented schematically in Figure 3. The penetration pattern is similar to a bottle nipple when using Ar, whereas when using He, the cross-sectional area has a parabolic penetration.

Fig. 3. Schematic representation in aluminum welds using different shielding gases

There are three basics metal transfer in GMAW process: *globular transfer*, *spray transfer* and *short-circuiting transfer* (Kou, 2003).

In the *globular transfer*, metal drops are larger than the diameter of the electrode, they travel through the plasma gas and are highly influenced by the gravity force. One characteristic of the globular transfer is that this tends to present, spatter and an erratic arc. This type of metal transfer is present at low level currents, independently of the shielding gas. However, when using CO2 and He, globular transfer can be obtained at all current levels.

On the other hand, *spray transfer* occurs at higher current levels, the metal droplets travel through the arc under the influence of an electromagnetic force at a higher frequency than in the globular transfer mode. In this transfer mode, the metal is fed in stable manner and the spatter tends to be eliminated. The critical current level depends of the material, the diameter of the electrode and the type of shielding gas.

In *short-circuiting transfer*, the molten metal at the electrode tip is transferred from the electrode to the weld pool when it touches the pool surface, that is, when short-circuiting occurs. The short-circuiting is associated with lower levels of current and small electrode diameters. This transfer mode produces a small and fast-freezing weld pool that is desirable for welding thin sections, out-of-position welding and bridging large root openings. Figure 4, shows the typical range of current for some wire diameters.

Welding of Aluminum Alloys 67

It is possible to use *Direct Current* (DC) or *Alternating Current* (AC) to weld by GTAW. In the case of DC, we can use direct polarity (electrode negative, DCEN) or reverse polarity

Direct polarity is the most commonly employed in GTAW. In this case, the electrode is connected to the negative pole of the heat source and the electrons are emitted from the electrode and they are accelerated as they travel through the arc (plasma). This effect produces a high heat in the workpiece and therefore gives a good penetration and a

DCEN DCEP AC

**+ + +**

(a) (b) (c)

Shallow weld surface cleaning

On the other hand, when reverse polarity is used, the electrode is connected to the positive pole of the heat source. Now the effect of the heating due to the bombardment of the electrons is higher in the electrode than that of workpiece, which results in a wide weld bead and shallower than that generated by direct polarity. In this case, due to high energy concentration in the electrode it is necessary to employ a thicker diameter and a cooling system to eliminate the electrode tip melting possibility. The bombardment effect by positive ions of the inert gas removes the oxide film and produces a cleaning effect on the welding surface. Therefore, reverse polarity can be used to weld materials that are resistant

to oxides such as aluminum and magnesium, if it is not required a high penetration.

When alternating current is used, is possible to obtain a good combination of oxides elimination (cleanliness) and penetration, as is presented in Figure 6. This polarity is the

There are several types of electrodes to weld by GTAW. These include pure tungsten and tungsten alloyed with thorium oxide (ThO2) or zirconium oxide (ZrO2), which are added to improve the arc ignition and to increase the life of electrode. In the last years some alloy elements have been incorporated, such as cerium and lanthanum, which also increase the life of the electrode and tend to decrease the risk of radiation that is produced when electrodes of high thorium content are employed. Zirconium electrodes are preferred for AC, because they present a higher melting point than pure tungsten or tungsten-thorium. During the welding process, it is assumed that the electrode tip is hemispherical type. This is a very important aspect, because the arc stability depends in a greater manner of tip geometry. There are electrodes of varies diameters, which can range from 0.3 to 6.4 mm. Table 1 presents the recommended current ratings for different diameters of electrodes using Ar shielding gas.

**+ + +**

Intermediate

(positive electrode, DCEP), Figure 6, shows the polarity effect on the weld.

relatively narrow weld shape.

**+ + +**

Fig. 6. Polarity in GTAW

Pool

Deep weld no surface cleaning

most employed to weld aluminum alloys.

Fig. 4. Typical welding current ranges for wire diameter and welding current

The principal advantages of GMAW process with respect to SMAW process are: (i) There is not production of slag. (ii) Is possible to perform welds in all welding positions. (iii) Rate deposition is roughly two times than SMAW. (iv) Quality of the welds is very good. (v) Is possible to weld materials with a short-circuiting transfer mode, which tends to improve the reparation and maintenance operations.

### **1.3 Gas tungsten arc welding (GTAW)**

This is a welding process that melts metal by heat employing an electric arc with a non consumable electrode. GTAW process employs an inert or active shielding gas, which protects the electrode and the weld metal. A schematic representation of GTAW process is showed in Figure 5. The arc functions as a heat source, which can be directly used for welding, with or without the use of filler materials. This process produces high quality welds, but the principal disadvantage is that the rate of deposition is slow and it limits the range of application in terms of thickness. For instance, in welding of aluminum alloys it is convenient to use this welding process in thickness no greater than 6 mm, since greater thicknesses require a large number of passes and the welding operation tends to be expansive and slow.

Fig. 5. Schematic representation of GTAW process

Globular/Spray

0 100 200 300 400 500 600 700

Welding current, A

The principal advantages of GMAW process with respect to SMAW process are: (i) There is not production of slag. (ii) Is possible to perform welds in all welding positions. (iii) Rate deposition is roughly two times than SMAW. (iv) Quality of the welds is very good. (v) Is possible to weld materials with a short-circuiting transfer mode, which tends to improve the

This is a welding process that melts metal by heat employing an electric arc with a non consumable electrode. GTAW process employs an inert or active shielding gas, which protects the electrode and the weld metal. A schematic representation of GTAW process is showed in Figure 5. The arc functions as a heat source, which can be directly used for welding, with or without the use of filler materials. This process produces high quality welds, but the principal disadvantage is that the rate of deposition is slow and it limits the range of application in terms of thickness. For instance, in welding of aluminum alloys it is convenient to use this welding process in thickness no greater than 6 mm, since greater thicknesses require a large number of passes and the welding operation tends to be

**b)**

0.8

Short-circuiting

Flow meter Regulator

Cable 2

Shielding gas

Base metal Weld pool

Welding direction

Welding metal Arc

Tungsteng electrode

Contact tube

Electrical conductor

Torch

Filler wire Shielding gas Gas passage

Pulsed Spray

Fig. 4. Typical welding current ranges for wire diameter and welding current

Pulsed Spray

1.2

Wire diameter, mm

reparation and maintenance operations.

**1.3 Gas tungsten arc welding (GTAW)** 

expansive and slow.

Welding direction

Filler wire

**a)**

Torch

Cable 1

Power source Workpiece

Fig. 5. Schematic representation of GTAW process

1.6

2.4

It is possible to use *Direct Current* (DC) or *Alternating Current* (AC) to weld by GTAW. In the case of DC, we can use direct polarity (electrode negative, DCEN) or reverse polarity (positive electrode, DCEP), Figure 6, shows the polarity effect on the weld.

Direct polarity is the most commonly employed in GTAW. In this case, the electrode is connected to the negative pole of the heat source and the electrons are emitted from the electrode and they are accelerated as they travel through the arc (plasma). This effect produces a high heat in the workpiece and therefore gives a good penetration and a relatively narrow weld shape.

Fig. 6. Polarity in GTAW

On the other hand, when reverse polarity is used, the electrode is connected to the positive pole of the heat source. Now the effect of the heating due to the bombardment of the electrons is higher in the electrode than that of workpiece, which results in a wide weld bead and shallower than that generated by direct polarity. In this case, due to high energy concentration in the electrode it is necessary to employ a thicker diameter and a cooling system to eliminate the electrode tip melting possibility. The bombardment effect by positive ions of the inert gas removes the oxide film and produces a cleaning effect on the welding surface. Therefore, reverse polarity can be used to weld materials that are resistant to oxides such as aluminum and magnesium, if it is not required a high penetration.

When alternating current is used, is possible to obtain a good combination of oxides elimination (cleanliness) and penetration, as is presented in Figure 6. This polarity is the most employed to weld aluminum alloys.

There are several types of electrodes to weld by GTAW. These include pure tungsten and tungsten alloyed with thorium oxide (ThO2) or zirconium oxide (ZrO2), which are added to improve the arc ignition and to increase the life of electrode. In the last years some alloy elements have been incorporated, such as cerium and lanthanum, which also increase the life of the electrode and tend to decrease the risk of radiation that is produced when electrodes of high thorium content are employed. Zirconium electrodes are preferred for AC, because they present a higher melting point than pure tungsten or tungsten-thorium.

During the welding process, it is assumed that the electrode tip is hemispherical type. This is a very important aspect, because the arc stability depends in a greater manner of tip geometry. There are electrodes of varies diameters, which can range from 0.3 to 6.4 mm. Table 1 presents the recommended current ratings for different diameters of electrodes using Ar shielding gas.

Welding of Aluminum Alloys 69

Tool pin

Fig. 7. Illustration of the friction-stir welding process, b) weld between aluminum sheets and

Rotation speed, rpm 0 1000 2000 3000 4000

Fig. 8. Relationship between rotational speed and peak temperature in FSW of AA6063 (Sato

Advancing side

Shoulder

Retreating side

Welding direction

**a)**

Joint

c) An actual tool with a threaded-pin

Maximun temperature, K

et al., 2002)

650

700

750

800

850

Weld


Table 1. Recommended electrode diameters and current range employed with Ar shielding gas

Table 2, presents the gases used as a protection, in GMAW and GTAW. A shielding gas is selected according to their ionization potential, density, degree of protection and the effect of oxides removal. For example, it is easier to ionize an Ar gas (15.7 eV) than a He gas (24.5 eV), and due to this effect the arc ignition tends to be more easy. Furthermore, the Ar density is higher than He and consequently the penetration of the weld bead is better.


Table 2. Gas shielding properties employed in GMAW and GTAW (Kou, 2003)

### **1.4 Friction stir welding (FSW)**

Friction-Stir Welding (FSW) is a solid-state, hot-shear joining process (Thomas et al.; 1991, Thomas & Dolby, 2003, Dawes & Thomas, 1996, Mishra & Ma, 2005). The process utilizes a bar-like tool in a wear-resistant material (generally tool steel for aluminum) with a shoulder and terminating in a threaded pin. This tool moves along the butting surfaces of two rigidly clamped plates placed on a backing plate. The shoulder makes a contact with the top surface of the plates to be welded. The heat generated by friction at the shoulder and to a lesser extent at the pin surface and it softens the material being welded. Severe plastic deformation and flow of this plasticised metal occurs as the tool is translated along the welding direction. The material is transported from the front of the tool to the trailing edge where it is forged into a joint. Figure 7 shows a schematic representation of FSW.

There are two principal parameters in FSW: tool rotation rate (ω, rpm) in clockwise or counterclockwise direction and the tool traverse speed (*v*, mm/min) along the line of joint. The rotation of the tool results in stirring and mixing of material around the rotating pin and the translation of the tool moves the stirred material from the front to the back of the pin and finishes welding process. Additionally, the angle of spindle or tool tilt and pressure are other important process parameters. A suitable tilt of the spindle towards trailing direction ensures that the shoulder of the tool holds the stirred material by threaded pin and move material efficiently from the front to the back of the pin. The heat generation rate,

1.0 20-50 1.6 50-80 2.4 80-160 3.2 160-225 4.0 225-330 5.0 330-400 6.4 400-550

Electrode diameter, mm Current, A

Table 1. Recommended electrode diameters and current range employed with Ar shielding

Table 2, presents the gases used as a protection, in GMAW and GTAW. A shielding gas is selected according to their ionization potential, density, degree of protection and the effect of oxides removal. For example, it is easier to ionize an Ar gas (15.7 eV) than a He gas (24.5 eV), and due to this effect the arc ignition tends to be more easy. Furthermore, the Ar density is higher than He and consequently the penetration of the weld bead is better.

> Molecular weigth, g/mol

Argon Ar 39.95 1.784 15.7 Carbon dioxide CO2 44.01 1.978 14.4 Helium He 4.00 0.178 24.5 Hydrogen H2 2.016 0.090 13.5 Nitrogen N2 28.01 1.25 14.5 Oxygen O2 32.00 1.43 13.2

Friction-Stir Welding (FSW) is a solid-state, hot-shear joining process (Thomas et al.; 1991, Thomas & Dolby, 2003, Dawes & Thomas, 1996, Mishra & Ma, 2005). The process utilizes a bar-like tool in a wear-resistant material (generally tool steel for aluminum) with a shoulder and terminating in a threaded pin. This tool moves along the butting surfaces of two rigidly clamped plates placed on a backing plate. The shoulder makes a contact with the top surface of the plates to be welded. The heat generated by friction at the shoulder and to a lesser extent at the pin surface and it softens the material being welded. Severe plastic deformation and flow of this plasticised metal occurs as the tool is translated along the welding direction. The material is transported from the front of the tool to the trailing edge where it is forged

counterclockwise direction and the tool traverse speed (*v*, mm/min) along the line of joint. The rotation of the tool results in stirring and mixing of material around the rotating pin and the translation of the tool moves the stirred material from the front to the back of the pin and finishes welding process. Additionally, the angle of spindle or tool tilt and pressure are other important process parameters. A suitable tilt of the spindle towards trailing direction ensures that the shoulder of the tool holds the stirred material by threaded pin and move material efficiently from the front to the back of the pin. The heat generation rate,

Table 2. Gas shielding properties employed in GMAW and GTAW (Kou, 2003)

Density, g/L Ionization

ω

, rpm) in clockwise or

potential, eV

gas

Gas Chemical

**1.4 Friction stir welding (FSW)** 

symbol

into a joint. Figure 7 shows a schematic representation of FSW. There are two principal parameters in FSW: tool rotation rate (

Fig. 7. Illustration of the friction-stir welding process, b) weld between aluminum sheets and c) An actual tool with a threaded-pin

Fig. 8. Relationship between rotational speed and peak temperature in FSW of AA6063 (Sato et al., 2002)

Welding of Aluminum Alloys 71

microstructural and mechanical conditions in welding of aluminum alloys, especially for

After a fusion welding process two principal zones are identified in the welded joints named: Fusion Zone (FZ) and Heat Affected Zone (HAZ) whereas in the case of FSW three different zones are formed: stirred zone (nugget), Thermo-Mechanical Affected Zone (TMAZ) and the HAZ (Mishra & Ma, 2005). These zones are showed in the macrographs of

Fig. 10. Principal zones in welding of aluminum, a) MIEA welding technique (Al-6061-T6)

In a fusion welding process, the heat input produces a fusion-solidification phenomenon, which is different to that obtained in the solidification of an ingot. (i) In an ingot, solidification begins with heterogeneous nucleation at the chill zone meanwhile in a weld pool the liquid metal partially wets the grains of the parent metal and epitaxial growth takes place from the partially melted grains of the parent metal (Davies et al., 1975). (ii) The rate of solidification in a weld pool, which depends on the traveling speed as well as the welding process, is by far faster than in an ingot. (iii) The macroscopic profile of the solid/liquid interface in welds progressively changes as a function of the traveling speed of the heat source whereas it exclusively depends on the time for an ingot. (iv) The movement of the liquid metal in a weld pool is greater than in an ingot due to the Lorentz forces which create turbulence within the molten metal (Grong, 1997). Figure 11 shows longitudinal views,

Electrode

Gravity force

FSW and MIEA.

**a)**

Backing plate

Figure 10.

**2.1 Microstructure** 

and b) FSW (Al-7075-T651)

**2.1.1 Weld metal microstructure in MIEA** 

Heat flow

Fig. 9. Schematic representation of MIEA welding technique

Heat flow Heat flow

temperature field, cooling rate, *x*-direction force, torque and power are totally depended of the welding speed, the tool rotation speed, the vertical pressure on the tool, etc. Figure 8, shows a relationship between rotational speed and peak temperature in FSW of a 6063 aluminum alloy (Nandan et al., 2008).

FSW enables long lengths of weld to be made without any melting taking place. This provides some important metallurgical advantages compared with fusion welding, i.e. no melting means that solidification and liquation cracking are eliminated; dissimilar alloys can be successfully joined; the stirring and forcing action produces a fine-grain structure. However, one disadvantage is that the keyhole (exit hole) remains when the tool is retracted at the end of the joint (Figure 7b).

Several alloys have been welded by FSW, they included the following aluminum alloys: 5083, 5454, 6061, 6082, 2014, 2219 and 7075 (Nandan et al., 2008).

### **1.5 Modified indirect electric arc welding technique (MIEA)**

Although, welding of aluminum alloys is relatively easy employing friction stir welding, when the thickness is thick a fusion welding process is usually required to join these materials. In the case of a fusion welding process, a large amount of heat input can be dissipated via heat conduction throughout the base material close to the welded zone. Typically, this thermal dissipation induces localized isothermal sections where the thermal gradient can have important and detrimental effects on the microstructure and therefore on the mechanical properties of the material constituting the heat affected zone (HAZ), specially in aluminum alloys hardening by artificial ageing (Myhr et al., 2004). In order to improve the mechanical and microstructural conditions of the welded joint in aluminum alloys, the Modified Indirect Electric Arc (MIEA), has been developed (Ambriz at al., 2006, Ambriz et al. 2008). This welding technique is based on a simple joint modification which provided several advantages with respect to the traditional arc fusion welding process, for instance:


MIEA welding technique employs the same equipment that is required to weld by GMAW. A schematic representation of the MIEA joint is present in Figure 9.
