**2.1.1 Weld metal microstructure in MIEA**

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,

Welding of Aluminum Alloys 73

Fig. 12. Schematic representation of the local and nominal crystal growth rate (Ambriz et al.,

The longitudinal macrostructures for the MIEA joint, Figures 11b-d, exhibit significant differences with respect to the multi-passes single V groove joint. Irrespective of the preheating condition, the local crystalline growth maintains an angle nearly constant in relation to the moving heat source. The virtual non existence of changes in growth direction means that the local and nominal rates of crystalline growth tend to be equal. This phenomenon yields a significantly different grain structure in the weld metal for the MIEA joint as compared to the structure observed for the single V groove joint. It leads, in fact, to a grain refining effect which is obviously affected by the initial preheating temperature of the

Figure 13 shows the grain structure at the bottom, mid height and top of the welds. These micrographs correspond to equivalent positions between welds and were captured at the same magnification. A dramatic change in the size and morphology of the grains is observed for the single V groove joint. Besides, some levels of porosity, as indicated by the arrows, are visible. The fine grain size present in the root pass is ascribed to the rapid cooling and/or to recrystallisation effects owing to subsequent welding passes which increased the heat input and caused grain growth toward the top of the weld. Microstructural examination of the fusion line revealed epitaxial growth from partially melted grains and columnar-dendritic grains. The micrograph in Figure 13 at the top of the single V weld shows that this solidification mechanism prevails between welding passes. On the other hand, the microstructures obtained for the MIEA joints do not exhibit major changes in morphology and size with regard to position. Thus, while the microstructures for the single V groove joint show that competitive growth occurs during solidification, the MIEA joint exhibit signs of heterogeneous nucleation which promotes grain refining. Figure 14 shows a micrograph obtained in the Scanning Electron Microscope showing heterogeneous nucleation in MIEA. Also, the levels of porosity in the MIEA joints decrease with preheating temperature (50, 100 and 150 ºC) and are comparatively lower than that obtained in the single V groove joint. Epitaxial solidification is also observed at the fusion

2010b)

joint.

which depict the direction of solidification of the welds, for a multi-pass welding and MIEA with different preheating conditions. The arrows indicate the displacing direction of the electric arc.

Fig. 11. Longitudinal top view of the weld metal grain structure at the mid plane for: a) single V groove, b)-d) MIEA

Figure 11a corresponds to the single V groove joint and it shows the crystalline growth of a columnar-dendritic structure at a given angle with respect to the direction of the heat source. This feature is determined by the traveling speed of the welding torch. In this instance, the rate of the local crystalline growth tends to be that of the welding process. This phenomenon is illustrated by means of a schematic representation in Figure 12.

It is possible to observe that the local crystalline growth, *RL*, is always larger than the nominal crystalline growth, *RN*, since there are directions in which growth occurs preferentially. Thus, the rate of crystalline growth tends to be the traveling speed of the heat source, *v*, when the angles are less pronounced (when 0 α → and φ → 0 ), according to the following equation (Grong, 1997).

$$R\_L = \frac{R\_N}{\cos \phi} = \frac{v \cos \alpha}{\cos \phi} \tag{1}$$

The changes in direction are readily appreciated for the longitudinal view shown in Figure 11a. Competitive growth toward the heat source is evident, giving rise to columnar grains; this characteristic is typical of arc fusion welding processes. Analyzing equation (1) along with Figure 12, it is apparent the increase of the rate of crystalline growth as a function of the changes in orientation of the crystalline growth with regard to the largest thermal gradient of the weld pool. It is clear thus that local crystalline growth is favored due to the prevailing high temperature conditions.

which depict the direction of solidification of the welds, for a multi-pass welding and MIEA with different preheating conditions. The arrows indicate the displacing direction of the

Fig. 11. Longitudinal top view of the weld metal grain structure at the mid plane for:

phenomenon is illustrated by means of a schematic representation in Figure 12.

source, *v*, when the angles are less pronounced (when 0

Figure 11a corresponds to the single V groove joint and it shows the crystalline growth of a columnar-dendritic structure at a given angle with respect to the direction of the heat source. This feature is determined by the traveling speed of the welding torch. In this instance, the rate of the local crystalline growth tends to be that of the welding process. This

It is possible to observe that the local crystalline growth, *RL*, is always larger than the nominal crystalline growth, *RN*, since there are directions in which growth occurs preferentially. Thus, the rate of crystalline growth tends to be the traveling speed of the heat

cos cos

φ

The changes in direction are readily appreciated for the longitudinal view shown in Figure 11a. Competitive growth toward the heat source is evident, giving rise to columnar grains; this characteristic is typical of arc fusion welding processes. Analyzing equation (1) along with Figure 12, it is apparent the increase of the rate of crystalline growth as a function of the changes in orientation of the crystalline growth with regard to the largest thermal gradient of the weld pool. It is clear thus that local crystalline growth is favored due to the

*RN <sup>v</sup> RL*

α

cos

α

φ

→ and

φ

= = (1)

→ 0 ), according to the

electric arc.

a) single V groove, b)-d) MIEA

following equation (Grong, 1997).

prevailing high temperature conditions.

Fig. 12. Schematic representation of the local and nominal crystal growth rate (Ambriz et al., 2010b)

The longitudinal macrostructures for the MIEA joint, Figures 11b-d, exhibit significant differences with respect to the multi-passes single V groove joint. Irrespective of the preheating condition, the local crystalline growth maintains an angle nearly constant in relation to the moving heat source. The virtual non existence of changes in growth direction means that the local and nominal rates of crystalline growth tend to be equal. This phenomenon yields a significantly different grain structure in the weld metal for the MIEA joint as compared to the structure observed for the single V groove joint. It leads, in fact, to a grain refining effect which is obviously affected by the initial preheating temperature of the joint.

Figure 13 shows the grain structure at the bottom, mid height and top of the welds. These micrographs correspond to equivalent positions between welds and were captured at the same magnification. A dramatic change in the size and morphology of the grains is observed for the single V groove joint. Besides, some levels of porosity, as indicated by the arrows, are visible. The fine grain size present in the root pass is ascribed to the rapid cooling and/or to recrystallisation effects owing to subsequent welding passes which increased the heat input and caused grain growth toward the top of the weld. Microstructural examination of the fusion line revealed epitaxial growth from partially melted grains and columnar-dendritic grains. The micrograph in Figure 13 at the top of the single V weld shows that this solidification mechanism prevails between welding passes. On the other hand, the microstructures obtained for the MIEA joints do not exhibit major changes in morphology and size with regard to position. Thus, while the microstructures for the single V groove joint show that competitive growth occurs during solidification, the MIEA joint exhibit signs of heterogeneous nucleation which promotes grain refining. Figure 14 shows a micrograph obtained in the Scanning Electron Microscope showing heterogeneous nucleation in MIEA. Also, the levels of porosity in the MIEA joints decrease with preheating temperature (50, 100 and 150 ºC) and are comparatively lower than that obtained in the single V groove joint. Epitaxial solidification is also observed at the fusion

Welding of Aluminum Alloys 75

Generally speaking MIEA joint yields homogeneous grain refined microstructures in the weld metal, having the average grain size well below than that obtained with the conventional single V groove joint. The differences in grain size and morphology between single-V groove and MIEA joints are expected to have a significant impact on the mechanical performance of the welds. Before dealing with this aspect, it is worth to elucidate about the possible mechanism that gives rise to a self-refining effect when welding

The intense plastic deformation and frictional heating during FSW results in the generation of a recrystallized fine-grained microstructure within the stirred zone (Mahoney et al., 1998). This is usually referred to as a weld nugget (or nugget zone) or dynamically recrystallized zone. Also, under the same FSW conditions, onion ring structure is observed in the nugget

> Advancing side Retreating side Pin shoulder side

> > Anvil side

Fig. 15. Optical image showing the macroscopic features (Nandan et al., 2008) in a

transverse section of FSW of 2195-T81 Al-Li-Cu alloy. Note the onion-ring and the adjacent

Depending on the processing parameter, tool geometry, temperature of the workpiece, and thermal conductivity of the material, various shapes of nugget zone have been observed. Basically, nugget zone can be classified into two types, basin-shaped nugget that widens near the upper surface and elliptical nugget. Sato et al. (Sato et al., 1999) reported the formation of basin-shaped nugget on friction FSW of 6063-T5 aluminum alloy plate. They suggested that the upper surface experiences extreme deformation and frictional heating by contact with a cylindrical-tool shoulder during FSW, thereby resulting in generation of basin-shaped nugget zone. On the other hand, Rhodes et al. (Rhodes et al., 1997) and Mahoney et al. (Mahoney et al., 1998) reported elliptical nugget zone in the weld of 7075-

In terms of grain size it is well know that FSW produces a fine structure, which is a direct function of the welding parameters like: tool geometry, chemical composition of the workpiece, temperature of the workpiece, vertical pressure and active cooling. For example,

with the MIEA joint technique.

zone, as is presented in Figure 15.

large upward movement of material

T651 aluminum alloy.

**2.1.2 Nugget zone in FSW** 

Onion ring laverting

Parental material

line of the MIEA welds, however, competitive columnar growth was restricted instead grain structures alike those observed in the centre of the weld metal (Figure 13) were present. The characteristics of solidification observed for the MIEA welds in Figure 13 are the result of heterogeneous nucleation which is based on the principle of the formation of a critical radii needed to achieve the energy of formation from potential sites for nucleation such as inclusions, substrates or inoculants (Ti or Zr) (Rao et al., 2008; Ram et al., 2000; Lin et al., 2003). For the MIEA welds, these sites are principally the sidewalls of the joint in conjunction with the content of Ti in the filler and base metal since the significant dilution of base metal favors incorporation of Ti into the weld pool.

Fig. 13. Optical micrographs in welding of 6061-T6 for multipass welding process and MIEA with three different preheating (50, 100 and 150 ºC)

Fig. 14. Scanning electron micrographs showing heterogeneous nucleation in welding of 6061-T6 aluminum alloy obtained by MIEA

line of the MIEA welds, however, competitive columnar growth was restricted instead grain structures alike those observed in the centre of the weld metal (Figure 13) were present. The characteristics of solidification observed for the MIEA welds in Figure 13 are the result of heterogeneous nucleation which is based on the principle of the formation of a critical radii needed to achieve the energy of formation from potential sites for nucleation such as inclusions, substrates or inoculants (Ti or Zr) (Rao et al., 2008; Ram et al., 2000; Lin et al., 2003). For the MIEA welds, these sites are principally the sidewalls of the joint in conjunction with the content of Ti in the filler and base metal since the significant dilution of

 Single V,T=25°C MIEA, T=50°C MIEA, T=100°C MIEA, T=150°C Fig. 13. Optical micrographs in welding of 6061-T6 for multipass welding process and MIEA

Fig. 14. Scanning electron micrographs showing heterogeneous nucleation in welding of

base metal favors incorporation of Ti into the weld pool.

with three different preheating (50, 100 and 150 ºC)

6061-T6 aluminum alloy obtained by MIEA

Top

Centre

Bottom

Generally speaking MIEA joint yields homogeneous grain refined microstructures in the weld metal, having the average grain size well below than that obtained with the conventional single V groove joint. The differences in grain size and morphology between single-V groove and MIEA joints are expected to have a significant impact on the mechanical performance of the welds. Before dealing with this aspect, it is worth to elucidate about the possible mechanism that gives rise to a self-refining effect when welding with the MIEA joint technique.
