**2.1.4 Heat affected zone**

Heat Affected Zone (HAZ) is present in fusion welding as well as in FSW processes. The wide of this zone is a direct function of the heat input and the thermal conductivity of the materials to be welded. Obviously the HAZ in FSW tends to be lower than that obtained in a fusion welding process. The HAZ is very important in welding of aluminum alloys, especially in alloys hardened by precipitation (artificial ageing), for instance 2024-T6, 2014- T6, 6061-T6 and 7075-T6. During artificial ageing in Al-Mg-Si alloys (6000 series), a high density of fine, needle-shaped β'' particles are formed uniformly in the matrix (aluminum, α). This precipitate is the dominating hardening phase, which is produced according to the following precipitation sequence (Dutta & Allen, 1991).

Super-Saturated Solid Solution (SSS) → Solutes clustering → GP zones (spherical) → β'' (needle)→β' (bar)→β

in FSW of 6061-T6 aluminum alloy is possible to obtain a grain size near to 10 μm (Liu et al., 1997). Figure 16 illustrates the characteristic microstructures in 2024 and 6061 aluminum alloys welds obtained by FSW. One of the principal parameters which affect the grain size in

Fig. 16. Representative 2024Al/6061 Al FSW microstructure comparison, a) 2024 Al base plate grain structure, b) 2024 Al lamellar weld zone grain structure and c) 6061 Al base plate

In FSW a Thermo-Mechanically Affected Zone (TMAZ) is formed between the parent metal and the nugget zone, as shown in Figure 10b. The TMAZ experiences both temperature and deformation during welding process. The TMAZ is characterized by a highly deformed structure. Although the TMAZ underwent plastic deformation, recrystallization does not occur in this zone due to insufficient deformation strain. However, dissolution of some precipitates is observed, due to the high-temperature. The extent of dissolution depends on

Heat Affected Zone (HAZ) is present in fusion welding as well as in FSW processes. The wide of this zone is a direct function of the heat input and the thermal conductivity of the materials to be welded. Obviously the HAZ in FSW tends to be lower than that obtained in a fusion welding process. The HAZ is very important in welding of aluminum alloys, especially in alloys hardened by precipitation (artificial ageing), for instance 2024-T6, 2014- T6, 6061-T6 and 7075-T6. During artificial ageing in Al-Mg-Si alloys (6000 series), a high density of fine, needle-shaped β'' particles are formed uniformly in the matrix (aluminum, α). This precipitate is the dominating hardening phase, which is produced according to the

Super-Saturated Solid Solution (SSS) → Solutes clustering → GP zones (spherical) → β'' (needle)→β' (bar)→β

grain structure (Li et al., 1999)

**2.1.4 Heat affected zone** 

**2.1.3 Thermo-mechanically affected zone** 

the thermal cycle experienced by TMAZ.

following precipitation sequence (Dutta & Allen, 1991).

FSW is the tool rotation, as was reported previously (Sato et al., 2002).

However, since these precipitates are thermodynamically unstable in a welding process, the smallest ones will start to dissolve in parts of the HAZ where the peak temperature has been above the ageing temperature (> 160 ºC), while the larger ones will continue grow (Dutta & Allen, 1991). Close to the weld fusion line full reversion of the β'' particles is achieved. At the same time, coarse rod-shaped β' precipitates may form in the intermediate peak temperature range. This microstructural transformation is showed in Figure 17.

Fig. 17. TEM bright field images of microstructures observed in the ‹100› Al zone axis orientation after artificial ageing and Gleeble simulation (Series 1), a) Needle-shaped β'' precipitates which form after artificial ageing, b) Mixture of coarse rod-shaped β' particles and fine needle-shaped β'' precipitates which form after subsequent thermal cycling to *T*p = 315 ºC (10 s holding time), c) Close up of the same precipitates shown in b) above, d) Coarse rod-shaped β' particles which form after thermal cycling to *T*p = 390 ºC (10 s holding time) (Myhr et al., 2004)

### **2.2 Mechanical properties 2.2.1 Microhardness**

In order to determine the effect of the welding process in aluminum alloys, a common practice is to perform a microhardness profile in a perpendicular direction to the weld bead, as is showed in Figure 18. Standard Vickers measurements are conducted with an appropriate penetration force and time, i.e. 1 N and 15 s. The indentation is measured and the hardness is calculated applying equation 2:

$$HV = 1.8544 \frac{P}{d^2} \tag{2}$$

Welding of Aluminum Alloys 79

welded joint are well-defined. It is clear that in the soft zone (HAZ) the hardness number range is between 0.55 to 0.7 GPa. This soft zone results from the thermodynamic instability of the β" needle-shaped precipitates (hard and fine precipitates) promoted by the high temperatures reached during a fusion welding process. Indeed the temperatures reached during the welding process are favorable to transform the β' phase, rod-shaped, according

The individual mechanical behaviour of the base metal, weld metal, HAZ and welded samples in as welded condition for 6061-T6 aluminum welds by MIEA is shown in Figure 21

From Figure 21, it can be observed that the experimental results for the base metal are in agreement with nominal values found in the literature for 6061-T6 alloy (American Society for Metals Fatigue and Fracture, 1996). Also, the base metal exhibits the best mechanical properties and well defined proportional limit. The tensile properties of the sample obtained from the HAZ presents a 41% and a 19 % reduction of the ultimate strength with respect to the base metal and weld metal respectively. The loss of mechanical strength, commonly referred to as over-aging, when welding a 6061-T6 alloy is a fairly well understood phenomenon and it is explained in terms of the precipitation sequence. During welding, however, the base metal adjacent to the fusion line is subjected to a gradient of temperature imposed by the welding thermal cycle. At certain distance from the fusion line, the cooling curve crosses the interval of temperatures between 383 to 250 °C in which the β*'* phase, rodshaped, is stable. It is thus the transformation of β*''* into β*'* the responsible of the decrease in hardening of the α matrix due to the incoherence of the β*'* phase caused by the

On the other hand, in the case of FSW for 6061-T6, the same effect (over-ageing) is observed, although in this case the welded specimens represents an ultimate strength of 70% of the base material (Moreira et al., 2007). The conventional stress-strain curves are presented in

to the transformation diagram for the 6061 alloy.

Fig. 20. Vickers hardness mapping over the welded zone

thermodynamic instability of β*''* in a welding process.

**2.2.2 Tensile properties** 

Figure 22.

as stress as function of strain graph.

where *HV* is expressed in MPa if *P* is given in N and *d*, the indent diagonal, in mm.

Fig. 18. Mesh definition for classical Vickers indentation measurements

Microhardness measurements give a general idea of the microstructural transformations and the variation of the local mechanical properties (Ambriz et al. 2011) produced after a welding process in aluminum alloys.

Fig. 19. Microhardness profile in aluminum alloys welds, a) 6061-T6 alloy welded by MIEA and b) 6082-T6 alloy welded by FSW (Moreira et al., 2007), note that 1HV=9.8×10-3 GPa

Figure 19 presents the Vickers hardness number profile in two different aluminum alloys welds obtained by MIEA and FSW. In both cases a significant difference for the hardness number of the weld material and HAZ with respect to the base material (∼ 1.05 GPa or 107.1 HV0.1) is observed. Also, at the limit between the HAZ and the base metal, we note the presence of a soft zone which is formed nearly symmetrically in both sides of the welded joints. It should be note that the hardness obtained in this zone represents roughly 57 % of the hardness number of the base material. This seems to indicate that the tensile mechanical properties after welding process will be greatly different. Figure 20 visualizes the location of the soft zone highlighted by the Vickers hardness profile represented in Figure 19a, by means of a hardness mapping. In this figure, the hardness values for each zone of the

Weld HAZ HAZ

70 mm

where *HV* is expressed in MPa if *P* is given in N and *d*, the indent diagonal, in mm.

2 mm

Fig. 18. Mesh definition for classical Vickers indentation measurements

(a) (b)

Fig. 19. Microhardness profile in aluminum alloys welds, a) 6061-T6 alloy welded by MIEA and b) 6082-T6 alloy welded by FSW (Moreira et al., 2007), note that 1HV=9.8×10-3 GPa

Figure 19 presents the Vickers hardness number profile in two different aluminum alloys welds obtained by MIEA and FSW. In both cases a significant difference for the hardness number of the weld material and HAZ with respect to the base material (∼ 1.05 GPa or 107.1 HV0.1) is observed. Also, at the limit between the HAZ and the base metal, we note the presence of a soft zone which is formed nearly symmetrically in both sides of the welded joints. It should be note that the hardness obtained in this zone represents roughly 57 % of the hardness number of the base material. This seems to indicate that the tensile mechanical properties after welding process will be greatly different. Figure 20 visualizes the location of the soft zone highlighted by the Vickers hardness profile represented in Figure 19a, by means of a hardness mapping. In this figure, the hardness values for each zone of the

Indentations

0.5 mm

Microhardness measurements give a general idea of the microstructural transformations and the variation of the local mechanical properties (Ambriz et al. 2011) produced after a

Base metal Base metal

9.5 mm 8 mm

welding process in aluminum alloys.

welded joint are well-defined. It is clear that in the soft zone (HAZ) the hardness number range is between 0.55 to 0.7 GPa. This soft zone results from the thermodynamic instability of the β" needle-shaped precipitates (hard and fine precipitates) promoted by the high temperatures reached during a fusion welding process. Indeed the temperatures reached during the welding process are favorable to transform the β' phase, rod-shaped, according to the transformation diagram for the 6061 alloy.

Fig. 20. Vickers hardness mapping over the welded zone
