**2.1.2 Nugget zone in FSW**

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 zone, as is presented in Figure 15.

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 large upward movement of material

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- T651 aluminum alloy.

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,

Welding of Aluminum Alloys 77

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)

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

> 1.8544 <sup>2</sup> *<sup>P</sup> HV*

*d*

= (2)

(Myhr et al., 2004)

**2.2 Mechanical properties 2.2.1 Microhardness** 

the hardness is calculated applying equation 2:

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

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 grain structure (Li et al., 1999)
