**2.1 Microstructure**

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 Figure 10.

Fig. 10. Principal zones in welding of aluminum, a) MIEA welding technique (Al-6061-T6) and b) FSW (Al-7075-T651)
