Microstructural analysis
Figure 4 shows the optical micrographs of the base materials and friction stir processed FSWed joints. The AA6082 base material showed an elongated coarse grain structure with mean grain diameters of 36.08 µm leaning towards the rolling direction (Figure 4(a)). The AA8011 base material microstructure also showed coarse elongated grains with a mean grain size of 29.36 µm. The microstructure of the FSPed AA6082/AA8011 and AA8011/AA6082 joints consisted of three zones namely TMAZ, the heat affected zone (HAZ) and NZ. It was observed that thegrain size was not affected at the HAZ region post FSP. The TMAZ grain structure consisted of deformed and elongated grains which are as result of the processing tool mechanical action [29]. The TMAZ is generally wearker than the nugget zone but stronger than the HAZ region due to grain size variations [11]. The nugget zones of both joints consisted of fine and equiaxed grain structure. This was due to the high temperature experienced by the NZ and the dynamic re-recrystallization which occurred during the processing [11, 25, 36]. The AA6082/AA8011 friction stir processed FSWed joint had a mean grain diameter of 3.98 µm while the AA8011/AA6082 was 5.87 µm.