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.