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S. S. Nayak et al.
here consisting of more subgrains and having more strain and dislocation defects.
Therefore, it is having high strength.
Figure 6.7a–c shows the EBSD IPF maps of different weld zones of the aluminium
substrate. It can be noticed that the grain boundary characteristics and their orientations have been changed in the FSWed regions. The HAZ has equiaxed and coarser
microstructures. The average aspect ratio of the microstructures was 0.55. The grain
size varied in a range of 2.5–36.30 µm and the average size was calculated as 19.6 µm.
The SZ of the weld confined of elongated microstructures with an average aspect
ratio of 0.49. The grain size varied in the range of 2.5–30 µm with an average value
of 12.2 µm. Histogram plot of base and welded regions’ grain size in aluminium
substrate are shown in Fig. 6.7g.
Further, the EBSD map of the transition region (i.e. TMAZ) between SZ and
HAZ revealed similar microstructures as the SZ. The average values of grain size and
their aspect ratio in TMAZ are 13.8 µm and 0.52, respectively. Thermo-mechanical
action during FSW causes the elongated microstructures at SZ and TMAZ. However,
frictional heat is the cause for the formation of equiaxed grains at HAZ.
FSW-led recrystallization and grain growth of the microstructures have been
explained in the subsequent section. Heat and deformation of the FSW modified
the microstructure size [43] and their orientations. Both are responsible for the DRX
and recovery in the aluminium substrate. Deformation due to the tool causes defects
(dislocations, point defects) in the lattice structure of aluminium. Dynamic recovery
occurs when an excessive amount of dislocations or point defects form in weld
regions. Later, dislocations get tangled and form a cell wall, which is the early sign
for the recovery. Further, as the welding proceeds, it adds dislocation and heat into
the lattice structure which makes easy cross-climb and slips for the dislocations. It
leads to the annihilations of dislocations into the existing cell walls, which later turn
into LAGBs and subgrains (misorientation angle <15°). The driving force for the
recovery is the reduction in strain energy (i.e. dislocations and point defects) of the
weld material [44]. Recovery forms the serrated grain boundaries which later gets
converted into strain-free grains after DRX. Recrystallization and grain growth take
place after recovery. The available dislocations and LAGBs in the weld region drive
both recrystallization and grain growth in the annealing stage of the weld.
Recrystallized and subgrains can be seen in IQ maps of weld regions depicted in
Fig. 6.7d–f. Grains having misorientation angle 2°–15° and beyond have been taken
as LAGBs and high angle grain boundaries (HAGBs), respectively. Grains having
misorientation angle 2°–5° are known as very low-angle boundaries as shown in
bottom right corner of Fig. 6.7. This division is based on the extent of misorientation between two grains. In simple words, LAGBs are comprised of an array of
dislocations and their properties are a function of misorientations, while HAGBs
are normally not independent of misorientation. The term min. denotes to minimum
misorientation angle and the term max. denotes to maximum misorientation angle
of the grain boundary as shown in bottom right corner of Fig. 6.7.
The recrystallized grains have been indicated by the square box and subgrains in
red and blue colour boundaries are circled in HAZ, TMAZ and SZ. After welding,
S. S. Nayak et al.
here consisting of more subgrains and having more strain and dislocation defects.
Therefore, it is having high strength.
Figure 6.7a–c shows the EBSD IPF maps of different weld zones of the aluminium
substrate. It can be noticed that the grain boundary characteristics and their orientations have been changed in the FSWed regions. The HAZ has equiaxed and coarser
microstructures. The average aspect ratio of the microstructures was 0.55. The grain
size varied in a range of 2.5–36.30 µm and the average size was calculated as 19.6 µm.
The SZ of the weld confined of elongated microstructures with an average aspect
ratio of 0.49. The grain size varied in the range of 2.5–30 µm with an average value
of 12.2 µm. Histogram plot of base and welded regions’ grain size in aluminium
substrate are shown in Fig. 6.7g.
Further, the EBSD map of the transition region (i.e. TMAZ) between SZ and
HAZ revealed similar microstructures as the SZ. The average values of grain size and
their aspect ratio in TMAZ are 13.8 µm and 0.52, respectively. Thermo-mechanical
action during FSW causes the elongated microstructures at SZ and TMAZ. However,
frictional heat is the cause for the formation of equiaxed grains at HAZ.
FSW-led recrystallization and grain growth of the microstructures have been
explained in the subsequent section. Heat and deformation of the FSW modified
the microstructure size [43] and their orientations. Both are responsible for the DRX
and recovery in the aluminium substrate. Deformation due to the tool causes defects
(dislocations, point defects) in the lattice structure of aluminium. Dynamic recovery
occurs when an excessive amount of dislocations or point defects form in weld
regions. Later, dislocations get tangled and form a cell wall, which is the early sign
for the recovery. Further, as the welding proceeds, it adds dislocation and heat into
the lattice structure which makes easy cross-climb and slips for the dislocations. It
leads to the annihilations of dislocations into the existing cell walls, which later turn
into LAGBs and subgrains (misorientation angle <15°). The driving force for the
recovery is the reduction in strain energy (i.e. dislocations and point defects) of the
weld material [44]. Recovery forms the serrated grain boundaries which later gets
converted into strain-free grains after DRX. Recrystallization and grain growth take
place after recovery. The available dislocations and LAGBs in the weld region drive
both recrystallization and grain growth in the annealing stage of the weld.
Recrystallized and subgrains can be seen in IQ maps of weld regions depicted in
Fig. 6.7d–f. Grains having misorientation angle 2°–15° and beyond have been taken
as LAGBs and high angle grain boundaries (HAGBs), respectively. Grains having
misorientation angle 2°–5° are known as very low-angle boundaries as shown in
bottom right corner of Fig. 6.7. This division is based on the extent of misorientation between two grains. In simple words, LAGBs are comprised of an array of
dislocations and their properties are a function of misorientations, while HAGBs
are normally not independent of misorientation. The term min. denotes to minimum
misorientation angle and the term max. denotes to maximum misorientation angle
of the grain boundary as shown in bottom right corner of Fig. 6.7.
The recrystallized grains have been indicated by the square box and subgrains in
red and blue colour boundaries are circled in HAZ, TMAZ and SZ. After welding,