3.2. OM, EBSD and TEM observation
Fig. 5 shows the OM structure of welded joint. A homogeneous microstructure formed in WNZ and less obvious “S-line” defects can be observed (arrowed in the figure). A distinction among WNZ and HAZ can be seen from the significant variation of grain size. Both AS-HAZ and RS-HAZ maintained the similar pancake-shape grain structure as BM, indicating that FSW heating input had little or no influence on the grain structure in HAZ. Between WNZ and HAZ, there was a transition zone, thermo-mechanically affected zone (TMAZ) characterized by a highly deformed rheological structure due to the effects of shoulder-driven flow and pin-driven flow. A clear boundary between TMAZ and HAZ can be found in AS of joint, while the boundary in RS was not clear. According to the microhardness arrangement map shown in Fig. 3, the softening band with minimum hardness value did not located in TMAZ but in AS-HAZ. Therefore, in current study, attention was focused on BM, WNZ and softening band in AS-HAZ.