3.1. Hardness and tensile properties tests
Fig. 3 shows the Vickers hardness arrangement in cross section of BM and FSWed joint. It reveals that an entire drop of hardness occurred after FSW as compared with BM. A roughly symmetrical micro-hardness distribution could be seen in welded joint, and two well pronounced softening bands with a distance of 10 mm from the joint center located in AS-HAZ and RS-HAZ, respectively. The softening band in AS-HAZ was about 5 mm in breadth, which was wider than the other one in RS-HAZ (3 mm). The average values of hardness in BM, AS-HAZ, RS-HAZ and WNZ were 186 HV, 124 HV, 132 HV, and 158 HV, respectively.