3.3 Strengthening mechanism of SZ
The grain orientation image of AA6061 BM is shown in Fig. 4 (a). After rolling process, the grain orientation images of shoulder affect zone (SAZ) and pin affect zone (PAZ) are shown in Fig. 4 (b) and (c). It was observed that the grain was remarkably refined from approximately 67 μm to approximately 1.6 μm after FSW. After rolled to 3 mm, the grain of PAZ and SAZ was slightly coarsened as shown in Fig. 4 (d) and (e).
The fine grain of as-welded SZ was attributed to the stirring effect from stir tool at the temperature lower than melting point. During rolling process, the grain rotation occurred[23-26]. With the effect of stress from rolls, the fine grain tended to rotate to a certain orientation. Therefore, the fine grains with a similar orientation merged and generated the coarse grain [22]. However, the grain size of rolled SZ has not been greatly improved.