4 Conclusions
In the present investigation, the AA6061 DS-FSW joint was fabricated. In order to improve the fatigue performance of the SZ, the joint was rolled to different thickness. The following major conclusion can be drawn from this experimental investigation and discussion.
(1) After welding, the fatigue life of SZ was remarkably shortened compared with that of BM. The weak dislocation strengthening effect contributed to the poor YS and UTS which contributed to the shortest life of as-welded SZ region.
(2) After rolling process, the dislocation strengthening effect was greatly improved. The hardness, YS, and UTS were gradually improved. The fatigue life of rolled SZ was gradually extended. When the plate was rolled to 3 mm and 2 mm, the fatigue life of SZ became longer than that of the original BM.
(3) The fracture of the SZ had three steps. The first was the crack initiation based on the debonding of lamellar structure. The second was the fatigue crack propagation. It contributed to the formation of fatigue striation on the fractured surface. The third was the complete fracture. When the crack in SZ expanded to a certain degree, the residual connection could not support the high stress. The instantaneous fracture occurred. In some SZ samples, the gradual fatigue crack propagation was not occurred as the instantaneous crack propagation occurred in the SZ which resulted in the instantaneous elongation.
(4) Rolling process was not able to make the lamellar structure disappear. Lamellar structure was consisted of thick coarse grain layer and thin fine grain layer. The thickness of fine grain layer was only approximately 10 μm. The thin fine grain layer had more GB and HAGB. It contributed to its greater hindering effect on dislocation. During fatigue test, the cracking tended to occur in fine grain layer as its serious hardening.