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.