FSW made a sound joint without voids, cracks or distortion.
Fig. 2 shows the macrostructure of the welded
joint and Fig. 3 illustrates details of the variation of
the microstructure corresponding to regimes indicated
by (a)–(d) in Fig. 2. The microstructure of the welded
joint is formally divided into four zones [12]: base material,
heat affected (HAZ), thermo-mechanically affected
(TMAZ) and stirred zone.
As shown in Fig. 3, from the base material to the
weld centre, the crystal grains diminish gradually. The
fine texture of base material has been slightly elongated
in the rolling direction, and the size of crystal grain
approximates to 35 lm (Fig. 3(a)). The weld nugget is
composed of fine-equiaxed recrystallized grains, which
are formed under the high temperature and high rate
of deformation in the weld nugget due to the pins stirring
[13], and the size of the crystal grain is about
8 lm (Fig. 3(c)). The TMAZ microstructure consists of
bent elongated grains near the nugget shown in the
micrograph (b) and (d), and divided into the retreating
side (RS) (Fig. 3(b)) and the advancing side (AS)
(Fig. 3(d)) depending on different microstructures.
Although both the advancing side and the retreating
side have a boundary with the weld nugget, the boundary
in the advancing side is clearer than that in the
retreating side.
3.2. Fatigue properties
Fatigue test results show that most failures