Fig. 3 Hardness of FSW joint: (a) Hardness distribution, (b) Hardness of
BM and SZs; Tensile properties of BM and SZs: (c) Stress-strain curves,
(d) Strength vs. elongation
The stress-strain curves of BM and SZs are shown in Fig. 3 (c). The
yield strength (YS), ultimate tensile strength (UTS), and elongation of
the BM and SZs were measured and are plotted in Fig. 3 (d). It was found
that the YS and UTS of the as-welded SZ was 197 MPa and 229 MPa,
respectively, and was smaller than that of BM. After rolling process,
the YS and UTS gradually increased to 352 MPa and 369 MPa, respectively.
Moreover, after rolled to 4 mm, the YS and UTS were all higher than that
of BM. The elongation of the as-welded SZ was approximately 20% and was
lower than that of BM (26%). After rolling process, the elongation of
SZ with a thickness of 2 mm gradually reduced to 6%.
Based on the hardness, strength, and fatigue life of BM and SZs. It was
concluded that the improvement in hardness and strength has a remarkable
influence on the fatigue performance of SZ. After rolled to 4 mm, the
hardness and strength of SZ has been remarkably enhanced compared with
the as-welded SZ. Therefore, the fatigue performance of SZ with a
thickness of 4 mm was also enhanced. However, in despite of the high
hardness or strength of SZ with a thickness of 4 mm compared with that
the original BM, the fatigue life of the SZ (4 mm) was still shorter
than that of BM. It would be discussed in section 3.5 .
When the SZ was rolled to 3 mm and 2 mm, the hardness and strength were
enhanced again compared with the SZ with a thickness of 4 mm. The
fatigue performance was also deeply enhanced and better than that of the
original BM.