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.