13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- However the hardness value in the outer-HAZ was slightly lower than that of the BT9 BM, meaning that a narrow soft zone appeared due to the appearance of β phase as shown in Fig.1(c). The aging treatment at 550C (Fig.4(a)) led to the elimination of the soft zone at the BT9 side and a slightly higher hardness in the FZ. The increase of hardness was mainly due to the presence of the fine α′ and precipitation of some secondary α phase in the β phase both in FZ and HAZ. These results are similar to those reported by Kabir et al. [9]. In addition, the microhardness values in the HAZ were also observed to decrease gradually as the distance increased from the FZ border line. This was attributed to the decreasing fine α′ and secondary α phase in the HAZ. However, after STA the hardness value in the FZ and HAZ was only slightly higher than or nearly the same as that of the BMs due to the coarse lamellar α + β structure (Fig.4(b)). It is of special interest to observe that the narrow soft zone in the outer-HAZ on the BT9 side disappeared as well after STA due to the decomposition of β phase. Figure 3. Microstructure of the post-welded joint after STA (a) HAZ of Ti-6Al-4V and FZ, and (b) FZ and HAZ of BT9 Figure 4. Hardness distribution of the welded joint in (a) as-welded and aging conditions, and (b) STA condition 3.3 Hysteresis loops The hysteresis loops of the first cycle and mid-life cycle at a total strain amplitude of 1.2% are plotted in Fig 5(a) and (b), respectively. It is seen that the initial tensile (or ascending) phase of the first quarter cycle represented indeed a tensile stress-strain curve tested at a strain rate of 1×10-2 s-1. Then the tensile yield strength (YS) of the post-welded joints can be determined from the initial 200 300 400 500 -9-6-3 0 3 6 9 Hardness, HV Distance from the weld centreline,mm As-welded joint Aging Ti-6Al-4V BT9 HAZ FZ HAZ 200 300 400 500 -8-6-4-2 0 2 4 6 8 Hardness, HV Distance from the weld centreline,mm STA BT9 Ti-6Al-4V FZ HAZ HAZ HAZ of Ti-6Al-4V FZ FZ HAZ of BT9 (b) (a) (a) (b)
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