Issue 35

Y. Besel et alii, Frattura ed Integrità Strutturale, 35 (2016) 295-305; DOI: 10.3221/IGF-ESIS.35.34 298 higher heat input is expected for weld-480 than weld-720. Indeed, the maximum temperatures measured at 7 mm away from the joint line were 377 and 339 °C for weld-480 and weld-720, respectively. The distribution of the JLR of weld-480 was wider on both advancing side (AS) and retreating side (RS) than that of weld- 720. As marked with the rectangle I in Fig. 3(a), the JLR was extended towards the edge of SZ on the retreating side. In weld-720 (lower heat input), the JLR distributed more around the weld center in SZ (Fig. 3(b)). This spatial distribution of JLR serves as an indicator for the degree of material flow and mixing during the FSW process. Seidel and Reynolds visualized the material flow in the weld using marker insert technique and observed increased mixing in the weld with a decreasing weld pitch which is defined as the tool advance per rotation [20]. They suggested that the higher heat input might result in the softening of a greater amount of material, and consequently more material flow in the weld zone. In our study, comparing the distribution of the JLR in each weld, it can be said that greatest material flow was brought about in weld-480, i.e. higher heat input. This finding is consistent with the results of Seidel and Reynolds. (a) weld 480 (b) weld 720 (c) stir-in plate weld Figure 3 : Macroscopic weld structure. Hardness and tensile test results Vickers hardness profiles of the different welds are shown in Fig. 4. In all weld conditions, softening occurred in SZ and hardness was mostly recovered in the neighboring TMAZ. The hardness in SZ of weld-720 was higher than that of the other two welds and the lowest hardness was observed in weld-480. This order reflects inversely the order of the heat input: i.e. higher heat input brought about greater softening in SZ. Tensile test results are summarized in Tab. 2. Fracture occurred in SZ in all welds. Tensile strength and yield stress of weld-480 are lower than those of weld-720. In particular, the degradation in elongation of weld-480 is significant: the reduction is more than 30% compared to weld-720, although SZ (i.e. the fracture site) in weld-480 was softer than weld- 720. Weld ID E-modulus [MPa] 0.2% yield stress  0.2 [MPa] Tensile strength  TS [MPa] Elongation  max [%] weld-480 70793 218 311 4.61 weld-720 72182 237 329 6.77 Table 2 : Tensile test results.