Figure 9 Thermal histories in correspondence of thermocouples: A, TC1; B, focus on TC1; C, TC2 and D, focus on TC2.
Figure 9 shows a good agreement between numerical and experimental results as general trend. In order to better highlight the differences between numerical and experimental curves, thermal histories curves at TC1 and TC2 have been cut at a time of 550 s and reported in Figures 9B and 9D, respectively. According to Figures 9A and 9B, the FE model provides, in correspondence of TC1, a small over-estimation of about 23 °C in the first peak value, corresponding to the first pass. This can be due to a displacement of the thermocouple within the plate hole, occurred during the first pass, leading to a not perfect contact between the thermocouple and the plate. This difference cannot be found during the second pass because the correct position of all thermocouples was checked at the end of the first pass. Moreover, according to Figure 9B, a time shift can be observed between numerical and experimental data. Such disagreement can be attributed to the welding speed, not perfectly constant along the whole weld seam, but, in proximity of thermocouple TC1, faster than the value reported in Table 3. As matter of the fact, the welding speed reported in Table 3 has been calculated as the ratio of the time spent for the first pass to the length of the weld seam.
Figure 10 shows the comparison of the numerical results with the experimental ones in correspondence of thermocouples located at the middle section of the plate during the whole process. Also here, in order to highlight the differences between numerical and experimental results, curves have been cut at 550 s and reported in Figures 10B, 10D, 10F and 10H. Excluding the initial experimental values of the curves, which are affected by high noise due to the electrical shock at the beginning of the welding process, it can be stated that the experimental results are in good agreement with the numerical ones. It can be observed that the FE model provides a small over-estimation of about 17 °C in correspondence of the second peak, during the second pass, at TC3 (Figure 10B), while there are slight differences in the peak values during the first pass at TC4 (Figure 10D) and TC6 (Figure 10H), characterized by an under-estimation of the numerical results. However, these differences are lower than about 5 %.