# Spectroscopic Approach for the On-Line Monitoring of Welding of Tanker Trucks
## 3 Experimental Results and Discussion  
### Field Trials  
An initial analysis was performed via welding trials in the production line under analysis to study the performance of the monitoring system. Nine different trials were designed by trying to perform correct seams as well as modifying different process parameters. Detailed information about these tests is presented in Table 2.  
As can be observed, the first two seams were performed with standard parameters and without perturbations intentionally applied to the process. However, some defects appeared during the X-ray off-line analysis. It is worth mentioning that all these defects were considered to be acceptable in terms of the associated standard for the tanker trucks. It is especially relevant that some porosities appeared in Weld Trial #2. Figure 3 shows the results of the studies performed with the captured plasma spectra. Figure 3a depicts the plasma RMS and color temperature profiles associated with both seams. It can be observed that, although there are some subtle variations, especially in Trial #2, the presence of defects is not clearly indicated by perturbations in these signals. This situation changes when H lines are employed to generate the output profiles. In Figure 3c,d, the ratio of the H I (hydrogen neutral atom) (656 nm) line to the Al I line at 396 nm is used. It is worth noting that, in this case, a clear difference appears between the results derived from weld Trials #1 and #2. The higher contribution of the H I line in Trial #2 seems to be correlated with the appearance of porosities in the seam detected by the X-ray inspection. Finally, Figure 3e,f show similar analyses, though using the ratio of Na I (589 nm) to Al I (670 nm). Again, the higher Na I contribution can be correlated with the detected porosities. \begin{table}
\begin{tabular}{c c c} \hline
**Weld Trial N\({}^{\circ}\)** & **Study Parameter** & **Defects (X-ray)** \\ \hline
1 & Correct welding & Undercuts/Ex. penetration \\
2 & Correct welding & Undercuts/Porosities \\
3 & Water inclusion & Undercuts/Porosities \\
4 & Oil inclusion & Undercuts/Porosities \\
5 & Solvent inclusion & Porosities/Ex. penetration \\
6 & Welding current/Feed Wire & Porosities \\  & Speed & Porosities \\
7 & Gas flow rate/Dry & Undercuts/Porosities \\  & compressed air & Undercuts/Porosities \\
8 & Joint preparation/Stand-off & – \\
9 & & \\  & & \\  & & \\ \end{tabular}
\end{table} 
Table 2: Welding trials. The experimental conditions for correct welding were: welding current, 220 A; process speed, 400 mm/min; protection gas flow rate, 22 L/min. In terms of the study derived from Figure 3, an interesting analysis can be performed with Trials #7 and #9 and the results shown in Figure 4. The former trial was performed with a lower protection gas flow rate (reduced from 22 to 20 L/min), and a dry compressed air torch was employed at the end of the seam to simulate a greater perturbation in the process. Trial #9 was designed to study the influence of process speed in the process dynamics: a higher speed was applied at the beginning, then it was reduced, and in the last half of the seam both water and oil were applied in the seam backing. Figure 4 presents the results derived from the analysis of Trial #7 and the final section of Trial #9, associated with contaminants in the weld backing. Regarding Trial #7, it might be expected that the H I contribution should be significant, considering the gas flow rate reduction. However, it seems that 20 L/min, although 2 L/min under the standard gas flow rate, is a sufficient condition for the measurement of the pressure. Figure 3: Output monitoring profiles for Weld Trials #1 and #2 using different spectroscopic parameters. Plasma RMS signal (thick blue line) and color temperature (thin green line) associated with (**a**) Trial #1 and (**b**) Trial #2. Ratio of H I line (656 nm) to Al I line (396 nm) (thick blue line) and 670 nm (thin green line) associated with (**c**) Trial #1 and (**d**) Trial #2.