# Spectroscopic Approach for the On-Line Monitoring of Welding of Tanker Trucks
## 3 Experimental Results and Discussion
### In-Line Production Analysis  
The previously discussed weld trials were necessary in order to perform an initial calibration of the system, especially with respect to the viability of detecting different perturbations and/or defects. However, such trials are typically far from the real production conditions, even if the same production line is employed. In the literature, it is quite common to find validations of different monitoring approaches by means of experimental tests or weld trials, for example, by simulating porosities [11], perturbations in the gas flow rate [28] and oxidation, slag or metal inclusions [29]. However, it is more difficult to find works in which the proposed solutions have been tested in real field tests in real production scenarios. The monitoring system, designed by SADIQ Engineering S.L. and the Photonics Engineering Group (University of Cantabria), described at the beginning of Section 2 has been used for months in the manufacture of tanks for tanker trucks in the facilities of Talleres Cobo Hnos S.L. in Guarnizo, Spain, and is presented in Figure 6 as it was installed in the production line under analysis. During this time, the system has stored all the performed seams and the results have been validated with X-ray inspection, especially when some specific problems have been detected. One problem in this regard lies in the requirement imposed by the associated standard of only using the X-ray inspection for a reduced length of the performed seams. However, as will be discussed below, the occurrence of specific defects is rather limited, and defective events are typically associated with contamination that implies porosities distributed along the seam. Figure 7 depicts the results derived from the analysis of the seams associated with three different tankers developed on different days. Each tanker is formed by up to 14 different seams, with a total welding length of more than 45 m. The vertical dotted lines in the graphs indicate the separations between different seams, depicted sequentially. Figure 7b,d,f show the corresponding H I profiles which serve as indications of the occurrence of atmospheric contamination or porosities derived therefrom. It can be appreciated that the level of H I participation, calculated as the ratio of the integration of the H I line at 656 nm to an average of the Al lines identified in the spectra, increases from Tanker #2 to #1, and becomes quite significant for Tanker #3. Analysis of the X-ray inspections of these seams revealed a very good correlation, detecting several porosities for Tankers #2 and #3. In fact, an analysis performed a posteriori on the data acquired for the system indicated that the welding for Tanker #3 had been performed with a protection gas flow rate of 16 L/min, inferior to the standard value of around 20 L/min, which explains the high contamination that can be seen in Figure 7f. It is also interesting to point out how in this case (Figure 7e) the electrical parameters, both the welding current and voltage, exhibit a noisier profile in comparison to the rest of the seams. Figure 7a,c,d present the welding voltage (y \(\approx\) 1), current (y \(\approx\) 2), gas flow rate (y \(\approx\) 3) and wire feed speed (y \(\approx\) 4) for these three tankers, with their corresponding values normalised to allow a suitable representation. It is also remarkable how, in spite of the low contamination detected, the voltage and current profiles are quite unstable in Figure 7c for Tanker #2. This might be explained by reference to other defects, such as lack of penetration, that were detected in this case by the X-ray inspection. Although the occurrence of defects apart from porosities is low, Figure 8 presents an example of a crack detected via X-ray in Seam #222940 (Tanker #2). As already discussed, while H profiles are suitable to detect the appearance of porosities, other defects might not be reflected in them.