# Spectroscopic Approach for the On-Line Monitoring of Welding of Tanker Trucks
## 2 Materials and Methods  
Taking into account the specific requirements of the process under analysis, a customised monitoring system was designed and developed to perform a suitable on-line monitoring of welding quality in the production line operating in the facilities of Talleres Cobo. All the data obtained by the sensors were captured by a computer running custom-developed software that stores data in a binary format with associated timestamps for each capture. The main sensor of the system is based on plasma optical spectroscopy and is complemented with traditional sensors to monitor the electrical, mechanical or environmental parameters of the process. Figure 1 is a scheme of the monitoring system, including the plasma optical spectroscopy-based system, as well as the welding voltage and current (closed loop Hall effect), wire feed speed (encoder-based, 800 pulses per revolution), protection gas flow rate (with temperature compensation), welding speed and position (encoder-based, 800 pulses per revolution) and ambient temperature and humidity sensors. The welding process has been illustrated in Figure 2.  
Figure 1: Schematic representation of the designed welding monitoring system incorporating the following sensors: (1) welding voltage, (2) welding current, (3) plasma optical spectroscopy-based, (4) protection gas flow rate, (5) wire feed speed, (6) welding speed and position and (7) temperature and humidity sensors. Figure 2: Images of the MIG welding process of Al-Mg alloy sheets. (**a**) Detail of the welding torch and the optical fiber used during the trials. (**b**) Preparation of the sheets before the welding process. The MIG (Metal Inert Gas) process has been designed to join sheets of aluminium alloys, in particular, AA5186 (H111), which contains a high level of magnesium. Table 1 shows the chemical composition of this material. This composition has been directly obtained from a certificate provided by the manufacturer of the alloys (Hydro). The main information for the on-line monitoring system was obtained via welding plasma optical radiation, which is captured by means of an optical fiber (Ocean Optics silica, 400 \(\upmu\)m core diameter) like the one depicted in Figure 2a. It is worth mentioning that in this case there were no input optics attached at the end of the fiber, i.e., the plasma radiation was directly acquired by the optical fiber and delivered to a CCD spectrometer located within the monitoring system's control unit. Although the numerical aperture of the chosen optical fiber limits the field of view, the plasma radiation captured by the spectrometer is sufficient for the performance of a suitable spectroscopic analysis. The spectroscopic studies were carried out with a low-cost spectrometer (Ocean Optics STS-VIS) with an optical resolution of 1.5 nm and a spectral range of 340 to 820 nm. An initial characterization of the optical response of the setup was performed, allowing an equalization of the spectra, thus avoiding the effects of the different attenuations of the optical fiber at different wavelengths, for example. The spectrometer was configured with an integration time of 42 ms. Given a welding speed of 400 mm/min, we obtained a spatial resolution of 0.28 mm for the analysis. The use of plasma optical spectroscopy for welding diagnostics is based on the correlation that exists between certain spectroscopic parameters and the quality of the associated seams. The profiles associated with these parameters tend to exhibit clear perturbations associated with specific defects, such as crater formation [11], lack of penetration [19], metallic inclusions [20] or porosities [21]. They may also be used to detect the operational mode (conduction/keyhole) of a laser welding process [22]. Plasma electronic temperature (Te) has been used in this way by several authors, such as [PERSON] et al. [23] and [PERSON] et al. [24].