## 2 Materials and Methods  
In this work, the welding of two austenitic stainless-steel (X6 CrNiTi18-10) low thickness tubes were carried out in an automated manner. The welding process was implemented following the European standard EN-10296-2:2005 [30], which is the specific standard of reference in this type of stainless-steel welded tubes, in order to generate different specimens for the two types of tubes. This European standard mainly specifies the mechanical properties of the welds, the chemical compositions of the materials, and the tests and criteria to ensure the quality and the geometrical features of the welds. For this research, a specific flow-work (Figure 1) was ad hoc established to analyze and optimize the essential parameters that affect the quality of the weld such as the weld root penetration and the welding bead width. This process is repeated twice (experiments 1 and 2). The specimens were manufactured specifically for this research using welding. The union was made between tubes of 63.1 mm (Figure 2a) and 63.8 mm (Figure 2b) in diameter, both with 1.5 mm of thickness. The tubes were fixed in welding assembly jig before the welding procedure, as can be seen in Figure 2c. In total, 18 welds were made: 9 for the first experiment and 9 for the second experiment. The experiment was performed twice to increase statistical validation. The tubes were joined using the GMAW process, for which a welding machine SKS LSQ5 + Q8p was used together with a Fanuc Arc Mate 120 iC cell, SKS P5S Uncoiler, Collision Box Power Joint SKS, Torch SKS 61-9-120-10, and Neck SKS 58-1-45-400. The robot speed established for this work, after several experimental tests to define this parameter, was 150 cm/min because it was the highest speed where a visually stable weld bead was observed. The shielding gases used during the process were Argon and O\({}_{2}\), with percentages of 98% and 2%, respectively. The additional material was an ESAB brand austenitic steel with 1 mm diameter, and the stick-out value adopted was 12 mm. As input parameters, the welding voltage, travel angle, and welding current were selected for this study. Figure 3 shows, schematically, the travel angle. Figure 1: Flowchart of the experimental work followed for this research. Figure 2: (**a**) Tube 63.1 mm in diameter, (**b**) tube 63.8 mm in diameter, and (**c**) welding position of the two tubes. Figure 3: Travel angle. Before carrying out the welding process, the tubes were measured to ensure that they were compliant. Their welding regions were then cleaned with, firstly, sandpaper, then a cotton cloth, followed by a solvent (acetone and ethylic alcohol), and, lastly, a clean cotton cloth. Finally, they were fixed in welding assembly jig as shown in Figure 2c. After joining the pieces, the macrographic test was performed in three different regions to identify the quality parameters (width and depth of the weld bead). For this, cuts were made in the weld beads at 45\({}^{\circ}\), 165\({}^{\circ}\), and 285\({}^{\circ}\) angles with a conventional cutting machine as shown in Figure 4.  
In the next step, the samples were polished in an LS1 remote polishing machine using sandpapers with 180, 320, and 600 grits. Finally, a chemical attack was carried out for better visualization of the cutting region of the weld bead. For the chemical attack, a solution of hydrochloric acid with nitric acid was used in a proportion of 1:10. Once this task was completed, 45 samples were obtained out of a possible 54. Some samples were discarded due to imperfections. The final samples were photographed in high resolution, avoiding lens distortions and aberration, and then evaluated by StructureExpert Weld from Struers [31]. The software identified the weld bead thickness and weld penetration depth among other parameters. Based on the data revealed by the StructureExpert Weld integrated software and the input parameters, the Taguchi method was applied to find the most relevant parameter.