# Parameter Optimization of Orbital TIG Welding on Stainless Steel Pipe
## 1 Introduction  
TIG welding is an arc welding process with a non-melting electrode [1]. TIG welding with the welding head to make a circular motion around the arc weld is called orbital welding, as shown in Figure 1. Orbital TIG welding is widely used in industries to weld pipes to transport materials such as oil, gas, and other liquids [2, 3, 4, 5]. Therefore, the strength of the orbital welding pipes is critical to prevent them from leaking under high pressure [6, 7, 8, 9]. There will be numerous risks to the public and environment if weld quality control is not ensured [10].  
Heat input plays a critical role in forming welding joints during the welding process. A suitable heat input parameter will give the weld complete penetration [11, 12, 13, 14]. If the heat input is too high, it will cause the weld to overheat, burn, and be of poor quality [15]. Conversely, if it is too low, the base metals will not be molten enough, leading to poor penetration depth of the weld joints. The heat input depends on the welding current, voltage, and travel speed. Many welding parameters can influence the quality of orbital TIG welding joints. For instance, wire feed speed and filler wire diameter affect the weld's heat input and formation process. Therefore, many authors have tried to optimize these factors to improve orbital TIG welding quality. [PERSON] et al. [16], for instance, appliedboth simulation and experimental investigations to study the orbital welding process of AISI 316L stainless steel. They surveyed the impact of welding current, welding speed, and standoff distance via the L-9 orthogonal array via the Taguchi method. The optimal input parameters for tensile strength are a welding current of 40 A, welding speed of 1.08 mm/s, and standoff distance of 1.6 mm. Using the Taguchi optimization method, [PERSON] et al. [17] examined the orbital pipe welding process of SS316L steel. By changing the welding parameters, they focused on the tensile strength, distortion, bead width, ovality, and tapers. The results revealed that at a welding current of 114.7 A, the tensile strength could reach 661.4 MPa with a low distortion level.  
Interestingly, [PERSON] et al. [18] also optimized the orbital welding process of AISI 1020 steel pipe using an artificial neural network and genetic algorithm. The authors tried to improve the welding joints' tensile strength and Rockwell hardness. The greatest ultimate tensile strength (UTS) that could be attained was 411.2 MPa, with a hardness of 95 HRB using the parameters of welding current of 110 A, welding voltage of 24 V, travel speed of 300 mm/min, and arc length of 3 mm. These experimental results were close to the optimal result, pointing to the advances in artificial neural network and genetic algorithm methods. [PERSON] et al. [19] reported the impacts of welding current and sequence on the mechanical characteristics, microhardness, and distortion of orbital pipe welding of SS316L steel. The results indicated a 51% decline compared to the original base metal. Moreover, increasing the welding sequence led to a reduction in the microhardness of the welding bead.  
However, the above studies did not mention the influence of the filler wire diameters on the weld, despite the fact that it can strongly affect the welding quality. This study evaluated weld quality with different welding currents, welding voltages, travel speeds, and welding wire sizes in the orbital welding process of SUS304 steel pipe. The welding joints were assessed through the tensile test and observed by a metallurgical microscope. The results illuminate the orbital welding process, especially with SUS304 pipe.