## 1 Introduction  
Friction stir welding (FSW) is a solid-state welding method developed by the Welding Institute (TWI) in the UK in 1991 that uses frictional heat to join two workpieces at temperatures below the base material's melting point. The basic principle is as shown in Figure 1. FSW is performed by pressing a cylindrical tool with a spiral pin rotated at high speed on the joint between the two fixed base materials. Heat is generated by friction between the rotating tool and the base material, which leads to a softenedregion around the joint, and the plastic flow caused by the rotating tool forcibly stirs the base materials on both sides of the interface to join the two base materials [1]. The benefits and advantages of FSW include high mechanical strength (around 90% of the base material) due to low distortion and residual stress as a result of welding with a lower heat input compared to conventional fusion welding and high-quality joints without defects such as pores and cracks [2]. FSW is also used to join lightweight alloys (Al, Mg, and Ti alloys), dissimilar materials, and materials that are difficult to join using conventional welding techniques. FSW is economical because it does not require consumable equipment such as filler metals and inert gas and it is eco-friendly because it does not emit harmful rays or substances, and the process can be standardized and automated by reducing the need for skilled welders. The disadvantages of FSW include exit holes after withdrawing the rotating tool, difficulties in joining complex 3D curved shapes, and need for backing plates to withstand the frictional pressure at the back of the joint. In addition, FSW is only used to join light metals or low melting metals due to the limitation of tool materials. Because of these characteristics, FSW has been mainly applied to the automotive industry in the past, but as its limitations have been supplemented through continuous research and development, FSW is now applied to a wide variety of other fields such as railways, military, aerospace/aviation, and shipbuilding [3]. Friction stir welding has generally been limited to butt welding, as shown in Figure 2a. This is because the welding area between the tool and the materials to be joined is the widest, and when the rotating tool presses the base material with a constant force during the welding process, it is easy to obtain a fixing force to prevent the base material from bending on the opposite side of the welding surface. Frictional heat is generated on the tool's shoulder side, pin lateral side, and the area where the pin bottom side meets the base material, and the frictional heat generated from the shoulder side accounts for 90% of the frictional heat generated by the tool. However, in the case of fillet joint FSW, the frictional heat decreases as the friction area between the tool shoulder and the base material decreases, as shown in Figure 2c. Therefore, several studies related to fillet joints have been performed in the past by conducting FWS on T-joint specimens to solve this problem, as shown in Figure 2b, but it is difficult to find cases applied to actual structures [4; 5; 6; 7; 8]. Even in the automotive industry, the application of FSW is limited to butt welding processes in general, but conventional fusion welding techniques such as metal inert gas (MIG) welding, tungsten inert gas (TIG) welding, and laser welding are used for fillet joint processes. These fusion welding techniques cause excessive welding  
Figure 1: Principle of friction stir welding (FSW) process: (**a**) Description of FSW progress; (**b**) general design shape of FSW tool. deformation and residual stress in addition to the phase change of the base material around the weld because the heat input is very high. Welding deformation can cause problems in the manufacturing process and the assembly with other structures, and reduce the mechanical strength due to high residual stress.