# A Literature Survey on Electrical-Current-Assisted Friction Stir Welding
## 4 Various Types of EA-FSW Tools  
The tool used in the FSW process plays the leading role in joining metals. In the EA-FSW process, the tools are generally divided into four groups. The first group comprises simple tools, and electrical current flows on them from a conductive conductor. This type of tool is simple and similar to the conventional FSW process. A schematic view of a simple EA-FSW tool is depicted in Figure 2a (Type a). This type of tool was an early idea for joining by the EA-FSW process. Due to the electrical resistance of the tool material and heat transfer of the joint line by the tool, this type of tool is not very popular, because the temperature of the tool significantly increases, and it is hard to use this type of tool for a long time [24]. The second type is the tool with a conductive coil or core inside the tool. A conductive core (such as copper) in this tool type is placed inside the FSW tool. The thickness and conductivity of the materials inside the tool determine the electrical current flow and electrical heat input inside the joint line. A schematic view of the EA-FSW tool with a conductive core is depicted in Figure 2b (Type b). The third type of tool is developed from the first type of tool. In this type, the electrical current flows from the tool body, but the tool is not solid. A hole is contained inside the tool, and a coolant flows inside this hole. This mechanism is applied to decrease the tool temperature during the joining process. The coolant could be liquid (water or soap water) or gas (nitrogen or hydrogen) [23]. The flow of coolant decreased the tool temperature during the EA-FSW process. Figure 2c (Type c) presents the schematic view of the third tool type. The fourth type is the modern type of tools, which is presented in Figure 2d (Type d). In this category, tool type two or tool type three is equipped with an electromagnetic or ultrasound vibration system to decrease the limitations of various joining configurations or materials by EA-FSW. The material used as an EA-FSW tool is another parameter that can affect the joint's electrical current and final quality [25]. It is essential that the electrical current passes from the tool material, but in the Type a tool, this has not had a significant effect on the final properties of the joint. The listed material used as an EA-FSW tool is presented in Table 1. A steel-based tool and tungsten-based tool are used during EA-FSW of similar joints. Table 1 presents the data ordering of the workpiece material. The first category is related to the EA-FSW joint of aluminum alloys and, after that, magnesium alloys and titanium alloys, and at the end, low-carbon steel and Inconel. In the available literature until this moment, limited research has been conducted on the EA-FSW process. From the process point of view, all tool types have been used in EA-FSW of similar joints, but when comparing them, tool Type a is the most popular. During EA-FSW, the rotational tool speed was kept constant, and just in low-carbon steel and Inconel joints, the rotational tool speed was selected as a variable. It seems the most critical factor during the EA-FSW process is the electrical current, for decreasing the strength of weldments, and the tool material should be conducive to desirable current resistance. The majority of  
Figure 2: EA-FSW Tools, (**a**) Type a, (**b**) Type b, (**c**) Type c, and (**d**) Type d.  
tools were tungsten-based and tool steel-based material. Tungsten-based tool materials were reported for welding titanium-based materials and Inconel materials. The maximum electrical current used during a similar EA-FSW joint was 720 A, and the lowest electrical current was reported to be 37.2 A. On the other hand, the most common current range was between 0 A and 150 A. The situation in the dissimilar joint is different from similar joint cases.