# A Literature Survey on Electrical-Current-Assisted Friction Stir Welding
## 1 Introduction  
Friction stir welding (FSW) is a solid-state joining process in development. This process was introduced by The Welding Institute (TWI) in 1991 for the similar joining of aluminum alloys [1]. In this technique, a hard-non-consumable tool rotates and penetrates the joint line [2]. The tool action is applied by a welding machine [3]. The welding machine exerts an axial force during the welding procedure, in order to be in touch with the workpieces [4]. Due to the presence of friction between the tool and welding materials, heat generates at the tool-workpiece interface [5]. This heat is the result of the sliding friction and deformation of the base metal [6]. Due to the available experimental test, the generated heat during the FSW process has not been increased above the base metals' melting points [7]. Although, in some cases, local melting was also reported for materials with a low melting point. The FSW process is also called a thermomechanical process, and this process has also been categorized in solid-state joining techniques [8]. FSW is developing rapidly, particularly in the application of hard-weldable materials such as aluminum alloys, magnesium alloys, zinc alloys, copper, and others that are not weldable or joined with specific welding processes [9]. The flexibility of the FSW process has allowed various dissimilar joints to be producible [10]. Research has shown that the joining of dissimilar metals, polymers, and even dissimilar joints between polymers and metals are reachable with the FSW process [11]. The advantages of the FSW process include low heat input, low changes in base materials after joining, no or little filler material, no fume and welding spatter, very little pre-processing before the joining procedure, less residual stress after joining, and lower levels of pores and cracks [12]. For these reasons, FSW has been rapidly used in various industries and has produced enormous amounts of economically efficient products [13]. FSW was utilized successfully for the joining of non-ferrous metalsat below the melting point [14]. Although, in the case of metals with a high melting point, using FSW faces some issues, such as choosing a tool material, tool wear, and energy consumption. The final joint's quality is related to the welded sample's ultimate tensile strength and hardness. The welding researchers try to achieve the highest ultimate tensile strength in the welded samples. The best properties belong to the weld with the lowest porosity and mechanical properties similar to higher weldments. The researchers found that the FSW process for joining high-melting-temperature materials or refractory metals such as ultra-high-strength steels, chromium alloys, Tantalum, and Tungsten alloys is not effective [15]. This means that, from the technical point of view, the tool wear during FSW is very high and cannot weld the raw materials. As a consequence, the final joint mechanical properties would be quite low. In some metals, such as Tantalum and Tungsten, it is not possible to conduct FSW because the tool should be more rigid with a higher strength than raw material. In this case, there is no report for FSW of Tantalum and Tungsten because these metals are used for FSW tools, not weldments [16; 17]. There are various literature sources that demonstrate the researchers used extra-hard materials for the FSW tool, changed joint geometry, and used specific tool shapes in order to decrease defect possibility in the joint line after the FSW process [17]. All these factors have drawbacks, such as using expensive materials for producing the tool, the limitation of using various tool shapes with a high risk of breaking during welding, and undesirable joint geometry [18]. Research in this area has shown that FSW of high melting temperature or hard materials needs support, and development in the conventional FSW process can improve the final joint [19].