# Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding--A Review
## 2 Friction Stir Welding (FSW)  
FSW involves the conversion of mechanical energy into thermal energy [3]. The conventional welding tool is composed of two fundamental non-consumable elements, which are the shoulder and the pin [75]. The pin is also known as probe. In FSW with conventional tool, the shoulder and the pin rotate together, being both responsible for generating enough frictional heat to soften the materials. The pin is also responsible for mixing the softened material, while the shoulder avoids its projection out of the welding zone [76]. Figure 1 illustrates the FSW process with a conventional tool and some of the main welding parameters. FSW is a simple process that follows a set of four well-defined stages, which are schematically shown in Figure 2. The first stage consists of the progressive penetration of the rotating tool into the material, until it reaches the desired depth. This first stage is known as the plunging stage. Then, there is a waiting step in which the tool remains in rotation without advancing, so that the material reaches the ideal operation temperature. This step is called the dwell stage and its duration is defined as the dwell time. In the third stage, called the welding stage, the translation movement of the tool occurs, giving rise to the joining of the plates after cooling the material. When the tool reaches the end of the defined path, the rotation and the translation movement stop. The last step is called the end of welding and tool retracting stage [76; 77]. After completing all the steps of the FSW process, the material must remain clamped to the structure for a few minutes to decrease the possibilities of distortions during cooling [33]. Some authors defend the need of a fifth step, the cooling stage, since the cooling conditions may affect the quality of the weld produced [78]. The welded joint usually presents a keyhole at the end of the welding path and material protrusion at the beginning [75]. Generally, the peak forces of the FSW process occur during the plunging stage, as the materials have not yet reached the operating temperatures [79; 80; 81]. FSW does not require the addition of material, protective atmosphere, or joint preparation [12]. Since during the process there are no emissions of toxic f  
Figure 1: Schematic illustration of FSW with conventional tool and its main welding parameters: (**a**) overview of the process and (**b**) detail on parameters related to tool penetration. Figure 2: FSW main steps: (**a**) plunging stage, (**b**) dwell stage, (**c**) welding stage, and (**d**) end of welding and tool retracting stage. can be said that this welding technique is safer and environmentally friendlier than many others [46]. Because the main source of heat of the FSW process is the friction between the base material and the rotating tool, this welding process can be classified as an economic and energy-efficient process compared to others that traditionally consume higher energy levels [82]. Even for large thickness, welding can be carried out with a single pass of the tool and the welds do not require any posttreatment. Serial production can be carried out using robots [45]. Despite being common to find voids and cavities within the weld seam, as a consequence of improper mixing and entrapped air, for example, these voids are not interconnected, meaning that the joint is normally watertight [45]. FSW allows the production of 3D complex structures [25] and can be applied to any joint configuration, although the most common is to weld in butt or lap joint configurations [26]. When FSW is performed in lap joint configuration, it is also known as friction stir lap welding (FSLW) [27]. Generally, the main source of heat during the FSW process is friction. The consequent plastic deformation of the material also contributes to the increase of the total amount of heat generated, but with much less impact than the heat produced by friction [75].