# Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding--A Review
## 2 Friction Stir Welding (FSW)
### FSW Main Welding Parameters
The dwell time controls the preheating of the materials before the actual welding. Poor calibration of this parameter severely affects the quality of the beginning of the weld seam. Short dwell times lead to the formation of weak joints due to the lack of heat generated. On the other hand, in the case of polymer-based materials, excessively long dwell times promote material degradation [33]. Through the macro and micro analysis of the cross section of the welds, it is noticed that the right and left sides consistently present different characteristics. The side of the material where the direction of the rotational speed is the same as the traverse movement of the tool is called the advancing side (AS). The side of the weld on which the direction of the rotational speed is opposite to the traverse movement of the tool is called the retreating side (RS) [84], as can be observed in Figure 1a. When welding polymeric base materials, most of the welding defects, such as voids and cracks, preferably occur on the RS [50; 51]. The dissimilar distribution of the heat generated, which is higher in the AS, makes the transportation of the material from the AS to the RS more difficult, preventing a good adhesion of the molten material to the base material on the RS [51]. As a result, most of the welds tend to fail on the RS. As mentioned before, FSW was initially developed for welding metallic materials such as aluminium and its alloys, which are normally difficult to weld by conventional welding methods [11; 12]. Normally, in the FSW process, the metallic materials do not  
Figure 3: Main pin geometries used in FSW: (**a**) straight cylindrical, (**b**) cylindrical threaded, (**c**) cylindrical grooved, (**d**) straight conical or tapered, (**e**) straight hexagonal, (**f**) straight square, and (**g**) straight triangular. achieve the melting temperature. Therefore, the welding process occurs with the metals in the solid state. This means that the formation of cracks and voids can be avoided, the thermal distortion of the metal after welding is significantly reduced, and the dimensional stability is improved. On the other hand, there is no consensus on the part of the authors regarding the state of the material during the FSW process for polymeric base materials. [PERSON] et al. [44] monitored the pin temperature while welding PE, PA6, and PVC, and concluded that the welding process occurred with the material in the solid state, because the temperature measured was always below the melting temperature of these polymers. However, [PERSON] [9] argued that FSW for polymers is not a process that occurs exclusively in the solid state. According to this author, FSW for polymeric materials is a welding process in which there are portions of solid material suspended in a molten polymeric matrix. Most of the latest publications refer that FSW of polymeric materials occurs exclusively in liquid state. [PERSON] et al. [85] and [PERSON] et al. [86] confirmed this thesis after verifying that most of the temperatures measured during the welding process of PE were above the melting point. In the case of composite welding, [PERSON] et al. [54] argued that the process must generate enough temperature to reach the melting of the composite to produce welds with good mechanical properties. On the other hand, excessive temperatures should be avoided as they can lead to the burning and deterioration of the composite materials, resulting in loss of mechanical strength [50; 54]. Thus, it is important to control the welding temperature, since it is proved that the joint strength is reduced when the decomposition point is exceeded [20]. The study of the FSSW and the RFSSW involves most of the main FSW parameters, such as rotational speed, plunge depth, dwell time, and axial force/pressure. However, the dwell time definition for the FSSW and RFSSW is different because there is no translation movement. Therefore, the dwell time ends up corresponding to the welding time. In the RFSSW, the term joining time is normally used instead of dwell time.