# Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding--A Review
## 2 Friction Stir Welding (FSW)
### FSW Main Welding Parameters  
Despite the simplicity of operation, the number of parameters involved that affect the quality of the welds makes the study of the FSW complex. For this reason, it is important to explain what each parameter corresponds to, in order to understand its influence on the process. The main welding parameters involved in the FSW are the rotational speed (\(\omega\)), welding speed (\(v\)), tilt angle (\(\alpha\)), plunge depth (\(p\)), and axial force (\(F_{z}\)). The rotational speed (\(\omega\)) quantifies the amount of rotation of the welding tool and it is measured in rotations per minute [rpm]. The welding speed (\(v\)), also known as feed rate, traverse speed, and joining speed, corresponds to the velocity of the linear movement of the welding tool along the joint and it is normally measured in millimetres per minute [mm/min]. The tilt angle (\(\alpha\)), or attack angle, corresponds to the inclination of the tool in relation to the normal vector on the material surface, and it is measured on degrees [\({}^{\circ}\)]. The plunge depth (\(p\)), or penetration depth, quantifies the penetration of the tool on the material and is measured in millimetres [mm]. Finally, the axial force (\(F_{z}\)) is the welding parameter that is used to measure the force exerted by the tool on the material during the welding process and is quantified in Newtons [N]. According to [PERSON] et al. (2014) [26], the axial pressure (\(P_{a}\)) is a parameter that has more representativity than axial force. However, the choice of evaluating force instead of pressure is related to the fact that the FSW equipment is usually parameterized by force. Regarding the plunge depth parameter, two different procedures are used to measure the penetration of the tool into the material, which results in different values being determined. Therefore, it is important to distinguish the plunge depth of the shoulder (\(p_{s}\)) and the plunge depth of the pin (\(p_{p}\)). The first is the maximum depth reached by the shoulder relative to the surface of the material, while the second is the maximum depth reached by the pin relative to the surface of the material. Figure 1b shows the two measurement procedures, thus allowing to clarify why the values obtained are different, although both are measuring the penetration of the tool. The dimension and geometry of the tool components also influence the quality of the welds. Therefore, they must be considered important parameters as well. Because most of the shoulders are circular, especially in the FSW with rotary shoulder tools, where the shoulder contributes to the total amount of frictional heat generated, the influence of the shoulder diameter (\(d_{s}\)) must be considered. However, in the case of FSW with stationary shoulder tools it is also common to use rectangular shoulders. Therefore, depending on the case, instead of the diameter of the shoulder, the length and width of this component should be considered. Furthermore, the pin length (\(l_{p}\)) and the pin diameter (\(d_{p}\)) are the other two important welding parameters. The pin length must be chosen depending on the thickness of the materials [7]. In terms of pin geometry (\(G_{p}\)), the geometries are normally cylindrical, tapered, or prismatic. The side surfaces of the pins can be straight, or contain irregularities, such as threads or grooves. These features are shown in Figure 3.  
With the development of the technology, new FSW variants have emerged. Some of them require the addition of external heat to the welding process, as mentioned before. Therefore, it became necessary to consider the tool temperature (\(T_{t}\)) and the material temperature (\(T_{m}\)) as important parameters for the characterization of the FSW process. The dwell time (\(t_{D}\)), also known as holding time, is another parameter that must be considered, but that it is often undervalued. It refers to the time between the moment that the tool reaches the desired plunge depth and the beginning of the translation movement.