# Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding--A Review
## 3 Friction Stir Spot Welding (FSSW)  
FSSW, also known as friction spot joining (FSJ), is a variant of FSW that is distinguished, mainly by the absence of translation movement [56] and, therefore, is associated with spot welding [60]. Developed by Mazda Motor Corporation and Kawasaki Heavy Industry, FSSW started to be implemented in the automotive industry to replace the traditional resistance welding of aluminium sheets [87], allowing a 40% reduction in equipment investment and a 99% reduction of energy costs [63]. The FSSW is mostly used in lap joint configurations.  
FSSW can be divided into three main stages, which are normally called plunging stage, stirring stage, and retraction stage [88], as illustrated in Figure 4. FSSW process starts at the plunging stage, with the penetration of the rotating tool in the material until it reaches the desired plunge depth [56]. The plunge rate must be controlled as the speed of this step influences the success of the entire process [87]. Upon reaching the desired depth, the first stage ends and the stirring stage begins. [64]. During the stirring stage, due to the frictional heat generation created by the high speed of the tool, the material to be joined softens. The rotational movement of the tool is also responsible for mixing the softened material [64]. Normally the stirring stage ends with the stop of the rotation motion, with the tool still in the weld. After a delay time exerting pressure on the material, the retraction stage occurs with the removal of the tool [87]. Removing the tool gives rise to the characteristic hole in the centre of the joint, due to the cooling of the material around the pin [64]. Although this is the most commonly reported procedure, especially in publications on FSSW of polymer-based materials, tool removal can be performed without stopping its rotation. Some authors refer to the existence of an initial preheating phase. However, the experimental results on unreinforced polymers showed that this step has virtually no influence on the welding quality [59; 60].  
From the analysis of the cross section of the weld region, it is possible to identify two geometric characteristics of the weld that make it possible to draw several conclusions about the quality of the FSSW: the thickness of the weld nugget (\(x\)) and the thickness of the upper sheet under the shoulder indentation (\(y\)) [64], both represented in Figure 5. The thickness of the weld nugget (\(x\)) is directly related to the area of the weld cross section (\(A_{s}\)). Therefore, the increase in one means the increase in the other. A larger welded area corresponds to a greater amount of joined material and therefore usually means a stronger joint [64]. The thickness of the upper sheet under the shoulder indentation (\(y\)), as the name implies, corresponds to the height of the upper plate after the FSSW process. This measure is important because the penetration of the shoulder into the upper plate leads to a decrease in its thickness. So, the reduction of this dimension could lead to a reduction of weld strength [64].