# Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding--A Review
## 1 Introduction
Recently, FSW is also being studied for the joining of polymers and FRTP composites to metals [17; 18; 19; 20]. The feasibility of joining unreinforced polymers by FSW was demonstrated in 1997 [21; 22]. This welding technique is only suitable for thermoplastics and not for thermosetting polymers. While thermoplastics increase their fluidity when exposed to an increase of temperature, and can be moulded at temperatures below the degradation point, the thermosets do not [23]. The literature shows that it is already possible to join several thermoplastics by FSW, such as acrylonitrile butadiene styrene (ABS) [24; 25; 26; 27], polyamide 6 and 66 (PA6 and PA66) [28; 29; 30], polycarbonate (PC) [31; 32], polyethylene (PE) [33; 34; 35], polylactic acid (PLA) [36], polymethylmethacrylate (PMMA) [37; 38; 39], polyphenylene sulfide (PPS) [40], polypropylene (PP) [41; 42; 43], and polyvinyl chloride (PVC) [44]. Since 2011, FSW is also being studied for the joining of FRTP composites reinforced with chopped fibres [45; 46]. Most of the publications found are related to PP reinforced with chopped GF [3; 5; 47; 48; 5; 45] or chopped CF [3; 5; 48; 49], but matrices of PA6 [50; 51; 52; 53], PC [54] and polyetheretherketone (PEEK) [40] have also been reported. The studies found in the literature are still very much focused on the feasibility of the technology. Even so, [PERSON] et al. [51] and [PERSON] et al. [52] obtained joint efficiencies of about 50% and 59%, respectively, both for the joining of PA6 reinforced with 30% of GF. These results show that the technology has a lot of potential. Thus, it is expected that with future studies on the optimization of the technology, it will be possible to obtain even better joint efficiencies. Within FSW of polymeric base materials, it is worth mentioning friction stir spot welding (FSSW) and refill friction stir spot welding (RFSSW) [55], which are variants of the conventional FSW technique that can be distinguished by the absence of linear movement of the tool. Although they only allow punctual joining, which results in stress concentration and does not guarantee watertight weld, they must be considered as promising welding techniques as well. FSSW of unreinforced polymers was demonstrated in 2008 when [PERSON] and [PERSON] [56] reported the success of lap joining PP plates. Since then, FSSW has been studied to join ABS [57; 58], PC [59; 60; 61], PE [62; 63; 64], and PMMA [65]. Regarding FSSW of FRTP composites, no publications were found. The feasibility of the joining of unreinforced polymers by RFSSW was demonstrated by [PERSON] et al. [66] in 2010, who successfully welded PMMA plates. The dissimilar joining of unreinforced polymers and FRTP composites by RFSSW was reported in 2014 by [PERSON] et al. [67; 68]. One year later, by successfully joining CF reinforced PA66 laminate plates, [PERSON] et al. [69] demonstrated the joining of FRTP composites by RFSSW. It is important to highlight the fact that this was the first publication found that referred to the joining of FRTP composites reinforced with continuous fibres in the literature. The objective of the current work is to show the potential of the FSW and its variants to join FRTP composites. To accomplish that, the FSW technique and two other important variants, the FSSW and the RFSSW, are presented and explained in a brief but complete way. Despite not having found any publication referring the joining of FRTP composites by FSSW, it is to be expected that in the near future this topic will be deeply investigated. Therefore, the FSSW technique will be shortly presented. Another reason that justifies the need to present the FSSW technique is that it helps to understand and distinguish the RFSSW technique. The FSW review is focused on polymers reinforced with chopped fibres, as no publications on continuous reinforcements were found. Still, [PERSON] et al. [7] defends that the FSW of FRTP composites of continuous reinforcements requires further investigation.