# Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding--A Review
## 1 Introduction  
The number of applications of the fibre-reinforced polymer (FRP) composites has been growing solidly as they are high performance materials that can be manufactured quickly and with low production costs [1]. Fibre-reinforced thermoplastic polymer (FRTP) composites have less sensitivity to damage when loaded statically or dynamically, and have greater potential for repair than composites of thermosetting matrix [2]. FRTP composites present good mechanical properties, low density, good resistance to chemicals, are recyclable and, theoretically, have infinite life in service [3]. Compared to common metallic materials, FRTP composites present superior specific strength and specific stiffness [4]. These materials are mostly intended for the application in the automotive, naval, aeronautic, and aerospace industries [5], as they allow the production of lightweight structures with high mechanical performance. Thus, their inclusion contributes to the reduction of fuel consumption and CO\({}_{2}\) emissions [6]. The two most common reinforcements used for the production of these composites are the glass fibre (GF) and the carbon fibre (CF) [3]. The conventional methods used to join FRTP composites are the mechanical fastening and the adhesive bonding [7]. Because mechanical joints can be non-permanent, they are very interesting for disassembly or maintenance purposes. However, mechanical fasteningis normally associated with the addition of mass and may require the existence of holes in the parts to be joined, which leads to high stress concentration and to composite damage, due to the interruption of the reinforcement fibres [7]. In addition, the majority of the mechanical joints are not watertight [8]. Contrary to mechanical fastening, the adhesive bonding only induces residual stress concentration. On the other hand, the adhesive bonding can be difficult to perform on FRTP composites [7], due to the reduced surface tension of these materials. This joining method requires joint preparation and a cure period. The cure period varies between a few seconds and many hours, it may require the application of heat and/or pressure, and it is normally associated with the emission of toxic fumes [9]. Fatigue stresses in an aqueous environment promote a faster degradation of the adhesives [10]. Since welding appears to have great potential to join FRTP composites, several welding technologies have been developed and tested in recent years. In addition to friction stir welding (FSW), ultrasonic welding (USW), laser welding (LW), and induction welding (IW) stand out. The USW allows joints with good mechanical strength if the fibres are not cut. However, the application is limited to reduced thicknesses. LW has been extensively investigated, as it is a fast and flexible process, and does not require direct contact between tools and joint. On the other hand, LW depends a lot on the optical properties of the materials and currently gives rise to the formation of porosity in the composite, caused by gases and the shrinkage of the molten polymer, which reduces the strength of the joints. In turn, IW requires long process times, presents difficulties related to heat distribution uniformity, and is not recommended to join large parts [7]. FSW is a joining technique that was developed in the British engineering organization The Welding Institute and was patented by [PERSON] et al. in 1991 [11; 12]. This process emerged with the goal of overcoming the difficulties of conventional welding techniques, when applied to lightweight alloys. Since the success of welding aluminium alloys has been demonstrated, FSW has been used in the welding of many other metals and their alloys, such as magnesium, titanium, copper, and steel [13]. FSW can additionally be used to join dissimilar metals [14; 15; 16]. Nevertheless, FSW is not a welding technique exclusive for the joining of metallic materials, as its application for the joining of polymers and polymeric matrix composites, for example, has already been demonstrated.