# Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding--A Review
## 5 FSW of FRTP Composites  
The inclusion of reinforcement fibres in a polymer means that many of its original properties change. This explains why FSW of unreinforced polymers is different from FSW of FRTP composites, even for polymers reinforced with discontinuous fibres. [PERSON] and [PERSON] confirmed this theory after finding that the optimal welding parameters for joining unreinforced PP sheets [41; 93] were different from those found for welding PP reinforced with 30 wt% of chopped GF [45]. Although prior knowledge on unreinforced polymers is a good starting point for the study of composite welding, direct conclusions should not be drawn from one to the other.  
The greatest difficulties that the conventional welding methods traditionally used to join FRTP composites face are normally related to poor fibre distribution. Conventional methods present an inability to maintain the continuity of the fibres along the transition between base material and weld seam [45], where the reinforcement fibres are easily oriented parallel to the weld seam [51], as shown schematically in Figure 7a. According to [PERSON] and [PERSON] [45], the interruption of the reinforcement distribution means that the joint strength is generally limited by the strength of the matrix. On the other hand, because FSW involves the mixture of the material from the different parts, the fibre distribution is significantly improved, which allows a more homogeneous fibre interlacing along the joint, as illustrated in Figure 7b. This improvement of the fibres interlacing mechanism allows the production of welds with greater strength than the matrix [45], thus overcoming the limitations of other welding methods. Despite the fact that FSW allows a better distribution and orientation of the fibres than conventional welding methods, there is a lack of studies in the literature that address how the different welding parameters affect these aspects. The use of tracer materials is very interesting for a better understanding of the material flow, as it is already proven in the FSW of polymers [94] and metals [95; 96; 97]. Therefore, the use of tracer materials is highly recommended for the study of material flow in future works on FRTP composites.  
One of the current disadvantages of the FSW process is the fibre breakage caused by the shearing forces of the welding tool [45]. This phenomenon in discontinuous reinforcements is much less harmful than in continuous, but it must be considered in both cases. New welding tool designs and correct optimization of the parameters could be the keys to solve the fibre breakage problematic, as well as to reduce or eliminate other welding defects.  
Figure 6: RFSSW with sleeve plunge configuration main steps: (**a**) initiation stage, (**b**) sleeve penetration stage, (**c**) weld seam compression stage and (**d**) tool retracting stage.  
The influence of each FSW parameter on the joining of FRTP composites reinforced with discontinuous fibres discussed in the literature will be presented below, one parameter at a time. The main welding parameters compared in each publication and the optimum parameters found are shown in the Table 1.