# Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding--A Review
## 5 FSW of FRTP Composites
### Rotational Speed and Welding Speed  
The analysis of variance (ANOVA) performed by [PERSON] et al. [49] showed that welding speed had the greatest impact on the performance of their welds with a contribution of about 79% and that rotational speed was the second with an influence of about 12%. In contrast, [PERSON] et al. [47], [PERSON] et al. [50], [PERSON] et al. [52], and [PERSON] et al. [54] observed that the rotational speed had a bigger contribution with an influence between 47% and 57%, while the welding speed showed an influence between 12% and 33%. Anyway, the ANOVA performed by the different authors shows that rotational speed and welding speed are the two parameters with most influence on the welding process, with a combined influence between 65% and 91% when compared to other welding parameters such as tilt angle [47; 49; 50], axial force [52], and tool geometry [54]. During FSW, they must be calibrated to ensure sufficient heat generation to soften the composite matrix [45] and to properly transport the softened material from the AS to the RS of the welds [47; 50].  
Excessively high rotational speeds, as well as low welding speeds, lead to the overheating of the materials and to excessive turbulent mixtures [3; 47]. The excessive turbulence caused by high rotational speeds is favourable to the formation of tunnel defects, as observed by [PERSON] et al. [46] and [PERSON] et al. [5]. According to [PERSON] et al. [50], high rotational speeds also contribute to the increase of plasticized materials being expelled out from the weld seam and to the burning of the polymeric matrix.  
In opposite direction, excessively low rotational speeds, as well as high welding speeds, lead to improper stirring of the softened material, poor vertical material flow, and insufficient heat generation, promoting the formation of wormhole defects and other common void defects, most likely in the RS, as observed by [PERSON] et al. [46], [PERSON] et al. [5] and [PERSON] et al. [50]. [PERSON] et al. [54] noted that the reduction of the rotation resulted in worsening of the surface finish, with the increase of the surface roughness.  
[PERSON] and [PERSON] [45] and [PERSON] et al. [51] stated that the shearing forces resulting from the rotational movement of the tool cause the fragmentation of the reinforcement fibres. According to [PERSON] and [PERSON] [45], higher rotational speeds lead to a higher degree of fibres breakage. Although their study does not consider the influence of the welding speed, it is expected that its reduction also favours the increase of the fragmentation of the reinforcement, since the fibres will be exposed for a longer period to the shearing forces of the welding tool. Furthermore, [PERSON] and [PERSON] [45] observed that the joint strength of the welds was influenced by the degree of fragmentation of the fibres, while [PERSON] et al. [51] did not notice a direct correlation between fibre shortening and weld strength. This can be explained since after the FSW the fibres of the composites of [PERSON] and [PERSON] [45] were below the critical length, and the fibres of the composite of [PERSON] et al. [51] were not.  
Figure 7: Schematic illustration of the fibre distribution on the cross section of joints produced by: (**a**) conventional welding methods and (**b**) FSW (based on [PERSON] and [PERSON] [45]).  
Consequently, they stated that an increase in the fibres length must be considered before welding FRTP composites by FSW, since there is a critical fibre length to respect in order to take full advantage of the reinforcement strength [45; 51].