# Joining of Fibre-Reinforced Thermoplastic Polymer Composites by Friction Stir Welding--A Review
## 5 FSW of FRTP Composites
### Pin Dimension and Geometry  
The pin dimension and geometry is another very important parameter on FSW. Changing the geometry or the size of the pin has a huge impact on the amount of heat generated by the tool and on the quality of the material mixture [48]. It also has a great impact on the surface finish and tensile strength of the welds [46]. The study of the influence of this parameter was considered by different authors. Table 2 resumes the different pin geometries compared and identifies which achieved better results in each publication.  
As Table 2 shows, although these authors compared tools of different geometries and dimensions, their observations converged, which allowed the deduction of good conclusions regarding the optimization of this parameter.  
Based on the work of these authors, it is clear that a circular pin geometry, either conical (tapered) [46,48,54] or cylindrical conical [3,47], is always recommended for welding FRTP composites by FSW, regardless of the composite matrix or reinforcement used. Prismatic geometries, such as the triangular pin geometry tested by [PERSON] et al. [46], do not generate enough heat since they have few points of contact.  
Regarding the type of lateral surface of the pins, the choice is also convergent since the publications refer to the imperative use of grooved [46] or threaded [3,47,48,54] pins, instead of simple lateral surfaces. The researchers pointed out that the use of threaded or grooved pins is imperative because they guarantee a larger contact area, the improvement of heat generation, and a better material mixing of the material, resulting in stronger welds [46,48].  
As stated before, the shearing forces of the welding tool are responsible for the fragmentation of the reinforcement fibres. To study the influence of the pin geometry on the fibre breakage, [PERSON] and [PERSON] [45] butt welded PP plates reinforced with 30 wt% of chopped GF with average length of 338 \(\upmu\)m, with two milling cutters of four and eight-tooth as welding tools. According to previous studies developed by them, FSW of unreinforced PP achieved better results with the eight-tooth milling cutter tool [41,93]. However, for welding reinforced PP, this tool led to a very high fibre fragmentation. Although the degree of fragmentation measured for the four-tooth tool was also high, the average length of the fibres measured was 194 \(\upmu\)m for this tool, which was about twice the length measured when the eight-tooth tool was used (103 \(\upmu\)m).  
In short, all these authors [3,46, 47, 48, 54] proved that the conical geometries with grooved or threaded lateral surfaces, such as the illustrated in Figure 8, produced stronger welds, without visible defects, a clean appearance, and uniform width. In contrast, the other pin geometries produced welds of lower strength and rough surfaces, as a result of the formation of superficial and internal defects, such as tunnel defects. Although [PERSON] and [PERSON] [45] presented a study on the influence of the pin geometry on fibre breakage, further investigation on the effect of different conical threaded or grooved tools on the degree of fragmentation of the reinforcement fibres is required and recommended.