## 4 Conclusions  
The study shows the technical and operational feasibility of a portable Friction Stir Welding device developed specifically for joining components manufactured by additive manufacturing (AM). Specifically, using thermoplastic polymers of the polylactic acid (PLA) type. After carrying out the tests and analyzing the results, the main findings from this study are as follows:  
* The combination of FSW and AM technologies for thermoplastics could be an efficient alternative for joining AM parts with complex geometries and in situ repairs.
* During the welding, the FSW process revealed two different phases: a transient stage and a steady-state stage. The latter is characterized by a homogeneous and predictable material flow, while the former shows irregular material distribution. The length of the transient stage was longer for specimens with higher infill densities.
* In samples with a lower infill (15%) and added filler, the weld zone was mostly composed of filler material, with only minimal melting of the base material. In these cases, the internal voids were easily occupied by the filler material flowing into them. In contrast, in the samples with 100% infill, the stirred material flowed more homogeneously, and no filler was required. Therefore, for low-density base materials, additional material must be supplied to achieve a good result, whereas it may not be necessary for high-density materials.
* In the Friction Stir Welding process of PLA components without material addition, a reduction in the total component mass occurred due to the formation of burrs.
* Increasing the feed rate resulted in a narrower welded joint. When the filler material flow rate was kept constant, higher welding speeds produced narrower weld lines to distribute the total added mass.
* Tool inclination plays a key role in the process, as confirmed by both the results obtained in this study and other researchers.
* The analysis of the welding direction highlighted that the welding direction must be set forward. The test performed with a negative inclination in the movement direction resulted in poor-quality welds with significant cross-sectional reduction. The forward direction increases friction and heat generation and consequently enhances material stirring.
* When the cylindrical pin and the truncated conical welding tool were compared, the truncated conical tool resulted in fewer burrs, reduced mass loss, and fewer defects, producing a more homogeneous weld line. Visual inspection also indicated a higher level of material stirring with the truncated conical tool, which presumably contributes to improved weld quality.
* The analysis of rotational speed showed that increasing it from 2000 rpm to 2500 rpm led to a noticeable improvement in weld quality.
* Overall, the results suggest that this portable device holds great promise for industrial applications that demand mobility, flexible operation, and strong, reliable welds on 3D-printed polymer parts. This is particularly relevant in fields like aerospace, automotive, and medical industries, where the integration of additive manufacturing and solid-state joining technologies is becoming increasingly important.
* Future work should focus on automating the FSW process for additive manufacturing polymers, which would improve control over welding parameters and quality. Additionally, integrating sensors for in situ monitoring could provide deeper insight into heat transfer, material behavior, and defect formation during welding.  
In conclusion, these findings highlight the potential of using FSW as a feasible joining method for polymer parts made via AM. This study lays the groundwork for future research aimed at industrial scale-up and the exploration of other types of polymers and processing conditions.