###### Abstract  
This study analyses the application of the Friction Stir Welding (FSW) process on polymeric materials manufactured by additive manufacturing (AM), specifically with polylactic acid (PLA). FSW is a solid-state welding process characterized by its low heat input and minimal distortion, which makes it ideal for the assembly of complex or large components made by additive manufacturing. To evaluate its effectiveness, a portable FSW device was developed for the purpose of joining PLA specimens made by AM using different filler densities (15% and 100%). Two tool geometries (a cylindrical and truncated cone) were utilized by varying the parameters of rotational speed, tilt angle, and feed rate. The results revealed two different process stages, transient and steady-state, and showed differences in weld quality depending on the material density, tool type, and material addition. The study confirms the viability of FSW for joining PLA parts made by AM and suggests potential applications in industries that require robust and precise joints in plastic parts, thereby helping hybrid manufacturing to progress. friction stir welding; fused deposition modeling; PLA; polymer welding; solid state welding 2025  
24 June 2025  
16 July 2025  
Accepted: 17 July 2025  
Published: 22 July 2025  
**Citation**: [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] Friction Stir Welding Process Using a Manual Tool on Polylactic Acid Structures Manufactured by Additive Techniques. _Appl. Sci._**2025**, 15, 8155. [[https://doi.org/10.3390/appj5158155](https://doi.org/10.3390/appj5158155)]([https://doi.org/10.3390/appj5158155](https://doi.org/10.3390/appj5158155))  
**Cogyrable**: 0225 by the authors. Cogyrable**: 0225 by the authors. solid-state welding (where no melting occurs) [4; 7]. Friction Stir Welding (FSW) belongs to the solid-state category and was developed by The Welding Institute (TWI) in the UK in 1991 [4; 5; 7]. Initially created for joining aluminum alloys, FSW has gained significant industrial popularity, especially for welding composite and polymeric materials [8; 9; 10; 11; 12]. Today, FSW is widely used in aerospace, marine, automotive, and railway industries, and more recently, in parts manufactured by additive manufacturing (AM) [13]. FSW can be implemented using a range of equipment, from modified CNC milling machines to purpose-built FSW platforms and robotic arms. The process can be implemented in manual, semi-automatic, and fully automatic modes [9; 11], depending on the production requirements. The manual method is cost-effective, straightforward, and offers flexibility, making it well suited for research and prototyping. However, it falls short in terms of the precision, repeatability, and scalability needed for industrial applications. Automated systems, on the other hand, provide better accuracy, process control, and consistent quality, which are essential for mass production, though they tend to be more expensive and less adaptable to quick changes during experimentation [14]. To carry out this project, a novel manual and portable friction welding device, with and without filler material, was designed and patented, as described in detail in the Section 2. Additive manufacturing (AM) has grown rapidly in recent years, partly due to the expiration of key related patents. AM involves creating three-dimensional objects by adding the material layer by layer [15; 16; 17; 18]. This process allows for design flexibility, customization, and the production of complex geometries that are difficult or impossible to achieve with traditional methods. AM has applications across various industries, including the automotive, aerospace, medical, and fashion sectors. Welding plays a significant role in the context of AM, particularly for post-processing, repairs, and part assembly.