## 4 Conclusions  
In this work, the feasibility of adhesion between aluminium and three polymer-based materials: ABS, PA, and PA + CF, was studied. The effect of the substrate surface roughness and the presence of fibers on the adhesion strength between each pair of materials was determined. The aluminum's substrate was mechanically abraded by the SP and GB techniques, giving rise to two types of surface roughness values in addition to that of the non-abraded metal, thus Ra = 0.16 \(\upmu\)m for the non-abraded surface, Ra = 0.32 \(\upmu\)m for the surface abraded by SP, and Ra = 1.65 \(\upmu\)m for the surface abraded by GB. Despite the fact that the use of SP for abrasion purposes has been extensively studied for different types of joints, its use in joints manufactured by FFF processes requires further discussion. The GB technique proved to be superior, leading to higher ULSS values. The following procedures depended on the choice of polymer. For ABS, a solution was manually applied on top of the aluminium substrate, followed by a short curing time before the ABS filament was deposited on top of that. For PA and PA + CF, the application of the primer was done by FFF, which was then remelted on top of a hot plate. The SEM's analysis of the cross section of the PMHM joints reveals the characteristic layered structure of the 3D printed polymer-based materials, and indicates a thickness of 0.125 mm for the aluminium-ABS single lap joints, validating the programmed 0.1 mm primer's thickness for the PMHM joints produced with PA and PA + CF and, thus, reinforcing the relevance of the FFF technology on the primer fabrication processes. The mechanical strength of PMHM single-lap joints produced by FFF increases for substrates with a higher surface roughness, the remelting of the primer (PA and PA + CF), and the higher stiffness of the polymer-based adherent. The mechanical strength of the studied single-lap joints was in good agreement with the reported values in the literature when the same levels of surface roughness of the substrate were considered. The results obtained in this work demonstrate the suitability of filaments with short carbon fibers in producing PMHM joints by FFF. The filaments with short carbon fibers might be a viable alternative for expensive continuous carbon filaments, also allowing their use in a broader range of 3D machines. The analysis of the fractured surface of the joints allowed for the correlation of the adhesion performance of each joint with the resulting failure mode. Depending on the surface roughness level of the aluminium and the polymer-based material, adhesive, cohesive or substrate failures modes were identified. Figure 7: Photograph of each single-lap joint’s failure mode: (**a**) aluminium-PA abraded by SP, (**b**) aluminium-PA + CF abraded by SP, (**c**) aluminium-ABS abraded by GB, (**d**) aluminium-PA + CF abraded by GB, and (**e**) aluminium-PA + CF abraded by GB. Overall, this work validated the FFF technique for a novel combination of materials, aluminium-PA and aluminium-(PA + CF). Aluminium-(PA + CF) single-lap joints exhibited the best mechanical properties, due to the inherent stiffness of the PA + CF and the application of the thermal treatment on the primer. The thermal treatment of the primer proved to be extremely advantageous when looking for a PMH component's fabrication. This is an element that can be easily added to a standard FFF process. Future works might include the introduction of an adhesive fillet by FFF in the end region of the joint and the studying of the aluminium-(PA + CF) single-lap joints, considering higher values for the surface roughness of aluminium. The advances in this technology could promote the utilization of FFF processes as a complementary manufacturing technique for lightweight components, such as those used in the transportation sector.