## 2 Materials and Methods
### Surface Preparation of the joints  
The adhesion between the aluminium and the 3D printed material in the joint region was improved by employing two consecutive strategies. The first strategy was to increase the surface roughness of the aluminium substrate through sandpaper polishing (SP) or grit blasting (GB) techniques, followed by the second strategy, which consisted of the application of a primer on top of the aluminium substrate prior to the 3D printing process. The SP process was applied using sandpaper with particles with an average diameter of 125, and the polishing direction was varied alternately between 0 and 180 in order to create a uniform pattern. The GB technique was performed on a RP-TOOLS 350 L machine with brown corundum particles (Al2O3) with an average granulate size of 256 and a pressure of 6 bar for a period of 20 s. During the GB process, each substrate was placed at an angle of 45 relative to the blasting nozzle, with a working distance of 200 mm. The blasting direction varied alternately between 0, 90, +45 and -45 in order to create a uniform blasting pattern [81]. The specimens were cleaned with isopropyl alcohol and compressed air after each of these mechanical abrasion processes. For aluminium-ABS joints, an ABS primer was manually spread with a paintbrush following the procedure of [PERSON] et al. [22; 23], as shown in Figure 3a,b. The ABS primer was obtained by the dissolution of small portions of the ABS filament (5 mm length) in acetone for 24 h, giving rise to a 25 wt.% ABS solution. On the aluminium-PA and aluminium-PA + CF's joints, the application of the primer was done by FFF, using PA and PA + CF, respectively, and is shown in Figure 3c. For the aluminium-PA and aluminium-PA + CF's joints, the primer was remelted on a hotplate at 270 C for 1 min in order to better infiltrate the cavities on the surface of the aluminium substrate, as shown in Figure 3d. Several FFF depositions were performed (aiming to produce single-lap joints) on the surfaces of the previously prepared aluminium substrates using SP and abrasion with the GB technique and the application of the primer. Table 2 shows the successful and unsuccessful results of \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Filament** & **Printing Temperature (C)** & **Infill Percentage** & **Deposition Speed (mm/s)** & **Layer Thickness (mm)** & **Raster Angle (°)** & **Number of Contours** \\ \hline ABS & 280 & 100 & 50 & 0.1 & \(\pm\)45 & 12 \\ \hline PA & 270 & 100 & 45 & 0.1 & \(\pm\)45 & 12 \\ \hline PA + CF & 260 & 100 & 30 & 0.1 & \(\pm\)45 & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: 3D printed parameters for each filament. the FFF depositions that aimed to make adhesive joints between the aluminium and the 3D printed materials. It should be noted that only samples where a primer was applied gave rise to successful single-lap joints with the adhesion between the aluminium substrate and the polymer-based adherent. For PA and PA + CF, both SP and GB techniques had satisfactory results, as shown in Figure 3e,f, while for ABS the GB technique was the only successful one. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Filament** & **Surface Treatment** & **Primer** & **Single-Lap Joint** \\ \hline \multirow{4}{*}{**ABS**} & \multirow{2}{*}{SP} & No & Fail \\  & & Yes & Fail \\ \cline{3-3}  & & & No & Fail \\ \cline{3-3}  & & GB & Yes & Success \\ \hline \multirow{4}{*}{**PA**} & \multirow{2}{*}{SP} & No & Fail \\  & & & Yes & Success \\ \cline{3-3}  & & GB & No & Fail \\ \cline{3-3}  & & GB & Yes & Success \\ \hline \multirow{4}{*}{**PA + CF**} & \multirow{2}{*}{SP} & No & Fail \\  & & & Yes & Success \\ \cline{3-3}  & & GB & No & Fail \\ \cline{3-3}  & & GB & Yes & Success \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Single-lap joints results: filaments, surface treatments and primer.