## 3 Results and Discussion
### Mechanical Properties  
The mechanical properties ULSS and E of the studied single-lap joints are displayed in Figure 6, and indicate the average values obtained from the experimental data. The raw data for all the tested specimens can be found in Supplementary Materials.  
From the data presented in Figure 6, we can observe an increase in ULSS when we consider the single-lap joints produced in aluminium treated with the GB technique. This increase in mechanical properties is probably linked to an increase in adhesion (by the mechanical interlocking effect) caused by a greater surface roughness of the aluminium substrate [54,88]. The same increase in the mechanical properties of single-lap joints (when higher values for the surface roughness are considered) was verified for aluminium-aluminium single-lap joins glued with a polyurethane (PU) adhesive [84], aluminium-ABS single-lap joints [23], aluminium-PA single-lap joins with alternate layers of PA, and PA reinforced with continuous CF [22], aluminium single-lap joins with polyphenylene sulfide (PPS) reinforced with CF [85], and aluminium-carbon fiber reinforced polymer (CFRP) single-lap joints glued with an epoxy adhesive [86] or with an epoxy adhesive with carbon  
Figure 5: SEM image of a cross section of a single-lap aluminium-PA + CF joint.  
Figure 6: Mechanical properties (ULSS and E) of PMHM joints.  
nanotubes (CNTs) [89]. It is also evident from the figure that the ULSS values of the single-lap joints increase when changed from ABS to PA, and from PA to PA + CF, indicating a better strength for the joints produced with PA and PA + CF, possibly due to a greater affinity of the polyamides with aluminium, and to a higher stiffness of the PA + CF. The single-lap joints made with PA + CF were also the ones that presented smoother surfaces and less shrinkage, features that can mitigate the delamination of the polymers from the aluminium substrate [88]. The values of ULSS for SP and GB aluminium-PA single lap joints were 2.5 and 3.8 MPa, respectively. These values are in line with the ones reported by [PERSON] et al. [84] for aluminium-aluminium single-lap joins glued with a PU adhesive (2.9 MPa), achieved for the same level of surface roughness of the substrate (Ra = 0.30 \(\upmu\)m), but below the value of 6.5 MPa reported by [PERSON] et al. [87], which was obtained with a higher surface roughness of the substrate, with Ra = 3.15 \(\upmu\)m.  
The ULSS values of single-lap joints, where the aluminium substrate was subjected to the GB technique, were 1.0, 3.8, and 6.6 MPa for the ABS, PA, and PA + CF adherents, respectively. The ULSS value for the aluminium-ABS single-lap joint is in accordance with that reported by [PERSON] et al. [83] (-1 MPa) and by [PERSON] et al. [23] (0.9\(-\)1.6 MPa), and below that reported by the same author in [22] (5.3 MPa) obtained for higher values of surface roughness values (Rc = 89.0 \(\pm\) 0.3 \(\upmu\)m) compared to those reported in this work (Rc = 6.20 \(\pm\) 1.67 \(\upmu\)m). For these aluminium-ABS joints, lower ULSS values might be the result of the primer's application process and the nonexistence of a thermal post-processing of the metallic adheren' prior to adhesion with the ABS top adherent, since the build plate temperature significantly influences the ULSS value, as demonstrated by [PERSON] et al. [83].  
The ULSS value for single-lap joints with PA + CF (6.6 MPa) can be compared with the value obtained by [PERSON] et al. [87] (6.5 MPa) and with that found by [PERSON] et al. [22] for single-lap joints made of aluminium with alternate layers of PA and PA with continuous carbon fibers (21.9 MPa). The discrepancy found in these results may be explained by multiple factors: a larger surface roughness of the substrate (as a vehicle for good adhesion among polymer/metal joints), and the use of continuous carbon fibers that will enhance the stiffness of the joint and increase its ULSS.