## 3 Results and Discussion
Considering the uncertainty of the results, we conclude that the optimal pattern pitch is near 500 \(\upmu\)m, which results in an approximately 100% increase in tensile shear load compared to that of no surface patterning, although the correlation between tensile shear load and pitch is marginal. The cross-section of the laser fusion-welded CFRP and zinc-coated steel was investigated using SEM imaging to understand the nature of the increased tensile shear strength up to \(N_{2}=3\) and decreased tensile shear strength at \(N_{2}=5\). The SEM images in Figure 8 show that at \(N_{2}=1\), the PEEK matrix has seeped into the shallow grooves formed by the pre-inscription during the laser fusion process forming a tight bond between the two dissimilar material, thereby increasing the bonding strength. However, as \(N_{2}\) is increased to 3, the images show that the recondensed particles become trapped inside of the pre-inscription, hindering the PEEK matrix from flooding the grooves. As  
Figure 5: (**a**) Pattern depth variation and (**b**) hump height variation w.r.t. the number of scan passes \(N_{2}\) under different nanosecond laser powers \(P_{2}=26\) W, 35 W, and 45 W. A laser scan speed of \(v_{2}=200\) mm/s and focal length of \(z_{2}=0\) mm were used. Figure 6: (**a**) Tensile shear load variation w.r.t. the number of scan passes \(N_{2}\) under different laser powers \(P_{2}=26\) W, 35 W, and 45 W. \(d_{2}=500\)\(\upmu\)m, \(v_{2}=200\) mm/s, and \(z_{2}=0\) mm were used. (**b**) Tensile shear load variation w.r.t. the pattern pitch \(d_{2}\). Laser power \(P_{2}=45\) W, \(N_{2}=1\), \(v_{2}=200\) mm/s, and \(z_{2}=0\) mm were used. is increased further to 5, the images show that some grooves formed by the pre-inscription have become too deep for the PEEK matrix to seep into the cavities during the welding process, leaving large vacancies in which gases may be trapped. Such results show that although all the joints with rectangular patterns result in a higher tensile shear load compared to that of non-patterned joints, there exist optimal parameters that maximize the bonding strength between the CFRP and zinc-coated steel. Figure 8: SEM images of the laser fusion cross-section of CFRP and galvanized steel w.r.t. the number of laser scans \(N_{2}\) to form the pre-inscribed rectangular patterns. The parameters used for this bonding are: \(P_{1}\) = 500 W, \(N_{1}\) = 2, \(z_{1}\) = 10 mm, \(d_{1}\) = 5 mm, \(P_{c}\) = 5 MPa, \(v_{1}\) = 10 mm/min, \(P_{2}\) = 45 W, \(N_{2}\) = 1, \(v_{2}\) = 200 mm/s, and \(z_{2}\) = 0 mm, \(d_{2}\) = 500 um. Figure 7: The SEM image of the fractured cross-section for the laser fusion-welded CFRP and zinc-coated steel joint with laser patterned grooves after the tensile shear test. Figure 9 shows the comparison of the tensile shear load test results for the CFRP and zinc-coated steel joint using four different kinds of methods: self-piercing rivet, laser fusion bonding with and without pre-inscribed patterning, and adhesive bonding. The results show that for this study, joints formed using non-patterned laser fusion bonding exhibit comparable tensile shear loads to self-piercing rivets with approximately 4 kN, while joints formed using patterned laser fusion bonding exhibit comparable tensile shear loads to adhesive bonding with approximately 8 kN. Therefore, it can be concluded that the pre-inscribed rectangular patterns on the zinc-coated steel sheets significantly improve (up to 100%) the tensile shear load of joints between CFRP and zinc-coated steel, and this is a result of an improved fusion between the two dissimilar materials due to the presence of pre-inscribed grooves.