## 3 Results and Discussion
the clamping pressure \(P_{c}\). \(P_{1}\) = 0.7 kW, \(z_{1}\) = 10 mm, \(d_{1}\) = 5 mm, \(N_{1}\) = 2, and \(v_{1}\) = 10 mm/min were used. In an effort to increase the bonding strength of the fusion welded joint between CFRP and the zinc-coated steel sheets, the surface of the steel, which overlaps with the CFRP sample, is pre-inscribed with a rectangular pattern using a nanosecond pulsed laser, as shown in Figure 4a. The laser pattern chosen for this study is a rectangular pattern with a varying line pitch represented by \(d_{2}\), as shown in Figure 4b. The magnified image in Figure 4c shows that when the surface of the steel is patterned using the nanosecond laser, the recondensed debris from the process forms a hump along the laser path. Figure 4d shows the SEM image of the laser patters as \(N_{2}\) is increased from 1 to 5. It shows that although the depth of the pattern consistently increases as \(N_{2}\) is increased, the width of the pattern is not consistent, yet decreases due to the increased amount of recondensed debris after \(N_{2}\) = 3. The pattern depth and the hump height are measured w.r.t. the number of scan passes \(N_{2}\), and the results are shown in Figure 5a,b. As expected, the pattern depth consistently increases with the increasing \(N_{2}\) value, since more material is removed as the number of scans is increased. The height of the hump was found to generally increase with increasing \(P_{2}\) values, but the trend is inconclusive for increasing \(N_{2}\) values due to large variations. Formation of the hump is undesirable, since it hampers tight contact between the materials to be welded, leading to increased gas encapsulation and lower weld qualities. Tensile shear load test results for the laser fusion-bonded CFRP and pre-inscribed rectangular surface patterned zinc-coated steel is shown in Figure 6a for varying \(N_{2}\) values and in Figure 6b for varying \(d_{2}\) values. The results clearly show that the tensile shear load is at maximum for \(N_{2}\) = 1 and decreases as \(N_{2}\) is increased to 3, and then is approximately maintained at a constant value. The figure also shows that the power of the nanosecond laser used to pattern the zinc-coated steel does not influence the tensile shear strength of the joint. Such results indicate that the pattern depth and the hump height resulting from the pattering laser does not correlate with the tensile shear strength. For example, the presence of the pattern indeed increases the tensile shear strength of the fusion-welded CFRP and zinc-coated steel. However, when the pattern depth becomes more than 75 \(\upmu\)m, it causes an adverse effect. Figure 7 shows the SEM image of the fractured cross-section of the welded joint after the  
Figure 4: (**a**) The rectangular grid pattern of the laser patterning on the zinc-coated steel surface with \(d_{2}\) = 500 \(\upmu\)m. (**b**) SEM image of the cross-section of the zinc-coated steel after rectangular patterning using nanosecond laser irradiation. (**c**) Magnified image of the cross-section of the zinc-coated steel after rectangular laser patterning. The hum height and the pattern depth are shown in the figure. (**d**) The SEM images of the zinc-coated steel after each laser scan. tensile shear test. The figure shows that CFRP residues are present within the laser-patterned grooves. This shows that the improved anchoring effect due to the laser pattern causes a partial cohesive fracture resulting in improved joining strength compared to that of the pure interfacial fractures observed in the joint with no patterns. The tensile shear load test results for the laser fusion-bonded CFRP and surface patterned zinc-coated steel are shown in Figure 6b for the varying pattern pitch \(d_{2}\) when \(N_{2}=1\). The results show that for all the pattern pitch sizes, the tensile shear load is increased compared to the samples with no surface pattern.