## 3 Results and Discussion  
To find the optimal processing conditions for the laser fusion bonding of CFRP and zinc-coated steel, a parametric study was performed by varying the welding parameters, and the tensile shear load test results of each case are shown in Figure 3. Figure 3a shows the tensile shear load with respect to (w.r.t.) the laser power \(P_{1}\) at different focal positions, \(z_{1}\) = 5, 10, 15 mm. The figure shows that for the samples used in this study, the optimal laser power is approximately 0.7 kW, resulting in a tensile shear load of 4.26 kN at \(z_{1}\) = 10 mm. The optimal laser power was chosen since the results show high tensile  
Figure 1: (a) Schematic diagram of the laser fusion welding of carbon fiber-reinforced plastic (CFRP) and zinc-coated steel and the welded sample. The laser fusion welding was done using a laser with a power of \(P_{1}\), a focal length of \(z_{1}\), and the number of passes as \(N_{1}\). (b) The custom-made jig design is also shown as a 3D rendering in which the samples are placed for the laser fusion welding process. Figure 2: Schematic of the laser surface pattered zinc-coated steel using a nanosecond pulsed fiber laser. The pre-inscribed rectangular pattern is formed by irradiation of a laser with a power of \(P_{2}\), the number of passes as \(N_{2}\), and a scan speed of \(v_{2}\). shear load test results for all the focal lengths with a minimum amount of errors. The figure also shows that the optimal \(z_{1}\) value is 10 mm, which results in a relatively high tensile shear load with relatively small variations. Next, the influence of the pitch \(d_{1}\) on the tensile shear load is investigated, and the results are presented in Figure 3b. The figure shows that the maximum tensile shear load of 4.26 kN is obtained for the weld pitch of \(d_{1}\) = 5 mm. The results show that as \(d_{1}\) is increased from 1 to 5 mm, the tensile shear load is increased. However, when \(d_{1}\) is increased above 5 mm, the results become unreliable, although the maximum tensile load is maintained at approximately 4.2 kN. The number of passes \(N_{1}\) in each weld line is found to be an important parameter, and the tensile load test results w.r.t. \(N_{1}\) are shown in Figure 3c. The results in the figure show that the tensile shear load of \(N_{1}\) = 2 is significantly higher than that of \(N_{1}\) = 1, since a single path is not enough to fully fuse the two materials at the optimized laser power of 0.7 kW. However, as \(N_{1}\) is increased beyond 2, the tensile shear strength is decreased, which is suspected to be the result of the increased thermal damage on the polymer matrix of the CFRP in the interfacial area. Finally, tensile shear load tests, for the samples using different clamping pressures denoted by \(P_{c}\) are conducted, and the results are shown in Figure 3d. The figure shows that the tensile shear load is maintained at approximately 4 kN up to \(P_{c}\) = 5 MPa. However, the reliability is reduced as it is further increased. The parametric study shows that for this study, the optimized laser processing parameters in laser fusion welding of CFRP and zinc-coated steel are: \(P_{1}\) = 0.7 kW, \(z_{1}\) = 10 mm, \(d_{1}\) = 3 mm, \(N_{1}\) = 2, and \(P_{c}\) = 5 MPa. We use these parameters for investigating the effect of pre-inscribed rectangular patterns on the joining strength of the two dissimilar materials. Figure 3: **(a)** Tensile shear load variation with respect to (w.r.t.) laser power, \(P_{1}\) for laser focal positions \(z_{1}\) = 5, 10, 15 mm. \(d_{1}\) = 5 mm, \(N_{1}\) = 2, \(P_{c}\) = 5 MPa, and \(v_{1}\) = 10 mm/min were used. **(b)** Tensile shear load variation w.r.t. the weld pitch, \(d_{1}\). \(P_{1}\) = 0.7 kW, \(z_{1}\) = 10 mm, \(N_{1}\) = 2, \(P_{c}\) = 5 MPa, and \(v_{1}\) = 10 mm/min were used. **(c)** Tensile shear load variation w.r.t. the number of passes \(N_{1}\). \(P_{1}\) = 0.7 kW, \(z_{1}\) = 10 mm, \(d_{1}\) = 3 mm, \(P_{c}\) = 5 MPa, and \(v_{1}\) = 10 mm/min were used. **(d)** Tensile shear load variation w.r.t.