## 3 Results and Discussion
### Identification of the Joining Parameters  
The laser parameters play a significant role in the failure mode of laser-joined metal and plastic hybrids. An adhesive failure occurs when the hybrid fails directly at the interface of the joint. Increased thermal input, resulting from high laser power and low scanning speed, can damage the plastic and lead to failure inside the plastic part while the overlap joint remains intact. This is known as a cohesive failure mode [33]. An adhesive failure mode is crucial for identifying the influence of surface pretreatment on the lap shear strength. The criteria to identify the optimal parameter set include the maximum lap shear strength in combination with an adhesive failure mode. The lap shear strength of the cold-rolled AISI 304-polyamide 6.6 laser-joined hybrids is presented in Figure 8.  
Figure 8: Lap shear strength of the cold-rolled AISI 304—Polyamide 6.6 hybrids. Figure 7: Flowchart of the results and discussion section. Up to a line energy of 150 J mm\({}^{-1}\), the lap shear strength values are low. Below 83 J mm\({}^{-1}\), some of the tested samples failed before the sample could be mounted. At 158 J mm\({}^{-1}\), an arithmetic mean of 2.85 MPa is achieved with an adhesive failure mode in every sample. With increasing line energy, the arithmetic mean rises. Additionally, the number of cohesively failed samples increased as well. At 166 J mm\({}^{-1}\), two of the five tested samples failed cohesively in the plastic side of the hybrid. At 250 J mm\({}^{-1}\), the highest arithmetic mean at 4.67 MPa could be achieved, and all five of the five tested samples failed cohesively. Therefore, the maximum lap shear strength with adhesive failure mode is achieved at 158 J mm\({}^{-1}\) with 2.85 MPa. A cross-section of the cold-rolled AISI 304-polyamide 6.6 is shown in Figure 9.  
In this cross-section, polyamide 6.6 is shown on top, while the steel is on the bottom--vice versa to the joining configuration. The joining process has no influence on the steel side of the hybrid. On the plastic side, different interactions are observed. At the interface, there are some voids. These voids can be attributed to the vaporization of the water content of the polyamide 6.6 or thermal degradation. Additionally, the heat-affected zone is visible due to a slightly brighter curve, overlapping on the right side of the joined area. On the top right side of the steel sample, some molten plastic is visible, which was pressed out of the overlap area during the joining process. The investigation of the cross-section shows wetting from the polyamide 6.6 melt on the cold-rolled AISI 304. However, the created voids can reduce the lap shear strength because they can function as inner notches. Figure 10 displays an adhesively and cohesively failed AISI 304-polyamide 6.6 hybrid. In Figure 10a, an adhesively failed cold-rolled AISI 304-polyamide 6.6 hybrid is shown, and in Figure 10b, a cohesively failed sample at a higher line energy is presented. The hypothesis is that with higher line energy, the thermal damage increases, which leads to a weakened plastic sample. This is also supported by the number of cohesively failed  
Figure 10: Failure mechanisms of the cold-rolled AISI 304—Polyamide 6.6 samples (**a**) adhesive failure and (**b**) cohesive failure mode. Figure 9: Cross-section of a cold-rolled AISI 304—Polyamide 6.6 (PA6.6) hybrid joined with 158 J mm\({}^{-1}\) with visible pores and the heat affected zone (HAZ). samples. At the line energy of 166 J mm\({}^{-1}\), only two of the five tested samples failed cohesively, while at 250 J mm\({}^{-1}\), every sample failed within the plastic. Also, the fraction of the plastic residue on the overlap joint increases, which can lead to a higher ratio of cohesive failure. The adhesively failed samples show plastic residues on the steel surface, suggesting the presence of chemical bonds between the polar groups and the oxides on the steel surface, as described in the Introduction section.