## 3 Results and Discussion
### Identification of the Joining Parameters
At line energies below 158 J mm\({}^{-1}\), only smaller areas or no residue at all were identified on the steel samples, indicating that a certain amount of thermal energy is needed to create these chemical bonds. Further investigation of the plastic residue was conducted through SEM analysis on a lap-sheared sample, as shown in Figure 11.  
Figure 11a depicts the boundary between the residue and the steel surface. The plastic residue was elongated in the direction of the lap shear test, indicating some form of ductility. Additionally, Figure 11b shows the surface of the steel at a higher magnification, revealing the plastic residue between the grains, which were flattened out by the cold rolling process. In summary, the optimal joining parameter for the indirect laser joining of polymide 6.6 and cold-rolled AISI 304 within the scope of this experiment was at a line energy of 158 J mm\({}^{-1}\). Due to the lower melting point of polypropylene compared to polyamide 6.6, the overall needed line energy to create sound joints is reduced. The lap shear strength of the laser-joined sandblasted AISI 304-polypropylene hybrids is illustrated in Figure 12.  
Figure 11: ETD images of a cold-rolled AISI 304–polyamide 6.6 lap-sheared sample, joined at a line energy of 158 J mm\({}^{-1}\): (a) Boundary between macroscopic residue and the steel surface; (b) microscopic view of the steel surface. Figure 12: Lap shear strength of the sandblasted AISI 304—Polypropylene hybrids. The lap shear strengths of the sandblasted AISI 304-polypropylene hybrids have a clear distribution in the investigated range. The maximal arithmetic mean is achieved at 81 J mm\({}^{-1}\) with 4.26 MPa. A slightly higher line energy can almost achieve the same value. At 88 J mm\({}^{-1}\), the arithmetic mean of the lap shear strength is 4.22 MPa. Increasing the line energy further or reducing it below 81 J mm\({}^{-1}\) results in a decrease in lap shear strength. Additionally, it is worth mentioning that no lap shear sample of sandblasted AISI 304 and polypropylene failed cohesively. Figure 13 shows a cross-section of a hybrid joined with 81 J mm\({}^{-1}\). In direct comparison with the polyamide 6.6 hybrids, there are no voids or pores visible, as expected due to the lack of dissolved water in polypropylene. The heat-affected zone is lower in height, has less visibility and spreads over the whole width of the sample. To investigate if residue was left on the sheared samples, a lap shear joined is shown in Figure 14 before and Figure 14 after the lap shear tests. Figure 14a shows the as-joined state of the sandblasted AISI 304-polypropylene sample. Due to the amorphous characteristic of the used polypropylene, the joined area has good visibility. Figure 14b shows the same sample that failed adhesively in the lap shear test. No residue is visible either on the steel or on the plastic side. Every lap-sheared sample at the line energy of 81 J mm\({}^{-1}\) failed adhesively, and no macroscopic residue was visible. Additionally, lap shear samples were joined at 81 J mm\({}^{-1}\) with cold-rolled AISI 304, and no bond could be achieved. In addition to the macroscopic analysis of the lap-sheared sample, SEM analysis was conducted on the same sample shown in Figure 8:0 The SEM analysis is depicted in Figure 8:1  
Figure 8:2 Sandblasted AISI 304–polypropylene samples: (**a**) joined area and (**b**) adhesive failure. Figure 8:3 Cross-section of a sandblasted AISI 304–polypropylene hybrid joined with 81 J mm\({}^{-1}\). Figure 15a,b present BSE images of the sandblasted AISI 304 part of the lap-sheared sample shown in Figure 8:4 Dark particles are embedded within a brighter matrix. EDS measurements on both phases showed that the brighter phase consists mainly of iron, chromium and nickel, which are the main content of AISI 304 steel. The darker phase consists mainly of oxygen and aluminum, indicating that these are corundum particles embedded in the surface during the sandblasting process.