## 4 Conclusions  
The aim of this investigation was to identify the interaction between a low-pressure plasma treatment and the lap shear strength of laser-joined metal and plastic hybrids. The focus was on understanding the correlation between surface free energy, lap shear strength and potential residue on the surface of the metal due to covalent bonds. The achieved results can be summarized as follows:  
* Nonpolar plastics, such as the polypropylene used, require sufficient mechanical adhesion when employed in a laser joining process with metals. No plastic residue was detectable on the fractured surface. The maximum lap shear strength of 4.26 MPa was achieved for sandblasted AISI304-polyppropylene laser-joined hybrids at a line energy of 81 J mm\({}^{-1}\).
* Polyamide 6.6 hybrids can be joined without mechanical adhesion on smooth surfaces. Adhesively fractured samples leave residues on the steel surface. Higher line energies can induce cohesive failure within the plastic sample. At 158 J mm\({}^{-1}\), the cold-rolled AISI 304-polyamide 6.6 hybrids achieved 2.85 MPa with an adhesive failure mode.
* Low-pressure plasma treatments with argon, oxygen and water as plasma gases increase the surface free energy significantly. The disperse and polar values calculated by the OWRK method were at the same level at 180 s treatment time, elevating the surface free energy by a factor of 2.4 for polypropylene and 2.3 for polyamide 6.6 to ca. 80 mN m\({}^{-1}\).
* The low-pressure plasma-treated polyamide 6.6 samples exhibited slightly varying lap shear strengths, ranging from +2.7% to \(-\)12.9%, depending on the plasma gas used. The variation was in the range of the untreated samples. The drying effect of the low-pressure treatment increased the number of cohesively failed samples. This also led to a more fractured appearance of the residue on the steel sample.
* The lap shear strength of the plasma-treated polypropylene samples decreased by 30.8% for argon plasma, 42.7% for water plasma and 53.3% for oxygen plasma. The decline was attributed to over-aging and the formation of low-molecular-weight oxidized materials on the surface.  
These findings contrast with [PERSON] et al.'s research [25], which identified a correlation between the surface free energy and the lap shear strength of laser-joined metal and plastic hybrids. In this study, the low-pressure plasma treatment either had no effect or negatively affected the lap shear strength. Furthermore, the creation of polar groups on the nonpolar polypropylene did not lead to the formation of covalent bonds between the plastic and the steel surface.  
Conceptualization, C.T.; methodology, C.T.; validation, C.T. and L.W.; formal analysis, C.T.; data curation, C.T.; writing--original draft preparation, C.T.; writing--review and editing, L.W. and P.F.; visualization, C.T.; supervision, W.T. and C.H.; project administration, W.T. and C.H.; funding acquisition, W.T. and C.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the German Research Foundation (DFG, TI 343/170-1, HO 4776/57-1).  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The authors declare no conflict of interest.