# Optimization of Laser-Assisted Polypropylene Aluminum Joining
## 4 Conclusions  
Based on this study on the laser joining of aluminum to polypropylene, several conclusions can be made:  
* Laser joining of Al to PP is not feasible without pre-treatment processes. Laser engraving a pattern on the Al surface greatly increases the joining capacity by mechanical interlocking for the tensile-shear configuration.
* Several factors influence joint strength. They can be categorized into laser engraving and laser joining parameters. For engraving, line width, density, and speed influence the pattern and tooth dimensions for the purpose of mechanical interlocking. While for joining, an optimized combination of spatial and temporal modulations of the beam improves the weld quality by minimizing the PP thermal degradation.
* Modulated power presents a direct relationship with the depth of the Al melt pool and, consequently, PP thermal degradation. With the setup and configuration used for this study, 280 W of modulated power is the threshold of PP not being thermally decomposed. At 290 and 300 W, PP degrades in the form of small, separated bubbles with less than 25 \(\upmu\)m in diameter, and the degradation of PP is escalated at 310 W and above, which significantly challenged the permeability and mechanical performance of the joints.
* The optimum engraving line width and density are 0.28 mm and 27.3%, respectively, to promote the mechanical interlocking between PP and Al. At a higher density, Al is damaged in the form of deep cavities that do not contribute to the joining process.  
Figure 16: Maximum shear load in the function of the engraving speed (mm/s) with the different failure modes.  
* The optimum engraving speed is 150 mm/s. Low speeds act similarly to high density and make Al prone to failure due to losing the cross-section.
* Finally, with the optimum setup for this study, a load of 1500 N, which corresponds to a strength of 20 MPa, is achieved for laser joining of 0.5 mm thick Al to natural PP. This optimum setup addresses the minimum weakening of the Al, regarding the mechanical performance, due to the laser engraving process, while the engraves are filled with PP in the presence of minimum thermal degradation.
* Achieving such a mechanical performance in LAMP joining via the development of mechanical interlocking for PP as a non-polar polymer can provide different applications in the automotive and packaging industries. It is worth mentioning that laser-based surface engraving with PW lasers is a fast, robust, and reliable process for such applications. However, the mechanical degradation of the metallic partner due to the loss of cross section should be addressed with some solutions such as using high-strength alloys.  
Methodology, M.A.E.; Software, A.M.; Validation, M.A.E. and A.M.; Formal analysis, A.M.; Investigation, A.M.; Resources, P.P.; Writing--original draft, M.A.E. and A.M.; Writing--review & editing, M.A.E.; Visualization, A.M.; Supervision, M.A.E. and P.P.; Project administration, M.A.E. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
The authors declare no conflict of interest.