# Optimization of Laser-Assisted Polypropylene Aluminum Joining
## 1 Introduction  
The reduction in CO\({}_{2}\) emissions is an important goal in research as it is one main cause of global warming threatening life on earth. An effective way to reduce weight is by switching to lighter materials, building thinner, or reducing the number of parts. For instance, a common strategy to reduce fuel consumption is the substitution of aluminum for steel in the automotive industry [1, 2]. Another interesting alternative to reduce the weight in vehicle applications is to implement new structural designs such as honeycombs instead of comparatively heavy conventional aluminum parts [3]. A recent approach is to employ the miscellaneous properties of metals and polymers in light-weight hybrid structures, hence advanced joining methods must be considered. Compared to traditional metal polymer joining methods such as mechanical fastening or adhesive bonding, Laser Assisted Metal-Plastic (LAMP) joining [4] has the advantage of not requiring other parts or substances (screws, glue, etc.) that can be harmful to the environment and require extra production steps [5]. The choice of polypropylene (PP) can be very interesting as it is a low-density, high-corrosion, and humidity-resistant material [6]. However, due to its non-polar characteristics and bad adhesion properties, natural PP cannot join aluminum without a pre-treatment for LAMP joining. It is due to this reason that no chemical bonding occurs between molten PP and the Al surface. Therefore, to address the poor adhesion of polypropylene surfaces, some processes such as plasma or flame treatments have been implemented [7, 8]. However, some limitations should be addressed such as the lack of durability in the plasma treatment of polymers due to the hydrophobic recovery [9]. Even though the LAMP joining of PP to aluminum (Al) requires an additional step, it still has a high automatization potential and can be faster than traditional methods, considering that the joint is liquid and gas sealed [10]. Promoting mechanical interlocking has been introduced as an effective way to improve the mechanical properties of metal-polymer laser-joined assemblies [11]. Laser-based surface treatments of the metal with a pulsed wave (PW) or continuous wave (CW) laser have been also investigated in this regard for different material combinations [12; 13]. However, the polymeric materials used in these studies are polar. In the case of LAMP joining for polar polymeric materials such as polyamide promoting, physicochemical bonding is also reported [14; 15]. Consequently, increasing the surface roughness or creating grooves on the metal surface to develop mechanical interlocking also contributes to physicochemical bonding thanks to the higher surface available for bonding between the materials. It might be a reason for the contradiction of the surface roughness effect as the significance of mechanical interlocking and physicochemical bonding is not defined, while they both contribute to the mechanical performance of such assemblies. Some reported the positive effect of surface roughness [16] and others found no correlation between surface roughness and shear strength [17]. Additionally, because of the geometry and properties of the metal samples used in these studies, the degradation of the metallic partner due to surface structuring is not addressed, except for a relatively deep laser structuring for the fiction press joining process [18] or the failure of protrusions due to low cross-section [19]. Therefore, it is interesting to evaluate the effect of the laser engraving process on the LAMP joining of a non-polar polymer, i.e., PP via promoting the mechanical interlocking with a minimized effect of physicochemical bonding. Another issue is the thermal degradation of the polymer at a high temperature, which is addressed by the optimization of laser joining parameters for the laser joining of Al to polyamide [15; 20]. Hence, thermal degradation needs to be evaluated for PP during the laser joining process.