Laser Welding of Metals to Polymers Under the Special Influence of Surface Pretreatment on Interfacial Thermal Transfer

Joining of metals to polymers is increasing among various industries because of its ability to produce lightweight hybrid products with tailored properties. Common joining methods, such as adhesive bonding and mechanical fastening, require adding a third material which might involve hazardous chemicals or add extra weight and stress concentration points to the component. On the other hand, Laser-Assisted Metal – Polymer joining (LAMP) is a contactless, rapid, highly precise and energy-efficient technique, that produces autogenous and miniaturized joints. It was already demonstrated that surface pretreatment prior to the welding process has a significant impact on the joint performance by modifying surface chemistry and topography, promoting chemical bonding and mechanical interlocking. This research aims at expanding the understanding of the effects of surface properties on the joint’s performance by investigating their influence on interfacial thermal transfer. While increased surface roughness of metallic partner is expected to improve LAMP joint performance by promoting mechanical interlocking, it is hypothesized that a smoother surface would improve the joint quality by enhancing the interfacial thermal transfer during the welding process, resulting in a larger area of molten polymer at the interface and a better joint performance. In this research, aluminum (Al1050) and titanium (Ti64) were joined with polyamide (PA6.6). Initially, laser welding parameters were optimized and kept constant during all surface pretreatments’ investigations. Preliminary surface pretreatments, using short-pulsed laser ablation and atmospheric plasma pretreatment, were conducted on Al1050 – PA6.6 to analyze the effects of surface composition and topography on joint quality and performance, and to optimize interfacial adhesion. Results show a correlation between increased surface oxidation and surface energy with improved interfacial adhesion and joint resistance to shear failure. Compared to plasma pretreated surfaces, laser ablation of metals results in a very rough surface which exhibits perfect wettability to both water and diiodomethane. This promotes mechanical interlocking and interfacial adhesion, resulting in a relatively stronger joint failing in a cohesive failure mode. Results confirm that an improvement of the assembly’s shear resistance to failure can certainly be achieved without an increase in surface roughness and interfacial interlocking, as observed in case of plasma pretreatment. Design of Experiments (DoE) techniques were utilized for both material combinations in order to optimize laser ablation process and to investigate the effects of pretreatment parameters on surface properties, interfacial thermal transfer, joint quality and resistance to failure. Laser ablation parameters influenced the surface topography with no significant effect on the surface composition, and all laser-ablated surfaces showed perfect wettability to both water and diiodomethane. While all ablated surfaces demonstrate cohesive failure mode, smoother ablated surfaces results in a better interfacial thermal transfer as indicated by the Thermal Contact Resistance (TCR) calculations and measurements, based on Cooper–Mikic–Yovanovich (CMY) model and layered Laser Flash Analysis (LFA) investigations, respectively. Results show that a smoother ablated surface results in better interfacial thermal transfer, melting a larger area of polymer which increases the joint quality and resistance to shear load.LaserSTAM