Influence of self-organizing microstructures on the wettability of molten plastic on steel for hybrid plastic-metal joints

Abstract

Multi-material construction enables new approaches to lightweight production by using the right material in the right spot. In order to combine different materials suitable joining techniques are required, e.g., for plastics and metals. The production of plastic-metal hybrid enables reducing dead weight while maintaining and preferably boosting the components performance. Common joining techniques like adhesive joining and riveting have specific disadvantages and direct welding of these materials fails due to their different physical and chemical properties. Another promising approach is a two-staged laser-based joining process. First, a laser source generates microstructures on the metallic joining partner to increase the boundary surface and create undercut cavities. In the second step plastic and metal are thermally joined together. Both joining partners are clamped together, the metal surface is heated up with a laser, and through heat conduction, the thermoplastic polymer matrix melts and flows into the cavities. After hardening, a connection is formed. For metal surface microstructuring, ultrashort pulsed lasers can be used to create a spongy topography. These self-organizing microstructures, so-called cone-like protrusions, have previously shown to have a big influence on the wettability of the surface. For hybrid joining, it is essential to have a complete filling of the microstructures with molten polymer in order to achieve high joint strengths, which can be achieved by a good wettability of the metal surface. In this contribution, the wettability of molten plastic (PA6 and PP) on microstructured steel surfaces is evaluated to identify the correlation between wettability and joint strength.