Laser-based surface pre-treatment for metal-plastic hybrids using a new process strategy

In order to join metal-plastic hybrid parts so as to achieve lightweight constructions, innovative joining technologies are essential. For this, thermal joining is a promising approach due to the absence of any joining agent. The joint strength can be increased by surface pre-treatment of the metal with continuous wave laser radiation. Large joining areas, however, are challenging in terms of productivity, efficiency and geometrical flexibility when using conventional beam trajectories. A more promising approach is the use of continuous beam trajectories, which can be easily combined with on-the-fly processing. More precisely, a circularly oscillating laser focal spot is geometrically independent of every possible in-plane load direction, and causes lower thermal distortion of the metal sheet than using conventional beam trajectories. Therefore, the circular beam oscillation was characterized for the processing of large areas. In this context, the resulting topography of the metal was analyzed with respect to the surface roughness and a design criterion was mathematically derived and experimentally validated. Additionally, the joint strength was compared to the strength resulting from conventional beam trajectories. For verification, steel substrates were joined with a carbon-fiber-reinforced thermoset. The laser-based pre-treatment using the circularly oscillating laser focal spot led to a similar joint strength, while being more productive when combined with on-the-fly processing, compared to conventional structuring trajectories. Consequently, the processing time can be significantly reduced.