Generation of undercuts using cw-laser structuring as a surface pre-treatment for thermally joined metal-polymer hybrids

Pulsed laser systems are often used for surface pre-treatment prior to thermal direct joining, as they enable the creation of undercuts but their process speeds are too low for industrial applications. Continuous wave (cw) laser structuring is a promising alternative that can achieve high area rates of up to 200 mm²/s, representing a 10- to 100-fold increase. Although this allows high strengths to be generated in the shear direction, the strength in the normal direction is significantly lower due to the lack of undercuts. To address the latter problem, this study investigates three laser structuring strategies known as ‘oblique irradiation’, ‘hollowing bottom’ and ‘remelting top’. They aim to create defined undercuts to enhance mechanical interlocking between cold-rolled DP1000 steel and carbon fibre-reinforced PA6. The strategies were evaluated through metallographic analysis and mechanical testing in tensile shear and cross-lap tensile configurations. Among the approaches, the “Hollowing bottom” strategy demonstrated superior performance, achieving a maximum tensile shear strength of 30.3 MPa. A subsequent optimisation loop of this structuring strategy based on the principle of ‘design of experiments’ (DoE) revealed that the morphology of the undercut and reproducibility have a significant influence on the bond strength, especially under peel load. This resulted in a 40% increase in head tensile strength compared to the reference and to demonstrate the clear benefits of undercut surface structures. The findings highlight the potential of optimized cw-laser structuring to improve joint reliability in lightweight hybrid applications.