Surface design and texturing of strip steel using nanosecond pulsed lasers for simulated roughness transfer and paint appearance

The final surface topography of flat sheet (galvanized) products is achieved using textured rolls during the skin-pass process. The topography is generally applied to improve the surface functionality of the sheets obtained, or reduce downstream environmental effects. This work analyzes the short-pulse laser texturing technique as an technology to improve a key functionality of galvanized metal sheets for the automotive sector: the paint appearance. For this purpose, deterministic patterns were selected from theoretical methods and produced on imprinting dies by nanosecond pulsed laser processing. The pulsed laser texturing technique has been shown to be highly controllable with suitable reproducibility and surface characteristics in terms of roughness (Ra), peak count (RPc) and waviness (Wa) values, as main factors connected to the desired paint appearance functionality. The roughness transfer capability during the skin-pass process was assessed by normal loading compression tests for different contact pressures and sheet materials. Additionally, the painting process applied in the production chain of automotive parts were reproduced at lab scale, following by the waviness and roughness characterization and validation of the topographic designs proposed. Experimental results show a strong dependence of transfer capability of textures as function of mechanical properties of the metal sheets. For the considered deterministic patterns meaningful differences in terms of transfer capability of Ra and Rpc to steel strip were found as function of the pattern topography reaching maximum transfer capability of 60% and 130% for Ra and Rpc respectively, for contact pressure range between 250 MPa and 500 MPa, corresponding to maximum absolute values of Ra = 1.5 μm and Rpc = 190 cm-1 on the steel strip. Concerning waviness, results revealed a significant decrease of waviness over all length scales considering Rpc>75cm-1 and 0.7 μm < Ra<1.5 μm, which involves an enlargement of the current process window with respect to current Electro Discharge Texturing process.