Influence of laser surface texturing on surface microstructure and mechanical properties of adhesive joined steel sheets

This work discusses the resultant microstructure of laser surface treated galvanised steel and the mechanical properties of adhesively bonded surfaces therein. The surface microstructure obtained at laser intensities between 170 and 1700 MW cm⁻² exhibit zinc melting and cavity formation. The wavy surface morphology of the treated surface exhibits an average roughness R_a between 1·0 and 1·5 μm, and a mean roughness depth R_z of 8·6 μm. Atomic force microscopic analyses revealed that the R_z inside the laser shot cavities increased from 68 to 243 nm when the incident laser intensity was increased from 170 to 1700 MW cm⁻². X-ray fluorescence analyses were used to measure Zn coating thicknesses as a function of process parameters. Both X-ray fluorescence and X-ray diffraction analyses demonstrated that the protective coating remains at the material surface, and the steel structure beneath was not affected by the laser treatment. Tensile tests under peel strength conditions demonstrated that the laser treated adhesively joined samples had resistance strength up to 88 MPa, compared to a maximum of only 23 MPa for the untreated surfaces. The maximum deformation for rupture was also greatly increased from 0·07%, for the original surface, to 0·90% for the laser treated surfaces.