Laser texturing of steel was investigated in WP4 with average powers exceeding 100 W. The optimum point for the repetition rate of 10 MHz was around 60 W. The heat accumulation can be reduced by reducing the overlap and enlarging the marking speed. The surface quality becomes better with decreasing overlap i.e. higher marking speed because the heat accumulation will also be decreased. However, for the overlaps of 25% and 12.5%, the single pulses become visible implying that the interlaced mode should be used to get a flat surface.
It was shown that the ablation process is, in principal, scalable up into the several 100 W regime, but it is limited by:
- Plasma shielding which occurs for copper and brass at the repetition rates of several MHz and increases for higher repetition rates. The effect becomes stronger as higher removed mass per pulse at the optimum point is.
- Heat accumulation which leads to bumpy surfaces. This effect becomes stronger if lower the heat conductivity or thermal diffusivity of the material is.
3-Pulse burst efficiency
In previous experiments, an increase of the specific removal rate for a three-pulse burst and copper was observed. A hypothesis concerning plasma shielding and change in the reflectivity of the surface due to its molten state was made. To foster this hypothesis, additional experiments with varying energy per pulse in the burst, other materials (gold and silver) and calorimetric measurements were performed and compared with previous results on copper and steel.
The behaviour of the second pulse can be explained by plasma shielding but for the increase in efficiency of the third pulse in case of copper, silver and gold alternative explanations have to be found. In APPOLO project, we can conclude that it is possible to achieve high specific removal rates with the three-pulse burst which can reduce the demands concerning the repetition rate by a factor of three.
Work on flat surfaces and stitching
The synchronised galvoscanner allows to combining two images. After the successful alignment of the two coordination system, the marking strategy in the intersection region is crucial for seamless stitching. The stitching strategies are already shown. In the last step, the best strategy was transferred to the setup including the synchronised galvoscanner and a linear axis. With a light microscope, almost no influence on the surface quality and optical effect can be found using the described strategy.
Partners: BUAS, IOM, LUMENTUM, NST, DG, SWG