For aluminum materials, the natural oxide layer is hygroscopic and the thickness increases with time. Therefore, removing this rough, contaminated oxide layer to expose the underlying aluminum may be sufficient to create sufficient contrast. Another more complicated factor is that the degree of melting or ablation of the underlying aluminum material can significantly affect the appearance of the mark.
Careful adjustment of the parameters of the laser produces a brighter surface that exhibits a higher contrast melting effect. By using a pulse energy of ~1 mJ, a deeper, more oxidized surface can be formed on the aluminum material, but if a low L* value is desired, a strong, non-fragile surface can be obtained, The appearance of the mark does not change as the viewing angle changes, requiring careful control of the process. Increasing the level of ablation to form a slightly rough surface also results in a surface that is darker in color, higher in absorbency, and has a larger L* value (Fig. 1). The surface dimensions shown were all <10 μm and the surface roughness (Ra) was much lower than <5 μm.
Figure 1: Dark grey aluminum surface treated with a 5ns, 75μJ laser, magnification: 200X
The removal of anodized coatings from aluminum surfaces is a widely used technique, and the same rules apply to the application of lasers on substrates - a strong meltability means a more reflective surface. Whether it is bare aluminum or anodized aluminum, the marking speed is high at 1-2m/s. Recently, laser marking techniques have been developed on specific anodized coatings, using a low nanosecond, sub-nanosecond fiber laser to achieve an L* value of <30, although the marking speed is much lower than in the above manner.
Figure 2: Surface effect of 0.8mm thick copper material treated with 0.15 nanoseconds and 1 nanosecond pulse