During the laser marking process called annealing, no material is taken away, which means there is no ablation. There is no tangible mark on stainless steel and other metals; however, a color change occurs through heating up the metal. Stainless steel, for example, changes color when it reaches temperatures between 392 and 572 degrees
Different colors can be achieved with different temperatures, though the goal is usually a “perfect black.” Perfect black markings are possible on stainless steel and titanium, but not on other metals. Annealing is also possible on any alloys that contain iron.
To anneal metal, it is important to only heat it instead of vaporize it. This can be achieved by reducing the energy density of the laser impact on the steel. The best way to do this is to go out of focus to increase the laser spot size, as the same energy on a much bigger surface will reduce the energy density. Heating up metal takes time, therefore, slow marking speeds should be used.
Using the conventional method of annealing, the laser is used out of focus (to get a large laser spot for heating the material) and the marking speed is set very low because heating takes time.
With this process, the user can create deep black (even partly-colored) and homogenous markings on stainless steel; however, due to the large spot and slow marking speed, there are also some negative effects:
- Fuzziness at small markings
- Long marking times
- Corrosion sensitivity because the intense heating of the material can cause the marking to corrode
-Josh Stephens, Trotec Laser
