Irradiation by the beam from a CO2laser has been found to be an effective means of marking a black anodized aluminum surface. In general, this process works on any dark anodized surface. The beam is scanned over the surface to form a desired pattern. Depending on the circumstances, this technique could be an attractive alternative to painting, machine engraving, and etching; in particular, if there is a need to create a precise and/or intricate pattern, then this technique can satisfy that need in less time and at less cost while yielding better results. For this purpose, one uses a laser that is not powerful enough to cut the aluminum workpiece but is powerful enough to bleach the anodized surface to a nearly white appearance. The precise nature and degree of the bleaching depends on the specific aluminum-alloy/anodized-coating combination and on the parameters of the laser beam and scan. Tests at different laser power levels, scanning speeds, and numbers of pulses per unit length of scan must be performed on a sample of the particular anodized aluminum to find the combination of parameters that yields the best results.
The laser-scanning mechanism is driven by the output of a graphics program running under any Windows operating system. From the perspective of the program, the laser-scanning mechanism is merely a Windows printer. The pattern to be applied is drawn by use of the program, making allowances for the width of the laser beam (analogous to the diameter of a cutting or engraving tool). The drawing is then used to perform a test on a specimen of the anodized material to be marked. The resulting pattern is measured and any necessary corrections are made in the drawing data file. The revised drawing file is then tested as before, measured, and corrected as necessary. The cycle of test, measurement, and correction is repeated until the test and measurement indicate success.
The technique has been used to mark anodized aluminum docking targets shaped as truncated cones on square bases (see figure). In this application, there is a need for some additional process steps (beyond the basic steps described above): It is necessary to measure the anodized aluminum workpiece to determine the heights of the target pattern surfaces for proper adjustment of the laser. The corrected target-pattern drawing file generated as described above is used to generate three additional files: one for the perimeter of the base, one for the target pattern on the base, and one for the target pattern on top of the truncated cone. The base-perimeter file is used to laser cut a hole in a sheet of poly(methyl methacrylate) or other suitable material on the laser work table; this hole is used to position the workpiece. The other two files are used to control the laser on the base and top surfaces, respectively.
This work was done by Roger Megason, Herbert Mitchell, and Marvin Williams of Johnson Space Center.