It is not recommended to etch amluminum due to its highly reflective property. Using the low power for etching the beam can easily be reflect away, becoming damaging to the machine or operator.
Would it help if there was a decent oxide layer on the aluminum to absorb the beam and thus create a 'light' etch?
Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.
The only decent way to etch aluminum with a CO2 laser is to have the aluminum anodized or use the "Etching Aluminum" made for laser engraving. This is the stuff the trophy and plaque places use. It usually has a thin layer of aluminum on top then another thin subtrate under that shows up as black once the top is vaporized. This is not what you want since your probably using 6061 for a structural purpose. Your cheapest and best bet would be to have it silk screened or decaled or if you need a permanent mark have it routed in. Trying to engrave regular aluminum will only damage your optics and equipment.
try using 2000watts at 1800 fr. with 70 % duty cycle at 180 IPM. I can't guarentee that your lens won't burn up after a few hours of this, but it works short term.