Keller's reagent might be expired so you can make new/fresh solution or you can use %0,5 HF reagent but the percentage %0,5 might be excessive for 6061 aluminum alloy so you have to decrease contact time.

You might try one of the Modified Keller's:

2.5 ml HNO3

1.0 ml HCl

1.5 ml HF

95 ml H2O

This tends to work a little better on 6061.

I think the above combination needs a little change:

2.5 mL HNO3

1.5 mL HCl

1 mL HF

95 mL H2O

Van Der Voort lists several "Modified" Keller's Reagents. This one seemed the most appropriate for 6061.