Aluminum is hard to do with Cermark. (There is another product called Alumamark that works easier.)
For Cermark you’re going to need a high DPI (600+) and slow speeds. Aluminum dissipates heat better than most of the Cermark-able metals so you’re also likely to get warping with it that thin. You want to make sure there’s no coating on the bracelet blanks - they may have a clear coat that’s not going to allow the Cermark to fuse properly. If so, sand them before spraying.
I use two light coats of Cermark with about a half to an hour dry time before lasering.