Im cutting 2mm aluminium circles at 600mm plus diameter & getting heavy dross. When I cut 1.6mm circles they are very clean on the edge, Im using the cut speed as recommended by hypertherm at 6600 136v
I'm not an expert but more power may help. Aluminium needs a lot more power than steel for a given thickness. Try contacting Jim Colt from Hypertherm. He hangs around on several forums including CNCZone and he really knows his stuff.