My first choice would be a very sharp 1/2" HSS two flute endmill. If you go smaller diameter, you'll have trouble doing full depth in one pass. HSS takes flex shock better than carbide and you don't wear cutting edges dull on AL, also much less $. Carbide will certainly work if you don't have trouble with breakage.
You may have trouble with AL welding to the cutter. If this happens, reduce your speed, increase your feed, and add coolant if you can.