From the 2008 catalog on the site:
Fuji utilizes three different aluminum alloys. A6 is our revolutionary quaternary phase alloy and contains magnesium, silicon, copper and other proprietary elements. It is up to 35% stronger than other, more conventional aluminum materials used in bicycle frame construction. This particular combination of elements, combined with a unique heat treatment process, actually improves weld zone strength by 30% and allows for thinner walled tubes – and therefore, a super lightweight frame - without sacrificing strength and durability.
Altair 2 and Altair 1 alloys both start with the finest grade aluminum, which is then drawn and shaped to Fuji’s specifications. Altair 2 differs slightly in that tubes made of this material are custom butted – a more intensive process where wall thickness, or gauge, is manipulated to save weight in some areas of the tube without sacrificing strength.