Yes, you MUST have 100% Ar.
Beware, mig welding aluminum is VERY difficult, buy lots of extra stock to practice on - also, you must change out the liner because the metal one will contaminate the aluminum wire (iron oxide and aluminum powder = thermite).
Get the kit which will have new drive wheels and the liner, and get the right gas, and don't plan on using any thin aluminum, I was never able to have fine enough control on my machine to get it to work well on anything less than 1/8" material. Also, you will probably buildup too much heat and burn through if you try to go far, weld about 1" at a time and let the material cool. Your machine and mine are just barely on the edge of being capable of doing this. It is nearly impossible to dial it in to a reliable point.