Select the shielding gas. Aluminum welding typically uses a shielding gas of pure argon. Steel welding typically uses a blend of argon and CO2, so you may need to replace any regulators that are made specifically for CO2.
Choose a MIG welding machine for aluminum. The welding machine must have more voltage so that it can form the welds more quickly. A 115-volt feeder is adequate for an aluminum work piece that's up to 1/8 inch thick, and a 230-volt machine can handle aluminum that's up to ¼ inch thick.
Use electrodes of the right alloy and thickness. Thinner electrode wire is more difficult to feed, while thicker wire needs more time to melt. In the case of aluminum, there is an extremely narrow range you should use. Electrode wire for aluminum should be 0.035 inches in diameter. The most common alloy is 4043 aluminum and harder alloys like 5356 aluminum may also be used occasionally.
Feed the electrodes wire with a feeding kit specifically designed for aluminum. These kits should include contact tips with larger holes than would be used for steel wire of the same diameter because aluminum expands more than steel when it's heated.
Weld as quickly as possible. Use a single, straight pass on each weld to prevent the entire work piece from overheating and possibly melting.