Brandyn

With welding any aluminum the biggest part is to get it clean without contamination. Use a fresh grinding wheel that hasn't been used on steel, or S.STL.

For repair work GTAW, (TIG) welding is probally the easiest method of welding aluminum. although SMAW (stick) and GMAW (mig) may also be used.

(the following pertains to GTAW welding Alum)
The amperage will depend on the thickness of the part, the thicker the part the more amperage required.

Two common methods to reduce the amperage are changing the gas to an argon/helium mix, the more helium in the mixture the hotter the arc will be, or preheating the part, or both depending on how thick the part is.

To choose a filler metal properly you will need to know your base alloy.
 
From there you can choose a rod.

If you post more information with the particulars of your job  be happy to make some suggestions.

Here is a link to the WELDPOOL website procedures for GTAW welding of aluminum http://home.istar.ca/~bsant/WeldProc/algtaw.html

Hope these help you, Rich

I HAVE A SMALL BOAT AND I NEED TO WELD THE SCAG BACK ON THE LOWER UNIT OF THE MOTOR.I HAVE A PORTABLE ARC WEDER AT WORK IS THERE A SPECIAL ROD I CAN BUY TO WELD ALUMINUM THE PART IS ONLY 1/4 OF AN INCH THICK PLEASE ADVICE.

Hi Tony B
If your component is aluminum and only 6mm thick-your best
bet would be ac~ TIG(GTAW) or oxy-acetylene if you have access to the right consumables.
Cheers - Trefor Williams.

CLEANLENESS IS NEXT TO GODDLINESS WHEN WELDING ALI" .THEN PROPER HEAT AND ROD IS NEEDED AS STATED BY "RICH".CLEANING IS MOST IMPORTANT SINCE ALI" IS SO PORUS.

Tony B: NO, you can't do it with what you have. There is no "special" rod that will allow you to stick-weld aluminum.

brandyn,
 you just need a big tig machine and a 10% silicon filler rod,at a pinch you could get away with  5% Si filler or use a mig and 4043 wire, depends as someone said on thickness,
cheers
Stinkefete