Clean your aluminum with acetone, to remove surface grease. Rinse the aluminum thoroughly with water, to get rid of acetone residue, then scrub it with a stainless steel brush.
Clamp your aluminum to the backing bars, which will act as a heat sink and protect sections of the aluminum that are not being welded from warping during the welding process.
Preheat your aluminum, in an oven, to between 275 and 500 degrees Fahrenheit. Ask the manufacturer of your base metal for specific directives on welding temperatures.
Weld the aluminum in a straight line. If using a TIG welder, as recommended, set the welder's adjustable balance control to obtain a larger amount of penetration and a narrower weld bead, instead of a wider weld bead and shallower penetration. Set your TIG welder to focus more heavily on the "Electrode Negative" portion of the cycle, instead of the "Electrode Positive" portion.
Move as quickly as possible through the aluminum, to avoid an excessive heat-treated zone, but be sure to obtain a welding pool of the size recommended by your aluminum manufacturer. If recommended, add filler metal, of the same mechanical characteristics and appearance as the aluminum, to the weld. This will shield the welding pool from developing metal oxides, and will reduce undesirable side effects of welding, such as warping.
Allow the weld several minutes to cool.