In order to weld aluminium you must use:
Pure argon or argon helium mix as the protection gas.
A welding wire with a composition suitable for the base material to be welded.
Note: If only a torch prepared for steel wires is available, it must be altered as follows:
Make sure that the cable is as short as practicable, if possible no more than 3 metres long.
Remove the brass liner nut, gas nozzle, contact tip and then take out the steel liner.
Insert a Teflon or carbon fibre liner making sure that it protrudes from both ends.
Screw the contact tip back on so that the liner butts up to it.
At the free end of the liner insert the liner nipple and O-ring and fasten with the nut without over-tighten.
Use wire feeder rollers suitable for aluminium wire.
Adjust the pressure exerted by the arm of the wire feeder group on the roller to the lowest possible setting.
Consider the use of a push-pull or spool on gun type welding gun to aid wire feed consistency.
Ensure that the contact tips are suitable for use with aluminium.
Make sure the material to be welded is clean.
Remember to fill the crater at the end of the weld to reduce weld defects.