In my limited 10000 hours of arc time experience, i have found, it behooves both processes to preheat for good results. Aluminum sucks the heat from the puddle to the surrounding material due to the heat conductive properties of the metal. I do still hold a valid American Welding Society cert for Aluminum, all thicknesses, all positions, all alloys, x-ray clear, for both Mig and Tig procedures. Just in case you're wondering where I'm coming from.
