Can't say enough about PRE-HEAT when welding Aluminum.
Aluminum melts around 1220 F (660.37C). and will continue to absorb heat to its boiling point.
There fore if you want to weld it you must PRE-HEAT. If you are welding on a large section of aluminum you may find you will need to preheat even if you have a 400 amp TIG torch.
Get a 600' F temp stick and preheat a large enough area to retard the heat sink with the torch you will be welding with (no jewlers torch you need lots of heat). If its a small project you may need to heat the entire work and what ever it rest upon. Fire bricks are great work rest for Aluminum (you don't need a conductor when Oxy/Acet welding).
If you do a good job of PRE-HEATING you will discover it welds very well and will be hard to tell from TIG (When you get some experience).
Keep your work clean. Use a neutral flame.
Do not lay your work on another heat sink like a big thick piece of plate (Steel or other wise) while welding.