Some tips for welding aluminum with under-powered welders

• Cleanliness is next to Godliness

1. Use only a clean stainless steel brush. Contamination with steel, grease etc. is deadly!
2. If grinding , for example to remove a bad weld, use a wheel specified for aluminum and don't grind anything else with it!
3. Use an Aluminum cleaner and de-oxydiser (available at better welding supply stores) rinse off with CLEAN water and dry with a clean rag.
4. If you make a mistake or two and have to grind off a weld, remove it completely! Bad welds trap contaminants which will outgass and mess up the next try.
   

• Pre-Heating the material is a good idea, especially if your material has a large surface area.

1. 500-700 degrees is good according to this tip on the Hobart Welding Board by Seldom. To get the temperature right you can use a pine sliver as he describes.
2. It may be completely impossible to weld if you don't pre-heat.
   

•  Maintaining a gas shield pocket is critical

1. You must use 100% Argon to weld aluminum. (There are some specialized applications which use a combination of inert gasses)
   
2. Make absolutely certain you have the correct gas flow. I found my gauge had been bent at some time and read much higher than it should have. Nasty slag with pellets of bird shit which do not bond at all!
3. The correct angle is critical - See the article at Miller Welding: Aluminum Welding Hints. This is not as easy as it seems at first! It's quite different from welding with steel. 
4. Always use the "Push" technique. You may not be able to get into the "Spray Mode" with a welder under 200 Amps capacity but you can still make good welds.
   

• The adjustment of wire feed rate and current is quite critical. Make some practice welds on scrap material to get it right.
  

Some places with more info on both Steel And Aluminum MIG welding