A little pre heating is needed before being able to put full amperage to the weld. This helps bring out all the air pockets and impurity's. There is no need to heat up the whole thing first like cast iron.

BTW stainless rod works well on cast iron.

I think aluminum is easy to weld with the right equipment and fun too. Sure does suck up a lot of heat though.

when i was welding alot of aluminum we would  hit it with a torch  ,,then weld the cast ,,or heavy alum, ,,then hit the welds with a airchisle with a round  or blunt end on it to destress the welds,,,,very awsome work you have done  i hope when you come down to fresno i can get underit and look at it  ,,just awsome