are you heating the castings up before you weld or just burning it in ,,,good job by the way
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.