Hi James,
Where I work, we do a lot of alum. castings that are 1/2" thick and 1.5" wide with a 1/4" bevel at the welded joint. Each piece is pre-heated and we found these castings weld best with 100 % Ar.. We MIG using 4043 .035 at 30cfh. The caster never revealed his composition to us, so we had some trial & error. The castings are very clean & bare when we get them, so we only have to brush with a SS brush at the joint. Our initial test showed 4043 gave much better fill than 5356 0r 5556. Since these have no stress or torque on them, we have suffered no failures with the welded joint. Also, look at the previous post bill4d posted for additional info.. Hope that helped a bit......Denny