Is it possible to weld dissimilar metals? Alum casting to cast iron for example?

No...and yes.

The basic answer is that you can only join metals that share physical characteristics, like brass and bronze are both copper alloys.

Steel, stainless steel, tools steel, cast iron can all be joined using various specialty filler metals.

Aluminum and steel are usually joining using "transition metal" plates.

Transition metals are dissimilar metals joined using high explosives. The joined plates are then sawed into strips. The steel side can be welded to a steel boat hull, and the aluminum side is welded an aluminum super structure.

These guys make the plates.

I just read an article in Welding Journal about a cold wire feed welding process that can join aluminum and steel. Freaky stuff. Very cutting edge.

Does anyone still do that ? I thought they were nearly all made with friction welding these days - much cheaper to run the process.

I surprised myself the other day when I managed to attach steel and aluminum. I used the Esab 31 alloy aluminum brazing rod and flux with no difficulties. This rod isn't like the aluminum soldering rods and does a real good job on some types of aluminum. However it won't work with cast aluminum at all (I think that the melting point of cast aluminum is under the rods melting point). It does work wonders on condenser coils which is where I did the aluminum to steel job.

Wayne Cook Shamrock, TX