[JFP in PA]

The only way I know to weld aluminum to steel is using a bimetallic transition metal insert, where one side is steel, the other aluminum. Bimetallic transition materials are available commercially in combinations of aluminum to such other materials as steel, stainless steel and copper. These inserts are best described as sections of material that are comprised of one part aluminum with another material already bonded to the aluminum. The method used for bonding these dissimilar materials together, and thus forming the bimetallic transition, are usually rolling, explosion welding, friction welding, flash welding or hot pressure welding, and not arc welding. The welding of these steel aluminum transition inserts can be performed by methods such as GMAW (Mig) or GTAW (Tig). You weld the steel side to the steel object, and then weld the aluminum side to the aluminum object.

I doubt most people operating out of their home garage are in a position to even attempt it..........