As one cannot weld the two so diffenet material direct, special bi-metal (Al+St) pieces/bars/plates are commercially available. They have been manufactured by explosive bonding, sometimes microscopic amounts of other metal layers have been added on the bonding for special properties. Anyhow, these bi-metal transition pieces act as adaptor between your parts; you weld your Al part to the Al-side of the piece, likewise your St part on the St-side of the piece. For example shipbuilding industry (esp. those building large passenger vessels and special craft) is widely using this kind of bi-metal parts for joining large aluminium structures to the steel hull, e.g. deckhouses, funnels or large upper deck structures of aluminium (Al used for weight saving purposes only) to the steel hull structures. Also the same type of connecting techniques is used in gas ships having their spherical cryogenic cargo tanks made of aluminium (Moss-type LNG Carriers) where the huge tanks are fixed to the steel foundation ("skirt") by welding which is possible only by such large bi-metal bars. The Al. gas tanks are maybe examples of the largest bi-metal transition joints (Al and steel parts are 40 to 70 mm by thickness, the transition pieces being up to 100 x 200 mm by cross section) whereas the smallest 'sizes' are for connecting material thicknesses of some millimetres only, the bi-matal bar then e.g. 10 x 20 mm. Round pad-like bi-metal transition pieces are available for welding an aluminium pillar onto a steel deck, so applications are many. The availability of bi-metal pieces (which allows welding) has superseded bolted connections between Al and steel which would require in most cases (at least in shipbuilding) a galvanic insulation between the different metals against corrosion. Bi-metal joints do not suffer from any kind of galvanic corrosion.
