Welding the cog

I think some techniques are in the brazing or soldering temp ranges but I'm no welder just a little knowledge (dangerous!).

Google brings this up for aluminum / steel welding http://www.eng-tips.com/viewthread.cfm?qid=148077&page=1

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.