You can't weld aluminum to steel, so solder, braze, weld, etc are no go. If both surfaces are a bit rough, brushed rather than polished shiny, I'd probably try a urethane-based glue in addition to nuts and bolts. Give it a couple of days to dry, since it drys by solvent evaporation, and the metal will block that except at the edges. A thin glue layer will be better than a thick one, and mechanical fasteners will be most effective around the edges since if the glue fails, it will be by peeling rather than sliding or straight pulling.