Check out the Kent White videos or some of the online stuff about oxy-acetylene welding aluminum. Don’t think you’ll have much luck welding steel to aluminum.
Another little comment. some of the rod you buy "lately" is not really "braze" it is more like solder. Lower temp. Decades ago a then boss was going to braze up a bunch of aluminum irrigation pipe, and got set up. Turns out that legit aluminum braze makes a VERY obnoxious, messy, impossible to see through flame because of the chemical interaction. There is a work around. It is called "cobalt blue." These are filter glasses which are also used in the glass blowing industry. They are the OPPOSITE of green welding glasses. True "cobalt blue" glasses do NOT block ultra blue spectrum of light. They block the infra red and let all the "high" end of light through. You must be careful with them, because say, in sunlight they can damage your eyes. But they do allow you to see through the messy flame
You can tell if you have "real" cobalt blue by going into a room lighted by fluorescent lighting. and with white walls or a white T shirt. Wearing the glasses will give you the effect of being in a "black light" bar. Your nails will glow, as will your T shirt or anything else white.
