On Friday, April 29, 2016 at 7:51:50 PM UTC-7, wrote:

I want to solder a steel tube into an aluminum one. I'm thinking that
if I tin the steel with the mostly tin lead free plumbing solder and
the aluminum with the mainly zinc aluminum solder I should then be
able to join them together

Fluxes for aluminum (fluorides) and for steel (acid) are very different.
Probably you'd have to tin the parts separately, then use
a milder flux while heating the whole assembly, and let solder
wick in from one end or the other of the joint. There are
many steps to this, it's probably easier to knurl the steel, apply some
compatible sealant/adhesive (silicone?) and press the assembly together.
That has the advantage, too, that you can anodize the aluminum beforehand.