I'm not all that familiar with the term aluminum solder. I do have
some of the low temp (750F?) aluminum rods that you occasioanlly see on
late night infomercials and are sold at auto swap meets and flea
markets. They work well on aluminum and pot metal and pretty much form
a soldered joint.

I've fabricated some objects out of discarded aluminum sign stock with
a combo of folded edges and seams filled with these low temp rods and
ground/sanded smooth. The work has held up well in a vibration prone
section of a motorcycle.

The rods I have don't need flux, but they do specify to not overheat.
OA is too hot. Simple propane torch is recommended.

Try your solder without the flux and use low heat like a propane torch
and see what happens. Make sure the joining surfaces are clean right
before starting. Clean aluminum usually means degreased with acetone
and wire brushed with a stainless brush never used for anything else.