There is a company called superior flux that sell aluminium flux cored solder wire that is based on either 99C or 96S solder which is 99% tin with 1% copper and 96% Tin with 4% silver respectively.

I have personally used these products to solder aluminium but you do need to also buy their aluminium cleaner and you need to solder at 300 degrees centigrade.

The fluxes are very corrosive so you will need to wear PPE and use the aluminium cleaner afterwards to clean off the corrosive flux residues.

Good luck and Ben Baskin at the website is very helpful.

Be aware that some aluminium grades will not solder at all.

The other alternative is to use zinc-tin solders but the solder is very prone to corrosion.