Soldering aluminum is very difficult, because... it instantly oxidizes upon contact with the atmosphere, even if it has just been cleaned. For soldering there are powerful acid fluxes, like the proven F-64A (radio engineering stores), but they are not available everywhere.
I came across a trick in a Soviet textbook to get around these difficulties. A solution of copper sulfate is taken (sold in fertilizers in the form of a blue powder), and then a few drops of the solution are applied to the cleaned and degreased aluminum. Carefully (so as not to touch the aluminum) a copper wire is dipped into the solution, to which the “+” of the voltage source is connected. The aluminum part is connected, accordingly, to the “-” of this source. The current passes through the vitriol solution and precipitates copper from it. The exposure time depends on the current and voltage (it can be a 1.5V AA battery or a 12V battery).
In my case it was a 12V battery and I didn’t even bother with the solution - I just dropped tap water on a few crumbs of vitriol
