Go with a flux. Very like the solder, the flux needs to be specialized for aluminum or for the combination of metals you intend to hitch. The easiest alternative is to invest in the flux from the identical source as your solder, as These are likely meant to work together. https://sergiouzhbk.widblog.com/82138638/helping-the-others-realize-the-advantages-of-types-of-solder-to-use-on-aluminum-radiator