Certain metals can be very difficult to solder. Aluminium is a typical example. The most common reason for this is a thin (a fraction of a nanometre) protective oxide layer. The trick to soldering these metals is to strip this layer, usually with an acidic flux, and creating an environment that prevents it from reformning by keeping the oxygen out.

There are special fluxes for such applications but a simple home-made trick is to use common Aspirin. It is acidic enough in the melted state to dissolve the oxide layer and, if used correctly, will keep the oxygen out. BUT - be very careful! Aspirin produces a lot of noxious fumes, make sure you have adequate ventilation. Also make sure that after soldering you clean everything that came into contact with it as it is a bit corrosive.