Low temp aluminum brazing rod has been around for a few years. One can see it at hobby and gun shows, usually with some good looking gal using it to put a couple of pieces of raw material together.
A good and not so expensive source of this rod is your local NAPA store. Good news is that most NAPA stores will sell it by the stick or at least fractional pound, but, some will only sell it by the complete package, which is expensive and will leave you with a longer-than-lifetime supply.
I've fixed a butchered Ruger .41 magnum trigger guard, and a "rotted" water line on a small aluminum boat, with the stuff. Used just a propane torch, but usually more heat is needed, as larger aluminum pieces suck the heat away from the brazing area.
The joint must be absolutely clean (filed, scraped, ect.)! An oxy-acetylene torch would do just fine, as in regular brazing on steel, though MAP gas torches work, too.
If one were repairing a crack in a side cover, for instance, he should vee groove the crack with a die grinder or Dremel tool and then lay in a bead of aluma-rod. Putting the cover on a stove burner (when the wife is not home!) works to keep the heat level up. Once done, the "weld" can then be filed down and polished to look like new.
