Steve. Welding and brazing are two different systems.
Welding actually melts the material into one piece (I am referring to open-flame, oxy-acetylene welding ....like with the two tanks). If a rod is used, it is usually of the same material.
Brazing is basically soldering using brass as the material being melted to join the two other pieces of metal. In brazing, like soldering, the brass is not melted by the flame, but rather by the metals being joined. They have been heated to the point of melting the brass when it is applied to them. This would warrant the two metals being joined to have a higher melting temperature than the brass. This being said, aluminum has a lower melting point than brass.
There are a couple products out there that are designed specifically for joining aluminum through heat. It works kind of like welding, but in a soldering sense. The trick to all of this with the aluminum(and regular brazing and soldering) is the special flux used. Flux cleans the area being joined and removes the oxidation from the surface; ensuring a clean and strong adhesion.
Try this special stuff out on some scrap first. It's a bit tricky. You can find it at Home Depot with the solders etc.
Hope this helps, and sorry for being too long winded.
