Aluminum Standards and Data provides definitions for welding and brazing:
Welding is the "joining of two or more pieces of aluminum by applying heat or pressure, or both, with or without filler metal to produce a localized union through fusion or recrystallization across the interface".
Brazing is the "joining of metals by fusion of nonferrous alloys that have melting points above 425 degrees C (800 degrees F) but lower than those of the metals being joined. This may be accomplished by means of a torch (torch brazing), in a furnace (furnace brazing), or by dipping a molten flux bath (dip or flux brazing)." According to the ASM Specialty Handbook: Aluminum and Aluminum Alloys: "Brazing, by definition, employs filler metal having a liquidus above 450 degrees C (840 degrees F) and below the solidus of the base metal. Brazing is distinguished from soldering by the melting point of the filler metal: solders melt below 450 degrees C (840 degrees F). Brazing differs from welding in that no substantial amount of the base metal is melted during brazing. Thus the temperatures for brazing aluminum are intermediate between those for welding and soldering. Also, brazed aluminum assemblies generally are between welded and soldered assemblies in strength and resistance to corrosion"
Since soldering is done below 450 degrees C (840 degrees F), aluminum filler alloys are not used in soldering aluminum. Instead, solders for aluminum alloys are often zinc, tin, cadmium and lead.