Non-ferrous metals

Abstract

Aluminium and aluminium alloys may be joined by arc-fusion welding, resistance welding, solid-phase welding, brazing and adhesive bonding. Low-melting aluminium-zinc solders have also been used to a limited extent. Fusion welding was first accomplished by the oxy-acetylene process but is now carried out by means of GTA and GMA welding and sometimes using coated electrodes. The power source for GTA welding is usually an a.c. transformer with high frequency re-ignition. When the electrode is positive a non-thermionic cathode (see Section 3.5.2.1) forms on the aluminium surface; the cathode spot wanders over the weld pool surrounding plate surface and removes most of the oxide. With d.c. electrode-positive most of the arc heat is absorbed by the tungsten electrode, and at normal welding currents this melts and is difficult to manage. With a.c. welding, a compromise is possible; during the electrode-positive half-cycle the oxide film is removed while during the electrode-negative half-cycle the workpiece is heated. However, d.c. electrode-positive may be used for special purposes. Welds may also be made with d.c. electrode-negative using helium as the shielding gas, but this too is for special applications. In general GTA welding is applicable to lighter sections than GMA welding.