Page 165 - Welding of Aluminium and its Alloys
P. 165

148    The welding of aluminium and its alloys


                             Tungsten electrode

                                             Cooling water



                     –VE
                                                   Plasma gas
                        Power
                        source
                     +VE      Nozzle
                                                 Shielding gas


                                        Workpiece

                     8.1 Principles of the plasma-TIG transferred arc torch. Courtesy of TWI
                     Ltd.


              from cutting being that no cutting gas is introduced to blow away the molten
              metal (Fig. 8.1).
                There are a number of advantages that plasma-TIG has compared with
              conventional TIG mainly because of the cylindrical and constricted plasma
              column.This provides less sensitivity to process variables than with the TIG
              process. The constricted plasma column means that the heat is confined to
              a smaller area than with TIG, enabling a very stable controllable arc to be
              produced at currents as low as 0.1A. It is possible to weld without keyhol-
              ing at thicknesses less than 2.5mm but there is little advantage to be gained
              in terms of productivity over TIG. The keyholing technique may be used in
              manual welding but it is more common to find it in mechanised or auto-
              mated applications.
                The plasma is strongly directional and can be pointed in any given direc-
              tion even at very low currents. The cylindrical plasma column means that
              heat input is constant irrespective of torch to workpiece distance, unlike
              TIG with its conical arc.The tungsten electrode is recessed inside the torch
              nozzle, making tungsten contamination an impossibility. There is  also an
              increase in weld quality with a reduced risk of porosity and distortion.
              Higher welding currents enable material as thick as 15mm to be welded
              positionally in a single pass with a square edge weld preparation using the
              keyhole technique although in the flat position the maximum thickness is
              limited to around 8mm without filler metal.
                The process may be used in a melt-in mode using techniques similar to
              those that would be used for TIG. The weld may be made autogenously in
              those alloys that are crack-resistant; filler wire may be added to those that
              are crack-sensitive. This wire can be added manually but torches are avail-
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