Alternating Current Gas Tungsten Arc Welding (GTAW)
A gas tungsten arc method commonly used for aluminium pipe welding uses alternating current (a-c), a tungsten electrode, and argon gas or a mixture of argon and helium. The arc provides good cleaning and excellent oxid film dispersion with this method. With some power supplies, high frecuency is necessary to aid arc initiation during reversal of current, thus stabilizing the arc. This process gives a clean, bright weld pool which is cleary visible and easily controlled.
The welding technique is not dificult for an experienced GTAW welder. Although pure tungsten electrodes may used, zirconiated tungsten electrodes are often used for alternating current GTAW. they have higher current capacity than pure tungsten electrodes of equal size. Thoriated tungsten electrodes should not be used because they give a wandering, unstable arc with a-c power.
Direct Current Electrode Negative (DCEN)
Another gas tungsten arc method uses direct current electrode negative, a tungsten electrode, and helium shielding gas. With this method, and the use of smaller electrodes of 1 or 2 percent thoriated tungsten or 1.5 or 2 percent lanthanated tungsten, a more concentrated than is obtanied. This results in deeper joint penetration than with a-c welding. The weld pool does not appear as fluid as with a-c arc. This method requires much more skill by the welder, much more thorough precleaning of the joint, and interpass cleaning, since there is not arc cleaning action by DCEN welding.
Gas Metal Arc Welding (GMAW)
Gas metal arc welding uses direct current electrode positive (DCEP).
Motor or engine driven generators, transformer rectifier machines or inverter power supplies may be used and may be either constant current (drooping) or constant voltage type. Constant speed wire feeders are most commonly used. With constant current (drooping) power sources the wire feeder control requires a scratch or slow-speed starting feature. With true constant current power sources, i.e., having a vertical volt-ampere characteristic curve, voltage feedback from the arc is necessary to control electrode speed. The method of metal transfer. Short circuiting or globular transfer are not apropriate for welding aluminium because excess weld splatter is generated and the weld is prone to lack of fusion defects.
Automatic Welding
Equipment has been developed for both machine and automatic gas shielded welding of aluminium pipe. The desing and operation of this type of equipment permit controlled use of the characteristic high speed of gas metal arc welding in the joining of pipe in the horizontal fixed position. Equipment of this type has been succesfully used on a number of pipe line jobs in the field. Constant voltage, constant current, and pulsed current power sources have proved succesful.
Automated equipment has also been developed for tungsten arc welding of aluminium pipe.
Jorge T. Reyna´s Blog (NDT, Welding, Concrete & Coatings)