Mechanised electro-gas welding
A technique described as electro-gas welding was developed by the Alcan Company in the late 1960s but seemed to drop out of favour in the late 1990s, which is surprising when the advantages of the process are considered. The weld may only be carried out in the vertical-up (PF) position but is capable of welding both square edge butt joints and fillet welds with throats of up to 20 mm in a single pass.
To operate successfully the process uses a long arc directed to the back of the penetration cavity. This provides a deeply penetrating arc that operates in the space above the weld pool. The pool fills the cavity below the arc, solidifying as the torch is traversed vertically up the joint line. The molten pool is retained in position and moulded to shape by a graphite shoe attached to and following immediately behind the welding torch.
The process utilises a drooping characteristic power source capable of providing 600 A at 100% duty cycle coupled to a water-cooled machine torch. The torch is mounted on a vertical travelling carriage at an angle of 15° from the horizontal. The gas shroud should be at least 25 mm in diameter and the tip of the contact tube should be flush with the shroud.
For butt welding the graphite shoe is made from a flat plate shaped with a groove to mould the cap, flared out towards the top of the shoe where the weld pool is formed. The fillet weld mould is provided with a pair of ‘wings’ set back to press against the plates to form the fillet. In both cases the shoe is held against the plates by spring pressure. The shoe must be long enough to hold the molten metal in place until it has solidified - in the region of 100 mm may be regarded as sufficient. It has been found that heating the shoe to 350 °C before commencing welding assists in preventing fouling of the shoe with parent metal.
During welding the arc must be prevented from arcing onto the weld pool or the graphite shoe. This requires careful control of the wire position and the wire feed speed, as a balance must be achieved between the volume of metal being fed into the pool, the volume of the mould and the traverse speed.