Gas Tungsten Arc Welding:
The idea of utilizing a gaseous shielding medium to defend both the arc and weld metal from contamination by the atmosphere is almost as old as the covered electrode. Roberts and van Nuys in the year of 1919, and others various years later, considered the problem and a variety of gases were proposed from the inert gases to hydrocarbons and hydrogen. In the year of 1930s, the interest started to centre on the inert gases but it was not until the year 1940 that experiments were begun at the Northrop Aircraft Co. of USA with the deliberate intention of creating a practical inert-gas welding method. The metal to be welded was melted through an arc struck from a tungsten electrode, in an atmosphere of the inert monoatomic gas helium.
The original equipment comprised the simple tungsten electrode torch and a d.c. generator. Arc beginning was by brushing the electrode on the work but it led to contamination of the electrode and a high-frequency spark generator was added up to the equipment so that an arc could be struck from the electrode without touching it on the work. At first electrode-negative and electrode-positive both polarity were utilized, although the negative polarity was favored because less heat was produced at the tungsten electrode, which remained comparatively cool.
Along the desire to weld thicker material, welding currents of over 100 A became essential and it was no longer possible to use the electrode-positive polarity because the tungsten electrode became so hot that molten tungsten dropped off into the weld pool. The higher welding currents also necessitated water cooling of the body of the torch due to the increased amount of heat conducted back along the electrode.
By the year 1944 it was recognized that electrode polarity was of greater importance than had appeared at primary. Up to that time the inert-gas arc process had been utilized principally on thin-gauge magnesium and stainless steel, but attempts had also been made to weld aluminium along which it was found essential to employ a flux. However, It was observed that oxide removal could be accomplished by the arc itself on electrode- positive a.c. or in d.c. welding, therefore making a flux unnecessary. Unless a certain minimum open-circuit voltage was available while welding aluminium with a.c., the oxide film was not broken down so that the a.c. was rectified and welding was impossible. By the year 1946, however, it had been found that the spark ioniser could be made to stabilise the a.c. arc. Slowly a preference emerged for argon over helium in manual welding, largely as a consequence of the smaller change in arc voltage with arc length while welding with argon. This made the procedure less critical from the welder's view point.
Once a begin has been made to the welding of aluminium by the inert-gas tungsten-arc method there started a period of fast development because of the new range of applications opened up. Although restricted for several years to welding sheet material at less than 150 A now there was a demand to go to higher currents. Ceramic ones replaced metal gas nozzles, in turn these being replaced by water-cooled metal nozzles when it was found that the ceramic nozzles had a restricted life. The water-cooled torch body and power lead was now necessary to give lightness and flexibility to the torch and, because the high-frequency ioniser was left on constantly, great attention had to be paid to insulation.
Although the high-frequency ioniser stabilised the arc, it did not influenced the inherent imbalance among the voltage on alternate half-cycles which resulted in a d.c.