The idea of the use of non-consumable electrodes with inert gas shield for welding was first considered as early as 1920. However, the first patent for use of tungsten electrode with argon or helium shielding was reportedly issued in U.S.A. in 1930. Presently TIG welding is one of the most well established processes which can not only weld all metals of industrial use but also gives the best quality welds amongst the arc welding processes.
The standard equipment for manual TIG welding machine is quite simple and consists mainly of a power supply source, a welding torch and the connecting cables and hoses for a gas and water supplies. The accessories like feed wire mechanism, arc voltage control and feed-back system, electrode positioning device, seam tracking device, oscillator, etc. are incorporated in the system when semi-automatic and automatic versions of the process are employed. The material used in the tungsten rods and the major consumables are the shielding gas and the filler wire, where used.
Both d.c. and a.c. power supply sources are used with the static volt ampere (V-I) characteristic of the drooping type, that is, of the constant current type. In this type of power source the change in current with the change in arc length is minimal. Thus, an inadvertent slight shift in the welder’s hand will not have any discern able affect on the performance of the process. That is why steeper the V-I characteristic curve the more satisfactory the power source and this is particularly so for manual TIG welding.
TIG WELDING TORCH
TIG welding torches are designed for manual or automatic operation. Torches used for manual welding are provided with handles while those for automatic operation are without handles.
There are two basic types of TIG welding torches, air cooled and the water cooled. The air cooled torches are used with currents upto 150A while the water cooled torches are used with currents upto 1000A. The water cooled torches are available for currents differing in steps of 50A, for examples,200A TIG torches are rated by the current carrying capacity normally at 100% duty cycle.
WIRE FEED MECHANISM
The wire feed mechanism for TIG welding consists of three components viz; wire drive mechanism, speed control, and the wire guide attachment. The drive mechanism consists of a geared motor to drive the grooved rolls to push the wire. The speed control mechanism is essentially a constant speed governor and the wire is usually guided from the roller to the guide attachment through a flexible conduit. An adjustable wire guide is attached to the welding head so as to maintain the correct position and the angle of approach with respect to the tungsten electrode, work surface and the joint seam.
Usually an inert gas like argon or helium is used as the shielding gas for TIG. From amongst the active gases only hydrogen and nitrogen are employed for limited application and that also by mixing with helium or argon. In general gas selection is based on the metal to be welded.
Mild Steel: TIG is employed for high quality welds in the mild steel for thickness upto 3 mm and for that argon is generally used as the shielding gas. For thicker section Ar or Ar-He mixtures are found satisfactory. For TIG of pipe, Ar is recommended both for shielding and purging.
Stainless Steel and High Temperature Alloys: For stainless steel and high temperature alloys argon is used for manual TIG while hydrogen upto 25% is added to argon or helium for mechanised welding of these alloys. Hydrogen increases the arc voltage and makes the arc run hotter. A higher voltage is found desirable for welding thicker sections and materials with higher thermal conductivity.
Aluminium: Argon is generally used for welding aluminium; however Ar-He mixtures are employed when bead geometry needs to be controlled at high welding currents. For manual TIG welding of aluminium, argon shielding with A.C. and H.F. stabilisation is found superior to use of helium shielding with D.C. Argon provides easy arc initiation, better cleaning action and superior weld quality in production welding than does helium.
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