TIG welding torch weldingtheory
The TIG torch can be air cooled or water cooled and of vastly different shapes and sizes dependant on access to the area to be welded and welding current required. TIG torch for use on equipment without a electric operated valve ( normally scratch start systems ) can have a finger operated gas valve fitted to the torch head.
If the operator is using a foot control unit, the torch will not need a switch fitted. For welding in difficult to get to areas, a flexible head torch can be used and bent to the best position for welding. In water cooled torches, the current cable is a bore copper conductor within a water carrying hose, this means the conductor can be greatly reduced in size and weight.
The gas shield are now invariably alumina ceramics and are available in a wide range of sizes. When access is difficult, it may be necessary to project the electrode well beyond the end of the gas nozzle, this may result in inferior gas shielding because of turbulence. This can usually be overcome by employing a Gas Lens System replacing the standard collet and collet body system, this producing improved directional and stability of the gas flow.
Connection to the power source can be via a special lug if the equipment has a stud output fitting, or a universal dinse type TIG adaptor if output fittings are dinse type sockets.
Electrodes for TIG welding are Pure Tungsten or a Tungsten oxide, generally 2 % Thoriated tungsten are used for DC welding and 2 % Zirconated tungsten are recommended for AC welding. The diameter of the electrode is chosen to match the current required.
tig welding gas flow control
The TIG process relies entirely on the shielding gas to protect the hot electrode and molten pool and it is therefore essential for good arc striking that the flow of gas is initiated and allowed to stabilise before the arc is struck. Preflow timers are commonly fitted to better TIG power sources.
Equally the gas shield must be allowed to flow after the arc is extinguished, to prevent oxidation of the electrode and cooling weld. Postflow timers are fitted to most TIG power sources.
Shielding Gas weldingtheory
The most commonly used gas for TIG welding is argon which can be used on all metals. Argon - Hydrogen mixtures containing 2 - 5 % Hydrogen are frequently used for stainless steel and nickel-base alloys having the advantage of producing cleaner welds, giving deeper penetration. Helium - Argon mixtures give deeper penetration, greater heat input and therefore faster welding because of the higher arc voltage than pure Argon, but arc striking may be more difficult than in Argon. These mixtures can be used for Aluminum and Copper Alloys
DC Output Pulse Control
For DC welding use, there is often a pulsing facility which allows the welding current to be switched between a low current ( say approximately 15 amps ), sufficient to keep the arc alight but not produce much heat and the main pulse current ( say 50 - 350 amps ), dependant on the design control of the following parameters can be adjusted to provide high quality welding.
Peak Current Valve
Peak Time
Base Current Valve
Base Current Time
Frequency of Pulses
The use of pulsed current greatly extends the control which can be exercised on the process allowing:
* Improved consistency in the under head of unbacked butt welds.
* The ability to overcome differences in heat sink and therefore to join thick to thin material.
* The ability to make cylindrical or circular welds without a build up of heat and an increase in weld width.
* The ability to produce stable TIG welds at very low level.
tig Welding Process
In most Arc welding processes, the arc is struck from a consumable electrode to the workpiece and metal has been melted from electrode, transferred across the arc and finally incorporated into the molten pool. TIG process employs on electrode made from high melting point metal, usually a type of TUNGSTEN, which is not melted. The electrode and the molten pool is shielded from the atmosphere by a stream of inert gas which flows around the electrode and is directed onto the workpiece by a nozzle which surrounds the electrode.
In TIG welding, the primary functions of the arc is to supply heat to melt the workpiece and any filler metal which maybe necessary. This filler metal is fed manually into the molten pool at its leading edge. The second function of the arc is to clean the surface of the molten pool and the immediately surrounding parent metal of surface oxide films and therefore no flux is required. The shielding gas MUST be inert with respect to the tungsten electrode and the choice is therefore more limited than with the MIG process.
When using DC the DCEN electrode negative polarity is almost invariably employed, ( if DCEP is used most of the heat is in the electrode not the workpiece, so if this polarity is used, very much larger electrodes MUST be used ). The cleaning function of the arc does not take place on the DCEN polarity, so metals forming refractory oxide surface
films such as aluminum cannot be readily welded on this polarity. For Aluminum the electrode positive polarity on which cleaning takes place, would therefore appear desirable. In fact on this polarity, more energy is dissipated at the electrode which therefore becomes overheated. Aluminum is therefore welded using AC and the cleaning action takes place on the electrode positive half cycles and weld penetration takes place on the electrode negative half cycle. Zirconated tungsten electrodes are used for AC welding because Zirconia helps the electrode to maintain the desired stable end.