11-04-2013, 03:33 PM
AN INTRODUCTION TO TIG WELDING
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General Principles
TIG (Tungsten Inert Gas) welding also known as GTA (Gas Tungsten Arc) in the USA and WIG (Wolfram Inert Gas)
in Germany, is a welding process used for high quality welding of a variety of materials, especially, Stainless Steel,
Titanium and Aluminium.
Equipment
DC or AC / DC Power Source
TIG Torch
Work Return Welding Lead
Shielding gas supply line, ( normally from a cylinder )
Foot Control Unit ( common option )
Power Source
TIG welding can be carried out using DC for Stainless Steel, Mild Steel, Copper,
Titanium, Nickel Alloys etc and AC for Aluminium and its Alloys and Magnesium.
Further information on the TIG Welding Process follows information on equipment
used in this document.
The Power Source is of a transformer design with or without a rectifier, with a
drooping characteristic (constant current power source).
The output is generally controlled by either a moving core within the main
transformer of the power source or by using electronic control of power thyristors.
DC power sources could be of 1 phase or 3 phase design, with an inductor to provide a smooth output. AC and AC /
DC Power Sources are of a single phase design.
Arc Starting Circuit
HF : - Sparks of high tension jump
across the gap between electrode and
workpiece rapidly to carry the welding
current across to start welding in DC
TIG welding, this will stop once the arc
is struck, in AC TIG welding, this will
normally continue to keep the arc alive
as the AC output changes from a
Positive half cycle to a Negative half
cycle and back again.
Lift Arc : - The electrode is touched onto the workpiece, the TIG Torch switch or
foot control switch is operated, the equipment circuits detect a short circuit on the
output and allow only a very low current typically 5 - 8 amps to flow. The
electrode is lifted off the workpiece, the equipment circuits now detect a voltage
between electrode and workpiece and welding current strikes across that very
tiny gap as the electrode lifts off and welding continues.
Scratch Start : - The electrode is scratched or dragged and lifted off the
workpiece, much the same as striking an electrode in MMA ( Stick ) welding.
Using the HF method. No cross contamination from electrode and workpiece
takes place as they never touch, with Lift arc correctly set and used, only minimal
cross contamination occurs because of the low current when electrode is in
contact with workpiece, scratch start TIG is a low cost option for general TIG
welding, but cross contamination can occur.
AC Waveform Balance
A pot can be fitted, when welding in AC mode the positive half cycle cleans the oxides on the Aluminium and the
Negative half cycle produces weld penetration during welding self rectification occurs and causes an imbalance of
the waveform, a balance control allows the operator to adjust the amount of time the cleaning or penetration takes in
each cycle.
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.
TIG Torch
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.