Tungsten Inert Gas (TIG) Welding


Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.

http://en.wikipedia.org/wiki/Gas_tungsten_arc_welding

In TIG welding, a tungsten electrode heats the metal you are welding and gas (most commonly Argon) protects the weld puddle from airborne contaminants. TIG welding produces clean, precise welds on any metal.

http://www.millerwelds.com/resources/tech_tips/TIG_tips/

An arc is struck between a tungsten electrode (non-consumable) and the sheet metal to be welded. An inert gas shields the arc from the ambient to prevent oxidation. A filler material is optional. Carbon steels, low alloy steels, stainless steels, most aluminum alloys, zinc based copper alloys can be welded using this process. TIG is quite suitable for welding dissimilar materials, but usual cautions of galvanic corrosion still apply. The TIG process is a slower process compared to the MIG process, but the quality of weld is cosmetically better. There is no weld spatter, and the quality of welds is higher than MIG welding.

http://www.efunda.com/processes/metal_processing/welding_inertgas.cfm

Operation

Direct or alternating current power sources with constant current output characteristics are normally employed to supply the welding current. For DC operation, the tungsten may be connected to either output terminal, but is most often connected to the negative pole. The output characteristics of the power source can have an effect on the quality of the welds produced. Shielding gas is directed into the arc area by the welding torch, and a gas lens within the torch distributes the shielding gas evenly over the weld area. In the torch, the welding current is transferred to the tungsten electrode from the copper conductor. The arc is then initiated by one of several methods between the tungsten and the workpiece.

Polarity in welding has to do with the direction of the current in the welding process. With direct current (DC) the welding circuit can either be straight, or reverse polarity. When the machine is set for straight polarity, the current flows from the electrode to the weld surface and creates considerable heat in the metal. When the machine is in reverse polarity, the current is backwards and is flowing from the metal to the electrode causing a grater concentration of heat at the electrode.


Operating Modes

The mode used is largely dependent on the parent material being welded.

DC Electrode Negative (DCEN)

In this mode the tungsten electrode is the negative pole in the welding circuit, the workpiece being the positive pole.

DCEN is the most common mode of operation and is widely used for welding all carbon, alloy and stainless steels, as well as nickel and titanium alloys. Copper alloys, with the exception of those containing aluminium in significant amounts, can also be welded with this polarity.

DC Electrode Positive (DCEP)

In this mode the tungsten electrode is the positive pole in the welding circuit, the workpiece being the negative pole.

DCEP is used for aluminium alloys when welding, with pure helium as the shielding gas, since this polarity has a strong cathodic cleaning effect capable of removing the tenacious aluminium oxide film from the surface. It may also be used for TIG welding magnesium alloys.

Alternating Current (AC)

In this mode the polarity of the tungsten electrode and the workpiece alternate between negative and positive at the frequency of the applied welding current.

AC polarity is used most commonly when welding aluminium and its alloys with pure argon or argon-helium mixtures to take advantage of the combination of the cyclic heating and cleaning action. It is also suitable for welding magnesium alloys and aluminium bronze.



TIG welding and polarity
TIG Welding Current and Polarity

TIG Electrodes

Type of Tungsten
(Alloy)

Color Code

Remarks

Pure

Green

Provides good arc stability for AC welding. Reasonably good resistance to contamination. Lowest current carrying capacity. Least expensive. Maintains a balled end.

Ceriated

CeO2

1.8% to 2.2%

Orange

Similar performance to thoriated tungsten. Easy arc starting, good arc stability, long life. Possible replacement for thoriated.

Thoriated

ThO2

1.7% to 2.2%

Red

Easier arc starting. Higher current capacity. Greater arc stability. High resistance to weld pool contamination. Difficult to maintain balled end on AC.

Lanthanated

La2O3

1.3% to 1.7%

Gold

Similar performance to thoriated tungsten. Easy arc starting, good arc stability, long life, high current capacity. Possible replacement for thoriated.

Zirconiated

ZrO2

0.15% to 0.40%

Brown

Excellent for AC welding due to favorable retention of balled end, high resistance to contamination, and good arc starting. Preferred when tungsten contamination of weld is intolerable.



TIG Torch Nozzles/Cups


Typical TIG cups
TIG cups are used to channel the shielding gas around the electrode and over the surface of the weld pool. The cup sizes are multiples of 1/16" so #4 would be 4/16 or ¼" #8 would be ½ inch. Standard TIG cups run from #3 (3/16") to #16 (1").

The cups are made out of aluminum oxide, lava, and glass. The pinkish ones are aluminum oxide and the lava cups are generally tan or grey (sorry, they're made out of clay material, not real lava). The clear cups are either Pyrex or quartz.

The gas outlet or orifice of a TIG nozzle is measured in 1/16” (1.6mm) increments. For example a No. 4 nozzle is 1/4”, or 4/16” in diameter. The gas nozzle orifice is located at the end of the nozzle farthest from the torch body. With the exception of a specialty torch, such as one for micro TIG welding, the smallest nozzle is the No. 3, 3/16” and the largest is a No. 16, or 1” nozzle.



The most common TIG cups are made of Alumina Oxide and are “pink” in color. Alumina is a high- temperature, nonconductive pink ceramic material. This material is injection-molded and mass-produced, which is why the alumina nozzles are less expensive than those made with other materials. Alumina nozzles are durable and good for general TIG welding applications. Extreme heat generated from high amperage applications can cause a wide temperature difference from the front of the nozzle (tungsten electrode) to the back (torch body) resulting in thermal shock, which can cause the nozzle to crack, or in extreme cases blow the orifice end off.



Lava nozzles are tan/gray colored, Lava is a high-temperature, nonconductive clay material that is machined on a lathe to make special sizes. This process lends itself to odd-shaped TIG cup like the long (L), extra-long (XL), and extra-extra-long (XXL) nozzles. Lava TIG nozzles work well in specialty TIG welding applications with high heat but don't work as well in confined areas with excessive reflective heat, which can cause the nozzle to expand and contract, and ultimately break.


Glass nozzles consist of two types: Pyrex a low-temperature, nonconductive glass material, and Quartz a high-temperature, nonconductive glass material. These glass materials are hand-blown to make nozzles for specialty TIG torches for micro welding, and for large purging nozzles. Because glass nozzles can’t be threaded, you need to convert your "standard" TIG torch with modified collets, collet bodies or gas lens collet bodies to accept a “push-on” glass nozzle.



Gas Lens

Basically a gas lens is a diffuser for the shielding gas. It's a screen or filter of some type that takes the ripples and swirls out of the shielding gas stream in order to provide more uniform gas coverage of the weld pool.




Gas lens

Shielding gas flow before installing a gas lens

Shielding gas flow after installing a gas lens



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