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TIG Welding Process

E.T.Masih

Development of TIG Welding

After the discovery of the electric arc in 1800 by Humphry Davy, arc welding developed slowly.

C. L. Coffin had the idea of welding in an inert gas atmosphere in 1890, but even in the early 1900s, welding non-ferrous materials like aluminum and magnesium remained difficult, because these metals reacted rapidly with the air, resulting in porous and dross-filled welds.

To solve the problem, bottled inert gases were used in the beginning of the 1930s.

GTAW - Gas Tungsten Arc Welding

This process was perfected in 1941, and became known as heliarc or tungsten inert gas welding.

In 1953, a new process based on GTAW was developed, called Plasma Arc Welding.

It affords greater control and improves weld quality by using a nozzle to focus the electric arc, but is largely limited to automated systems.

Whereas GTAW remains primarily a manual, hand-held method.

GTAW Welding Operation

Gas Tungsten Arc Welding (GTAW) is frequently referred to as TIG welding. 

TIG welding is a commonly used high quality welding process. 

TIG welding has become a popular choice of welding processes when high quality, precision welding is required. 

In TIG welding an arc is formed between a non-consumable tungsten electrode and the metal being welded.

Gas is fed through the torch to shield the electrode and molten weld pool.  If filler wire is used, it is added to the weld pool separately. 

In TIG Tungsten is used as electrode

TIG Welding Benefits

• Superior quality welds

• Welds can be made with or without filler metal

• Precise control of welding variables (heat)

• Free of spatter

• Low distortion

Shielding Gases

• Argon

• Argon + Hydrogen

• Argon/Helium

Helium is generally added to increase heat input (increase welding speed or weld penetration). 

Hydrogen will result in cleaner looking welds and also increase heat input, however, Hydrogen may promote porosity or hydrogen cracking.

GTAW Welding Limitations

• Requires greater welder dexterity than MIG or stick welding

• Lower deposition rates

• More costly for welding thick sections

Weld Discontinuities

• Undercutting

• Tungsten inclusions

• Porosity

• Weld metal cracks

• Heat affected zone cracks

TIG Welding Problems

• Erratic arc

• Excessive electrode consumption

• Oxidized weld deposit

• Arc wandering

• Porosity

• Difficult arc starting

Welding Discontinuities

Some examples of welding discontinuities are shown below. 

Evaluation of the discontinuity will determine if the discontinuity is a defect or an acceptable condition:

Incomplete Fusion

Incomplete Fusion - A weld discontinuity in which fusion did not occur between weld metal and fusion faces or adjoining weld beads.

Undercut & Overlap

Undercut - A groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal.

Overlap - The protrusion of weld metal beyond the weld toe or weld root.

Overlap & Incomplete Joint Penetration

Under-fill - A condition in which the weld face or root surface extends below the adjacent surface of the base metal.

Incomplete Joint Penetration - A joint root condition in a groove weld in which weld metal does not extend through the joint thickness

Guide

Partial joint penetration groove welds are commonly specified in lowly loaded structures. 

However, incomplete joint penetration when a full penetration joint is required, as depicted above, would be cause for rejection. 

A fix for an incomplete penetration joint would be to back gouge and weld from the other side.  

Another acceptable partial penetration joint is shown below.

Partial Penetration Joint

Partial penetration joint on the right without discontinuities is an acceptable condition where appropriate. 

Appropriate engineering decisions need to be applied to determine what type of joint should be specified for a given application.

Weld Cracking

Above are several different representations of weld Cracking

Convex Fillet

Above is a representation of a convex fillet weld without discontinuities.

GTAW

TIG Welding Equipment

GTAW torch with various electrodes, cups, collets and gas diffusers

GTAW Torch, Disassembled

GTAW Torch

GTAW Torch parts

1. Welding Torch

2. Tungsten Electrod

3. Welding Power Supply

4. Shielding Gas Source

GTAW welding torches are designed for either automatic or manual operation and are equipped with cooling systems using air or water.

The automatic and manual torches are similar in construction, but the manual torch has a handle while the automatic torch normally comes with a mounting rack.

The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, can be varied on some manual torches according to the preference of the operator.

Air cooling systems are most often used for low-current operations (up to about 200 A), while water cooling is required for high-current welding (up to about 600 A).

The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply.

The internal metal parts of a torch are made of hard alloys of copper or brass in order to transmit current and heat effectively.

Description of Torch

Power supply

Gas tungsten arc welding uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant even if the arc distance and voltage change

This is important because most applications of GTAW are manual or semiautomatic, requiring that an operator hold the torch.

Examples of TIG Weld

Electrodes

The electrode used in GTAW is made of tungsten or a tungsten alloy, because tungsten has the highest melting temperature among pure metals, at 3,422 °C (6,192 °F).

As a result, the electrode is not consumed during welding, though some erosion (called burn-off) can occur.

The diameter of the electrode can vary between 0.5 millimeter and 6.4 millimeters (0.02–0.25 in), and their length can range from 75 to 610 millimeters (3–24 in).

Electrodes

ISOClass

ISOColor

WP Green

WC20 Gray

WL10 Black

WL15 Gold

WL20 Sky-blue

WT10 Yellow

A number of tungsten alloys have been standardized by the International Organization for Standardization and the American Welding Society in ISO 6848 and AWS A5.12, respectively, for use in GTAW electrodes, and are summarized in the adjacent table. Pure tungsten electrodes (classified as WP or EWP) are general purpose and low cost electrodes.

Safety

Like other arc welding processes, GTAW can be dangerous if proper precautions are not taken.

The process produces intense ultraviolet radiation, which can cause a form of sunburn and, in a few cases, trigger the development of skin cancer.

Flying sparks and droplets of molten metal can cause severe burns and start a fire if flammable material is nearby, though GTAW generally produces very few sparks or metal droplets when performed properly.

SafetyIt is essential that the welder wear suitable protective clothing, including leather gloves, a closed shirt collar to protect the neck.

Suitable welding helmet to prevent retinal damage or ultraviolet burns to the cornea, often called arc eye.

Due to the absence of smoke in GTAW, the arc appears brighter than shielded metal arc welding and more ultraviolet radiation is produced.

Transparent welding curtains, made of a polyvinyl chloride plastic film, dyed in order to block UV radiation, are often used to shield nearby personnel from exposure.