What is TIG Welding (GTAW)? πŸ‘¨β€πŸ­


Introduction

TIG is the process of electric arc welding with a non-consumable tungsten electrode or tungsten alloy under a gas shield of inert gas or inert gas mixtures. Adding material (also known as a dipstick, see below) may or may not be used.

How the process works

Welding by the TIG (or GTAW) process is the joining of metals by heating and melting them with an electric arc established between a non-consumable tungsten electrode and the workpiece.

Protection during welding is achieved with an inert gas or mixture of inert gases, which also has the function of transmitting the electric current when ionized during the process.

Welding can be done with or without filler metal. When made with filler metal, it is not transferred through the arc, but is melted by the arc. The electrode that conducts the current is a pure tungsten rod or alloy of this material (the filler metal 'rod' is not an electrode, it does not transmit current).
The arc area is protected from atmospheric contamination by the shielding gas, which flows from the gun nozzle.

The gas removes air, eliminating contamination of the molten metal and the heated tungsten electrode by the nitrogen and oxygen present in the atmosphere. There is little to no splash and smoke.

The weld layer is smooth and uniform, requiring little or no further finishing.

TIG welding can be used to perform high quality welds on most metals and alloys. There is no dross and the process can be used in all positions This process is the slowest of the manual processes.

Although it is the slower process (less productive), it is often used to root welds when there is no access to weld on the other side.

The preference for it is because the welder has great power to manipulate the welding and this allows to make a high quality weld. Of course, with great freedom of handling, the TIG process also ends up demanding great skill and training on the part of the welder.

Welding Equipment

TIG welding is usually a manual process, but it can be mechanized and even automated. Just to exemplify the mechanization, it is common, for example, to use the TIG process ("hot wire" method) to carry out the anti-corrosion coating (clad).

The equipment you need ter:
  • An electrode holder with gas passage and a nozzle to direct the protective gas around the arc and a claw mechanism to contain and energize a tungsten electrode, called a torch (or gun).
  • A supply of shielding gas.
  • A flowmeter and gas pressure reducer.
  • A power source, with identical volt-ampere characteristics as the coated electrode.
  • A high frequency source.
  • A supply of cooling water if the gun is water cooled.
The variables that most affect this process are the electrical variables (current voltage and characteristics of the energy source).

They affect the amount, distribution and control of heat produced by the arc and also play an important role in its stability and ultimately in the removal of refractory (heat resistant) oxides from the surface of some light metals and their alloys.

The tungsten electrodes used in TIG welding are of various classifications and the requirements for these are given in the AWS A 5.12 standard, basically we have:
β€’ EWPPure tungsten (99.5%)
β€’ EWCe-2Tungsten with 1,8 to 2,2% of CeO2;
β€’ EWLa-1Tungsten with 0,9 to 1,2% of La2O3;
β€’ EWTh-1Tungsten with 0,8 to 1,2% of Th02;
β€’ EWTh-2Tungsten with 1,7 to 2,2% of Th02;
β€’ EWGTungsten (94.5%) with the addition of some unidentified elements.
The addition of thorium and zirconium to tungsten allows it to more easily emit electrons when heated. The Ewth-2 type electrode is the most used, but some companies have placed restrictions on its use due to work safety issues.

Consumables – filler metals and gases

A wide variety of metals and alloys are available for use as filler metals in the TIG welding process.

Filler metals, if used, are normally similar to the metal being welded (good practice).

The shield gases most commonly used for TIG welding are argon, helium or a mixture of these two gases. Argon is often preferred over helium because it has several advantages:
  • Smoother arc action and no turbulence.
  • Least arc voltage for a given current and arc length.
  • Greater cleaning action when welding materials such as aluminum and magnesium, in alternating current.
  • Less cost and greater availability.
  • Lower gas flow for good protection (in flat position).
  • Better resistance to cross air current.
  • Easier arc initiation (due to lower ionization potential).
On the other hand, the use of helium used as a shielding gas, results in a higher arc voltage for a given arc length and current than argon, producing more heat, and thus is more effective for welding thick materials ( especially high conductivity metals such as aluminum).

However, since the density of helium is lower than that of argon, higher gas flow rates are usually required to obtain a more stable arc and adequate protection of the weld pool when welding in the flat position.

Due to the need for high flow and the higher cost of helium (in relation to argon), argon gas ends up being the most used in Brazil.

Features and uses

TIG welding is a very suitable process for thin thicknesses due to the excellent control of the weld pool (electric arc). The process can be applied in places that do not require filler metal (usually limited to low thickness stainless steel).

This process can also join thick walls of steel and metal alloy sheets and tubes. It is used for both ferrous and non-ferrous metal welding. Root passes of carbon steel and stainless steel pipelines, especially those for critical applications, are often welded using the TIG process.

Although TIG welding has a high initial cost and low productivity, these are compensated for by the possibility of welding many types of metals, of thicknesses and in positions not possible by other processes, as well as by obtaining welds of high quality and resistance.< br />
TIG welding makes it possible to weld aluminum, magnesium, titanium, copper and stainless steels, as well as metals that are difficult to weld and others that are relatively easy to weld such as carbon steels. Some metals can be welded in all positions, depending on the welding current and the skill of the welder.

The current used in TIG welding can be alternating or direct. With direct current you can use direct or reverse polarity.

However, since forward polarity produces minimal electrode heating and maximum base metal heating, smaller electrodes can be used, achieving even greater penetration depth than that obtained with reverse polarity or alternating current.

When low penetration is desired, the situation that leads to minimal heating of the base metal should be chosen, using reverse polarity or alternating current.

In aluminum welding, the current used is alternating, requiring a high frequency device that is normally built into the equipment.

Despite the mentioned advantages, it is important to remember that TIG welding, in order to be successful, requires exceptional cleaning of the joints to be welded and extensive training of the welder.

One consideration to keep in mind is the cone angle of the tungsten electrode tip, as taper affects weld penetration. However, this preparation only occurs for welding with direct current of direct polarity.

If the curvature of the electrode tip is decreased (sharper tip), the width of the bead tends to increase and penetration decreases. The tip becomes too sharp, the electric current density increases and the tip of this tip can reach temperatures above the melting point of the electrode, when it will then detach from the electrode and form part of the fusion pool, constituting after its solidification an inclusion of tungsten in the weld metal (metallic inclusion).

The thickness range for TIG welding (depending on current type, electrode size, wire diameter, base metal, and chosen gas) is from 0.1 mm to 50 mm.

When the thickness exceeds 5 mm, precautions must be taken to control the temperature rise in multipass welding. The deposition rate, depending on the same factors listed for thickness, can vary from 0.2 to 1.3 kg/h.

Preparing and cleaning joints

Preparing and cleaning joints for TIG welding requires all the care required for coated electrode welding and more:
  • The cleaning of the chamfer and edges must be to the shiny metal, in a band of 10 mm, on the inner and outer sides.
  • When deposition of the weld root, protection must be used, by means of inert gas, on the other side of the part. This gas injected into the root of the joint is called Purge. For carbon steels, protection from the inside of the joint (purge) is not necessary, with the notable exception of using a special consumable (inconel for example).

Process-induced discontinuities

With the exception of the inclusion of slag, most of the discontinuities listed for the other welding processes can be found in TIG welding. It is important for the welding inspector to know that:

Lack of fusion

It can happen if we use an improper welding technique. Arc penetration in TIG welding is relatively small. For this reason, for TIG welding, joints (or chamfers) suitable for the process must be specified. By suitable I mean larger chamfer angle.

Tungsten inclusion

They can result from accidental contact of the tungsten electrode with the weld pool: the hot end of the tungsten electrode can melt, turning into a drop of tungsten which is transferred to the weld pool, thus producing an inclusion of tungsten in the weld pool. Solder. Whether or not these inclusions are acceptable depends on the code that governs the service being run.

Porosity

It can occur due to inadequate cleaning of the chamfer or impurities contained in the base metal or a deficiency in the gas supply.

Cracks

In TIG welding they are usually due to hot cracking (the responsability of the engineer). What people do to "make a crack" is put a little copper in the carbon steel to be welded. After welding, the region that had the impurity (copper) usually cracks.
Longitudinal cracks can occur in deposits made at high speed.

Crater cracks

Most of the time, improper welding currents are due. The cracks due to hydrogen (cold cracking), when they appear, are due to moisture in the inert gas.

Conditions for personal protection

In TIG welding, the amount of ultraviolet radiation released is quite large. Parts of the skin directly exposed to such radiation burn quickly, which requires precautions; eye protection is essential.

Another aspect of these radiations is their ability to decompose solvent, releasing very toxic gases. Hence, in confined environments, we must take care that there are no solvents in the vicinity.

Learn Welding

Would you like to learn about welding? Check my Quick Welding Course.

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Materials: What is TIG Welding (GTAW)? πŸ‘¨β€πŸ­
What is TIG Welding (GTAW)? πŸ‘¨β€πŸ­
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