Before the invention of Gas Tungsten Arc Welding (GTAW), difficulties were faced for welding nonferrous metals like aluminum and magnesium since their reaction with the air was very rapid, resulting in porous weldments. The present-day GTAW process was initially developed in 1941 using tungsten electrodes and helium as an inert gas. The then inventor named the process ‘Heliarc.’ The American Welding Society named this process GTAW; however, it is popularly known by TIG (Tungsten Inert Gas) welding.
Working process of Gas Tungsten Arc Welding GTAW/TIG?
Gas tungsten arc welding (GTAW), popularly known by the name tungsten inert gas (TIG) welding is an arc welding process; however, the difference between GTAW and other arc welding processes (SMAW or GMAW) is that GTAW uses a non-consumable tungsten electrode. An inert shielding gas protects the welding arc, the tungsten electrode, and the molten weld pool from atmospheric air and contaminations. Filler metal is typically used (except in the case of very thin metal parts).
In this process, welding is done using the heat generated by the electric arc between the non-consumable tungsten electrode and the workpiece for the melting of the faying surfaces. The tungsten electrode is used only for maintaining an arc, and a separate filler wire is used to add the metal to the weld pool. Typically argon or helium or a mixture of argon and helium is used as the shielding gas. GTAW produces fewer fumes compared to GMAW or SMAW. The workpiece metals are normally reactive metals such as stainless steel, aluminum, magnesium, etc. Reactive metals are metals that get affected by the presence of oxygen and other gasses in the air.
When the welder does TIG welding manually, he/she has to engage both hands, one hand for the welding torch and the other hand for the filler wire, and both hands should move in perfect coordination to achieve good welding. TIG welding needs very good coordination between the eye and hands and also good practice. The welder must maintain a short arc while avoiding physical contact between the workpiece and the electrode. TIG welding is an arc welding process that operates at more than 6000º F (3315º C). The welder maintains a constant gap (1.5 to 3mm) between the workpiece and the electrode tip and tilts the torch a bit backward (around 8-15º from vertical). The welder manually adds the filler metal at the front of the weld pool.
There are different methods to strike the arc, including using a high-frequency generator. Once the welder strikes the arc, the welder moves the torch in small circles to create the welding pool, and the size of the welding pool depends on the electrode diameter and the current value. The triggering of the arc also starts the flow of the inert shielding gas. GTAW/TIG welding process gives the welder more control over the weld area than the other welding processes, and this helps a skilled welder to produce weldments of excellent quality. It is necessary to maintain perfect cleanliness of the equipment and metals used in TIG welding (they must be free of oil, dirt, moisture, and any other impurities) to achieve a good quality weld since the impurities make the weld porous.
Different methods like cleaning the surface with commercially available solvents, water jet cleaning, and stainless steel wire brush, etc., can be adopted. GTAW/TIG welding in an area of high and volatile air movement may need an increased flow of the shielding gas, which increases the process cost; hence, this process is not suitable for outdoor work.
So, the typical features of GTAW/TIG welding are:
- A non-consumable tungsten electrode is used to start and maintain the welding arc.
- An inert gas, normally argon or helium or a mixture of argon and helium, is used as a shielding gas to protect the welding arc, electrode, and the molten weld pool from atmospheric air and contaminations.
- This process is very good for welding reactive metals (stainless steel, aluminum, magnesium, and their alloys) for critical applications like aerospace, chemical, and food processing machinery.
GTAW/TIG welding generally uses a constant current type of power source, and the welding current ranges from 3 to 200 amps or 5 to 500 amps, and the welding voltage ranges from 10 to 35 volts (considering a 60% duty cycle). You may observe that GTAW works at a much lower voltage compared to SMAW or GMAW process. GTAW/TIG welding can work at low current, and hence the heat generated is low and low heat affected zone. This is an advantage when welding thin metal parts where the demand is to keep low heat.
GTAW/TIG welding can work with either direct current (DC) or alternating current (AC). There are two options in DC; the electrode can be connected to the positive terminal of the power source (DCEP) or connected to the negative terminal (DCEN). When DCEN is used, 70% of the heat generated is at the workpiece (anode) and 30% at the electrode (cathode), and this is exactly the reverse for DCEP. The high concentration of heat at the workpiece (DCEN) leads to increased weld penetration, and a high concentration of heat at the electrode (DCEP) means the welding torch needs more cooling. A high concentration of heat at the electrode (DCEP) leads to cleaning the oxide layer from the workpiece surface when the large positive ions from the electrode strike the workpiece surface. TIG welding with DCEP is a preferred method for welding reactive metals like aluminum.
What is the buttering layer?
When welding dissimilar steels such as stainless steel with carbon steel or alloy steel for critical and high-temperature applications, there is a need to develop the buttering layer before doing the actual welding.
Let us see this in detail. You have a stainless steel part and a carbon steel part, and they are to be welded. If you proceed to weld straight away, the carbon particles from the high carbon zone (carbon steel part) will enter into the low carbon zone (stainless steel), leading to carbide precipitation and cracks. To overcome come this, buttering is used. Buttering means to first weld on the carbon steel part to create a stainless steel surface (layer) and then weld the carbon steel part with the stainless steel part.
What are the methods of arc initiation in GTAW/TIG?
Different methods to ignite arc in GTAW/TIG:
- Touch start method using a carbon block as scrap metal.
- Using a high-frequency unit, high voltage (3000 to 5000 volts), and high frequency (100 to 2000 kHz.).
- Low current pilot arc.
1. Touch start method
Touch the tungsten electrode on a scrap carbon block, maintain the arc for a brief period to enable electron emission, and then the arc can be started easily on the required spot of the workpiece. The negative side of this method is, the electrode can pick up carbon particles to form tungsten carbide to contaminate the electrode. This affects the shape of the electrode tip and its life.
2. Using a high-frequency unit
The high-frequency unit supplies high voltage pulses at high frequency only for the initiation of the arc, and then it is replaced by the normal supply of current. The advantages of this method are- initiation of a clean welding arc and the longer life of the tungsten electrode. The high frequency generates unusually high electromagnetic emission, and it can cause hindrance in the nearby electronic equipment and instruments. Necessary precaution is to be taken to protect such equipment.
3. The low current pilot arc
An auxiliary power source supplies the power for the low current pilot arc, or it can be started using the scratch technique. This is a reliable and efficient method and is normally used with DC.
Quality issues in GTAW/TIG
The amount of heat input can affect the quality of the weld. Low input of heat due to the low welding current or excessive welding speed can reduce the weld penetration. On the other hand, a high level of heat input may cause wider weld bead, too much penetration, and spatter. Also, if the gap between the welding torch and the workpiece surface is more, the shielding gas loses its effect and causes porosity in the weld.
Each welding torch has a current rating, and if the value of the current exceeds its rated capacity, it may lead to tungsten inclusion in the weld (called tungsten spitting). The tungsten electrode may become contaminated if the welder by mistake allows its contact with the molten weld pool, leading to an unstable arc. In such cases, the tungsten electrode tip should be reground and cleaned to remove the contamination.
Equipment used in GTAW process
The GTAW/TIG welding system has four major parts:
- Power source.
- Welding torch with tungsten electrode and gas nozzle with facility for air or water cooling.
- Inert shielding gas.
- Controls for the movement of the welding torch, manual, semi-automatic, or automatic.
Other tools and equipment are:
- Welder safety gear.
- Welding cables with clamp for connecting to the workpiece and welding torch.
- Good quality stainless steel wire brush.
1. GTAW / TIG Power source
GTAW/TIG uses a constant current power supply to keep the current (or welding arc heat) relatively constant even when there is a variation in the arc length and voltage. This is an important aspect since most of the GTAW applications are either manual or semi-automatic and in both cases, the welder has to operate the welding torch. Keeping a steady arc length will be difficult with a constant voltage power supply, and variation of arc length cause variations in heat, which is not desirable. Hence, a constant voltage supply is not preferred.
The polarity used in GTAW/TIG depends mainly on the metal to be welded. DCEN is regularly used for welding steels, nickel, titanium, etc. DCEN is also used when doing automatic TIG welding of aluminum or magnesium with helium as the shielding gas. The electrode (connected to the negative terminal) produces heat by electrons emission; the electrons travel through the arc, initiating thermal ionization of the shielding gas, which increases the workpiece metal’s temperature. The flow of the ionized shielding gas is toward the electrode, resulting in oxides building on the surface of the weld pool.
DCEP is not common in TIG welding and is mainly used for shallow welds since the heat generated on the workpiece metal will be less. In DCEP, the flow of electrons will be from the workpiece metal to the electrode, and the electrode will reach a high temperature. To counter this, a large size tungsten electrode is used for DCEP. When the electrons move towards the electrode, the ionized shielding gas moves into the workpiece, clean the oxides, and other impurities from the workpiece, which improves the weld quality.
An alternating current (AC) is generally used for TIG welding aluminum and magnesium, either manually or in semi-automatic mode. AC has the advantages of both DCEP and DCEN since it makes the electrode and the workpiece to alternate between negative and positive charges. This results in the electrons flow shifting directions all the time and preventing overheating of the electrode while sustaining the heat in the workpiece metal.
2. GTAW / TIG welding torch
TIG welding torch has three main parts:
- Non-consumable tungsten electrode.
- Gas nozzle.
The collet is used to hold different sizes of tungsten electrodes. The gas nozzle enables the inert gas flow to form a protective shielding over the welding arc, tungsten electrode, and molten weld pool to protect them from atmospheric air and impurities. There can be a hand switch on the manual TIG welding torch to control the welding current. The metal parts inside the welding torch are typically made of electrical and thermal conductive hard alloys of copper or brass. The external body of the welding torch is made from heat-resistant plastic with insulating properties, and it provides insulation from heat and electricity to protect the welder.
GTAW/TIG welding torches are designed for manual operation as well as automation and have a provision for an air or water cooling system. Both the manual and automatic torches can look similar in their construction; however, the manual type comes with a handle, and the automatic type has a mounting arrangement. The GTAW welding torches are rated based on their current handling capacity, and this rating affects the speed of welding and hence the production rate. The low current (up to 150 amps) rated welding torches usually are air-cooled, and the higher current rated (more than 150 amps) ones are water-cooled. The welding torch is connected to the power source through a cable, and to the shielding gas cylinder through a hose and another connection to the compressed air/water for cooling.
The size of the gas nozzle should be in perfect coordination with the size of the molten weld pool; the bigger the weld pool higher the quantity of gas required. The bore (inside diameter) of the nozzle can be approximately three times the size of the electrode. The nozzle is of heat-resistant material (alumina, ceramic, fused quartz, etc.); fused quartz gives good visibility. The gas nozzle tends to undergo wear and tear due to the welding arc and its nearness to the heat zone, and the worn-out nozzles can no longer be able to jet out a uniform stream of shielding gas, and hence it should be replaced.
Electrode, shielding gases, and filler metal used in GTAW process
A. GTAW / TIG Electrode
The electrode material used in GTAW/TIG welding is tungsten since the melting point of tungsten (3420º C) is the highest in pure metals. Even though the tungsten electrode is called a non-consumable electrode, what it means is, it is not consumed as a filler metal. Otherwise, the tungsten electrode is consumed over a period due to normal wear and erosion. The diameter of the tungsten electrode can be 0.5 to 6.4 mm.
Tungsten electrodes used for DC TIG welding are generally made of pure tungsten. A very small percentage of thorium will improve welding arc starting (additives like lanthanum and cerium oxide are also used in place of thorium). The electrode diameter and its tip shape depend on the value of the welding current. The lower the current, the smaller is the electrode diameter and its tip angle. In AC TIG welding, the temperature of the electrode will be much higher, and the tungsten electrode with added zirconia is used to lessen the erosion of the electrode.
Tungsten electrodes with tip angles of 60º, 90º, and 120 º are in use. However, a smaller tip angle will be very sharp and tend to create localization of heat, which leads to the rapid decrease in the life of the electrode due to its degradation. Hence, a tungsten electrode with a conical tip of a large angle or a spherical tip is preferred for a smooth arc. Tungsten electrode with a ball-shaped tip has more life, but the conical tip will allow deep penetration and narrow weld beads. Sometimes, even a flat-faced tip is used.
If you want to improve the life of the tungsten electrode, it is necessary to control the temperature of the tungsten electrode (to keep it within a safe limit). Two factors govern the electrode temperature:
- Electrical resistance heating by the flow of current.
- The arc transferred from the electrode tip.
To control the electrical resistance, heating the tungsten electrode is coated with zirconium, thorium, and lanthanum, and such electrodes are called modified tungsten electrodes. The coatings on the pure tungsten electrodes improve their electrical conductivity and their capability of releasing electrons and make them suitable for use with AC and DCEP. This also results in better arc stability.
B. GTAW / TIG shielding gas
Argon and helium are the popularly used inert shielding gasses for high-quality weldments of reactive and ferrous metals (stainless, aluminum, magnesium, etc.). The selection of a shielding gas depends on:
- Metal to be welded and its thickness.
- Criticality of the weldment (its application).
- Cost factor.
Even though both argon and helium are inert shielding gasses, they have many differences. The ionization capability of helium is much higher than argon. Hence, using helium as shielding gas leads to higher arc voltage than argon. For the same arc length, helium creates a higher arc voltage than argon, which means the helium arc is hotter than the argon arc. Because of this, helium is preferred when welding thick metal (high melting point) plates at a faster speed. Also, helium has better thermal conductivity than argon; hence it transfers the heat from the arc to the workpiece more effectively, increasing the welding speed.
Helium poses difficulties related to arc stability and arc initiation compared to argon, and this behavior is mainly due to the better ionization capability of helium over argon. The minimum arc voltage and associated current for helium are higher than the minimum arc voltage and associated current for argon. Argon is 1.35 times heavier than air and approximately ten times heavier than helium (at the same pressure and temperature). The difference between the density of air and the shielding gasses determines the required flow rate of particular shielding gas to protect the weld pool, weld arc, and the tungsten electrode from the atmospheric air and contamination.
When the shielding gas comes out of the welding torch nozzle, if it is heavier than air, it settles immediately over the weld pool and the welding arc to form a protective shield. On the other hand, if the shielding gas is lighter than air, it tends to move up and needs more flow rate to form a protective shield. Since argon is heavier than air and much heavier than helium, the required flow rate of argon for GTAW welding is much less than helium. Hence, for effective shielding, the needed flow rate of helium (12 to 22 liters/minute) is 2 to 3 times of argon (4 to 12 liters per minute).
Different factors that influence the flow rate of the shielding gas are:
- The gap between the tungsten electrode and the nozzle.
- Size of the molten weld pool.
- The gap between the nozzle and the workpiece.
- The turbulence of the atmospheric air movement.
The flow rate of the shielding gas should be optimum (not high or not low). A higher flow of shielding gas leads to poor stability of the arc and higher atmospheric contamination due to the suction effect. Lower flow leads to insufficient or ineffective shielding. Sometimes small percentages of hydrogen, nitrogen, or oxygen are added to inert gas argon to improve the arc voltage, increasing the weld penetration and speed of welding. Helium (up to 25%) is added to argon to get the best of both:
- The helium arc has good thermal conductivity.
- Argon provides better arc initiation and stability.
Advantages of argon as a shielding gas over helium for general-purpose quality weldments are:
- Better arc initiation.
- Easily available and the cost is less.
- Good cleaning action when used as AC/DCEP.
- Allows shallow penetration, required for welding thin sheets.
All weldments for critical applications are done with inert gasses only. Helium is preferred over argon when high heat generation is required.
C. GTAW / TIG Filler wire
Filler wire is required for filling the groove when welding thick (steel, stainless steel, aluminum, etc.) plates by GTAW for critical applications like aerospace, nuclear, chemical, and food processing machinery industries. Filler wire is normally not used when welding thin sheets. Filler wire is added to the molten weld pool manually, or there can be a wire feeding mechanism.
The selection of proper filler wire metal is essential for good welding. In many cases, the welding may produce cracks even when you select a filler metal of the same composition as the workpiece metal. Hence, it is prudent to select a filler metal after a thorough consideration of:
- Requirement of the mechanical properties.
- Metallurgical compatibility between the workpiece and filler metal.
- Tendency/history of cracking of the workpiece metal during welding.
Example-for welding of aluminum alloys, aluminum filler wire containing 5 to 10% of silicon is used (this filler wire has a low melting point and high fluidity). Aluminum magnesium (5%) filler wires are also used. To increase the welding speed (rate of deposition), different options may be adopted; such as multiple head filler wire feed system that can feed more than one wire to increase the rate of weld deposition; however, a hot wire feeding unit can be a better option (the unit supplies filler wire heated to its below-melting temperature).
Variants of GTAW/TIG welding process
1. Pulse welding
Pulse TIG welding is a variant of GTAW/TIG welding, and in this process, the welding current pulses vary between a high and a low level. A high-level current or peak current is used for melting, and the low level or base current has two functions, sustaining the arc with low heat and enable the solidification of the weld pool. If the peak current is for more time, it allows more time for heating and melting, and if the base current is for more time, it will enable more time for cooling and solidification of the weld pool. The choice of peak current depends on the thickness of the workpiece metal and its thermal conductivity and the selection of the base current on solidification and cooling rate of the weld metal.
The duration (time) of the peak and base current establishes the pulse frequency, and this pulse frequency is set based on the requirement of heat input and the degree of weld pool control.
Base current varies typically from 10 to 25% of the peak current and depends on the thickness of the workpiece metal. The peak current value is set at 150 to 250% of the steady current used in normal GTAW for the same thickness of workpiece metal. The choice of peak current duration depends on the weld pool size and the required penetration. The duration of the base current decides the rate of solidification. The pulse current is decreased to reduce the penetration, and the pulse current duration is reduced to reduce the size of the weld pool.
Pulse TIG welding enables the welding of thin metal sheets using very low heat. This overcomes the issues faced in conventional GTAW/TIG welding, such as melt through, distortion, tolerance, and heat-affected zone (HAZ). In pulse TIG welding, the weld bead is made up of continuous overlapping weld spots if welding is done using low-frequency pulsing.
Advantages of pulse TIG welding, better mechanical properties of the weld metal due to a fine grain structure (for both low and high frequencies). This positive effect is predominant when the value of pulse frequencies is below 10.
Disadvantages of pulse TIG welding, very low pulse frequency leads to the formation of pores. Long base current duration causes fast solidification and insufficient opportunities for the gasses to escape from the weld pool.
2. Hotwire welding
This method is based on the theory of using preheated filler metal in TIG welding to reduce the required heat input (from the TIG system) and improve the weld deposition rate. Electrical resistance heating is generally used for preheating the filler wire, and AC is the preferred power source.
Hotwire TIG welding is generally used for ferrous metals and nickel alloys. This method is not preferred for welding aluminum and copper since their filler metal needs heavy current for electrical resistive heating.
Application areas for the GTAW process
- GTAW/TIG welding is widely used in aerospace, chemical, and food processing machinery industries for its high-quality welding, specifically for thin workpieces of aluminum, stainless steel, and other metals. The weldments used in these industries are critical in nature and should be without impurities.
- TIG welding is often used for the root welding of thicker metals, and the subsequent welding passes can be continued with other welding processes. This is specifically useful for welding pipes.
- TIG welding is used for the welding of dissimilar metals like carbon steel with stainless steel.
- TIG welding is the commonly preferred process for the maintenance rework of tools and dies.
- GTAW/TIG weldments have corrosion resistance and resistance to cracks. This makes GTAW a preferred process for nuclear engineering and operations like sealing of spent fuel containers before burying them and parts used in nuclear engineering.
- GTAW/TIG welding is good for applications that need a smooth and good-looking weld bead.
- GTAW usually is preferred for welding thin sheets of stainless steel and nonferrous metals (aluminum, copper alloys, etc.). GTAW has advantages over other welding processes (SMAW & GMAW) for welding thin sheets, and it will have more control to achieve high-quality welds. GTAW is a preferred welding method for welding aluminum sheets of thickness less than 1 mm.
Advantages and limitations of the GTAW process
- GTAW/TIG welding produces better quality welds (without contaminations) compared to other welding processes like SMAW, MIG welding, etc.
- The welder has better control over the welding variables in GTAW compared to the other welding processes.
- GTAW/TIG welding is excellent for root pass welding due to its good penetration.
- GTAW/TIG welding is a popular process for welding aluminum, stainless steels, and nickel-based alloys.
- GTAW/TIG welding is helpful for welding reactive metals such as titanium and zirconium.
- Due to the low current used, GTAW/TIG welding is good for welding thin metal parts.
- GTAW/TIG welding produces good quality welds of almost all the metals used in engineering, aerospace, chemical, and other industries.
- GTAW/TIG welding produces less smoke compared to SMAW, GMAW, etc.
- The GTAW/TIG welding has good versatility, and it is an all-position welding process widely used for the welding of Nickel/Cobalt alloys.
- GTAW can be adapted for automation, or can be done manually, and can be used for production or repair welding jobs.
- The GTAW process requires good coordination of eyes and hands, and the welder commonly uses both hands during welding. GTAW needs high skill, and the training period is much longer compared to other welding processes.
- The rate of weld metal deposition or speed of welding is low compared to other welding processes.
- Tungsten inclusion in the weld metal can occur if the tungsten electrode touches the weld pool.
- GTAW/TIG welding process is more expensive compared to other welding processes.
TIG welding tips
- GTAW/TIG welding operate at high temperature; hence it is useful for welding metals with a high melting point.
- Do not allow touching of the tungsten electrode with the weld pool since this can contaminate the weldment.
- If you accidentally allowed the electrode to touch the weld pool, immediately take it out and either regrind the tip and clean it before using or take a clean electrode from your box and continue the welding.
- Keep all the TIG welding equipment, workpiece, tungsten electrode, filler wire, and surroundings clean, ensuring quality weldment without contamination.
- Make yourself comfortable and sit when doing TIG welding, and it will be good if your arms are supported.
- Avoid doing GTAW/TIG welding in a place where the wind movement is violent or fast. Such atmospheric conditions disturb the shielding gas protection.
GTAW / TIG Safety considerations
All arc welding processes, including GTAW/TIG, can be dangerous if necessary safety precautions are not taken. The welder has to deal with electric current, electric arc, hot components, welding fumes, thermal radiations, etc. He/she must take all the safety precautions and wear safety gear for protection. The welder has to wear leather hand gloves, long sleeve jackets, shoes, good quality welding helmets (with flip-able welding glasses), and a mask (if there is no provision of built-in protection from fumes in the helmet).
During GTAW/TIG welding, the brightness of the welding arc can break down the atmospheric air into ozone and nitric oxides. The presence of ozone can affect the human lungs. The welding enclosure should have good ventilation for the quick exit of the toxic gases formed during welding. Also, the welding enclosure must not contain inflammable/combustible items like fuel, oil, paper, etc.
The production of smoke is less in GTAW/TIG compared to SMAW/GMAW, and due to this, the welding arc is more visible and brighter, and the welder is subjected to ultraviolet light. The ultraviolet can strongly affect the eyes and act like the sunlight on the human skin causing strong sunburns. Conventional standard welding helmets have dark plates on the front to prevent exposure to thermal radiation. The latest helmet designs have a liquid crystal-type faceplate that automatically darkens when exposed to the welding arc.
The place of GTAW/TIG welding should have a suitable fire extinguisher nearby.
GTAW/TIG welding is a necessary process and is specifically helpful for quality weldments required in critical applications like aerospace, chemical, and nuclear engineering industries. With the research and development in this area, we hope that this process will become much faster and better to make it economical for use in more and more industries.