1. Characteristics of Manual Tungsten Inert Gas (TIG) Welding Process
(1) Working Principle
Tungsten Inert Gas (TIG) welding is a method of gas shielded welding that uses a tungsten rod as an electrode and argon as a shielding gas.
An electric arc is generated between the tungsten electrode and the workpiece, and the stream of argon gas from the welding torch forms a tightly sealed layer in the arc area.
This isolates the electrode and the molten metal pool from air, preventing its intrusion. The arc heat is used to melt the base metal and filler wire to form a molten pool, which solidifies into a weld seam after cooling.
Argon, being an inert gas, does not chemically react with the metal, hence it adequately protects the molten metal pool from oxidation.
Argon also doesn’t dissolve into the molten metal at high temperatures, which prevents the formation of gas holes in the weld seam. Thus, the protective effect of argon is both effective and reliable, producing high-quality weld seams.
During welding, the tungsten electrode does not melt, hence TIG welding is also referred to as non-consumable electrode arc welding. Based on the power source used, TIG welding is divided into direct current (DC), alternating current (AC), and pulsed types.
(2) Process Characteristics
1) Advantages of TIG welding compared to other arc welding methods
a. Superior Protection
The high-quality weld seam is due to argon’s non-reactivity with metals and its insolubility in them. The welding process is essentially a simple process of metal melting and crystallization, resulting in a purer and higher quality weld seam.
b. Minimal Deformation and Stress
The argon gas stream compresses and cools the arc, concentrating the arc heat, which results in a narrow heat-affected zone. This minimizes deformation and stress during welding, making it particularly suitable for thin sheet welding.
c. Easy Observation and Operation
As it’s an open arc welding process, it is easily observable and operable, especially suitable for all-position welding.
The arc is stable, with minimal spatter, and there’s no need for slag removal after welding.
e. Easy Control of the Molten Pool Size
Since the filler wire and electrode are separate, the welder can effectively control the size of the molten pool.
f. Wide Range of Weldable Materials
a. Higher Equipment Cost;
b. High Ionization Potential of Argon, Difficult Arc Ignition, requiring high-frequency arc ignition and stabilization devices;
c. TIG Welding Produces 5-30 times more UV light than manual arc welding, generating harmful ozone for the welder, hence reinforced protection is required;
d. Wind Protection Measures are needed during welding.
3) Application Scope
TIG welding is a high-quality welding method and is widely adopted across various industries.
It’s particularly beneficial for chemically active metals that are difficult to weld using other arc welding techniques, but can easily achieve high-quality weld seams with TIG welding.
Additionally, in the welding of pressure pipes made from carbon steel and low-alloy steel, TIG welding is increasingly being used for root pass welding to improve the quality of the welded joints.
2. Parameters of Manual Tungsten Inert Gas (TIG) Welding
The process parameters for manual TIG welding include: type and polarity of the power source, diameter of the tungsten electrode, welding current, arc voltage, argon gas flow rate, welding speed, diameter of the nozzle, distance from the nozzle to the workpiece, and the length of the tungsten electrode protrusion.
The correct selection and rational combination of these parameters are essential for satisfactory welding quality.
1) Joint and Groove Types
TIG welding is mainly used for thin sheet welding of thickness less than 5mm. Joint types include butt, lap, corner, and T-joints. For sheets less than 1mm thick, flanged joints can also be used. When the plate thickness is more than 4mm, V-grooves should be used (for pipe butt joints of 2-3mm, V-grooves are necessary). U-grooves may also be used for thick-walled pipe butt joints.
2) Pre-welding Cleaning
Pre-welding cleaning is critically important for ensuring joint quality in TIG welding. Under inert gas protection, the molten metal does not undergo significant metallurgical reactions and oxidation and contaminants cannot be removed by deoxidization.
Therefore, prior to welding, the surfaces of the workpiece grooves, both sides of the joint, and the filler wire should be cleaned with an organic solvent (gasoline, acetone, trichloroethylene, carbon tetrachloride, etc.) to remove oil, moisture, dust, and oxide films.
For materials where the surface oxide layer has a strong bond with the base layer, such as stainless steel and aluminum alloy, mechanical methods should be used to remove the oxide layer.
Typically, stainless steel wire brushes or copper wire brushes, fine grinding wheels, or sanding belts are used.
3) Type and Polarity of the Power Source
The type and polarity of the power source can be selected based on the material of the workpiece, as shown in the table below.
Selection of Power Source Type and Polarity
|Power supply type and polarity
|Welded metal material
|DC direct connection
|Low carbon steel, low alloy steel, stainless steel, copper, titanium and their alloys
|DC reverse connection
|Suitable for melting electrode argon arc welding of various metals, tungsten electrode argon arc welding is rarely used
|Aluminum, magnesium and their alloys
When using direct current electrode positive (DCEP), the workpiece is connected to the positive pole, which is at a higher temperature, suitable for welding thick workpieces and metals that dissipate heat quickly.
The tungsten rod is connected to the negative pole, which is at a lower temperature, which can increase the permissible current and minimize the wear of the tungsten electrode.
With direct current electrode negative (DCEN), the tungsten electrode is connected to the positive pole, which results in high electrode wear, so it is rarely used.
In alternating current tungsten inert gas (AC TIG) welding, during the half-wave where the workpiece is negative and the tungsten electrode is positive, the cathode has the effect of removing the oxide film, referred to as the “cathode cleaning” effect.
When welding aluminum, magnesium and their alloys, which have a dense high-melting-point oxide film on their surface, if this oxide film cannot be removed, it will cause defects such as incomplete fusion, slag inclusion, wrinkling on the weld surface, and internal porosity.
The half-wave where the workpiece is positive and the tungsten electrode is negative can cool the tungsten electrode to reduce wear. Therefore, AC TIG welding is commonly used to weld highly oxidizing aluminum, magnesium, and their alloys.
4) Tungsten Electrode Diameter
The diameter of the tungsten electrode is mainly selected based on the thickness of the workpiece, the size of the welding current, and the polarity of the power source.
Improper selection of the tungsten electrode diameter can result in an unstable arc, severe tungsten rod wear, and tungsten inclusion in the weld. (Tungsten Electrode Composition: As an electrode, the tungsten electrode is responsible for conducting current, igniting the arc, and maintaining the arc.
Tungsten is a refractory metal (melting point 3410±10℃) with high temperature resistance (boiling point 5900℃), good electrical conductivity, and a strong ability to emit electrons, which makes tungsten rods suitable for use as electrodes.)
5) Welding Current
The welding current is mainly selected based on the thickness of the workpiece and the spatial position. Both too large and too small welding currents can result in poor weld formation or welding defects.
Therefore, within the range of permissible welding currents for different tungsten electrode diameters, the welding current must be correctly selected, as shown in the table below.
Permissible Current Ranges for Different Diameter Tungsten Electrodes (with Oxides)
|Tungsten Electrode Diameter
|Direct Current Arc Welding
|Direct Current Reversal
Shape of the tungsten electrode tip and current range
|Diameter of Tungsten Electrode
|Cutting Edge Angle
|Direct Current Rectification
|Constant Direct Current
6) Arc Voltage
The arc voltage is determined by the arc length. As the voltage increases, the weld width slightly increases while the penetration decreases.
By coordinating the welding current and arc voltage, the shape of the weld can be controlled. When the arc voltage is too high, it is easy to produce lack of fusion and the argon protection effect worsens.
Therefore, the arc length should be minimized as much as possible without causing a short circuit. The usual range of arc voltage for tungsten argon arc welding is 10-24 volts.
7) Argon Gas Flow
To reliably protect the welding area from air pollution, there must be a sufficient flow of protective gas. The larger the argon gas flow, the stronger the ability of the protective layer to resist the influence of flowing air.
However, when the flow rate is too large, not only will argon be wasted, but the protective gas flow may also form turbulence, bringing air into the protected area and reducing the protective effect.
Therefore, the flow rate of argon should be selected properly. The flow rate of gas can generally be determined by the following empirical formula:
Q = (0.8 – 1.2) D
- Q is the argon gas flow rate, L/mm
- D is the nozzle diameter, mm.
(Argon purity: Different metals require different purities of argon. For example, for welding heat-resistant steel, stainless steel, copper and copper alloys, the purity of argon should be greater than 99.70%; for welding aluminum, magnesium and their alloys, the purity of argon should be greater than 99.90%; for welding titanium and its alloys, the purity of argon should be greater than 99.98%. The purity of domestically produced industrial argon can reach 99.99%, so purification is generally not considered in actual production.)
8) Welding Speed
When the welding speed increases, the argon gas flow should also increase accordingly. If the welding speed is too fast, due to the air resistance affecting the protective gas flow, the protective layer may deviate from the tungsten electrode and the weld pool, thereby deteriorating the protective effect.
At the same time, the welding speed significantly affects the formation of the weld. Therefore, an appropriate welding speed should be selected.
9) Nozzle Diameter
When the nozzle diameter is increased, the gas flow should be increased at the same time. At this time, the protection area is larger and the protective effect is better.
But when the nozzle is too large, not only will the consumption of argon increase, but the torch may not be able to reach in, or it may obstruct the welder’s line of sight and make it difficult to observe the operation.
Therefore, the nozzle diameter for general tungsten argon arc welding is best between 5-14mm.
In addition, the nozzle diameter can also be selected according to the empirical formula:
D = (2.5 – 3.5) d
- D is the nozzle diameter (usually the inner diameter), mm;
- d is the tungsten electrode diameter, mm.
10) Distance from the Nozzle to the Workpiece
Here, we are referring to the distance between the end face of the nozzle and the workpiece. The smaller this distance, the better the protective effect.
Therefore, the distance between the nozzle and the workpiece should be as small as possible, but if it’s too small it makes operation and observation inconvenient. Therefore, the usual distance from the nozzle to the workpiece is between 5-15mm.
11) Tungsten Electrode Extension Length
In order to prevent the arc heat from damaging the nozzle, the end of the tungsten electrode protrudes outside the nozzle. The distance from the end of the tungsten electrode to the nozzle face is called the tungsten electrode extension length.
The smaller the tungsten electrode extension length, the closer the distance between the nozzle and the workpiece, and the better the protective effect, but if it’s too close, it will hinder the observation of the weld pool.
Usually, when welding a butt joint, a tungsten electrode extension length of 3-6mm is better. When welding a fillet joint, a tungsten electrode extension length of 7-8mm is better.
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