Gas Tungsten Arc Welding (GTAW) , also known by the acronym TIG (Tungsten Inert Gas), is an arc welding process that uses a non-consumable tungsten electrode. The arc is established between this electrode and the workpiece, while an inert gas (argon, helium, or a mixture) shields the weld zone from atmospheric contamination. Filler metal, when required, is added manually in the form of a rod or wire.
This process is governed by CSA W117.2 (Safety in Welding, Cutting, and Allied Processes) for safety aspects, and by the qualification standards CSA W47.1 (welding of steel) and CSA W47.2 (welding of aluminum) for the certification of companies and welders.
Distinctive Characteristics of GTAW
Characteristic
GTAW
Comparison with SMAW (Shielded Metal Arc Welding)
Electrode
Non-consumable tungsten
Consumable, coated
Filler metal
Separate, added manually
Included in the coating
Shielding gas
Argon, helium, mixtures
None (the coating generates the gas)
Arc control
Excellent
Moderate
Bead quality
Superior, clean
Good, with slag
Positions
All
All
Typical applications
Tubing, thin sheet, non-ferrous metals
Structural steel, heavy piping
GTAW is the process of choice for materials requiring high metallurgical cleanliness: stainless steels, aluminum, magnesium, titanium, copper, and nickel alloys. It allows precise control of heat input, which minimizes distortion and deformation.
Operating Principle
The Electric Arc and Its Stabilization
The GTAW arc is an electric arc established between a tungsten electrode (melting point ≈ 3,410 °C) and the workpiece. Tungsten is chosen for its exceptional resistance to temperature. The arc is maintained in an inert gas atmosphere that does not react chemically with the weld pool.
The current density in the GTAW arc is high, producing a plasma temperature between 5,000 °C and 30,000 °C depending on the amperage and the gas used. Energy transfer to the workpiece occurs primarily through conduction and convection of the plasma.
Arc Initiation
Three arc initiation methods are used:
15.Scratch start: the electrode briefly touches the workpiece and is then withdrawn. This method risks contaminating the electrode with the base metal. It is used only for troubleshooting or on simple equipment.
16.High-frequency (HF) start: a high-voltage (2,000 to 10,000 V) and high-frequency (250 kHz to 1 MHz) generator ionizes the gap between the electrode and the workpiece without contact. This is the most common method for alternating current (AC) and direct current (DC).
17.Lift arc start: the electrode touches the workpiece at low amperage, then is lifted; the arc establishes at low voltage. This method avoids contamination and reduces electromagnetic interference.
Polarity and Its Effects
Polarity
Penetration
Surface Cleaning
Application
**DCEN** (electrode negative)
Deep
None
Steels, stainless steel, titanium, copper
**DCEP** (electrode positive)
Shallow
High
Thin sheet (rare)
**AC** (alternating)
Medium
High (alternating)
Aluminum, magnesium
In DCEN (direct current, electrode negative), approximately 70% of the heat is generated at the workpiece, producing deep penetration. The electrode remains relatively cool, allowing higher currents to be used.
In DCEP, 70% of the heat is generated at the electrode, causing it to overheat. This polarity is rarely used, except for welding very thin sheet where low heat input is desired.
In AC, the alternating half-cycles combine the penetration of DCEN with the cathodic cleaning effect of DCEP. During the positive half-cycle (electrode positive), gas ions bombard the workpiece surface, breaking up the aluminum oxide layer (melting point ≈ 2,070 °C, while aluminum melts at 660 °C). This effect is essential for welding aluminum.
AC Balance
Modern GTAW power sources allow adjustment of the AC balance (balance control), expressed as a percentage of the electrode negative (EN) half-cycle. A typical setting is 65 to 75% EN. A higher percentage provides more penetration but less cleaning; a lower percentage provides more cleaning but less penetration and a hotter electrode.
AC Frequency
AC frequency (60 Hz base, up to 250 Hz on advanced machines) influences arc shape. A higher frequency constricts the arc, improves directionality, and reduces bead width. It is particularly useful for welding aluminum in the vertical or overhead position.
Equipment and Consumables
Welding Power Source
The GTAW power source must provide constant current (a drooping or slope characteristic). The essential parameters are:
Welding current (A): the main setting, determines heat input.
Arc voltage (V): a function of arc length, typically 10 to 15 V.
Up-slope: gradual current increase at start-up to avoid thermal shock.
Down-slope: gradual current decrease at stop to avoid craters and hot cracking.
Crater fill current: reduced current maintained for 1 to 3 seconds to fill the final crater.
Pre-flow and post-flow gas: gas must start 0.5 to 1 second before arc initiation and continue for 5 to 15 seconds after arc extinction to protect the electrode and the cooling weld pool.
GTAW Torch
The torch is air-cooled (up to approximately 200 A) or water-cooled (beyond that). Its components:
Torch body: electrically insulated, cooled.
Collet body: holds the electrode, ensures electrical contact.
Collet: clamps the electrode.
Nozzle: ceramic (standard), metal, or quartz (high temperature). Nozzle diameter determines gas coverage.
Gas lens: a device that equalizes gas flow, allowing the nozzle to be held further from the workpiece (up to 20 mm) while maintaining effective shielding.
Tungsten Electrodes
Electrodes are classified by their composition. The color code is standardized according to AWS A5.12:
Type
Designation
Color Code
Characteristics
Application
Pure tungsten
EWP
Green
Low current capacity, limited stability
AC, aluminum (rare)
Thorated (2%)
EWTh-2
Red
Excellent electron emission, long life
DCEN, steels, stainless steel
Ceriated (2%)
EWCe-2
Grey
Good stability, low radioactivity
DC and AC, versatile
Lanthanated (1.5%)
EWLa-1.5
Gold
Excellent stability, easy starting
DC and AC, versatile
Zirconiated (1%)
EWZr-1
Brown
Resists contamination, good on AC
AC, aluminum
Important: Thorium is slightly radioactive. Grinding thoriated electrodes must be done on a dedicated grinder with ventilation, and gloves must be worn. Ceriated and lanthanated electrodes are non-radioactive alternatives.
Electrode Sharpening
Sharpening is done in the longitudinal direction (parallel to the electrode axis). Grinding striations must be parallel to the axis to stabilize the arc. The sharpening angle influences penetration:
Narrow angle (15 to 20°): narrow arc, deep penetration, suitable for thick sheet.
Wide angle (30 to 60°): wider arc, shallower penetration, suitable for thin sheet.
The electrode diameter must match the current used:
Diameter (mm)
DCEN Current (A)
AC Current (A)
1.0
15 – 80
10 – 60
1.6
70 – 150
50 – 100
2.4
150 – 250
100 – 160
3.2
250 – 400
150 – 210
4.0
400 – 500
200 – 275
Shielding Gases
The gas must be inert (does not react with the weld pool). Common choices:
Argon (Ar): the most common. Density 1.38 times that of air, providing good coverage. Stable arc, effective cathodic cleaning on AC. Used for all metals.
Helium (He): lighter than air, requires a higher flow rate (1.5 to 2 times). Higher heat input, deeper penetration. Used for thick copper and thick aluminum.
Ar-He mixtures: combine argon stability with helium heat. Examples: 75% Ar / 25% He, 50% Ar / 50% He.
Hydrogen (H₂): added to argon (up to 5%) for austenitic stainless steels. Increases welding speed and penetration. Never on ferritic or martensitic steels or non-ferrous metals (risk of hot cracking).
Typical gas flow rates: 8 to 15 L/min for argon, depending on nozzle diameter and ventilation conditions. Excessive flow creates turbulence that draws in ambient air; insufficient flow does not provide adequate shielding.
Filler Metal
Filler metal must be compatible with the base metal. Rods are typically 1.6 mm, 2.4 mm, or 3.2 mm in diameter. For carbon steel, use ER70S-2, ER70S-3, or ER70S-6 rods. For stainless steel, ER308L (304 stainless steel), ER309L (dissimilar metals), ER316L (316 stainless steel). For aluminum, ER4043 (4043 alloy, good fluidity) or ER5356 (5356 alloy, better mechanical strength).
Preparation and Welding Procedures
Joint Preparation
Cleanliness is paramount in GTAW. Any contamination (oil, grease, paint, oxide, moisture) causes defects: porosity, inclusions, cracks.
Steels and stainless steel: degrease with solvent, brush with a dedicated stainless steel brush (never a brush that has been used on carbon steel).
Aluminum: degrease, then remove the oxide layer by brushing with a stainless steel brush or by chemical etching. Brushing must be done just before welding, as the oxide reforms quickly.
Titanium: rigorous cleaning, degreasing, and welding under extended gas shielding (trailing shield, back purging).
Joint Geometry
Joint Type
Thickness (mm)
Preparation
Gap (mm)
Butt, thin sheet
≤ 3
Square edge
0 – 1
Butt, medium sheet
3 – 6
V-groove (60°)
1 – 2
Butt, thick sheet
> 6
V or U-groove, root pass
2 – 3
T-joint
All
Square edge or groove
0 – 2
Operating Technique
72.Torch position: tilt of 70 to 80° from the workpiece (slightly pushing). The filler rod is held at 10 to 20° from horizontal, on the opposite side from the torch.
73.Arc length: maintain an arc length equal to the electrode diameter (approximately 1.5 to 3 mm). An arc that is too long disperses heat and reduces gas shielding.
74.Adding filler metal: the rod is dipped into the weld pool, never above the arc. Remove the rod from the gas shielding zone after each dip to prevent oxidation.
75.Travel: continuous and steady movement. For butt joints, a slight weaving motion may be necessary on thick sheet.
76.End of weld: use down-slope to gradually reduce the current, then maintain post-flow gas for 5 to 15 seconds.
Welding in Position
GTAW can be used in all positions. In the vertical (3G) and overhead (4G) positions, heat input must be reduced (lower current) and travel speed increased. Filler metal is added in smaller amounts to prevent sagging.
Welding Parameters and Calculations
Heat Input Calculation
Heat input is a critical parameter, particularly for high-strength steels and stainless steels. It is calculated as:
Important: For steel, the thermal transfer efficiency is approximately 60 to 70% for GTAW (compared to 80 to 90% for GMAW). Some standards require multiplying the result by the efficiency factor (for example, 0.6 for GTAW per API 1104).
Travel Speed
Travel speed is determined by the desired bead width and the metal thickness. A rule of thumb: the bead width should be 2 to 3 times the electrode diameter. Speed is adjusted to achieve complete penetration without excessive reinforcement.
Number of Passes
For a given thickness, the number of passes depends on the process capability. In GTAW, a single pass can weld up to approximately 3 mm without filler metal (autogenous fusion) and up to 6 mm with filler. Beyond that, multiple passes are required.
Welding Defects and Remedies
Defect
Probable Cause
Remedy
**Porosity**
Contamination (oil, moisture), insufficient gas, excessive flow (turbulence), current too high
Clean, adjust flow rate, reduce current
**Hot cracking**
High stresses, unsuitable filler metal, unfilled crater
Use down-slope, choose appropriate filler metal
**Tungsten inclusion**
Electrode contact with the weld pool, current too high, electrode too pointed
Correct technique, reduce current, re-sharpen
**Lack of penetration**
Current too low, sharpening angle too narrow, speed too fast
Increase current, widen angle, slow down
**Bead oxidation**
Insufficient gas shielding, post-flow too short, nozzle too far away
Current too high, incorrect polarity (DCEP), electrode too thin
Reduce current, check polarity, choose a larger diameter
Applicable Standards and Codes
CSA W117.2 — Safety in Welding
This standard defines safety requirements for all welding processes. Key points for GTAW:
Ventilation: GTAW produces ozone and nitrogen oxides. Local ventilation (source extraction) is required, particularly in confined spaces.
Eye protection: shade No. 10 to 12 lens for GTAW (the arc is more intense than SMAW at equivalent current). Welders nearby must use screens.
Thoriated electrodes: handle with gloves, wash hands after handling, grind under ventilation.
Electrical equipment: check grounding, cables in good condition, insulated electrode holder.
CSA W47.1 — Certification of Welding Companies (Steel)
This standard requires welders to be qualified according to qualified welding procedures (WPS) . For GTAW, the essential variables include:
The process (GTAW)
The type of current and polarity
The base metal and its thickness
The filler metal (classification)
The welding position
The shielding gas (composition and flow rate)
A welder qualified in GTAW on thin sheet (≤ 3 mm) is not automatically qualified for thick sheet. Qualification ranges are defined by the standard.
CSA W47.2 — Certification of Welding Companies (Aluminum)
For aluminum, qualification requires demonstrating mastery of the AC process with appropriate balance and frequency. Typical aluminum defects (hydrogen porosity, hot cracking) are specifically evaluated.
CSA B149.1 — Natural Gas and Propane Installation Code
For welding piping transporting natural gas, welds must be made according to qualified procedures and by certified welders. Inspection requirements (radiography, destructive testing) are defined in this code.
Canadian Electrical Code, Part I, Chapter V
This code applies to the electrical installations of welding power sources. Requirements include:
Rule 8-200: conductor capacity — welding cables must be sized for the maximum output current.
Rule 10-204: grounding of equipment — the power source must be grounded in accordance with the requirements.
Rule 14-100: overcurrent protection — circuit breakers or fuses sized according to the power source capacity.
Specific Applications
Stainless Steel Welding
GTAW is the reference process for stainless steel. Critical points:
Use a dedicated brush (never carbon steel).
Gas: pure argon or argon + 2 to 5% hydrogen (austenitic only).
Control heat input to avoid sensitization (carbide precipitation at grain boundaries between 425 °C and 870 °C), which reduces corrosion resistance.
For austenitic stainless steels, the interpass temperature must not exceed 150 °C.
Back purging with argon to prevent oxidation (formation of "soot" or "sugar").
Aluminum Welding
Alternating current (AC) with 65 to 75% EN balance.
Electrode: zirconiated (EWZr-1) or lanthanated (EWLa-1.5), 2.4 to 3.2 mm diameter.
Ultrasonic testing: detects lack of fusion and cracks.
Dye penetrant testing: detects surface cracks.
Magnetic particle testing: for ferromagnetic materials only.
Common Pitfalls to Avoid
155.Confusing DCEN and DCEP: in GTAW, the standard polarity is DCEN (electrode negative). DCEP overheats the electrode and gives shallow penetration. Aluminum uses AC, not DCEN.
156.Neglecting post-flow gas: cutting the gas immediately after arc extinction causes oxidation of the electrode and crater. Post-flow must be 5 to 15 seconds.
157.Using thoriated electrodes without precautions: thorium is radioactive. Grinding must be done under ventilation, with gloves, and residues must be disposed of as hazardous waste.
158.Welding aluminum with direct current: without the cathodic cleaning effect of AC, the aluminum oxide layer (melting point 2,070 °C) does not melt and remains on the surface, causing inclusions and lack of fusion.
159.Confusing gases: CO₂ is never used in GTAW (it is active and contaminates the tungsten). Hydrogen must never be used on carbon steel, aluminum, or copper (hot cracking).
160.Forgetting back purging: for stainless steel and titanium, the back side of the joint must be shielded with inert gas, otherwise it oxidizes and loses its corrosion resistance.
161.Calculating heat input without the efficiency factor: for GTAW, the factor is approximately 0.6. A calculation without this factor underestimates the actual heat input.
162.Using a carbon steel brush on stainless steel: this contaminates the surface with iron particles that cause corrosion.
163.Ignoring qualification requirements: each company must have qualified WPS and certified welders per CSA W47.1 or W47.2. A welder qualified on one procedure is not qualified for all.
164.Neglecting ventilation: GTAW produces ozone, an irritating and toxic gas. In confined spaces, ventilation must be forced and a safety attendant must be present.
Summary
GTAW uses a non-consumable tungsten electrode, an inert gas, and separate filler metal. It offers superior arc control and excellent bead quality.
Polarities: DCEN (steels, stainless steel, titanium), DCEP (rare), AC (aluminum, magnesium). AC balance and frequency are essential parameters for aluminum.
Electrodes are classified by composition (pure, thoriated, ceriated, lanthanated, zirconiated) with standardized color codes. Sharpening is done longitudinally.
Gases: argon (standard), helium (penetration), Ar-He mixtures, hydrogen (austenitic stainless steel only). Typical flow rate is 8 to 15 L/min.
Preparation is critical: absolute cleanliness, degreasing, brushing appropriate to the material.
Heat input is calculated: (V × A × 60) / (speed × 1000), with an efficiency factor of 0.6 for GTAW.
Standards: CSA W117.2 (safety), CSA W47.1 (steel), CSA W47.2 (aluminum), CSA B149.1 (gas), Canadian Electrical Code, Part I, Chapter V.
Common defects: porosity, hot cracking, tungsten inclusion, lack of penetration, oxidation.
Specific applications: stainless steel (thermal control, back purging), aluminum (AC, cleaning), titanium (extended shielding, color control).
Self-Assessment Questions (Exam-Type)
176.What is the primary role of the shielding gas in GTAW?
a) Cool the electrode
b) Protect the weld pool from the atmosphere
c) Transport the filler metal
d) Increase welding speed
181.To weld 6 mm thick aluminum, which polarity do you use?
a) DCEN
b) DCEP
c) AC
d) DCEN with helium
186.What is the sensitization temperature range for austenitic stainless steels?
a) 100 – 200 °C
b) 425 – 870 °C
c) 900 – 1,200 °C
d) 1,200 – 1,500 °C
191.A welder uses a 2.4 mm electrode with 300 A in DCEN. What is the risk?
a) Electrode overheating
b) Lack of penetration
c) Porosity
d) No problem
196.What is the typical gas flow rate for a 10 mm nozzle with argon?
a) 2 – 5 L/min
b) 8 – 15 L/min
c) 20 – 30 L/min
d) 35 – 50 L/min
201.Heat input is calculated as 1.2 kJ/mm without the efficiency factor. What is the actual heat input for GTAW?
a) 0.72 kJ/mm
b) 1.2 kJ/mm
c) 1.8 kJ/mm
d) 2.0 kJ/mm
206.Which type of electrode is recommended for AC welding of aluminum?
