GTAW

Overview

Definition

Gas Tungsten Arc Welding (GTAW), also known as Tungsten Inert Gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas (argon or helium).

Process Characteristics

• High quality welds
• Excellent control over welding parameters
• Clean process (no slag)
• Can be used with or without filler metal
• All position welding capability
• Suitable for thin materials
• Wide range of materials

Process Principle

Basic Setup Components

1. Power Source

   • Constant current (drooping characteristic)
   • AC or DC capability
   • High frequency unit for arc starting

2. Torch (Welding Gun)

   • Holds tungsten electrode
   • Delivers shielding gas
   • Water-cooled or air-cooled

3. Tungsten Electrode

   • Non-consumable
   • High melting point (3,410°C)

4. Shielding Gas Supply

   • Gas cylinder with regulator
   • Flowmeter
   • Hoses

5. Filler Metal (optional)

   • Separate rod fed manually
   • Not consumed by arc

6. Foot Pedal or Hand Control (optional)

   • Controls welding current during welding

Working Mechanism

1. Arc is established between tungsten electrode and workpiece
2. Inert gas flows through torch, shielding weld area
3. Arc heat melts base metal, forming weld pool
4. Filler metal (if used) is fed into weld pool
5. No flux or slag produced
6. Clean, precise weld results

Tungsten Electrodes

Electrode Materials

1. Pure Tungsten (EWP)

• Color code: Green
• Composition: 99.5% tungsten minimum
• Used with: AC welding
• Applications: Aluminum, magnesium
• Characteristics: Good arc stability on AC

2. Thoriated Tungsten (EWTh-1, EWTh-2)

• Color code: Yellow (1%), Red (2%)
• Composition: 1% or 2% thorium oxide
• Used with: DCEN (straight polarity)
• Applications: Steel, stainless steel, copper alloys
• Characteristics:
  • Excellent arc starting
  • High current capacity
  • Long life
  • Better electron emission
• Note: Thorium is radioactive (low level) - handle with care

3. Ceriated Tungsten (EWCe-2)

• Color code: Orange
• Composition: 2% cerium oxide
• Used with: AC or DC
• Applications: General purpose, thin materials
• Characteristics:
  • Good arc starting at low currents
  • Non-radioactive alternative to thoriated
  • Longer life than pure tungsten

4. Lanthanated Tungsten (EWLa-1, EWLa-2)

• Color code: Gold (1%), Blue (2%)
• Composition: 1% or 2% lanthanum oxide
• Used with: AC or DC
• Applications: All materials
• Characteristics:
  • Excellent arc starting
  • Non-radioactive
  • Good alternative to thoriated
  • Stable arc

5. Zirconiated Tungsten (EWZr)

• Color code: Brown
• Composition: 0.15-0.40% zirconium oxide
• Used with: AC welding
• Applications: Aluminum, magnesium
• Characteristics:
  • Better than pure tungsten for AC
  • Resists contamination
  • Maintains ball shape on AC

Electrode Diameter

• Common sizes: 0.5, 1.0, 1.6, 2.4, 3.2, 4.0, 4.8, 6.4 mm
• Selection based on welding current

Current Carrying Capacity (approximate):

• 1.0 mm: 10-75 A (DCEN), 10-60 A (AC)
• 1.6 mm: 50-150 A (DCEN), 40-130 A (AC)
• 2.4 mm: 100-250 A (DCEN), 90-210 A (AC)
• 3.2 mm: 200-400 A (DCEN), 180-320 A (AC)

Electrode Preparation

For DCEN (Straight Polarity)

• Grind to a point
• Included angle: 15-30° for thin materials, 60-90° for thick materials
• Grind lengthwise (parallel to electrode axis)
• Flat tip (0.5-1 mm) prevents tip melting

For AC

• Ball end preferred
• Ball forms naturally during welding
• Diameter of ball: 1.5 × electrode diameter
• Can pre-ball by running high current on scrap

Grinding Precautions

• Use dedicated grinding wheel for tungsten
• Grind lengthwise, not circumferentially
• Avoid contamination
• For thoriated tungsten: use ventilation, avoid dust inhalation

Shielding Gases

Gas Functions

1. Shield molten metal from atmosphere
2. Affect arc characteristics
3. Influence heat transfer
4. Affect weld bead shape and penetration

Common Shielding Gases

1. Argon (Ar)

• Most commonly used
• Atomic weight: 39.9
• Characteristics:
  • Good arc stability
  • Smooth arc
  • Good cleaning action on AC
  • Lower heat input than helium
  • Narrower, deeper penetration pattern
• Applications: All materials, especially thin sections
• Cost: Moderate

2. Helium (He)

• Atomic weight: 4.0
• Characteristics:
  • Higher arc voltage
  • Higher heat input
  • Wider, shallower penetration
  • Less cleaning action
  • Requires higher flow rates
• Applications: Thick materials, high-speed welding, copper, aluminum
• Cost: Expensive

3. Argon-Helium Mixtures

• Common ratios: 75% Ar / 25% He, 50% Ar / 50% He
• Combines advantages of both gases
• Better heat input than pure argon
• Better arc stability than pure helium
• Applications: Thick aluminum, copper alloys

4. Argon-Hydrogen Mixtures

• Typical: 95% Ar / 5% H₂ or 98% Ar / 2% H₂
• Hydrogen increases heat input
• Applications: Stainless steel (austenitic), nickel alloys
• Not for: Carbon steel, aluminum (causes porosity)

Gas Flow Rate

• Typical range: 5-20 L/min
• Depends on:
  • Torch cup size
  • Welding current
  • Joint configuration
  • Drafts/wind conditions

General Guidelines:

• Small cup (6-8 mm): 5-8 L/min
• Medium cup (10-12 mm): 8-12 L/min
• Large cup (16-20 mm): 12-20 L/min

Effects:

• Too low: inadequate shielding, oxidation, porosity
• Too high: turbulent flow, air entrainment, porosity

Welding Current and Polarity

Direct Current Electrode Negative (DCEN / Straight Polarity)

Characteristics:

• 70% of heat at workpiece
• Deep, narrow penetration
• Pointed electrode
• No cleaning action

Applications:

• Carbon steel
• Stainless steel
• Copper and copper alloys
• Titanium
• Most metals except aluminum and magnesium

Advantages:

• Deep penetration
• Narrow weld bead
• High welding speed
• Stable arc

Direct Current Electrode Positive (DCEP / Reverse Polarity)

Characteristics:

• 70% of heat at electrode
• Shallow, wide penetration
• Electrode overheats quickly
• Strong cleaning action (removes oxides)

Applications:

• Rarely used
• Limited to very thin materials

Disadvantages:

• Low current capacity
• Rapid electrode consumption
• Shallow penetration

Alternating Current (AC)

Characteristics:

• Heat distribution balanced
• Cleaning action during electrode positive half-cycle
• Penetration during electrode negative half-cycle
• Ball-shaped electrode tip

Applications:

• Aluminum
• Magnesium
• Materials with refractory oxides

Advantages:

• Oxide cleaning action
• Balanced heat input
• Good for aluminum and magnesium

AC Wave Forms:

Conventional Sine Wave

• Standard AC
• Requires high frequency for arc stability

Square Wave

• Modern inverter power sources
• Better arc stability
• Adjustable balance control
• Adjustable frequency

Balance Control:

• Adjusts ratio of electrode positive to electrode negative time
• More EN (electrode negative): deeper penetration, less cleaning
• More EP (electrode positive): more cleaning, less penetration
• Typical: 60-70% EN, 30-40% EP

Frequency Control:

• Standard: 50-60 Hz
• High frequency: 100-400 Hz
• Higher frequency: narrower arc, better control, less wandering

Arc Starting Methods

1. Scratch Start

• Electrode scratched on workpiece
• Simple but risks contamination
• Can damage electrode tip
• Not recommended for critical applications

2. Lift Arc (Touch Start)

• Electrode touches workpiece at low current
• Lifted to establish arc
• Current increases to welding level
• Less contamination than scratch start

3. High Frequency (HF) Start

• High voltage, high frequency pulse ionizes gap
• No contact required
• Clean start, no contamination
• Standard method for quality work
• Required for AC welding

HF Modes:

• Continuous: HF runs throughout welding (for AC)
• Start only: HF only for starting (for DC)

Welding Parameters

1. Welding Current

• Primary heat control parameter
• Selection based on:
  • Material type and thickness
  • Joint design
  • Electrode diameter
  • Welding position

Typical Ranges:

• Thin materials (< 1 mm): 10-50 A
• Medium (1-3 mm): 50-150 A
• Thick (> 3 mm): 150-400 A

2. Arc Voltage

• Typically 10-20 V
• Determined by arc length
• Arc length: 1.5-3 mm (approximately electrode diameter)
• Longer arc: wider bead, less penetration
• Shorter arc: narrower bead, deeper penetration

3. Travel Speed

• Typical: 2-8 mm/s
• Affects heat input and bead shape
• Too slow: excessive heat, wide bead, burn-through
• Too fast: narrow bead, incomplete fusion, undercut

4. Electrode Extension (Stick-out)

• Distance from electrode to cup
• Typical: 3-6 mm
• Longer extension: better visibility, less gas coverage
• Shorter extension: better gas coverage, less visibility

5. Torch Angle

• Longitudinal (travel angle): 5-15° push or drag
  • Push: better gas coverage, wider bead
  • Drag: better visibility, narrower bead
• Transverse (work angle): depends on joint type
  • Butt joint: 90°
  • Fillet: 45°

Filler Metals

Filler Rod Specifications

• Supplied as straight rods
• Typical length: 900 mm (36 inches)
• Diameters: 1.6, 2.4, 3.2, 4.0, 4.8 mm
• Must match base metal composition

Filler Metal Selection

• Match base metal composition
• Consider dilution with base metal
• May need different composition for dissimilar metals
• AWS specifications: ER70S-x (steel), ER4043 (aluminum), ER308L (stainless)

Filler Addition Technique

• Feed at 10-15° angle to weld pool
• Add to leading edge of pool
• Dip and withdraw (not continuous feed)
• Maintain rhythm: move torch, add filler, repeat
• Keep filler in gas shield

Welding Techniques

Manual GTAW Technique

Hand Position

• Torch in dominant hand
• Filler rod in other hand
• Rest hands on workpiece for stability
• Use finger walking technique for long welds

Torch Manipulation

• Stringer bead: straight line, no weaving
• Slight weave: small side-to-side motion (< 3× electrode diameter)
• Walking the cup: rest cup on workpiece, pivot torch

Filler Addition

• Dip filler into leading edge of pool
• Withdraw before it melts completely
• Maintain consistent rhythm
• Don't let filler touch electrode

Starting the Weld

1. Position torch at start point
2. Initiate arc (HF start preferred)
3. Establish weld pool
4. Begin adding filler metal
5. Start travel

Crater Filling

• Critical to prevent crater cracks
• Methods:
  • Reverse direction briefly
  • Add extra filler at end
  • Use current decay (slope-out)
• Never break arc abruptly

Current Control

• Foot pedal: variable control during welding
  • Start with low current
  • Increase to welding current
  • Decrease at end (crater fill)
• Hand control: similar function on torch
• Fixed current: set on power source

Pulsed GTAW

Principle

• Current alternates between high (peak) and low (background) levels
• Creates series of overlapping spot welds

Parameters

• Peak current (Ip): melting current
• Background current (Ib): maintains arc, allows cooling
• Peak time (tp): duration of peak current
• Background time (tb): duration of background current
• Pulse frequency (f): pulses per second

Formula:

f = 1 / (tp + tb)

Advantages

• Better control of heat input
• Reduced distortion
• Improved penetration control
• Better for thin materials
• Easier out-of-position welding
• Refined grain structure

Applications

• Thin materials (< 2 mm)
• Aerospace applications
• Precision welding
• Out-of-position welding

Advantages of GTAW

1. Weld Quality

   • High quality, clean welds
   • No slag or spatter
   • Excellent appearance
   • Minimal post-weld cleaning

2. Control

   • Precise control of heat input
   • Independent control of heat and filler addition
   • Excellent for thin materials

3. Versatility

   • Wide range of materials
   • All positions
   • With or without filler metal

4. Visibility

   • Clear view of weld pool
   • No smoke or fumes obscuring vision

5. Material Range

   • Ferrous and non-ferrous metals
   • Reactive metals (titanium, zirconium)
   • Dissimilar metals

Limitations of GTAW

1. Low Productivity

   • Low deposition rate (0.5-2 kg/h)
   • Slow process
   • Not economical for thick materials

2. Skill Requirement

   • Requires high operator skill
   • Coordination of both hands
   • Long learning curve

3. Wind Sensitivity

   • Shielding gas easily blown away
   • Requires wind protection for outdoor use
   • Drafts can cause porosity

4. Equipment Cost

   • Higher initial cost than SMAW
   • More complex equipment
   • Gas supply required

5. Limited Thickness

   • Not economical for thick materials (> 10 mm)
   • Better processes available for production welding

Applications

Typical Uses

• Aerospace industry (critical welds)
• Nuclear industry
• Pipe welding (root passes)
• Thin sheet metal fabrication
• Precision welding
• Repair and maintenance
• Food and pharmaceutical equipment

Material Applications

• Aluminum and aluminum alloys (AC)
• Stainless steel (DCEN)
• Carbon steel (DCEN)
• Titanium (DCEN)
• Magnesium (AC)
• Copper and copper alloys (DCEN, often with helium)
• Nickel alloys (DCEN)

Thickness Range

• Minimum: 0.5 mm (with proper technique)
• Maximum: 10 mm (single pass), unlimited (multi-pass)
• Optimal: 0.5-6 mm

Automated and Mechanized GTAW

Mechanized GTAW

• Torch mounted on carriage or manipulator
• Automatic travel
• Manual setup and monitoring
• Consistent quality
• Higher productivity than manual

Automatic GTAW

• Fully automated
• Programmed parameters
• Automatic filler wire feed
• Used for: pipe welding, tube mills, production welding

Orbital GTAW

• Torch rotates around fixed pipe
• Used for: tube-to-tubesheet welding, pipe welding
• Applications: nuclear, pharmaceutical, semiconductor industries
• Produces consistent, high-quality welds

Special GTAW Techniques

Hot Wire GTAW

• Filler wire preheated by electrical resistance
• Increases deposition rate
• Reduces heat input to base metal
• Applications: surfacing, thick materials

Narrow Gap GTAW

• Special joint design with narrow gap
• Reduces filler metal required
• Used for thick materials
• Requires special torch design

Weld Quality and Defects

Common Defects

1. Tungsten Inclusions

   • Tungsten particles in weld
   • Causes: electrode contact with pool, excessive current
   • Prevention: avoid contact, proper current, replace damaged electrode

2. Porosity

   • Causes: inadequate gas shielding, contamination, moisture, drafts
   • Prevention: proper gas flow, clean materials, wind protection

3. Oxidation/Discoloration

   • Causes: inadequate shielding, premature gas shutoff
   • Prevention: proper gas coverage, post-flow time (5-15 seconds)

4. Incomplete Fusion

   • Causes: insufficient heat, improper technique, contamination
   • Prevention: adequate current, proper manipulation, clean surfaces

5. Undercut

   • Causes: excessive current, wrong angle, fast travel
   • Prevention: proper parameters, correct technique

Quality Improvement

• Proper material cleaning (degrease, remove oxides)
• Correct gas selection and flow rate
• Appropriate welding parameters
• Skilled operator
• Wind protection
• Proper electrode preparation and maintenance

Safety Considerations

Specific GTAW Hazards

• Intense arc radiation (UV and IR)
• Ozone and nitrogen oxide generation
• High frequency electromagnetic radiation
• Inert gas asphyxiation hazard
• Electric shock

Safety Measures

• Proper welding helmet (shade #10-#14)
• Adequate ventilation
• Avoid HF interference with pacemakers
• Ensure adequate oxygen in confined spaces
• Proper grounding
• Dry gloves and clothing

Economics and Productivity

Deposition Rate

• Manual: 0.5-2 kg/h
• Mechanized: 1-3 kg/h
• Lower than other arc welding processes

Operating Factor

• Manual: 0.2-0.4
• Mechanized: 0.4-0.6

Cost Considerations

• Higher equipment cost
• Lower deposition rate
• Higher labor cost per kg deposited
• Justified by: quality requirements, material type, thickness
• Most economical for: thin materials, critical applications, difficult materials

When to Use GTAW

• Quality is critical
• Thin materials
• Difficult-to-weld materials
• No post-weld cleaning desired
• Precision required
• Root passes in pipe welding