GMAW

Introduction

GMAW (Gas Metal Arc Welding) = Arc welding process using a continuously fed consumable wire electrode and shielding gas

Also Known As:

• MIG (Metal Inert Gas) - when using inert gases (Ar, He)
• MAG (Metal Active Gas) - when using active gases (CO₂, gas mixtures)

Process Principle

1. Continuous wire electrode fed through welding gun
2. Electric arc between wire and workpiece melts both
3. Shielding gas protects weld pool from atmosphere
4. No flux required (unlike SMAW)
5. Semi-automatic or automatic operation

GMAW Equipment

1. Power Source

• Constant voltage (CV) type
• DC power (usually DCEP - electrode positive)
• Voltage range: 15-35V
• Current range: 50-600A

2. Wire Feeder

• Feeds electrode wire continuously
• Speed control: 50-500 mm/s
• Push or pull mechanism
• Wire spool holder

3. Welding Gun (Torch)

• Holds contact tip
• Delivers shielding gas
• Trigger control
• Air-cooled: Up to 200A
• Water-cooled: Above 200A

4. Contact Tip

• Conducts current to wire
• Guides wire
• Copper alloy
• Must match wire diameter

5. Gas Supply System

• Gas cylinder
• Regulator and flowmeter
• Hoses
• Flow rate: 10-25 L/min

6. Wire Electrode

• Solid wire (most common)
• Flux-cored wire (FCAW variant)
• Diameters: 0.6, 0.8, 1.0, 1.2, 1.6 mm

Shielding Gases

Inert Gases (MIG)

Argon (Ar):

• Most common for non-ferrous metals
• Smooth arc, low spatter
• Good for aluminum, stainless steel
• Narrow, deep penetration

Helium (He):

• Higher heat input than argon
• Wider, shallower penetration
• More expensive
• Used for thick sections

Ar-He Mixtures:

• Combines benefits of both
• Better heat input than pure Ar
• Common: 75% Ar + 25% He

Active Gases (MAG)

Carbon Dioxide (CO₂):

• Economical
• Deep penetration
• More spatter than inert gases
• Used for carbon steel
• Pure CO₂ or mixed with Ar

Ar-CO₂ Mixtures:

• Most common for steel
• Typical: 75-95% Ar + 5-25% CO₂
• Less spatter than pure CO₂
• Good arc stability

Ar-O₂ Mixtures:

• Small O₂ addition (1-5%)
• Improves arc stability
• Better wetting
• For stainless steel

Tri-Mix (Ar-CO₂-O₂):

• Three-gas mixture
• Optimized properties
• For stainless steel

Metal Transfer Modes

1. Short Circuit Transfer (Dip Transfer)

Characteristics:

• Wire touches weld pool, short circuits
• Arc extinguishes momentarily
• Metal transfers during short circuit
• 50-200 short circuits per second

Parameters:

• Low current: 50-150A
• Low voltage: 15-20V
• Small wire: 0.6-1.0 mm

Advantages:

• Low heat input
• Good for thin materials
• All position welding
• Less distortion

Disadvantages:

• More spatter
• Shallow penetration
• Slower deposition rate

Applications: Thin sheet metal, out-of-position welding

2. Globular Transfer

Characteristics:

• Large droplets form at wire tip
• Transfer by gravity
• Irregular transfer
• Spatter

Parameters:

• Medium current: 150-250A
• Medium voltage: 20-25V
• CO₂ shielding gas

Disadvantages:

• High spatter
• Irregular arc
• Flat position only

Applications: Limited use, transition region

3. Spray Transfer

Characteristics:

• Fine droplets spray across arc
• Continuous, smooth transfer
• Axial spray pattern
• Requires argon-rich gas (>80% Ar)

Parameters:

• High current: >200A (above transition current)
• High voltage: 25-35V
• Larger wire: 1.0-1.6 mm

Advantages:

• High deposition rate
• Deep penetration
• Low spatter
• Smooth bead

Disadvantages:

• High heat input
• Flat and horizontal positions only
• Not for thin materials

Applications: Thick sections, high-speed welding, flat position

4. Pulsed Spray Transfer

Characteristics:

• Alternating high (pulse) and low (background) current
• One droplet per pulse
• Spray transfer at lower average current

Parameters:

• Peak current: Above transition
• Background current: Below transition
• Pulse frequency: 30-400 Hz

Advantages:

• Spray transfer benefits at lower heat
• All position welding
• Suitable for thin materials
• Low spatter

Applications: Aluminum, stainless steel, all positions

GMAW Process Parameters

Current and Voltage

Current: Controlled by wire feed speed

• Higher wire speed → higher current
• Self-regulating (constant voltage)

Voltage: Set on power source

• Controls arc length
• Higher voltage → longer arc, wider bead
• Lower voltage → shorter arc, narrower bead

Wire Feed Speed (WFS)

• Primary control parameter
• Determines deposition rate
• Typical: 50-500 mm/s (2-20 m/min)

Relationship: I = k × WFS

Where k depends on wire diameter and material

Travel Speed

• Affects bead size and penetration
• Too fast → narrow bead, poor fusion
• Too slow → excessive buildup, burn-through

Stick-out (Electrode Extension)

• Distance from contact tip to arc
• Typical: 10-15 mm
• Affects heating and deposition

Shielding Gas Flow Rate

• Typical: 10-25 L/min
• Too low → porosity
• Too high → turbulence, gas waste

Advantages of GMAW

1. High productivity: Continuous wire feed, no stops
2. No slag: No chipping required
3. All positions: With proper parameters
4. Good visibility: Clear view of weld pool
5. High deposition rate: Especially spray transfer
6. Minimal cleanup: Little spatter (with proper settings)
7. Versatile: Wide range of materials and thicknesses
8. Easy to learn: Semi-automatic operation
9. Deep penetration: Good fusion

Limitations of GMAW

1. Equipment cost: More expensive than SMAW
2. Not portable: Requires gas cylinders, power source
3. Wind sensitive: Shielding gas blown away outdoors
4. Complex equipment: More components to maintain
5. Wire feeding issues: Possible bird-nesting, jamming
6. Initial setup: More parameters to set

GMAW Variants

FCAW (Flux-Cored Arc Welding)

• Tubular wire with flux core
• Self-shielded FCAW: No external gas (flux generates shield)
• Gas-shielded FCAW: External gas + flux
• Higher deposition rate
• Better for outdoor use (self-shielded)

Pulsed GMAW

• Pulsed current waveform
• Better control
• All-position spray transfer

Materials Welded

Carbon Steel: Most common, CO₂ or Ar-CO₂ Stainless Steel: Ar-CO₂-O₂ or tri-mix Aluminum: Pure Ar or Ar-He Copper Alloys: Ar-He mixtures Nickel Alloys: Ar or Ar-He

Applications

Automotive: Body panels, frames Fabrication: Structural steel, tanks, vessels Shipbuilding: Hull construction Manufacturing: General fabrication Robotics: Automated welding lines

GMAW Defects and Causes

Porosity:

• Insufficient gas coverage
• Contaminated base metal
• Drafts/wind

Lack of Fusion:

• Low current/voltage
• Fast travel speed
• Wrong angle

Excessive Spatter:

• High voltage
• Wrong gas mixture
• Dirty wire

Burn-through:

• Excessive heat input
• Slow travel speed
• Thin material

Undercut:

• High voltage
• Fast travel speed
• Wrong angle

Comparison: GMAW vs SMAW