1.2 GMAW — Gas Metal Arc Welding (MIG/MAG)
Key Takeaways
- GMAW uses a continuously fed solid wire electrode with external shielding gas and a constant voltage (CV) power source
- Four transfer modes: short-circuit (all positions, low heat), globular (flat only, high spatter), spray (high production), and pulsed-spray (all positions, controlled heat)
- MIG uses inert gas (argon/helium); MAG uses active gas (CO₂ or argon/CO₂ mix)
- 75% Ar / 25% CO₂ (C25) is the most common carbon steel shielding gas mixture
- ER70S-6 is the most popular solid wire for carbon steel — high silicon and manganese for improved wetting
- GMAW deposition efficiency is 90–95%, significantly higher than SMAW (60–65%)
1.2 GMAW — Gas Metal Arc Welding (MIG/MAG)
Gas Metal Arc Welding (GMAW), commonly known as MIG (Metal Inert Gas) or MAG (Metal Active Gas) welding, is a semi-automatic process that uses a continuously fed solid wire electrode and an external shielding gas to protect the weld pool.
Process Fundamentals
GMAW feeds a solid wire electrode from a spool through a wire feeder and welding gun. The arc is struck between the wire tip and the workpiece. An externally supplied shielding gas flows through the gun nozzle to protect the molten weld pool from atmospheric contamination.
Key Distinction — MIG vs. MAG:
- MIG (Metal Inert Gas): Uses inert gases (argon, helium) — common for non-ferrous metals (aluminum, stainless steel)
- MAG (Metal Active Gas): Uses active gas mixtures (CO₂, or argon + CO₂) — common for carbon steel
GMAW Equipment
| Component | Function |
|---|---|
| Power source | Constant voltage (CV) |
| Wire feeder | Feeds solid wire at a controlled rate |
| Welding gun | Delivers wire, current, and shielding gas |
| Shielding gas cylinder & regulator | Provides and controls gas flow |
| Contact tip | Transfers electrical current to the wire |
| Gas nozzle | Directs shielding gas around the arc |
GMAW Transfer Modes
GMAW operates in four distinct metal transfer modes, each with different characteristics:
| Transfer Mode | Voltage/Current | Shielding Gas | Characteristics | Best For |
|---|---|---|---|---|
| Short-circuit (short-arc) | Low V, Low A | CO₂ or Ar/CO₂ | Wire touches puddle, re-arcs repeatedly (50–200 times/sec); low heat input | Thin material, root passes, all positions |
| Globular | Medium V/A | CO₂ | Large droplets fall by gravity; significant spatter | Flat/horizontal only, rarely specified intentionally |
| Spray | High V, High A | Ar-rich (≥80% Ar) | Small droplets stream across arc; high deposition | Flat/horizontal, thick material, high production |
| Pulsed-spray | Pulsed high/low | Ar-rich (≥80% Ar) | Alternates peak and background current; spray-like with lower average heat | All positions, medium-to-thick material |
GMAW Wire Classification (AWS A5.18 — Carbon Steel)
Example: ER70S-6
| Character | Meaning |
|---|---|
| E | Electrode (can carry current) |
| R | Rod (can be used as filler without current) |
| 70 | Minimum tensile strength in ksi |
| S | Solid wire |
| 6 | Chemical composition designator |
Common solid wires:
- ER70S-3 — general-purpose, clean mild steel
- ER70S-6 — most popular; higher silicon and manganese for improved wetting and deoxidation, tolerates light mill scale
Shielding Gas Selection
| Gas | Properties | Typical Application |
|---|---|---|
| 100% CO₂ | Deep penetration, high spatter, lower cost | Carbon steel (short-circuit, outdoor) |
| 75% Ar / 25% CO₂ (C25) | Good penetration with reduced spatter | Carbon steel (most common all-around mix) |
| 90% Ar / 10% CO₂ | Low spatter, good bead appearance | Carbon steel (spray transfer) |
| 100% Argon | Smooth arc, low penetration | Aluminum, stainless steel |
| Ar / He mixtures | Increased heat, deeper penetration | Thick aluminum, copper alloys |
For the Exam: Know that GMAW uses a constant voltage (CV) power source (opposite of SMAW's constant current). Also know the four transfer modes and which shielding gases enable each mode.
Advantages and Limitations
| Advantages | Limitations |
|---|---|
| High deposition rate (5–20+ lbs/hr) | Requires external shielding gas (affected by wind) |
| Continuous wire feed — less downtime | More equipment than SMAW (feeder, gas, gun) |
| High deposition efficiency (90–95%) | Nozzle/tip maintenance required |
| Less operator skill than SMAW | Gun access can be limited in tight spaces |
| All-position with short-circuit or pulsed modes | Porosity risk if gas coverage is disrupted |
| Easily automated or robotized | Higher equipment cost |
What type of power source does GMAW require?
Which GMAW transfer mode allows welding in all positions and uses low voltage and current?
What is the most common shielding gas mixture for GMAW on carbon steel?
In the wire classification ER70S-6, what does the "S" designate?