MIG/MAG Welding
The MIG/MAG welding process was initially developed in the USA in the late 1940s for the welding of aluminum alloys.
Gas Metal Arc Welding
The latest Welding Standards now refer to the process by the American term GMAW
• The process uses a continuously fed wire electrode
• The weld pool is protected by a separately supplied
shielding gas
• The process is classified as a semi-automatic welding
process but may be fully automated
• The wire electrode can be either bare/solid wire or flux
cored hollow wire
MIG/MAG process variables
Welding current
Welding Polarity
Arc voltage
Travel speed
Gas Metal Arc Welding
Types of Shielding Gas
MIG (Metal Inert Gas)
• Inert Gas is required for all non-ferrous alloys (Al, Cu, Ni)
• The most common inert gas is Argon
• Argon + Helium used to give a „hotter‟ arc – better for thicker joints and alloys with higher thermal conductivity
MIG/MAG – shielding gases
Type of material – Shielding gas
Carbon steel – CO2 , Ar+(5-20)%CO2
Stainless steel – Ar+2%O2
Aluminium – Ar
MIG/MAG shielding gases
Argon (Ar):
higher density than air; low thermal conductivity the arc
has a high energy inner cone; good wetting at the toes; low
ionization potential
Helium (He):
lower density than air; high thermal conductivity uniformly
distributed arc energy; parabolic profile; high ionisation
potential
Carbon Dioxide (CO2):
cheap; deep penetration profile; cannot support spray
transfer; poor wetting; high spatter
MIG/MAG shielding gases
Gases for dip transfer:
- CO2: carbon steels only: deep penetration; fast welding
speed; high spatter levels
• Ar + up to 25% CO2: carbon and low alloy steels: minimum spatter; good wetting and bead contour
• 90% He + 7.5% Ar + 2.5% CO2:stainless steels: minimises undercut; small HAZ
• Ar: Al, Mg, Cu, Ni and their alloys on thin sections
• Ar + He mixtures: Al, Mg, Cu, Ni, and their alloys on thicker sections (over 3 mm)
Gases for spray transfer
- Ar + (5-18)% CO2: carbon steels: minimum spatter; good wetting and bead contour
• Ar + 2% O2: low alloy steels: minimise undercut; provides good toughness
• Ar + 2% O2 or CO2: stainless steels: improved arc stability; provides good fusion
• Ar: Al, Mg, Cu, Ni, Ti and their alloys
• Ar + He mixtures: Al, Cu, Ni and their alloys: hotter arc than pure Ar to offset heat dissipation
• Ar + (25-30)% N2: Cu alloys: greater heat input
Gas Metal Arc Welding
Types of Shielding Gas
MAG (Metal Active Gas)
• Active gases used are Oxygen and Carbon Dioxide
• Argon with a small % of active gas is required for all steels
(including stainless steel) to ensure a stable arc & good
droplet wetting into the weld pool
• Typical active gases are
Ar + 20% CO2 for C-Mn & low alloy steels
| Ar + 2% O2 100% CO2 | For stainless steels |
It can be used for C-steels
MIG/MAG Gas Metal Arc Welding
Electrode orientation
MIG / MAG – self-regulating arc
Electrode and metals
Terminating the arc
• Burnback time
– delayed current cut-off to prevent wire freeze
in the weld end crater
– depends on WFS (set as short as possible!)
MIG/MAG – metal transfer modes
Globular transfer
- Transfer occurs due to gravity or short circuits between drops and weld pool
- Requires CO2 shielding gas
- Metal transfer occurs in large drops(diameter larger than that of electrode) hence severe spatter
- Requires high welding current/arc voltage, and a high heat input process. Resulting in high residual stress and distortion
- Non-desired mode of transfer
Spray transfer
- Transfer occurs due to the pinch effect. There is NO contact between the wire and weld pool!
- Requires argon-rich shielding gas
- Metal transfer occurs in small droplets, a large volume weld pool
- Requires high welding current/arc voltage, and a high heat input process. Resulting in high
residual stress and distortion - Used for thick materials and flat/horizontal position welds
Dip transfer
- Transfer occurs due to short circuits between wire and weld pool, high level of spatter, and need inductance control to limit current raise
- Can use pure CO2 or Ar- CO2 mixtures as shielding gas
- Metal transfer occurs when the arc is extinguished
- Requires low welding current/arc voltage, and a low heat input process.Resulting in low residual stress and distortion
- Used for thin materials and all position welds
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