MIG/MAG welding (ISO 4063 — Process 131/135) is the most widely used industrial welding method, operating with continuously fed filler wire under shielding gas. It is the first choice for structural steel, excavator chassis and crane fabrication thanks to its high deposition rate and automation compatibility.
However, MIG/MAG quality depends heavily on parameter settings. An incorrect current–voltage combination directly leads to defects such as porosity, lack of fusion or undercut.
Key Parameters and Their Interactions
| Parameter | Typical Range | Effect |
|---|---|---|
| Welding Current (A) | 80–450 A | Increasing current raises deposition rate and penetration; excess causes undercut |
| Arc Voltage (V) | 16–38 V | Determines arc length; too low causes short-circuiting, too high increases spatter |
| Wire Feed Speed (m/min) | 3–18 m/min | Directly linked to current; set according to wire diameter |
| Travel Speed (cm/min) | 15–60 cm/min | Affects heat input and bead width |
| CTWD — Contact Tip to Work Distance | 10–25 mm | Increasing CTWD raises resistive heating and destabilises the arc |
| Gas Flow Rate | 12–20 l/min | Too low: inadequate shielding; too high: turbulence and porosity risk |
Shielding Gas Selection
For MAG welding (active gas) on carbon steel, the most common mixture is 80% Ar + 20% CO₂. Increasing CO₂ content raises penetration but also increases spatter. For stainless steel, the typical mixture is 98% Ar + 2% CO₂ or 97% Ar + 3% N₂.
- 100% CO₂ — Most economical, deepest penetration; but high spatter and rough bead appearance.
- Ar + 15–25% CO₂ — Standard for series structural fabrication. Balanced penetration and clean bead.
- Ar + 1–3% O₂ — For arc stability on stainless steel.
Transfer Modes
The combination of arc voltage and current determines how filler metal transfers to the base metal:
- Short-Circuit (Dip Transfer): Low current and voltage. Ideal for thin plate and positional welding. Low heat input.
- Globular: Intermediate parameters. Heavy spatter; not preferred in practice.
- Spray Arc: High current and voltage. Highest deposition rate; applicable only in flat (PA/PB) positions.
- Pulsed: Electronic control achieves spray transfer in all positions. Minimum spatter, controlled heat input. Preferred for stainless steel and aluminium.
Practical note: Under EN 1090 EXC3, heat input (kJ/mm) must be calculated and recorded in the WPS. Formula: Q = (U × I × 60) / (v × 1000) × k (thermal efficiency factor).
Common Defects and Solutions
Frequently encountered defects in MIG/MAG welding and their root causes:
- Porosity: Insufficient shielding gas (flow rate, wind, dirty surface). Solution: check gas flow, reduce CTWD, clean the surface.
- Lack of Fusion: Low current or high travel speed. Solution: increase current, reduce travel speed.
- Undercut: Excessive current or incorrect torch angle. Solution: reduce current, keep torch angle below 15°.
- Spatter: High CO₂ content or dirty contact tip. Solution: optimise gas mixture, clean equipment.
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