The Welding Arc — Principle, Striking and Parameters
Introduction
The electric arc is the heat source for SMAW (111), MIG/MAG (131/135), TIG (141) and SAW (121). It generates temperatures between 5,000°C and 30,000°C, melting the base metal and filler.
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Principle
The gas between electrode and workpiece becomes electrically conductive through ionisation, creating a plasma channel.
Arc zones
| Zone | Temperature | Function |
|---|---|---|
| Cathode zone | ~3,000°C | Electron emission |
| Arc column (plasma) | 5,000-30,000°C | Current conduction |
| Anode zone | ~3,500°C | Electron reception |
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Arc Striking Methods
- Tap start: Touch and lift — SMAW and MIG/MAG
- Scratch start: Scratch like a match — SMAW with cellulosic
- HF (High Frequency): Non-contact spark — TIG
- Lift-Arc: Touch at low current, then lift — TIG alternative
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Arc Length
| Length | Voltage | Effect |
|---|---|---|
| Short | Low | Deeper penetration, less spatter |
| Medium (ideal) | Normal | Stable arc, good fusion |
| Long | High | Unstable arc, more spatter, porosity |
Rule of thumb (SMAW): Ideal arc length ≈ core wire diameter.
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Arc Blow
Deflection of the arc by asymmetric magnetic fields. Solutions: switch to AC, reposition earth clamp, use two earth clamps, reduce arc length, demagnetise workpiece.
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Heat Input
HI = (V × I × η) / v
| Process | Efficiency (η) |
|---|---|
| SMAW (111) | 0.80 |
| MIG/MAG (131/135) | 0.80 |
| TIG (141) | 0.60 |
| SAW (121) | 0.99 |
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Current Types
| Type | Abbreviation | Main use |
|---|---|---|
| DC electrode positive | DC+ / DCEP | SMAW basic, MIG/MAG |
| DC electrode negative | DC− / DCEN | TIG, SMAW rutile |
| Alternating current | AC | SMAW rutile, TIG aluminium |
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*Article based on AWS Welding Handbook training materials. Published on WeldFC Academy.*