Welding Processes
Arc Welding, MIG, TIG & Resistance Welding — Comparison, Heat Input, HAZ & Defects
Last Updated: March 2026
Key Takeaways 📌
- Welding permanently joins metals by melting the joint area — with or without filler metal.
- SMAW (stick welding): most versatile, portable, all-position. Manual, slower.
- GMAW (MIG): continuous wire feed, shielding gas. High speed, good for production.
- GTAW (TIG): non-consumable tungsten electrode, highest quality. Slow, for precision work.
- Resistance welding: Heat from electrical resistance at the joint. Fast, no filler. Used for sheet metal (spot welding).
- Heat input: H = ηVI/v (J/mm) — controls weld bead size, HAZ width, and distortion.
- HAZ (Heat Affected Zone): Region next to the weld that is altered by heat but not melted — critical for weld quality.
1. Welding Fundamentals
Welding is a joining process that produces coalescence (fusion) of materials by heating them to the welding temperature, with or without the application of pressure and with or without filler material. The result is a permanent joint that is often as strong as or stronger than the base metal.
Welding processes are classified by their heat source:
| Category | Heat Source | Examples |
|---|---|---|
| Arc welding | Electric arc | SMAW, GMAW (MIG), GTAW (TIG), SAW, FCAW |
| Resistance welding | Electrical resistance (I²R heating) | Spot, seam, projection, butt welding |
| Gas welding | Oxy-fuel flame | Oxy-acetylene welding |
| Solid-state welding | Pressure/friction (no melting) | Friction, ultrasonic, diffusion, explosion welding |
| High-energy welding | Focused beam | Laser, electron beam welding |
2. SMAW — Shielded Metal Arc Welding (Stick Welding)
SMAW uses a consumable electrode coated with flux. The arc melts both the electrode and the base metal. The flux coating produces a shielding gas and slag to protect the weld pool from atmospheric contamination.
| Feature | Detail |
|---|---|
| Electrode | Consumable, flux-coated |
| Shielding | Flux coating → gas + slag |
| Current | AC or DC, 50–300 A |
| Advantages | Most versatile, portable, all-position, works outdoors, low equipment cost |
| Limitations | Manual only, slower, frequent electrode changes, slag removal needed |
| Applications | Construction, maintenance, repair, pipelines, shipbuilding |
3. GMAW — Gas Metal Arc Welding (MIG)
MIG welding uses a continuous consumable wire electrode fed through a welding gun, with an external shielding gas (argon, CO₂, or mixture) to protect the weld pool. No slag is produced.
| Feature | Detail |
|---|---|
| Electrode | Continuous consumable wire (auto-fed) |
| Shielding | External gas (Ar, CO₂, Ar+CO₂) |
| Current | DC (electrode positive), 100–500 A |
| Advantages | High deposition rate, fast, semi-automatic/automatic, no slag, clean welds |
| Limitations | Not suitable outdoors (wind blows shielding gas), higher equipment cost |
| Applications | Automotive, sheet metal fabrication, production welding, robotics |
4. GTAW — Gas Tungsten Arc Welding (TIG)
TIG welding uses a non-consumable tungsten electrode to produce the arc. Shielding is provided by an inert gas (argon or helium). Filler metal, if needed, is fed separately by hand.
| Feature | Detail |
|---|---|
| Electrode | Non-consumable tungsten (does not melt) |
| Shielding | Inert gas (Ar or He) |
| Filler | Separate rod (optional — can weld without filler) |
| Current | AC (for Al, Mg) or DC (for steel, stainless), 5–300 A |
| Advantages | Highest quality welds, precise control, no spatter, no slag, any metal |
| Limitations | Slowest arc process, most operator skill required, expensive |
| Applications | Aerospace, nuclear, food/pharma equipment, thin sections, root passes |
5. Resistance Welding
Resistance welding generates heat through electrical resistance at the joint interface (Q = I²Rt). Pressure is applied simultaneously. No filler metal or flux is needed.
Heat Generated
Q = I²Rt
Where: I = welding current (A), R = resistance at the joint (Ω), t = time (s)
| Type | Description | Application |
|---|---|---|
| Spot welding | Two electrodes clamp overlapping sheets; current flows through the contact point | Automotive body panels, appliances |
| Seam welding | Rotating wheel electrodes produce continuous or intermittent weld along a seam | Fuel tanks, cans, ducts |
| Projection welding | Raised projections on one part concentrate current at specific points | Nuts/bolts to sheets, wire mesh |
| Butt welding | Two parts pressed end-to-end; current heats the interface | Rods, bars, rails, chain links |
6. Process Comparison
| Feature | SMAW (Stick) | MIG | TIG | Spot Welding |
|---|---|---|---|---|
| Electrode | Consumable + flux | Consumable wire | Non-consumable W | Cu electrodes |
| Shielding | Flux → gas + slag | External gas | Inert gas | None needed |
| Speed | Slow | Fast | Slowest | Very fast |
| Quality | Good | Good | Highest | Good (lap joints) |
| Automation | Manual only | Semi/fully auto | Manual/auto | Fully automatic |
| Outdoor use | Yes | No (wind) | No (wind) | N/A (factory) |
| Filler needed | Yes (electrode) | Yes (wire) | Optional | No |
| Skill level | Medium | Low–Medium | High | Low (machine) |
7. Heat Input & HAZ
Heat Input (Arc Welding)
H = ηVI/v
Where: H = heat input (J/mm), η = arc efficiency (0.6–0.9 depending on process), V = arc voltage (V), I = welding current (A), v = travel speed (mm/s)
| Process | Typical Arc Efficiency η |
|---|---|
| SMAW | 0.70–0.85 |
| GMAW (MIG) | 0.75–0.90 |
| GTAW (TIG) | 0.60–0.70 |
| SAW | 0.85–0.95 |
The Heat Affected Zone (HAZ) is the region of base metal adjacent to the weld that was heated enough to change its microstructure but did not melt. The HAZ width is proportional to heat input — higher heat input → wider HAZ → potentially weaker zone. In heat-treated steels, the HAZ may be softer (over-tempered) or harder (quenched) than the base metal, depending on cooling rate.
8. Weld Joint Types
| Joint Type | Configuration | Common Use |
|---|---|---|
| Butt joint | Two parts aligned edge to edge | Plates, pipes (most common structural joint) |
| Lap joint | Two overlapping parts | Sheet metal, spot welding |
| T-joint | One part perpendicular to another | Structural frames, stiffeners |
| Corner joint | Two parts meeting at a corner (L-shape) | Boxes, frames, enclosures |
| Edge joint | Edges of two parallel plates joined | Sheet metal flanges, lightweight structures |
9. Weld Defects
| Defect | Cause | Remedy |
|---|---|---|
| Porosity | Trapped gas (moisture, contamination) | Clean base metal, dry electrodes, proper shielding |
| Lack of fusion | Insufficient heat, poor technique | Increase current, proper joint prep, correct angle |
| Lack of penetration | Root gap too small, low current | Increase root gap, increase current, reduce speed |
| Undercut | Excessive current, wrong angle | Reduce current, correct travel speed and angle |
| Slag inclusion | Slag trapped between weld passes | Clean slag between passes, proper technique |
| Spatter | Excessive current, wrong polarity | Adjust parameters, check gas flow |
| Distortion | Non-uniform heating and cooling | Pre-heat, balanced welding sequence, fixtures |
| Hot cracking | High sulphur/phosphorus, high restraint | Control chemistry, reduce restraint, preheat |
| Cold cracking (H₂) | Hydrogen + hard HAZ + residual stress | Low-hydrogen electrodes, preheat, PWHT |
10. Common Mistakes Students Make
- Confusing MIG and TIG: MIG has a consumable wire electrode and is fast. TIG has a non-consumable tungsten electrode and produces the highest quality but is slowest. Remembering “MIG = Metal Inert Gas (wire melts), TIG = Tungsten Inert Gas (tungsten doesn’t melt)” helps.
- Forgetting arc efficiency in heat input: H = ηVI/v, not just VI/v. The efficiency η accounts for heat lost to radiation and convection. Different processes have different η values.
- Thinking HAZ is part of the weld metal: The HAZ is base metal that was heated but NOT melted. The weld metal (fusion zone) is the region that actually melted and resolidified. HAZ and fusion zone are different regions with different microstructures.
- Not recognising that resistance welding uses no filler: Spot welding, seam welding, and projection welding join the base metals directly by melting the interface using I²R heating. No filler wire or rod is used.
11. Frequently Asked Questions
What is the difference between MIG and TIG welding?
MIG (GMAW) uses a continuously fed consumable wire electrode — it is faster, easier to learn, and ideal for production work. TIG (GTAW) uses a non-consumable tungsten electrode with optional separate filler — it is slower but produces the highest quality welds with the most control. TIG is preferred for thin materials, aerospace, and critical applications; MIG is preferred for speed and automation.
What is the Heat Affected Zone (HAZ)?
The HAZ is the region of base metal adjacent to the weld fusion zone that was heated enough to alter its microstructure (grain growth, phase changes, tempering) but did not melt. The HAZ can be harder or softer than the original base metal depending on the material and cooling rate. It is often the weakest part of a welded joint and is where many weld failures originate.