Arc welding: SMAW, GMAW, GTAW and SAW

Why it matters

Arc welding builds pressure vessels, ships, bridges, pipelines and car bodies. Choosing between stick, MIG, TIG and submerged arc — and setting the power source, polarity, current and speed correctly — decides penetration, deposition rate, distortion and whether the joint passes inspection.

Key ideas

The arc. An arc is a sustained electrical discharge through ionised gas between an electrode and the work, at roughly 10–40 V and tens to hundreds of amperes. Arc voltage rises with arc length; a common linear model is V_arc = A + B·l. In DC welding about two-thirds of the heat is released at the anode, so polarity matters:

• DCEN (straight polarity): electrode negative, work positive — more heat in the work, deeper penetration, cooler electrode (used in GTAW of steel).
• DCEP (reverse polarity): electrode positive — more heat at the electrode, higher melting rate, shallower penetration, and a cathodic cleaning action that breaks up oxide films (used in GMAW, and why aluminium is TIG-welded on AC).
• AC alternates both and avoids arc blow.

Power-source characteristics.

• Drooping (constant-current) sources — for manual processes (SMAW, GTAW): a change in arc length changes voltage a lot but current only a little, so the welder's hand tremor hardly affects heat. Modelled as a straight line V = OCV − (OCV/SCC)·I, where OCV is open-circuit voltage and SCC is short-circuit current. The operating point is where this line meets the arc characteristic.
• Flat (constant-voltage) sources — for continuous-wire processes (GMAW, FCAW, SAW): with constant wire feed, a longer arc lowers the current, the burn-off rate falls and the arc shortens again — self-regulation of arc length.

SMAW (manual metal arc, stick). Consumable flux-coated electrode. The coating gives a gas shield, a slag cover, arc stabilisers (K, Na compounds), deoxidisers and alloying elements. Cheap, portable, all-position, outdoor-friendly; but low duty cycle (electrode changes, slag removal) and limited to electrode length. Electrodes must be kept dry (hydrogen cracking).

GMAW (MIG/MAG). Continuous bare wire fed through the torch with an external shielding gas — argon or helium (MIG) for non-ferrous metals, CO₂ or Ar–CO₂ mixes (MAG) for steels. Usually DCEP and a constant-voltage source. Metal transfer modes: short-circuit (low current, thin sheet, all positions), globular, spray (above a transition current, high deposition, flat position) and pulsed spray. High productivity and easily automated or robotised; gas shield is disturbed by wind.

GTAW (TIG). Non-consumable tungsten electrode (often thoriated, ceriated or lanthanated), inert gas (Ar, He), filler rod added separately if needed. DCEN for steels and titanium; AC for aluminium and magnesium. Gives the cleanest, best-controlled welds on thin sheet, root passes and reactive metals, but is slow. Tungsten touching the pool causes tungsten inclusions; high-frequency start avoids this.

SAW. Continuous wire with the arc buried under granular flux; unfused flux is recycled. Very high currents (often 300–2000 A), high deposition rate, deep penetration, no visible arc or spatter, smooth bead. Limited to flat and horizontal positions and mainly to thick steel (ship plates, pressure vessels, pipes, columns). Thermal efficiency is the highest of the arc processes because the flux blanket traps heat.

Other arc processes to know. Flux-cored arc welding (tubular wire with flux inside), plasma arc welding (constricted arc, keyhole mode), electroslag and electrogas welding for very thick vertical joints, stud welding.

Formulas

H = η · V · I / v H = net heat input per unit length (J/mm), η = arc (heat-transfer) efficiency (typically ~0.9–0.99 SAW, ~0.7–0.85 SMAW and GMAW, ~0.6–0.75 GTAW; take from your data book), V = arc voltage (V), I = current (A), v = travel speed (mm/s). With v in mm/min: H (kJ/mm) = η · V · I · 60 / (1000 · v).

V_arc = A + B · l Linear arc characteristic: A (V) and B (V/mm) are constants, l = arc length (mm).

V = OCV − (OCV / SCC) · I Drooping power-source line (V in volts, I in amperes). Arc power P = V·I is maximum at I = SCC / 2, V = OCV / 2, so P_max = OCV · SCC / 4.

m = ρ · (π d² / 4) · f Wire melting (burn-off) rate: ρ = density (kg/m³), d = wire diameter (m), f = wire feed speed (m/s). Deposition rate = deposition efficiency × melting rate.

A_bead = η_d · (π d² / 4) · f / v Cross-sectional area of deposited metal (mm²) for wire feed f and travel speed v in the same units.

Worked examples

Example 1 — heat input (standard). SMAW at V = 25 V, I = 200 A, travel speed 300 mm/min, arc efficiency 0.8.

1. Gross heat input = V · I · 60 / (1000 · v) = 25 × 200 × 60 / (1000 × 300) = 300 000 / 300 000 = 1.0 kJ/mm.
2. Net heat input H = η × 1.0 = 0.8 × 1.0 = 0.80 kJ/mm.

Example 2 — operating point and maximum arc power (GATE level). A drooping DC source has OCV = 80 V and SCC = 600 A. The arc characteristic is V_arc = 20 + 4l (l in mm).

1. Source line: V = 80 − (80/600)·I = 80 − 0.1333·I.
2. For l = 4 mm: V_arc = 20 + 4 × 4 = 36 V.
3. Operating current: 36 = 80 − 0.1333·I, so I = (80 − 36) × 600/80 = 330 A; arc power = 36 × 330 = 11.88 kW.
4. Maximum power occurs at I = SCC/2 = 300 A, V = OCV/2 = 40 V, P_max = 80 × 600/4 = 12.0 kW.
5. Arc length for maximum power: 40 = 20 + 4l, so l = 5 mm.

Example 3 — GMAW deposition. Steel wire d = 1.2 mm, feed 8 m/min, density 7850 kg/m³, deposition efficiency 0.95, travel speed 400 mm/min.

1. Wire area = (π/4)(1.2)² = 1.131 mm².
2. Volume melted = 1.131 × 8000 = 9048 mm³/min.
3. Melting rate = 9048 mm³/min × 7.85 × 10⁻³ g/mm³ = 71.0 g/min = 4.26 kg/h.
4. Bead area = 0.95 × 9048 / 400 = 21.5 mm².

Common mistakes

• Forgetting the 60 when speed is in mm/min, or the 1000 when the answer is wanted in kJ/mm.
• Dropping the arc efficiency when the question asks for net heat input.
• Mixing up polarities: DCEN gives deeper penetration in GTAW; DCEP gives higher melting rate and oxide cleaning.
• Saying GTAW uses a consumable electrode, or that SMAW needs an external gas cylinder.
• Pairing a constant-voltage source with manual SMAW, or a drooping source with GMAW.
• Calculating arc power at OCV or SCC — at those points current or voltage is zero.

For GATE PI

Expect numericals on heat input with efficiency, operating point from a linear power-source line and a linear arc characteristic, maximum arc power, change in current when arc length changes, wire melting and deposition rate. Concept MCQs cover polarity effects, source characteristics and self-regulation, electrode coatings, shielding gases, metal transfer modes and matching process to application.

Quick check

1. Which polarity is used for TIG welding of steel, and why?
2. A source has OCV 60 V and SCC 400 A. What is the maximum arc power?
3. Why does GMAW use a constant-voltage source?
4. Name two functions of an SMAW electrode coating other than shielding.
5. 30 V, 250 A, 5 mm/s, η = 0.9: net heat input?

Answers: 1. DCEN — more heat goes into the work for deeper penetration and the tungsten stays cool. 2. 60 × 400/4 = 6 kW. 3. With constant wire feed it gives self-regulation of arc length. 4. Arc stabilisation, slag formation, deoxidation, adding alloying elements (any two). 5. 0.9 × 30 × 250/5 = 1350 J/mm = 1.35 kJ/mm.