Arc, gas and resistance welding; heat input

Why it matters

A car body is held together by thousands of resistance spot welds, exhaust systems and axle housings are MIG-welded, and chassis frames and repair work rely on arc welding. The quality of every one of these joints depends on how much heat goes in and how fast it goes in: too little and the joint lacks fusion, too much and the heat-affected zone becomes coarse, brittle or distorted. Heat input and arc-power numericals are standard exam fare.

Key ideas

Fusion welding basics. Heat melts the edges of the parts (and usually a filler), the pool solidifies into the fusion zone, and next to it a heat-affected zone (HAZ) is heated enough to change its microstructure without melting. In hardenable steels a fast-cooling HAZ can form martensite and crack (hydrogen cracking), which is why carbon equivalent, preheat and low-hydrogen electrodes matter.

Arc welding processes.

• SMAW (shielded metal arc / stick) – consumable flux-coated electrode; the coating stabilises the arc, makes shielding gas and slag, and can add alloying elements. Portable and versatile; slow, with slag removal.
• GMAW (MIG/MAG) – continuously fed bare wire, shielding by inert gas (Ar, He – MIG) or active gas (CO₂, Ar–CO₂ – MAG); constant-voltage power source with self-regulating arc length. Workhorse of automotive fabrication.
• GTAW (TIG) – non-consumable tungsten electrode, inert gas, separate filler if needed; highest quality, used for thin sections, aluminium and stainless steel.
• SAW (submerged arc) – arc buried under granular flux; very high current and deposition, high thermal efficiency, flat position only, thick plates.
• Plasma arc – constricted arc, very high energy density, keyhole welding.
• Polarity (DC): DCEN (electrode negative, "straight") puts more heat into the work → deeper penetration, used for TIG on steel. DCEP (electrode positive, "reverse") puts more heat at the electrode and gives a cleaning action on oxide films; AC is used for TIG welding of aluminium to combine both.

Arc and power-source characteristics. Arc voltage rises roughly linearly with arc length. A drooping (constant-current) source suits manual processes (SMAW, TIG): changes in arc length barely change current. A constant-voltage source suits GMAW and SAW: the current adjusts the burn-off rate to hold arc length. With a linear drooping characteristic the arc power is maximum when the operating voltage is half the open-circuit voltage.

Gas welding (oxy-acetylene).

• Neutral flame (O₂ : C₂H₂ ≈ 1 : 1) for most steels. Carburising (reducing) flame (excess acetylene) for high-carbon steel, hard-facing and some non-ferrous metals. Oxidising flame (excess oxygen) for brasses and bronzes.
• Primary combustion at the inner cone (C₂H₂ + O₂ → 2CO + H₂) gives roughly one-third of the heat and the highest temperature, about 3100–3300 °C. Secondary combustion with atmospheric oxygen in the outer envelope gives the rest.
• Low power density: wide HAZ and slow welding, but cheap and portable; also used for cutting (oxy-fuel cutting of steel) and brazing.

Resistance welding. Heat is generated by I²R at the faying (contact) surfaces, where contact resistance is highest, while electrodes apply force. No filler, no shielding, very fast and easily automated.

• Spot (car bodies), seam (rotating wheel electrodes, fuel tanks), projection (embossed projections localise current – nuts and studs), flash and upset butt welding (end-to-end joining of rods, rails, rings).
• Heat depends on current, time and resistance; resistance in turn depends on electrode force, surface condition and the materials. Aluminium and copper are hard to spot-weld because of low resistance and high conductivity.
• Copper-alloy electrodes are used because they conduct heat away from the electrode–sheet interface.

Other fusion processes. Thermit welding (Al + iron oxide exothermic reaction – rail joints), electron-beam welding (vacuum, deep narrow welds), laser welding (tailor-welded blanks for car bodies), electroslag welding (very thick sections).

Common defects. Porosity (moisture, inadequate shielding), slag inclusion, lack of fusion and incomplete penetration (low heat, high speed), undercut (excess current or voltage), cracking (hot cracks from sulphur/phosphorus and restraint; cold cracks from hydrogen + hard HAZ), distortion and residual stress.

Formulas

H = η·V·I / v Net heat input per unit length (J/mm); η = arc (heat-transfer) efficiency (process-dependent – take from data; for example SAW is high, around 0.9 or more, and GTAW lower), 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).

A_m = η_m·H / u_m Melted cross-section (mm²); η_m = melting efficiency, u_m = heat to melt a unit volume of metal (J/mm³, about 10 J/mm³ for steel – take from data).

V_arc = a + b·l Arc characteristic; l = arc length (mm), a and b constants (V and V/mm).

V_s = OCV·(1 − I / I_sc) Linear drooping source; OCV = open-circuit voltage (V), I_sc = short-circuit current (A). Maximum arc power P_max = OCV·I_sc / 4 at V = OCV/2.

H_r = I²·R·t Resistance welding heat (J); R = total resistance at the joint (Ω), t = weld time (s).

Worked examples

Example 1 (standard) – SMAW heat input. V = 25 V, I = 200 A, travel speed v = 5 mm/s, arc efficiency η = 0.8.

1. H = η·V·I/v = 0.8 × 25 × 200/5 = 800 J/mm.
2. With melting efficiency 0.5 and 10 J/mm³ to melt steel: A_m = 0.5 × 800/10 = 40 mm² of fused cross-section.

Example 2 (GATE level) – arc length and maximum power. The arc obeys V = 20 + 4l (V, l in mm). The power source has OCV = 80 V and short-circuit current 1000 A with a linear characteristic.

1. Source: V = 80(1 − I/1000) = 80 − 0.08I.
2. For l = 5 mm: V_arc = 20 + 20 = 40 V. Equate: 40 = 80 − 0.08I ⇒ I = 500 A.
3. Arc power P = 40 × 500 = 20.0 kW.
4. Check maximum: P_max = OCV·I_sc/4 = 80 × 1000/4 = 20 kW, reached at V = 40 V – so l = 5 mm is the optimum arc length. At l = 3 mm or 7 mm the power is only 19.2 kW.

Example 3 – spot weld energy. I = 10 kA, t = 0.2 s, R = 100 µΩ. H_r = I²Rt = (10⁴)² × 10⁻⁴ × 0.2 = 2000 J. A nugget 6 mm in diameter and 2.5 mm thick holds 70.7 mm³; at 10 J/mm³ it needs 707 J, so only about 35 % of the generated heat forms the nugget – the rest is conducted into the sheets and electrodes.

Common mistakes

• Mixing mm/min and mm/s in heat input (a factor of 60).
• Forgetting the arc efficiency when "net" heat input is asked, or using it when "gross" input is asked.
• Using DCEP for deep penetration in TIG; DCEN gives deeper penetration and keeps the tungsten cooler.
• Using a constant-voltage source for SMAW or a drooping source for MIG.
• Taking R in resistance welding as the bulk sheet resistance; the contact resistance at the faying surface dominates.
• Arithmetic slips with I²: 10 kA squared is 10⁸ A².

For GATE ME

Expect heat input and melting-efficiency numericals, arc characteristic plus power-source intersection (current, power, optimum arc length), resistance welding heat and nugget energy, and matching questions on processes, flames, polarity, power sources, electrode coatings and defects. Practise solving the arc–source intersection quickly and checking units.

Quick check

1. Which flame is used for welding brass?
2. Which polarity gives the deepest penetration in TIG welding of steel?
3. Heat input for 30 V, 150 A, 250 mm/min, η = 1, in kJ/mm?
4. What kind of power source does MIG welding use?
5. For OCV = 60 V and I_sc = 600 A (linear), what is the maximum arc power?

Answers: 1. Oxidising flame; 2. DCEN (straight polarity); 3. 1.08 kJ/mm; 4. Constant-voltage; 5. 9 kW.