Resistance, gas and solid-state welding

Why it matters

A car body carries several thousand resistance spot welds; railway rails are joined by thermit or flash welding; aircraft and battery makers rely on friction stir and ultrasonic welding for aluminium and copper that arc welding handles poorly. Oxy-fuel equipment is still the workhorse for repair, brazing and cutting in the field. Knowing how each process creates heat — or avoids melting altogether — tells you what it can join and what will go wrong.

Key ideas

Resistance welding uses heat from the electrical resistance of the workpieces and, above all, of their contact interface, under electrode force. No filler, flux or shielding gas.

• Total resistance between electrodes = electrode–sheet contacts + bulk resistance of both sheets + sheet–sheet interface. The interface resistance is the largest at the start, so the nugget forms there. Copper-alloy electrodes have low resistance and are water-cooled, keeping the electrode faces cool.
• Weld cycle: squeeze → weld (current on) → hold (force kept while the nugget freezes) → off.
• Force matters both ways: too little force raises contact resistance and causes surface burning, expulsion (spatter) and porosity; too much lowers resistance and heat, giving a small nugget and deep electrode indentation.
• Spot welding (overlapping sheets, a few kA to about 20 kA, a fraction of a second); seam welding (rotating wheel electrodes giving overlapping spots — leak-tight seams in tanks); projection welding (embossed projections localise current, several welds in one stroke, nuts and studs onto sheet); flash and upset butt welding (bars, rails, chain links; flash welding burns off the interface before upsetting); high-frequency resistance welding of pipe seams.
• Highly conductive metals (copper, aluminium) need much higher current because their low resistance generates little heat.

Gas (oxy-fuel) welding. Acetylene burns with oxygen in two stages:

• Primary (inner cone, from cylinder oxygen): C₂H₂ + O₂ → 2CO + H₂ — gives the hottest zone, about 3100–3200 °C at the cone tip.
• Secondary (outer envelope, using oxygen from the surrounding air): 2CO + H₂ + 1.5 O₂ → 2CO₂ + H₂O.
• Complete combustion needs 2.5 volumes of O₂ per volume of C₂H₂; about 1 comes from the cylinder and 1.5 from the air.
• Neutral flame (O₂ : C₂H₂ ≈ 1 : 1) for steels, copper and aluminium; carburising (reducing) flame (excess acetylene, feather beyond the cone) for high-carbon steel, hard-facing and nickel alloys; oxidising flame (excess oxygen, short hissing cone) for brass and bronze, where the oxide film reduces zinc fuming.
• Low power density (about 10 W/mm²): slow, wide HAZ, distortion; but portable, cheap, needs no electricity, and doubles as a cutting and brazing tool. Flux is needed for non-ferrous metals and cast iron.
• Thermit welding: aluminium powder reduces iron oxide (Fe₂O₃ + 2Al → 2Fe + Al₂O₃) releasing enough heat to produce superheated molten steel, poured into a mould around the joint — rails in the track, heavy repairs.

Solid-state welding joins metal without melting either part. Clean metal surfaces brought to atomic contact bond; heat and deformation help by breaking oxide films and increasing diffusion. Advantages: no cast structure or solidification defects, small or no HAZ (heat is still generated in hot processes, so a narrow HAZ exists), dissimilar metals can be joined (Al–Cu, Al–steel), little distortion.

• Friction welding: one part rotates against the other under axial force; friction heats the interface to the plastic range, rotation stops and a forging force consolidates the joint, expelling oxides in a flash. Inertia friction welding uses a flywheel's stored energy.
• Friction stir welding (FSW): a rotating non-consumable pin tool with a shoulder traverses the butt line, stirring plasticised metal — aluminium ship decks, aerospace panels, battery trays.
• Ultrasonic welding: high-frequency (about 20–40 kHz) shear vibration under modest clamping force scrubs off oxides — thin foils, wires, battery tabs.
• Explosive welding: a detonation drives one plate onto another at high speed, giving a wavy metallurgical bond — cladding of plates.
• Diffusion bonding: pressure and high temperature held for a long time in vacuum — titanium aerospace parts.
• Cold (pressure) welding and roll bonding: heavy deformation at room temperature — clad coins, bimetal strips. Forge welding: the blacksmith's hammer.

Formulas

H = I² · R · t H = heat generated (J), I = welding current (A), R = effective resistance between electrodes (Ω), t = weld time (s). Strictly H = ∫ I²R dt since R changes during the weld; the product form assumes average values.

H_m = U_m · V_n and η = H_m / H U_m = unit melting energy (J/mm³; about 10 J/mm³ for steel), V_n = nugget volume (mm³), η = fraction of generated heat used to form the nugget.

V_O₂ (cylinder) ≈ V_C₂H₂ for a neutral flame; V_O₂ (total) = 2.5 · V_C₂H₂ for complete combustion.

T = (2/3) · μ · p · π · R³ and P = T · ω Friction-welding torque for uniform interface pressure: μ = friction coefficient, p = axial pressure (Pa), R = bar radius (m), T in N·m; ω = angular speed (rad/s), P = power (W).

Worked examples

Example 1 — spot weld energy (standard). Two steel sheets are spot welded with I = 10 000 A for t = 0.25 s; effective resistance R = 100 µΩ. The nugget is a disc 6 mm in diameter and 2.5 mm thick. U_m = 10.3 J/mm³.

1. H = I²·R·t = (10 000)² × 100 × 10⁻⁶ × 0.25 = 10⁸ × 10⁻⁴ × 0.25 = 2500 J.
2. Nugget volume V_n = (π/4) × 6² × 2.5 = 70.7 mm³.
3. Heat to melt it H_m = 10.3 × 70.7 = 729 J.
4. Fraction used for melting η = 729/2500 = 0.29 (29 %) — the rest is conducted into the sheets and electrodes.

Example 2 — friction welding (GATE level). Two steel bars of 30 mm diameter are friction welded at 1000 rpm with axial pressure 60 MPa and μ = 0.3. Find the torque and the power at the interface.

1. R = 15 mm. T = (2/3)·μ·p·π·R³ = (2/3) × 0.3 × 60 N/mm² × π × 15³ mm³.
2. π × 15³ = 10 603 mm³; T = 0.2 × 60 × 10 603 = 127 235 N·mm = 127.2 N·m.
3. ω = 2π × 1000/60 = 104.72 rad/s.
4. P = T·ω = 127.2 × 104.72 = 13 320 W ≈ 13.3 kW.

Example 3 — oxy-acetylene gas consumption. A torch burns 0.5 m³/h of acetylene with a neutral flame.

1. Cylinder oxygen ≈ 1 × 0.5 = 0.5 m³/h.
2. Total oxygen for complete combustion = 2.5 × 0.5 = 1.25 m³/h, so 0.75 m³/h is drawn from the surrounding air.

Common mistakes

• Writing P = I²Rt and calling it power; I²R is power, I²Rt is energy.
• Forgetting to square the current, or not converting µΩ to Ω (1 µΩ = 10⁻⁶ Ω).
• Saying high electrode force increases heat; it lowers contact resistance and therefore heat.
• Taking total oxygen (2.5:1) as the cylinder ratio for a neutral flame; cylinder ratio is about 1:1.
• Using an oxidising flame on steel (oxidises the pool) or a neutral flame on brass (zinc fumes off).
• Claiming solid-state welds have no heat-affected zone at all; friction and diffusion welds have a narrow one.

For GATE PI

Expect numericals on I²Rt heat, nugget melting energy and efficiency, the effect of changing current or time on heat (current enters squared), friction-welding torque and power, and gas-consumption ratios. Conceptual MCQs ask which process suits a given joint (rails, nuts on sheet, Al–Cu battery tabs, cladding), flame types and their uses, the role of electrode force, and which processes are solid-state.

Quick check

1. Spot-weld current is raised by 20 % with everything else fixed. By how much does the heat rise?
2. Which flame is used for welding brass?
3. Name a resistance process that makes leak-tight continuous seams.
4. Why are spot-welding electrodes made of copper alloy and water-cooled?
5. Which solid-state process uses a rotating non-consumable pin tool?

Answers: 1. 1.2² = 1.44, so 44 % more. 2. Oxidising flame. 3. Seam welding. 4. Low resistance and cooling keep heat at the sheet interface and prevent the electrode sticking or wearing. 5. Friction stir welding.