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Resistance spot welding heat and efficiency

Short answer

Resistance spot welding passes high current through overlapping sheets, generating heat at the interface to form a nugget. Applications: car bodies, sheet‑metal assemblies. Heat generated I²Rt = 9000² × 120 × 10⁻ ⁶ × 0.25 = 2430 J. Heat to melt nugget = 0.25 g × (0.5 × 1500 + 270) J/g = 255 J. Efficiency = 10.5 %.

Explain the principle of resistance spot welding with a sketch and state its applications. In a spot welding operation a current of 9000 A flows for 0.25 s through a joint with a resistance of 120 micro-ohm. The weld nugget has a mass of 0.25 g of steel, with specific heat 0.5 J/g K, melting rise of 1500 K and latent heat of fusion 270 J/g. Find the heat generated and the efficiency of the process.

Principle

Two overlapping sheets are clamped between copper alloy electrodes, and a large current passes for a short time. The joint resistance generates heat by the Joule effect, H=I2RtH = I^2 R t, which melts a small nugget at the interface. The pressure is held while the nugget solidifies, forming a spot weld.

   electrode (Cu)   <- force
        |  |
  ======XXXX======  sheets (nugget X)
        |  |
   electrode (Cu)   <- force

Steps: squeeze, weld (current on), hold, release.

Applications: car bodies, metal furniture, utensils, electric boxes, wire mesh, thin sheet steel assemblies; high production and easily automated.

Heat generated

H=I2R t=(9000)2×120×10−6×0.25=8.1×107×1.2×10−4×0.25=2430 J\begin{aligned} H &= I^2 R\,t \\ &= (9000)^2\times 120\times 10^{-6}\times 0.25 \\ &= 8.1\times 10^{7}\times 1.2\times 10^{-4}\times 0.25 = 2430\ \text{J} \end{aligned}

Heat needed for the nugget

Heat to raise 1 g by 1500 K and melt it:

q=c ΔT+L=0.5×1500+270=1020 J/gq = c\,\Delta T + L = 0.5\times 1500 + 270 = 1020\ \text{J/g} Hnugget=0.25×1020=255 JH_{nugget} = 0.25\times 1020 = 255\ \text{J}

Efficiency

η=2552430×100=10.5%\eta = \frac{255}{2430}\times 100 = 10.5\%

The rest of the heat is lost by conduction into the water-cooled electrodes and the surrounding sheet.

Answer: heat generated = 2430 J; heat used to melt the nugget = 255 J; efficiency = 10.5%.

✓ Answer checked by Claude Opus 5.5 (high effort)

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