Question

A resistance spot-welding operation is performed on two pieces of 1.5 mm thick sheet steel using 12000 amps current for a duration of 0.20 second. The electrodes are 6 mm in diameter at the contacting surfaces. Resistance is assumed to be 0.0001 ohms and the resulting weld nugget is 6 mm in diameter and 2.5 mm thick. The unit melting energy for the metal is 12 J/mm³. What portion of heat generated was used to form the weld?
1. 29.4%
2. 70.6%
3. 58.8%
4. 41.2%

Answer 1

Correct Answer - Option 1 : 29.4%

Concept:

The portion of heat generated was used to form the weld is given as, \(\frac{Heat ~required ~to ~melt }{Heat ~generated ~in ~welding}\times100=\frac{H_m}{H_g}\times 100\)

Heat generated in welding (H_g) = I2Rt and

Heat required to melt required volume (H_m) = volume of nugget (V) × energy required to melt unit volume (H)

Volume of nugget = \(\frac{\pi }{4}{d^2}h\)

Calculation:

Given:

I = 12000 Amp, R = 0.0001 ohms, t = 0.2 sec, d = 6 mm, h = 2.5 mm, H = 12 J/mm³

where, I = current, R = resistance, t = time duration, d = diameter of nugget, h = height of nugget, H = unit melting energy 

Volume of nugget (V) = \(\frac{\pi }{4} \times {6^2} \times 2.5 = 70.68\) mm³

Heat required to melt required volume (H_m) = volume of nugget × energy required to melt unit volume

= 70.68 × 12 = 848.16 J

Heat generated in welding (H_g) = 12000² × 0.0001 × 0.2 = 2880 J

Portion of heat used = \(\frac{{848.16}}{{2880}}\times100=0.294 \) = 29.4%