GATE 2021 ME (ME1) – Question 47

Casting, Forming and Joining Processes · Welding, brazing, soldering and adhesive bonding · 2 marks · Numerical answer

The resistance spot welding of two 1.55 mm thick metal sheets is performed using welding current of 10000 A for 0.25 s. The contact resistance at the interface of the metal sheets is 0.0001 Ω. The volume of weld nugget formed after welding is 70 mm³. Considering the heat required to melt unit volume of metal is 12 J/mm³, the thermal efficiency of the welding process is _____ % (round off to one decimal place).

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Correct answer: 33.43 to 33.77

Explanation

Electrical energy is $I^2Rt=10000^2(0.0001)(0.25)=2500$ J. Useful nugget-melting energy is 70×12=840 J.

Efficiency = $100(840/2500)=\mathbf{33.6\%}$. Sheet thickness is unnecessary once nugget volume is supplied.