a) You have been assigned to design a robotics spot welding experiment for low carbon steel sheets. You found an experimental power source for spot welding process which is designed to deliver current as a ramp function of time: I = 100,000 t, where I = amp and t = sec. At the end of the power-on time, the current is stopped abruptly. You have decided to use this power source for you experiment. The sheet metal being spot welded is found to have unit melting energy = 10 J/mm³ and resistance R = 85 micro-ohms. The desired weld nugget size is: diameter = 4 mm and thickness = 2 mm (assume a disc-shaped nugget). You assumed that 25% of the energy generated from the power source will be used to form the weld nugget. In order to set up the process, it is required to determine the power-on time the current must be applied in order to perform this spot welding operation. What is the power-on time?

b) Why is the quality of welds produced by submerged arc welding very good?

c) Which materials can be friction stir welded, and which cannot? Explain your answer.

d) List the joining methods that would be suitable for a joint that will encounter high stresses and will need to be disassembled several times during the product life, and rank the methods.