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This is a super project and one that most of us need quite often. I would suggest either a fuse or a circuit breaker to prevent a flammable possibility.
75 amps will certainly melt something, but 1.5 volts isn't going to give you any penetration of the metal which is necessary for a strong weld.
Nice job. I believe I will include a fuse of some type in mine. I know just about everything you would plug into it would have a fuse but better safe than sorry ;)
It's a nice design and simple to put together, but I believe I would include a panel fuse holder to the circuit. If used as presented here, there is no protection from an overload or short circuit other than wires melting. Nice work