I followed some youtube videos 2 years ago, took an microwave 1.2kw xformer, destroyed / removed the secondary coil, took an zero awg gauge wire, managed to get 3 full turns in it, i had around 3.1 volts at maybe 400 amps ... made the solder clamp with interchangeable cooper needles.
Made an pulse driver, some schematics on google
voila an home made spot welder, did manage to spot weld good awg size metal plates.