If you are looking to build a spot welder using a microwave transformer, it is indeed possible. Spot welders are used to join two pieces of metal together by applying a high current for a short duration to create a weld spot.
To convert a microwave transformer into a spot welder, you would need to modify the transformer by rewinding the secondary coil with fewer turns of thicker wire to increase the current output. You would also need to create electrodes to deliver the current to the metal pieces you want to weld.