TL;DR: normal sand casting, using a microwave to heat up the metal inside an insulated silicon carbide crucible.

It's not, like, using the microwave to sinter a metal-powder 3D print or anything special like that.

...And also breaks instantly as soon as it's put under load. The process is for sure very interesting, but a simple casting like this is not the correct method for making something like a wrench. It'd be useful for plenty of other parts, though.