I'm aware of two ways to 3d-print in metal that are fairly close to FDM.

The first uses a two stage process. First stage is to print a clay made up almost but not entirely of metal particles. You use a paste extruder, such as this one from thingiverse, loaded with a metallic clay such as this (which evidently sees a lot of use by jewelry makers). Second stage is to fire the piece in a kiln. You end up with a metal piece.

The second uses an arc welder as the hot end of the extruder. Welding wire is fed to the business end the same way it's done in a normal arc welder. Lots of current is pumped through and hey-presto: 3d welded metal. Article here.