I believe there are two ways of printing to metal. One way that I've had done myself is to have a regular print done (in my case Light Jet) and then it's mounted to a sheet of aluminium. You can either mount it with the aluminium showing around the edges, like a matt. Or you can mount the print right to the edge of the metal. Both work well, though you need to be careful if you are leaving metal showing since aluminium is so soft. It scratches easily.
When mounted right to edge, and then hung with a few cm of spacer behind, it becomes a powerful way to present a large image.
The second way involves printing right on the metal. I've not had it done, but someone described how an outfit in Victoria BC does it. Apparently they've set up an inkjet system where the head doesn't just move back and forth, but also along the substrate. So, the metal stays stationary, and the inkjet moves from one to the other. All you have to do is prepare the surface of the substrate to receive the inkjet spray.
Hope this helps.
cheers