Pretty much anything can be 3d printed. Lay people see 3d printing as depositing layers of plastic, in reality "additive manufacturing" can be done with nearly any material these days. I have seen additive manufacturing of plastics, concrete, ceramics, paper, metals, and many others. The super drago engines are 100% 3D printed from a high performance alloy, I believe Inconel. Further additive manufacturing can produce geometries that would be nearly impossible (especially with good control of surface finishes and voids) with traditional methods. Internal features that would not be possible to machine can be printed, including completely contained internal hollow features to reduce weight. Below is a video of my personal favorite machine, it combines plasma deposition additive manufacturing (which can be used for most metals) and traditional high speed machining to make parts that otherwise would be impractical to make.
[video=youtube_share;ndx-amkf4qU]https://youtu.be/ndx-amkf4qU[/video]