  3D printing of plastics, ceramics, and metals has existed for several decades
and has revolutionized many areas of manufacturing and science. Printing of
metals in particular has found a number of applications in fields as diverse as
customized medical implants, jet engine bearings, and rapid prototyping in the
automotive industry. Whilst many techniques can be used for 3D printing metals,
they commonly rely on computer controlled melting or sintering of a metal alloy
powder using a laser or electron beam. The mechanical properties of parts
produced in such a way have been well studied, but little attention has been
paid to their electrical properties. Here we show that a microwave cavity
(resonant frequencies 9.9 and 11.2 GHz) 3D printed using an Al-12Si alloy
exhibits superconductivity when cooled below the critical temperature of
aluminium (1.2 K), with a performance comparable to the common 6061 alloy of
aluminium. Superconducting cavities find application in numerous areas of
physics, from particle accelerators to cavity quantum electrodynamics
experiments. The result is achieved even with a very large concentration of
non-superconducting silicon in the alloy of 12.18%, compared to Al-6061, which
has between 0.4 to 0.8%. Our results may pave the way for the possibility of 3D
printing superconducting cavity configurations that are otherwise impossible to
machine.
