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The Phys.org article Six degrees of nuclear separation contains the following sentence in the introduction paragraph for background
Astronauts now print their own parts in space to repair the International Space Station.
I know there have been tests of 3D printing on the ISS, but has it been used to build a replacement part that has actually been installed and used there?
As far as I could find, there have been no cases of 3d printed parts being installed as direct replacements to ISS parts.
There is however a history of tools being printed (but not necessarily used) and some instances of "functional prints" (parts which aren't for fun, decoration, or technology validation which stay in orbit permanently).
In 2014, NASA started off space 3d printing by printing a torque-ratchet. After that, This article shows another use of 3d printing on the ISS. In it, a wrench was printed however it's unclear whether this was actually used by any astronauts or just as a demo piece.
More recently, a 3d printer with plastic recycling capabilities was installed in the ISS however it too is only intended for technology validation. This video, which provides lots of information about 3d printing on the ISS also specifically states that all items from this printer are intended to be returned to Earth.
An example of "functional prints" includes radiation covers which were printed for radiation sensors in the BEAM module of the ISS. Additionally, parts for a cubesat were printed aboard the ISS and subsequently released from the ISS.
Personally, I wouldn't be surprised if to find a variety of 3d-printed bric-a-brac aboard the ISS such as pencil holders or cable management solutions. Having spent lots of time around 3d printers in personal, educational, and commercial settings, I've found that people always seem to find something to print, even if the printed object serves no purpose. I don't expect the ISS to be any different.
In conclusion:
While I could not find any direct evidence of 3d printed parts being used to repair a broken element aboard the ISS, there are cases of 3d printed parts being used in a functional capacity and not only for technology validation or testing purposes.
I'm pretty sure the quote is an overstatement. Though I imagine that over a long enough time, this might become reality.
I've checked my usual references, and while there are 3D printers on the ISS, which can create parts that can "theoretically" be used for something, I haven't found anything specifically answering the proposed question. From this source and this one, the general state of the art seems that printing in low-g is at demonstration stage. And those refer to plastic 3D printing, not metal 3D printing.
Furthermore, from the standards of manned spacecraft quality assurance, you cannot use these parts for mission critical systems or devices, as they would not be properly tested before integration. They can be used for small experiments, which are not mission critical nor life threatening. Some of those experiments might be inspecting the parts themselves to validate the 3D printing technology.
