When a high-power laser boils molten metal, it often generates deep and narrow gaps. These gaps are known as keyholes, and it had long been suspected that they were related to defects in the finished 3-D-printed part. The exact relationship between keyholes and porosity was not, however, fully understood. Researchers led by Tao Sun of the University of Virginia and two colleagues have now used high-energy X-rays derived from the Argonne National Laboratory’s Advanced Photon Source to image how keyholes and pores form in a titanium alloy during laser powder bed fusion.