Optical emission sensing for laser-based additive manufacturing - What are we actually measuring?

Numerous sensing modalities have been utilized to monitor metal additive manufacturing, thus assessing process stability and build quality. One common directed energy deposition sensing method is coaxial "melt pool"imaging, wherein a camera mounted coaxially with the laser-focusing optics views the laser-interaction zone via a dichroic mirror. This work demonstrates that coaxial "melt pool"imaging does not always provide reliable, as-solidified, pool geometry measurements. First, coaxially acquired images captured using a charge injection device camera for a range of laser powers, translation speeds, and powder flow rates are compared to optical profilometry-measured true deposition geometries. Furthermore, coaxial "melt pool"images and plume geometries, measured using a bandpass filtered CCD camera, show significant, abrupt "melt pool"geometry changes, which correspond to plume geometry changes. Additionally, interbuild comparisons demonstrate a linear relationship between "melt pool"and plume geometry. The authors conclude that coaxial imaging, as commonly implemented, is an often unreliable melt pool geometry measurement because plume emissions in and around the laser-interaction zone may obscure the melt pool.