Mark BarfootA Solution for Laser-Beam Quality Analysis During Powder-Bed Fusion Production
April 19, 2021Comments
This article was adapted from a white paper authored by EWI applications engineer Jacob Hay, and reprinted here with permission.
Laser powder-bed fusion (LPBF) additive manufacturing (AM) machines use a rapidly traveling laser focal spot (<100-µm spot diameter) to create overlapping melt tracks on a powder-covered build platform. The melt (or weld) quality and repeatability depend on the laser-spot size, shape and quality. Successful delivery of the large amount of weld metal needed during LPBF requires an understanding of the laser-beam quality and consistency at multiple locations on the build platform.
A few common factors that can affect the size, shape and quality of the laser-beam focal spot on the work surface include the LPBF machine’s optical setup, thermally induced distortion of beam-delivery optics and contamination of the scanner cover window. These critical factors require manufacturers to confirm the laser focal-spot characteristics of each LPBF machine and compare those characteristics at multiple locations on the build platform before certifying each machine for production in critical component manufacturing.
Using the EWI system, the laser beam traverses over known locations as it would during the normal build (Fig. 2). The ability to evaluate the laser beam in its scanning (moving) state provides more reliable and realistic results. The scanning motion used over each measurement location can be expanded to conduct one scan pass over each pinhole location before returning to the first pinhole for a second pass, and so on. This practice fully distributes the heat load on the scan lens, similar to the actual build process. 
For development purposes, EWI has built a single-point prototype system simulating these functions (Fig. 3) and tested it with promising results. The system includes a beam profile-sampling aperture with damage thresholds comparable to other laser-measurement devices; a high-speed photodetector; custom data-analysis software; and the ability to locate the measurement site anywhere on the LPBF platform. To date, all testing has been completed with a single measurement location, but the additional locations could be included with the use of a combining fiber. Each individual fiber would attach to a measurement-location pinhole assembly and combined before entering a single measurement sensor. 
Each profile dataset has been analyzed for focal-point size and distribution using custom data-analysis software (Fig. 4) and the results evaluated to confirm the function of the LPBF beam-analysis system. EWI researchers used filters and pinholes to manually adjust sensor-signal strength. This process could be improved with upgraded data-acquisition equipment. The manual test setup resulted in low signal strength when moving away from the focus location, due to changes in light intensity. Beam caustic calculations were performed within measurable ranges by manipulating the LPBF-bed Z axis.
