Detection of contamination of protective windows in Laser Powder Bed Fusion by means of a line scan camera

Abstract

In Laser Powder Bed Fusion (LPBF), there are high demands on the cleanliness of the protective window (PW). During processing, however, process-by-products may accumulate on the PW surface and thereby change the process boundary conditions. In this work, a methodology to assess the condition of PWs in a LPBF machine using a line scan camera is presented. Image processing is used to detect the degree of contamination on the PW surface by using a thresholding algorithm. The line scan camera results are further validated by ex-situ microscopic images of the contaminated surface of the PW. The higher resolution of these images enables the quantification of particle count and size distribution. Moreover, the influence of the contamination degree of the PWs on the beam quality is investigated. A laser focus shift of up to 22.6 mm against the direction of beam propagation within a caustic measurement as well as a minor decrease in relative density of 0.08 pp in 316L samples were detected due to the contamination. Based on this work, methods for monitoring the health condition of PWs in LPBF can be derived.