Structure-borne acoustic sensing with a recoater blade for in-situ protrusion detection and localization in powder bed fusion

Powder recoating is the first step of every powder bed fusion (PBF) layer, and material protrusions rising above the nominal powder surface can obstruct the recoater and compromise an entire build. Here we demonstrate a novel method for protrusion detection and localization from the structure-borne acoustic signal of the recoater blade itself. Two piezoelectric elements are bonded to the ends of a thin comb-shaped stainless-steel blade, so that the blade acts as the diaphragm and the elements transduce the dynamic strain at its surface. Spectral analysis shows that the recorded signal is dominated by flexural waves excited when a tooth is deformed, with a dominant frequency corresponding to the second cantilever mode of a single tooth. Times of arrival were picked per channel with a median-referenced energy-ratio detector, and the resulting time difference of arrival scales linearly with the difference in source-to-sensor distance across the blade width. Using this calibration, artificial protrusions raised in 0.1 mm increments were detected and localized, in agreement with optical images of the powder bed.