In situ process monitoring by optical microphone for crack detection in Laser Metal Deposition applications

Presented at the 11th CIRP Conference on Photonic Technologies (LANE 2020) © 2020 The Authors. Published by Bayerisches Laserzentrum GmbH We introduce an effective application of a novel laser-based optical microphone for in-situ monitoring of cracks during laser metal deposition (LMD) processes. The system relies on XARION’s technology, a membrane‐free optical microphone based on a Fabry–Pérot laser interferometer, that captures broadband airborne acoustic emission up to 1 MHz such as the sudden acoustic emission signals produced by the released energy of cracks. A set of experiments of two different applications, materials and geometries were conducted at AIMEN for evaluation: thin walls of stainless steel and thin deposited layers of tool steel. Through analyzing acoustic STFT-based spectrograms, an approach for identification of cracks and classification of signals is introduced. Process emission of 200 kHz and crack-type signals up to 600 kHz were detected for both cases. NDT methods (X-ray and Eddy Current) and microscopy investigation of defects validate this acoustic sensing system to monitor cracks in laser cladding and other additive manufacturing applications