Microphone signal specialities in laser powder bed fusion: single-track scan and multi-track scan

Acoustic emission is a promising candidate among online process monitoring methods in laser powder bed fusion. Although previous studies have demonstrated the feasibility for acoustic-based monitoring, an underlying understanding of the acoustic specialities related to laser–powder interactions in the enclosed chamber is scarce. This research employed a microphone to collect the acoustic emission signal during laser printing processes. Two contents are contained in the raw data: static pressure variation and acoustic signal. The features of each content are investigated and it follows several significant findings. First, more violent fluctuations of ambient gas static pressure in the chamber are due to the high temperature gradient of ambient gas and recoil pressure over the laser-material interaction zone, which typically occurs in a very low frequency band (<22.4 Hz). Second, spectral analysis for the acoustic signals has indicated that standing wave patterns are commonly occurrences in the chamber. The fundamental frequency of the relevant acoustic emission strongly depends on the number of melt tracks scanned per unit time. Additionally, with investigations of single-track scan and multi-track scan, it is certain that more powder particles participated in laser melting can result in larger acoustic emission energy in a relative high frequency range (>10 kHz). For powder bed additive manufacturing, these results offer a guidance for signal processing and machine learning to dig out the signal signatures of a specific defect in terms of acoustic monitoring.