Investigating the link between keyhole formation and acoustic signatures

To tackle the issue of quality assurance in powder bed fusion–laser beam (PBF–LB), in situ acoustic monitoring has emerged as a promising cost effective solution. However, as most work focuses on data driven approaches, there is still a lack of a clear understanding of the physical link between the recorded acoustic signal and the events inside the melt pool. In order to investigate this link the formation of keyholes is examined using synchrotron-based high-speed X-ray imaging accompanied with measurements from an optical microphone. It is hypothesized that the vapor gas generation is the main source of the airborne acoustics, and an amplitude modulation based phenomenological model is created in order to explain the (unstable) keyhole acoustic emissions. Based on these insights, a signal processing pipeline is constructed to detect the formation of (unstable) keyholes inside the melt pool. Furthermore, it is shown that when employing a pulsed laser, an additional amplitude modulation step is introduced which aids with the classification accuracy as it makes it easier to distinguish between the background noise and the signal generated by the keyhole. [Work supported by FWO fellowship 1SE9325N.]