Acoustic signal monitoring using audible cracking sounds for efficient in-situ crack detection in laser directed energy deposition of hard surfaces

Laser directed energy deposition (LDED) is a promising way for creating hard surfaces like ceramic-reinforced metal matrix composites (MMC), but it faces a significant challenge in identifying crack formation during the process. As an emerging solution, acoustic signal monitoring is easy to be integrated within the process, and significantly reduces the time needed to detect micro-cracks in as-built MMC surfaces. This study reports on cracking sounds produced while employing LDED with SiC particles on a stainless steel 316 L substrate, examining the sound characteristics across time and frequency domains. Different sound sources in LDED are analyzed in the frequency domain, specifying the suitable frequency range for crack monitoring. Interestingly, the in-process micro-cracking on the hard surfaces produces a distinct audible 'ping' sound typically ranging between 12000 and 16000 Hz. By recording this sound, an efficient approach is proposed to identify crack generation during the rapid cooling in the LDED process of hard materials.