Metrological acoustic emission approach for in-process depth measurement in water-jet guided laser drilling of nickel-based superalloy

With the continuous advancement of performance requirements for aero-engines, the fabrication of film cooling holes on turbine blades has become critical to engine performance. Water-jet guided laser (WJGL) technology, which combines the advantages of water-jet cooling and nanosecond laser ablation, offers a high-precision solution for micro-hole machining in superalloys. However, when machining hollow thin-walled structures, laser penetration can easily cause damage to the opposite wall. To mitigate such penetration-induced damage, this study establishes an acoustic emission (AE)-based metrological approach for ablation depth measurement during WJGL drilling. Spectral subtraction was first applied to reduce acoustic noise and enhance the signal-to-noise ratio of the measurement data. Single-spot ablation tests under varying power levels were then performed to calibrate the effective ablation intensity threshold (55 dB). The time- and frequency-domain processing of AE responses was conducted as quantitative measurement stages to extract traceable parameters correlated with material-removal depth. Based on these parameters, a support vector regression (SVR)-based measurement estimator was developed to determine depth in real time. The results show that this method enables accurate and repeatable depth measurement during WJGL drilling. Furthermore, measuring AE signals during penetration revealed that the signal intensity drops below 50 dB upon complete hole penetration, a characteristic independent of machining parameters. The proposed method provides a reliable means for in-process drilling depth measurement in WJGL, confirming high accuracy and reproducibility.