Hybrid metal-composite structures are widely used in aerospace industry but the machining challenges, especially at high cutting speeds, limit their employability due to higher production costs and tool wear. Abrasive Water jet (AWJ) is a viable high speed alternative, to conventional milling but the process is limited by the kerf roughness and striations, burrs and delamination. Due to difference in machining behavior of ductile metal and brittle composite, the resulting surface quality is sensitive to the use of non-optimal process parameters, and in-process faults and defects. This study explores the AWJ process monitoring in machining Ti6Al4V-CFRP stacks through Acoustic emission (AE) signals. AWJ Machining experiments were conducted with varying pressure (200, 275, 350 MPa) and traverse feed rate (15, 7.5, 10 mm/s) for two stacking configurations — Ti/CFRP and CFRP/Ti. AE signals acquired at each of these material layers were correlated with the process parameters. Signal RMS was calculated in time domain and a correlation was found with the jet power-to-speed ratio (Ė/u). Further, the AE signals were studied in frequency and simultaneous time-frequency domain. A characteristic frequency, based on an estimation of abrasive particle impacts, was calculated (222.5 kHz). This characteristic frequency band was used to monitor the surface quality. Among other features, maximum power spectral density (PSD) was found superior. Overall, a strong correlation was established between the Abrasive Water Jet process variables, AE signals and the resulting surface quality using the time-frequency spectrogram with the proposed characteristic frequency banding.

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