In–Situ Acoustic Process Monitoring of Interfacial Events in a Moving–Tool Ultrasound–Assisted Additive Manufacturing Process

Ultrasound–assisted additive manufacturing forms bonds through rapid interfacial events that are difficult to observe in situ, particularly because the deposition tool moves continuously. This study presents an energy–based acoustic framework for detecting and timing these interfacial events, demonstrated on the Resonant–Assisted Deposition (RAD) process. Rather than measuring absolute energy, the method treats the signal power of each channel as a relative indicator of acoustic energy and tracks how wave propagation is altered in structure–borne and airborne pathways, recorded by two AE sensors on the deposition head and substrate, and an airborne microphone. Because the interface acts on all pathways at once, its activity appears as a common–mode variation of the head and substrate signal powers, distinct from the opposing trade–off seen during free tool motion, and this signature locates the interfacial activities within deposition cycles. Two time–domain features segment the window: the Interfacial Slipping Duration (ISD), associated with the slip stage, and the Interfacial Consolidation Duration (ICD), the subsequent bond formation stages; together they give the interfacial–activity duration $T_{IA}$. The measured ISD is consistent with an independently reported slip stage thermal response, and the same signal–derived windows confine the time–frequency analysis to the interval where ultra–harmonic interfacial content appears. Across controlled changes in excitation frequency and substrate temperature, the features remain extractable and respond in distinguishable ways, indicating repeatability and sensitivity. The method provides a physically interpretable, fixed–sensor basis for characterizing the timing of interfacial events in a moving–tool process, intended for in–situ process monitoring.