Metamaterial acoustic lens-enabled noncontact real-time monitoring of mechanical properties of metal structures manufactured by additive friction stir deposition

Abstract

Achieving high fidelity in solid-state additive manufacturing requires a precise understanding of transient material behaviors and defect formation mechanisms. This study introduces a pioneering noncontact, in-situ ultrasonic monitoring system enabled by a metamaterial acoustic lens for additive friction stir deposition (AFSD). The metamaterial lens facilitates super far-field detection at a working distance of 120 mm, ensuring thermal resilience and enhanced spatial resolution, enabling minute crack detection with subwavelength resolution (∼0.8$\lambda$). Numerical simulations of stress-induced variations in bulk modulus and acoustic wave propagation were validated experimentally, revealing a strong correlation between transient acoustic behaviors and material properties. Unlike conventional machine feedback or ex-situ inspections, this method uncovers subtle inconsistencies and asymmetries—such as localized thermal softening, tensile stress gradients, and feeding irregularities—during real-time deposition. Four common manufacturing flaws (over-tensile stress, lack of heat, insufficient deposition, thermal anomalies, and inconsistent feeding) were systematically analyzed, demonstrating the system's sensitivity and multifunctionality. This innovative approach bridges the gap between in-situ diagnostics and ex-situ nondestructive testing, paving the way for advanced quality control in solid-state manufacturing processes. The presented methodology is extendable to other friction-stir and solid-state metal fabrication techniques by enhancing sensitivity and integrating process control, offering transformative potential for industrial and research applications