Improving ultrasonic inspection reliability of as-built WAAM components with rough surfaces using solid composite couplants

Wire and arc additive manufacturing (WAAM) components typically exhibit stepped, high-roughness as-built surfaces, which present challenges for quality assurance and nondestructive inspection in manufacturing environments. Surface machining is commonly required prior to ultrasonic testing, increasing production time and cost. This study proposes a solid carbon-based wax composite couplant to enable stable ultrasonic inspection directly on rough WAAM surfaces without additional surface finishing. Paraffin wax was identified as the optimal base matrix due to its superior acoustic transmission characteristics and was subsequently modified with carbon nanotubes, carbon powder, and graphene. Ultrasonic attenuation experiments demonstrated that incorporating 0.8 wt% graphene significantly improved signal stability, increasing the back-wall echo amplitude from 43 dB to 81 dB. Validation using defect blocks containing lack of fusion, slag inclusion, and porosity confirmed reliable defect detection on surfaces with roughness exceeding 140 μm. The proposed graphene–wax composite couplant allows direct inspection of as-built WAAM components, reducing the need for post-processing while improving inspection reliability. This approach offers a practical pathway for integrating ultrasonic quality assurance into WAAM production workflows.