The demand for cost-efficient manufacturing of complex metal components has driven research for metal Additive Manufacturing (AM) such as Wire + Arc Additive Manufacturing (WAAM). WAAM enables automated, time- and material-efficient manufacturing of metal parts. To strengthen these benefits, the demand for robotically deployed in-process Non-Destructive Evaluation (NDE) has risen, aiming to replace current manually deployed inspection techniques after completion of the part. This work presents a synchronized multi-robot WAAM and NDE cell aiming to achieve (1) defect detection in-process, (2) enable possible in-process repair and (3) prevent costly scrappage or rework of completed defective builds. The deployment of the NDE during a deposition process is achieved through real-time position control of robots based on sensor input. A novel high-temperature capable, dry-coupled phased array ultrasound transducer (PAUT) roller-probe device is used for the NDE inspection. The dry-coupled sensor is tailored for coupling with an as-built high-temperature WAAM surface at an applied force and speed. The demonstration of the novel ultrasound in-process defect detection approach, presented in this paper, was performed on a titanium WAAM straight sample containing an intentionally embedded tungsten tube reflectors with an internal diameter of 1.0 mm. The ultrasound data were acquired after a pre-specified layer, in-process, employing the Full Matrix Capture (FMC) technique for subsequent post-processing using the adaptive Total Focusing Method (TFM) imaging algorithm assisted by a surface reconstruction algorithm based on the Synthetic Aperture Focusing Technique (SAFT). The presented results show a sufficient signal-to-noise ratio. Therefore, a potential for early defect detection is achieved, directly strengthening the benefits of the AM process by enabling a possible in-process repair.

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Welds are currently only inspected after all the passes are complete and after allowing sufficient time for any hydrogen cracking to develop, typically over several days. Any defects introduced between passes are therefore unreported until fully buried, greatly complicating rework and also delaying early corrections to the weld process parameters. In-process inspection can provide early intervention but involves many challenges, including operation at high temperatures with significant gradients affecting acoustic velocities and, hence, beam directions. Reflections from the incomplete parts of the weld would also be flagged as lack-of-fusion defects, requiring the region of interest (ROI) to adapt as the weld is built up. The collaborative SIMPLE (SIngle Manufacturing PLatform Environment) project addresses these challenges by incorporating robotic inspection within a robotic tungsten inert gas (TIG) welding cell. This has been accomplished initially with commercial off-the-shelf ultrasonic phased arrays, but is flexible enough to adapt to future developments with solutions suitable for higher temperatures. The welding and inspection robots operate autonomously. The former can introduce deliberate defects to validate the latter, which uses 5 MHz 64-element phased arrays on high-temperature wedges to generate sector scans after each weld pass. The results are presented, confirming that the challenges have been addressed and demonstrating the feasibility of this approach.

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Residual Stress (RS) in engineering components can lead to unexpected and dangerous structural failures, and thus represent a significant challenge to quality assurance in both welding and metal additive manufacturing (AM) processes. The RS measurement using the ultrasonic method is based on the acoustoelasticity law, which states that the Time-of-Flight (ToF) of an ultrasonic wave is affected by the stress field. Longitudinal Critically Refracted (LCR) waves have the highest sensitivity to the stress in comparison with the other type of ultrasonic waves. However, they are also sensitive to the material texture which negatively affects the accuracy of the RS measurement. In this paper, a Phased Array Ultrasonic Testing (PAUT) system, rather than the single element transducers which are traditionally used in the LCR stress measurement technique, is innovatively used to enhance the accuracy of RS measurement. An experimental setup is developed that uses the PAUT to measure the ToFs in the weld, where the maximum amount of tensile RS is expected, and in the parent material, stress-free part. The ToF variations are then interpreted and analyzed to qualify the RS in the weld. The same measurement process is repeated for the Wire Arc Additive Manufacture (WAAM) components. Based on the results, some variations between different acoustic paths are measured which prove that the effect of the residual stress on the ultrasonic wave is detectable using the PAUT system.