[2B7] Automated thermal compensation for in-process ultrasonic additive and weld inspection
C MacLeod, E Foster, N Sweeney, E Nicolson, D Lines, E Mohseni, K Tant, S Pierce and A Gachagan
University of Strathclyde, UK
In-process ultrasonic inspection of 3D printed metal additive components and welded joints, at the point of manufacture or in-service repair, offer clear benefits in terms of productivity, reduced rework, cost and ultimately quality of the final build. However, in-process ultrasonic inspection of the uppermost layer of an additive or welded build, as it is deposited, does not ensure defect-free fabrication, as defects can initiate and grow in sub-top surface layers due to thermal stresses, impurities, hot and cold work and time.
A challenge of in-process ultrasonic volumetric imaging of such components is the effect on wave velocity from thermal gradients and varying microstructures across the component due to build geometry, density, features, arc power and post-arc time. Ultimately, if not accounted for, these variations can give rise to significant incorrect defect size and position, in the order of millimetres in common metallic materials, and hinder the accuracy and ultimate potential of in-process inspection for real-time defect-free process control.
In this work, we propose a strategy to compensate for the velocity variations encountered during welding due to thermal fields and use this to image defects in-process more reliably. Firstly, a model of the thermal field as a function of the welding parameters is presented and, using established relationships between the material’s temperature and elastic moduli, is mapped to a velocity field in the inspection plane. A fast marching method for modelling wavefront propagation through the inhomogeneous velocity field is then deployed to estimate the travel times from each phased array element to each point in the imaging domain. These thermally compensated delay laws are then directly fed into the total focusing method (TFM) for enhanced defect imaging of a common weldment configuration featuring a 2 mm-diameter side-drilled hole.
When a thermal gradient is present, TFM images exhibit an error of defect location of up to 4 mm. Applying the proposed thermal compensation strategy allows for accurate relocation of the defect and improves the signal-to-noise ratio (SNR) by up to 3 dB.
A challenge of in-process ultrasonic volumetric imaging of such components is the effect on wave velocity from thermal gradients and varying microstructures across the component due to build geometry, density, features, arc power and post-arc time. Ultimately, if not accounted for, these variations can give rise to significant incorrect defect size and position, in the order of millimetres in common metallic materials, and hinder the accuracy and ultimate potential of in-process inspection for real-time defect-free process control.
In this work, we propose a strategy to compensate for the velocity variations encountered during welding due to thermal fields and use this to image defects in-process more reliably. Firstly, a model of the thermal field as a function of the welding parameters is presented and, using established relationships between the material’s temperature and elastic moduli, is mapped to a velocity field in the inspection plane. A fast marching method for modelling wavefront propagation through the inhomogeneous velocity field is then deployed to estimate the travel times from each phased array element to each point in the imaging domain. These thermally compensated delay laws are then directly fed into the total focusing method (TFM) for enhanced defect imaging of a common weldment configuration featuring a 2 mm-diameter side-drilled hole.
When a thermal gradient is present, TFM images exhibit an error of defect location of up to 4 mm. Applying the proposed thermal compensation strategy allows for accurate relocation of the defect and improves the signal-to-noise ratio (SNR) by up to 3 dB.
