Thesis

Phased-array ultrasonic residual stress measurement in metal additive manufacturing : development, validation, and microstructure-aware interpretation

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Awarding institution
  • University of Strathclyde
Date of award
  • 2026
Thesis identifier
  • T18216
Person Identifier (Local)
  • 202293421
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Qualification Name
Department, School or Faculty
Abstract
  • Residual Stress (RS) remains a major challenge for metal Additive Manufacturing (AM), in large-scale processes such as Wire Arc Additive Manufacturing (WAAM). Steep thermal gradients and repeated thermal cycling can cause distortion, cracking and reduced structural performance. Existing RS measurement methods are often destructive, surface-limited or impractical for automated manufacturing. A non-destructive, subsurface-sensitive method is therefore required. The thesis describes the development of Phased-Array Ultrasonics for Residual Stress measurement (PAURS). PAURS employs phased-array transducers to produce Longitudinal Critically Refracted (LCR) waves and makes use of acoustoelastic calibration in order to convert variations in Time-of-Flight (ToF) into RS measurements. Full Matrix Capture (FMC) allows averaging multiple paths and assessing the quality of each path, increasing measurement redundancy and reducing variability. PAURS+ is also examined as a microstructure-aware extension of PAURS. PAURS was assessed across three studies. Measurements from as-deposited and inter-pass-peened WAAM Ti-6Al-4V samples were compared with those obtained using the Contour Method (CM) and Xray Diffraction (XRD); these methods differ in penetration depth and the gauge volumes. Additive Friction Stir Deposition aluminium was also investigated for variation in microstructure. In the WAAM studies, PAURS identified central tensile RS peaks that decreased with distance from the Heat-Affected Zone, qualitatively agreeing with CM and XRD. PAURS indicated a reduction in peak tensile RS from approximately 660 MPa in the as-deposited specimen to 530 MPa following inter-pass peening. Quantitative differences were attributed partly to penetration depth, gauge volume and reference condition. FMC provided path redundancy and reduced variability following path-quality assessment. The PAURS+ study found that LCR ToF exhibited limited sensitivity to the investigated microstructural transition, while attenuation and backscatter showed clearer microstructural contrast. Overall, PAURS demonstrates potential for non-destructive, repeatable and automated shallow-subsurface RS assessment. Further development is required before in-process deployment and quantitative separation of stress and microstructure.
Advisor / supervisor
  • Javadi, Yashar
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DOI

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