Doctoral Dissertations
Date of Award
12-2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Materials Science and Engineering
Major Professor
Katharine L. Page
Committee Members
Sudarsanam Suresh Babu, Katharine Lynn Page, Jeffrey Bunn, Dayakar Penumadu
Abstract
This work investigates how plastic strain accumulates—and how microstructure evolves—in solid-state additive manufacturing, using Additive Friction-Stir Deposition (AFSD) as a representative case study. The central motivation is that solid-state welding generates sharp plastic-strain gradients at sub-millimeter length scales, which in turn produce fine and spatially complex residual elastic strain continua that drive distortion and cracking during deposition and post-deposition heat treatment. These plastic and elastic strain gradients develop at length-scales that challenge state-of-the-art characterization and, as a result, limit model calibration and part-scale qualification. I address this gap by developing and applying diffraction-based methods to quantify these gradients in stored plastic strain and residual elastic strain. Firstly, I utilize energy-dispersive X-ray diffraction tomography with a stop-action AFSD build to quantify the evolution of microstructural heterogeneity—stored plastic strain, crystallographic texture, and residual elastic strain—at 200 μm resolution across an entire component. This approach links fundamental process asymmetry (i.e. advancing and retreating behavior) to microstructural heterogeneity and provides high-throughput, part-scale datasets that are well matched to calibrating and validating computational models (e.g., crystal plasticity, finite element, etc.). Secondly, I show that the evolution of these fine-scale plastic strain gradients is responsible for the layer-by-layer buildup of residual stresses. This is demonstrated by combining bulk-scale methods with fine-scale strain mapping (contour method and laboratory X-rays, respectively). The results show that fine-scale gradients in plastic strain accumulate behind the tool, which equilibrate across the component and strongly shape the bulk-scale residual stress distribution. Thirdly, I demonstrate how one-standard-deviation peak-fit errors obtained from diffraction measurements underestimate the true uncertainty of residual stress in components with steep gradients in stored plastic strain, crystallographic texture, and residual elastic strain. To address hidden biases in diffraction measurements, I introduce a tensor-consistent calibration procedure that inflates reported measurement uncertainties until each measured strain within a common volume satisfies the strain-transformation law within the expectations of one standard deviation errors. In the case of AFSD, these uncertainties must be inflated by 2× to 3×, depending on the strength of local microstructure and residual elastic strain gradients. Collectively, these contributions drive methods development at synchrotron and neutron diffraction facilities, which enable more representative qualification of solid-state additive processes and validation of predictive models.
Recommended Citation
Franz, Cole, "Understanding Accumulated Plastic Strain in Solid-State Additive Manufacturing and its Influence on Residual Stress Evolution. " PhD diss., University of Tennessee, 2025.
https://trace.tennessee.edu/utk_graddiss/13597