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Yang, Shenliang

University of British Columbia

As a widely used metal additive manufacturing technology, directed energy deposition (DED) provides a cost-effective solution for repairing and remanufacturing high-performance metallic components. Nevertheless, their industrial applications are limited by excessive geometric distortion (caused by high-gradient bulk residual stress) and poor surface quality. To achieve desired surface finishes, post-machining of DED is usually performed, which introduces additional surface residual stress. These residual stresses can compromise structural integrity, fatigue life, and corrosion resistance. Guided by three critical and practical research questions—(1) how to efficiently predict DED-induced bulk residual stress, (2) how to accurately simulate surface residual stress created by machining, and (3) how DED-induced bulk stress interacts with machining to generate final surface stress—this thesis develops physics-based models to understand the formation mechanism of residual stresses in combined DED and machining processes. First, analytical models are proposed to determine thermo-mechanical responses in single-material and functionally graded DED processes. These models employ finite difference methods for predicting temperature cycles, modified Green’s functions to quantify thermal stress, and radial return methods to update plastic stress, which features the material addition as the heat source moves in each deposition layer in DED. The frameworks effectively demonstrate how DED deposition conditions and material gradients influence bulk residual stress. Second, finite element method (FEM) models are presented to predict machining-induced surface residual stress in conventionally wet machining processes and high-speed machining processes with serrated chip generation. The results quantitatively illustrate the mitigation of surface tensile residual stress due to applying cutting fluids and the fluctuation of surface residual stress caused by periodic shear localization in chips. Lastly, the FEM machining model is extended by incorporating initial stress states and microstructure features to simulate post-machining DED processes. Results suggest that pre-existing residual stress significantly influences final residual stress, while the finer the grain size the more compressive residual stress on the post-machined surfaces. By revealing the formation mechanisms of residual stresses in DED and machining processes, this thesis offers practical guidance for planning the combined additive manufacturing and post-machining technologies, thereby fostering the adoption of advanced manufacturing technologies in mission-critical industries.

Text

2025-05-20

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/91164

Mechanical Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/