This thesis investigates the use of laser shock peening (LSP) to improve mechanical properties, electrochemical behavior, and stress corrosion cracking (SCC) resistance in laser powder bed fusion (LPBF) stainless steel. The thesis begins by introducing metal additive manufacturing and reviews the current technological frontiers of LSP before elucidating the fundamentals behind the imaging, experimental, and theoretical frameworks used in the subsequent chapters.
The experimental work is roughly divided into two parts; the first half is dedicated to study of the plasticity response augmentation by LSP in anisotropic stainless steel. The prevalence of back stress hardening occurring in anisotropic metal parts causes reduced fatigue life under random loading. LSP is known to improve fatigue life by inducing compressive residual stress and has been applied with promising results to AM metal parts. It is here demonstrated that LSP may also be used as a tool for mitigating tensile back-stress hardening.
This discussion is initially applied to rolled and annealed 304L stainless steel which is shown to exhibit material anisotropy. Back stress is extracted from hysteresis tensile testing for treated and untreated samples. Analysis of plasticity response by orientation imaging microscopy (OIM) and finite element analysis (FEA) describes back stress and residual stress development during tensile testing and LSP treatment. The research indicates LSP's potential to address manufacturing design challenges caused by yield asymmetry due to back stress and is thus next applied to additively manufactured 316L. The microstructure and texture in additively manufactured metal lead to anisotropic hardening behavior. Comparison of LSPed and as-built LPBF samples shows LSPed samples processed along the build direction demonstrate significant back-stress reduction. Electron backscatter diffraction (EBSD) illuminates grain morphologies' role, while crystal plasticity finite element (CPFE) modeling reveals mechanisms underlying back-stress reduction across different build orientations and crystal planes.In the second half of the thesis, LSP’s effect upon LPBF 316L material performance in corrosive environments is investigated.
This effort begins with analysis of LSP’s improvement to electrochemical and wetting behavior of as-built LPBF surfaces. The corrosion performance of LPBF stainless steel varies between studies and build parameters, thus motivating the search for postprocessing methods that enable wetted surface applications. The study examines electrochemical properties before and after LSP, measuring pitting potential, electrochemical impedance, contact angle, surface free energy, and surface finish. LSP imparts surface improvement which is attributed to morphology and chemistry alterations as well as compressive residual stress. LPBF stainless steel is also particularly susceptible to SCC due to surface-level tensile residual stress. The final study demonstrates LSP's ability to enhance SCC behavior in LPBF stainless steel by increasing time to crack initiation. Analyses of residual stress, texture, dislocation distribution, hardness, microstructure, and fracture surfaces are conducted to understand the mechanisms underlying SCC improvement. Dynamic crack modeling supports observed outcomes, linking residual stress and failure modes to LSP's effects.
This work highlights LSP's potential as a versatile tool for enhancing the performance and reliability of LPBF stainless steel components in demanding engineering applications. Further, it identifies the key relevance of the anisotropy of LPBF material structure to mechanical behavior and also to the effectiveness of LSP surface processing.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/x9h9-6n95 |
| Date | January 2024 |
| Creators | Over, Veronica Helen Marquez |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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