Date of Award

8-2023

Document Type

Dissertation

Degree Name

Ph.D.

Organizational Unit

Daniel Felix Ritchie School of Engineering and Computer Science, Mechanical and Materials Engineering

First Advisor

Maciej Kumosa

Keywords

Additive manufacturing, Electron beam melting (EBM), Fatigue, Fracture, Powder bed fusion (PBF), Ti-6Al-4V

Abstract

For applications in the aerospace field, selection of materials for a given design requires an understanding of critical properties, like fatigue and fracture, in addition to static mechanical and physical properties. With the ongoing advancements in metallic additive manufacturing techniques and the interest in applying the process to aerospace applications, there is a clear need to fully characterize properties. Arguably, the most attractive alloy for applications in aerospace is the Ti-6Al-4V alloy. The current dissertation examines the mechanical properties of the alloy, made by the Electron Beam Melting (EBM) Powder Bed Fusion (PBF) method. As illustrated in this work, the EBM Ti-6Al-4V properties vary depending on processing conditions, post-processing treatments (e.g. hot isostatic pressing: HIP), specimen orientation, and chemical content of the powder feedstock.

The current dissertation adds significantly to the literature on EBM Ti-6Al-4V, showing that the powder feedstock chemistry (especially oxygen) must be controlled, and engineers must account for the anisotropic behavior of the material as well as inherent material defects and the effects of HIP post-processing when designing for fracture and fatigue critical applications. First, the effects of powder feedstock oxygen content were characterized for a wide range of oxygen mass fraction (0.13 % to 0.47 %) by performing Charpy impact testing and resistance curve (J-R) testing to determine fracture toughness, all while including the effects of specimen orientation and HIP post-processing. The fracture trends showed significant reduction in toughness at the highest oxygen levels, with clear anisotropic effects, and a significant improvement in toughness after HIP. Next, the effects of orientation and HIP post-processing on fatigue performance were characterized by generating Stress-Life (S-N) curves and Fatigue Crack Growth Rate (FCGR) curves. The fatigue study showed limited anisotropy and a strong improvement in performance after HIP due to the closure of internal voids and lack-of-fusion defects. Finally, drawing on the characterization work, a Fracture Mechanics (FM)-based fatigue life model was utilized to compare predictions to as-tested results. Overall, this study demonstrated how an in-depth understanding of material performance properties could be utilized in an efficient FM-based fatigue model for damage tolerant additively manufactured designs.

Copyright Date

8-2023

Copyright Statement / License for Reuse

All Rights Reserved
All Rights Reserved.

Publication Statement

Copyright is held by the author. User is responsible for all copyright compliance.

Rights Holder

William A. Grell

Provenance

Received from ProQuest

File Format

application/pdf

Language

English (eng)

Extent

170 pgs

File Size

6.9 MB

Discipline

Mechanical engineering, Materials Science

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