Abstract
Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that fabricates parts by selectively scanning consecutive powder layers with an electron beam. Additive manufacturing technologies are increasing in importance for aerospace and medical applications, where the demand for a fundamental understanding and predictability of static and dynamic material properties are high. Ti-6Al-4V is the most widely used and studied alloy for this technology, and is the focus of this work in its Extra Low Interstitial (ELI) variation. The layered manufacturing of metallic components by EBM creates a unique directional microstructure, and consequently, anisotropic properties. Microstructure evolution, and its influence on mechanical properties of the alloy in the as-fabricated condition, has been documented by various researchers. However, fatigue crack propagation and the effects of the directional structure have not been sufficiently studied, imposing a barrier for this technology's potential extension to high-integrity applications. In this study, fatigue crack growth (FCG) both parallel and perpendicular to the build directions was studied for different stress ratios and crack growth stages. The interaction between the directional as-fabricated EBM microstructure and FCG was investigated and compared to that of the equiaxed β annealed microstructure obtained by annealing above the β transus temperature. The FCG threshold, ΔKth, was analytically modeled for the two relative crack propagation directions at different stress ratios, and FCG microstructural mechanisms were established for all three regions of crack propagation.
Original language | English |
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Pages (from-to) | 263-280 |
Number of pages | 18 |
Journal | Engineering Fracture Mechanics |
Volume | 176 |
DOIs | |
State | Published - May 1 2017 |
Funding
This research was performed under the Additive Manufacturing program of the Integrative Material Design Center (iMdc) at Worcester Polytechnic Institute, in collaboration with the Additive Manufacturing Demonstration Facility of Oak Ridge National Laboratory, and sponsored by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. Ph.D. candidates Yuwei Zhai and Anthony Spangenberger of the iMdc also collaborated actively in the material characterization performed during this study.
Keywords
- Additive manufacturing
- Electron beam melting
- Fatigue crack growth mechanisms
- Threshold modeling
- Ti-6Al-4V