Design, Fabrication, and Analysis of Additively Manufactured Functionally Graded Materials
Open Access
- Author:
- Bobbio, Lourdes Del
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 20, 2021
- Committee Members:
- Zi-Kui Liu, Major Field Member
Hojong Kim, Major Field Member
Allison Beese, Chair & Dissertation Advisor
Robert Voigt, Outside Unit & Field Member
John Mauro, Program Head/Chair - Keywords:
- additive manufacturing
functionally graded materials
CALPHAD
Ti-6Al-4V
Stainless Steel
Equilibrium calculations
Scheil-Gulliver simulations - Abstract:
- Functionally graded materials (FGMs) are a type of material systems with intentional changes in composition or processing within a single component. Metallic FGMs have many potential applications ranging from aerospace to nuclear fields. By fabricating functionally graded materials using additive manufacturing, it is possible to avoid traditional joining techniques such as welding, which can result in localized points of weakness where the material is joined together. An FGM fabricated via AM will have incremental changes in composition as a function of position, which can result in corresponding changes various thermal, mechanical, or other properties that can be specially tailored for specific applications where variations in properties within one component is advantageous. Because of complex thermal history resulting from the AM process it is important to study how this process affects the micro- and macro-structure as well as physical properties of the resulting component. Analysis the elemental and phase composition changes that occur as a result of the compositional changes along the length of an FGM component leads to the development of a viable path between stainless steel and titanium that is free of detrimental phases that could negatively impact the structural integrity of the component. Using both experimental and computational methods, various binary and ternary FGM systems are examined in this thesis including: stainless steel 304L (SS304L and Ni-20Cr (NiCr); Ti-6Al-4V, V, and SS304L; 420 stainless steel (SS420) and V and Ti-6Al-4V, and SS420 to V. These FGMs were experimentally using characterization techniques such as scanning electron microscopy, energy dispersive X-ray spectroscopy, electron back-scatter diffraction, and X-ray diffraction. The SS420/V, SS420 to V to Ti-6Al-4V, and Ti-6Al-4V to V to SS304L were compared with each other to evaluate the differences in sigma phase nucleation and growth present in all three samples. Thermal history analysis indicated that, at favorable elemental compositions, sigma phase growth increased at locations that experienced higher temperatures for longer amounts of time. For the SS304L/NiCr FGM, no detrimental phase formation occurred, but uniaxial tensile tests showed that the NiCr-rich region of the sample was brittle due to the fcc/bcc eutectic phase composition in that region. Equilbrium calculations and Scheil-Gulliver simulations were used to evaluate ten different ternary systems to determine the optimal gradient path between stainless steel and Ti-6Al4V and it was found that Ni, Cr, and V were all necessary elements to be used as intermediate elements. Using this is information, an FGM grading linearly from 316 stainless steel to Ni-20Cr to Cr to V to Ti-6Al-4V was fabricated and experimentally evaluated. No detrimental phases formed at any location along the gradient. The expected transition from fcc to bcc was observed via EBSD and XRD within the NiCr/Cr gradient region, as was predicted by Scheil simulations. Future work could explore the use of non-linear gradient pathways through the Cr-Fe-Ni-Ti-V composition space that would allow for more compositional flexibility and less intermediate material usage.