ADDITIVE MANUFACTURING OF FUNCTIONALLY GRADED MATERIALS BETWEEN FERRITIC AND AUSTENITIC ALLOYS
Open Access
- Author:
- Zuback, James Scott
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 22, 2019
- Committee Members:
- Tarasankar Debroy, Dissertation Advisor/Co-Advisor
Tarasankar Debroy, Committee Chair/Co-Chair
Todd Palmer, Outside Member
Allison Michelle Beese, Committee Member
Reginald Felix Hamilton, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- Additive manufacturing
Functionally graded materials
Steels
Hardness
Diffusion
Ferritic alloys
Austenitic alloys
Superalloys - Abstract:
- Ferritic steels and austenitic alloys are used extensively as structural materials in power generation facilities such as fossil-fired and nuclear power plants. Currently, these alloys are joined using common fusion welding processes. However, dissimilar metal welds between ferritic and austenitic alloys are susceptible to premature failure due to a variety of simultaneously operating metallurgical issues, including carbon migration, localized creep strain, and coefficient of thermal expansion mismatch. Although improvements have been made to alloy selection and weld design over the past few decades, the root causes that ultimately lead to failure persist. As many of the power plants responsible for the worldwide production of electricity have exceeded or are nearing their designed lifetimes, an engineering solution that directly addresses the underlying metallurgical challenges is of great interest. Dissimilar metal welds between ferritic and austenitic alloys are an attractive application for the design of a functionally graded material system capable of preventing premature failure. Functionally graded materials represent a class of advanced materials designed to achieve a function by locally controlling density, composition, or microstructure to engineer site-specific properties. Problems arising from abrupt changes in chemical composition, microstructure, and properties in dissimilar welds can be overcome by implementing a functionally graded material that gradually transitions from a ferritic steel to an austenitic alloy. Additive manufacturing is well-suited for the design and fabrication of spatially dependent material combinations to achieve specific functions by grading chemical composition in a layer by layer manner. Carbon migration in dissimilar metal welds between ferritic and austenitic materials has been identified as a major cause for poor creep performance and premature failures in nuclear applications. Steep composition gradients and abrupt microstructural changes result in a large thermodynamic driving force that facilitates carbon diffusion away from the ferritic material, leading to negative impacts on creep strength. An appropriately graded transition joint effectively reduces the driving force for carbon diffusion by lowering the carbon chemical potential gradient. Theoretical calculations show that negligible amounts of carbon diffusion occur under typical service conditions in a functionally graded material between 2.25Cr-1Mo steel and Alloy 800H compared to its dissimilar weld counterpart. The functionally graded material fabricated using laser-based directed energy deposition was then experimentally tested for its effectiveness in reducing carbon diffusion and was shown to significantly outperform the dissimilar weld. Microstructural characterization indicated that a full composition gradient from 2.25Cr-1Mo steel and Alloy 800H may not be necessary, and a shorter transition joint will suffice. When the microstructure becomes fully austenitic, there is little change in microhardness and further compositional grading provides no benefits in reducing carbon diffusion. Additionally, a soft zone formed at the beginning of the functionally graded material near the ferritic steel, leading to an abrupt change in properties. If this region is excluded from the functionally graded material, however, the designed function of the transition joint for reducing carbon diffusion is negatively impacted. This finding indicates that tradeoffs between the function and unexpected microstructure exist during the design and fabrication of functionally graded materials. Lack of fusion defects, which are detrimental to properties and performance, formed in specific regions of the functionally graded material. The formation of defects was traced to changes in molten pool geometry due to variations in chemical composition. The presence of minor alloying elements changes the activity of surface-active elements, namely oxygen, in discrete regions throughout the functionally graded material. The thermodynamic activities of surface-active elements significantly impact the magnitude and direction of liquid metal flow during processing due to surface tension driven effects. The simulated molten pool geometries were found to be deep and narrow for compositions close to the ferritic steel and become shallower and wider as composition is graded towards the austenitic alloy. To avoid lack of fusion defects, process parameters such as laser power or hatch spacing may need to be adjusted in-situ to account for the variations in molten pool geometry. Changes in chemical composition during compositional grading can lead to the formation of unexpected secondary phases that can significantly alter mechanical properties and sometimes lead to build failures. The precipitate morphology in additively manufactured nickel base superalloys with slight changes in chemical composition was investigated with the goal of establishing connections between chemical composition and precipitate type. Although the as-deposited alloys exhibited similar precipitate distributions, the differences in precipitate type and morphology were striking. Variations in minor alloying elements were found to be an important driver for secondary phase formation in both the as-deposited and hot isostatic pressed conditions.