“Plasma-Facing Materials by Design and Rapid Prototyping via … · 2017-06-06 · While monolithic fracture toughness of even unirradiated W is about 2.5 times lower than even

“Plasma-Facing Materials by Design and Rapid Prototyping via Additive Manufacturing” CH Henager, Jr., RJ Kurtz, PNNL, Richland, WA

and GR Odette, UCSB, Santa Barbara, CA

Context The scientific challenges associated with controlled nuclear fusion are extraordinary. For plasma-

materials interactions, the material surfaces directly in contact with the fusion plasma suffer extreme perturbations due to the continual energetic bombardment of plasma particles that both exhaust heat and "recycle" the hydrogen fuel. The surfaces are rapidly reconstituted and altered by this interaction. For example, a surface atom may be removed and redeposited over a billion times in a single year. Simultaneously the surfaces impose strict boundary conditions for the fusion plasma, making for a highly non-linear, evolving coupled physical system. The neutron radiation damage to the materials and structures involve atomic and meso-scale physical processes that span more than 20 orders of magnitude in time scale and over 8 orders of magnitude in length scale.

Three scientific grand challenges, at least, constitute key hurdles to be resolved in order to reliably establish fusion energy as a viable power source from a materials science perspective: 1) Taming the plasma-materials interface, 2) Conquering nuclear degradation of materials and structures, and 3) Harnessing fusion power (tritium science, chamber technology and power extraction). Tritium must be handled at an unprecedented scale in fusion. Flow rates of many kilograms per day must be effectively processed over an incredible range of temperatures, pressures and material conditions, while observing stringent accountancy and environmental release constraints [1].

This white paper focuses on a specific materials issue found in the high-heat flux region of the divertor, namely the use of tungsten (W) as a solid plasma-facing component.

Technology to be Assessed As indicated in the FESAC Report on “Opportunities for Fusion Materials Science and Technology

Research Now and During the ITER Era”, February 2012 [1], the “leading FNSF/DEMO candidate solid material to meet the variety of PFC material requirements is tungsten due to its projected erosion resistance, high melting temperature and high thermal conductivity”. The following research recommendations were identified as: 1) Identify and characterize suitable tungsten based materials in appropriate plasma, thermal and radiation damage environments; and 2) Develop engineering solutions for tungsten PFCs with high pressure helium gas coolant.

The technology to be assessed includes: 1. Design of tough W-based composites using oxidation resistant alloys 2. Additive manufacturing methods to manufacture test coupons of these designs 3. Rapid prototyping to accelerate design testing and evaluation for optimization 4. Explore the use of Direct Metal Laser Sintering (DMLS) methods in 3D printing of

components with designed composite microstructures and cooling channels W-based composites can be designed based on ductile phase toughening theories [2-6] and fiber-

based composite theories [7, 8]. The majority of composite designs can be produced by powder processing making them amenable to DMLS via 3D printing [9]. W-alloys that are more oxidation resistant are currently being evaluated and can be adapted for powder processing to be included in this approach. 3D printing using DMLS lends itself to rapid prototyping via accelerated testing provided the correct test methodologies can be adopted and a standard test coupon and test faculties exist. The use of CFD and FEM codes to design appropriate W-based PFCs with optimized cooling channels and strength will enable rapid design evaluations and optimization that is currently missing from fusion materials work [9-14].

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Application of the technology

Forms of W, or W-alloys, are the leading candidate plasma facing materials for the divertor and first wall armor for tokamak fusion reactors. W can be a solid structural component, part of a multi-material hybrid, or a coating depending on the design and application. Although W has a number of favorable properties as a PFC, it has long been recognized that its mechanical and oxidation properties are poor. Oxidation resistance is likely to be required for licensing in case of oxygen ingress causing radioactive WO3 and tritium releases. However, mechanical property studies are revealing that any form of pure W will suffer severe irradiation embrittlement, even with intrinsic unirradiated fracture toughness values of about 8 MPa√m, which is unacceptably low.

The technology discussed here seeks to develop an optimized W-based component with oxidation resistance, adequate fracture toughness, and optimized helium cooling designs all using rapid prototyping from 3D printed DMLS structures for use in fusion reactors as plasma-facing components, specifically in the divertor region.

Objectives and Challenges

The first challenge is to define the requirements for W-based PFC components. As examples, we might assume that objectives for PFC materials are microstructural designs of W-based composite alloys with adequate fracture toughness, of at least about 20 MPa√m, combined with passivating oxidation resistance, at least 104-times better than pure tungsten. While monolithic fracture toughness of even unirradiated W is about 2.5 times lower than even this modest objective, it is clear that composites, with a variety of reinforcements, can in principle meet or exceed the assumed 20 MPa√m goal, even if the W-matrix toughness suffers severe irradiation embrittlement [15-17]. New oxidation resistant alloys have been proposed that may achieve 4 orders of magnitude greater oxidation resistance than elemental W [18-20], but have poor mechanical properties, as anticipated. A corollary challenge is limits on alloying and reinforcement materials both in terms of activity limits, irradiation stability and long term chemical compatibility.

The second challenge is to create a rapid prototyping methodology combined with the materials by design paradigm to explore additive powder manufacturing routes to W-based test components. This stage will require a dedicated DMLS system that can be used to create W-alloy prototypes for testing. These prototypes will include powder alloy chemistry for oxidation resistance, W-composite designs for fracture toughness (suggested to be based on ductile phase toughening), functional gradation to a copper-based alloy for heat removal [21, 22], and conformal cooling channels [23, 24].

Design variables

The primary focus needs to be developing an oxidation-resistant, high-toughness W-based composite using materials-by-design methodologies and exploring additive manufacturing techniques. This must be accomplished while retaining the favorable features of W, including low sputtering yields, high thermal conductivity, and reasonable neutron activation levels. This represents an incredible challenge for a W-based PFC system based on current technology. While research has progressed, there is currently no integrated development of a composite material based on oxidation resistant W-alloys to achieve the necessary property combination. Further, the effects of new material combinations on sputtering yield, thermal conductivity, neutron activation and radiation damage tolerance remain to be determined. What is needed is a more rapid, integrated approach using materials by design principles to achieve advanced state-of-the-art PFC structures that can be tested and evaluated faster than current technology.

In developing optimized W-based composites several design tools that can be applied. One set of tools deals with ductile phase toughening (DPT) models that can be used to design particle and laminate toughened W-composites (See Figure 1a) [2-6]. A second set of tools are fiber bridging models that can be used to design W-composites reinforced with W or SiCf (or other) fibers (See Figure 1b) [7, 8].

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(a) (b) Figure 1. Shown in (a) is a schematic of ductile phase toughening and in (b) is a fiber toughening schematic. Each of these schematic descriptions is backed up with full mechanics-based theory.

Ductile-phase toughening (DPT) requires maintaining W as the majority matrix phase but reinforcing it with a second but ductile phase, so that fracture toughness is controlled by the deformation of the ductile phase. That is, cracks in the W-based composite must be forced to dissipate energy by plastic work in the ductile phase. The physical arrangement of the two phases, the amount and size of the ductile phase reinforcements, and the properties of the ductile phase are all design variables. Oxidation resistant W-matrix phases, like W-Cr-Y-type alloys, must be developed in tandem and integrated into the composite. All of this can be done in the context of powder metallurgy processing [25-28] so that DMLS methods using 3D printing are strongly favored as both design tools and rapid prototyping methods [9]. Figures 2 and 3 show schematically the ability of 3D printing to achieve conformal cooling channels using DMLS methods directly in the component and to create complex structures from CAD inputs.

Figure 2. Schematic diagram showing 3D printed conformal cooling channels in a nominal component. Figure from https://3dprintingindustry.com/news/sliced-3d-printing-digest-klm-airlines-waseda-university-milacron-holdings-weta-workshop-108410/. See also [29].

Figure 3. Image showing the ability to go directly from complex CAD designs to 3D printed prototypes using an Inconel IN718 microtruss strut design as an example [30].

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Risks and uncertainties Although we have a preliminary scientific foundation to design of tough W composites, radiation

damage resistant reinforcements and oxidation resistant W matric alloys, the integrated engineering design and manufacturing tools for creating and qualifying W-based PFC components are lacking. One uncertainty in achieving success is that we do not understand the behavior of a multi-material system in the ultra-harsh fusion environment. There is also uncertainly in the fusion environment itself, including for off normal events like plasma disruptions. Further, while high performance composites and robust design may be achievable, the manufacturing technology for turning them into actual PFC components does not exist. However, we believe that additive manufacturing approaches may provide new opportunities to develop better W-based composites, for example that could take advantage of functional grading, while at the same time providing a practical route to manufacturing complex PFC components. The use of DMLS is established but making tungsten products with the required densities may be a risk at this time. There is also the lack of a robust ductile phase that can withstand the harsh fusion environment, although a strong reduced-activation ferritic/martensitic steel (RAFM) is a starting place.

Maturity W-based PFC technology is at about TRL-5 for laboratory testing by the international community

reflecting many critical aspects of fusion environments, like high, cyclic heat fluxes at prototype PFC scale. However, coordinated separate effects testing of things like radiation damage and degradation, tritium permeation and retention and oxidation resistance, all at the coupon scale are needed. Further, single effects and radiation damage testing still suffers the lack of a realistic fusion neutron source. The resulting single and multiple effects databases must be integrated in component performance assessments, based on advanced, multiphysics-multiscale, large scale computational simulations, involving a huge number of degrees of freedom. A TRL-3 to 5 can be expected for the experimental proof of concept, with a goal of reaching TRL-6 as the fusion community readies for ITER and for component testing.

Technology development for fusion applications Figure 4 schematically shows the 6-fold set of issues, or property considerations, for tungsten as PFC

that require optimization. At the moment, thermal and sputtering properties are believed to be adequate, as are the D/H/T retention properties. Thus, oxidation and mechanical properties are the main focus of the needed technology development effort. However, new W-based alloys and composites must consider, and develop a corresponding understanding, of all 6 property sets. For example, W-Cr-Y alloys have been developed with much improved oxidation resistance compared to pure tungsten, but the effects of these alloy additions on the other 5 property sets had not yet been determined. Another is the effects of damage, such as delamination, on the thermal properties of W-based composites.

Figure 4. Schematic of tungsten property/performance envelopes for fusion energy systems, such as

PFCs for tokomaks (diagram is courtesy of J. Coenen, KIT).

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References 1. Opportunities for Fusion Materials Science and Technology Research Now and During the ITER

Era, DOE/SC-0149, DOE/SC-0149, 2012, Germantown, Maryland 20874-1290, U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences,

2. Heathcote, J., G.R. Odette, and G.E. Lucas. A finite element study on constrained deformation in an intermetallic/metallic microlaminate composite. in Layered Materials for Structural Applications. Symposium, 8-11 April 1996, 1996, Pittsburgh, PA, USA, Mater. Res. Soc, vol., pp. 183-8

3. Venkateswara Rao, K.T., G.R. Odette, and R.O. Ritchie, "Ductile-reinforcement toughening in γ-TiAl intermetallic-matrix composites: effects on fracture toughness and fatigue-crack propagation resistance", Acta metallurgica et materialia, 1994, vol. 42, no. 3, pp. 893-911.

4. Rowe, R.G., et al., "Microlaminated high temperature intermetallic composites", Scripta metallurgica et materialia, 1994, vol. 31, no. 11, pp. 1487-1492.

5. Odette, G.R., et al., "Ductile phase toughening mechanisms in a TiAl-TiNb laminate composite", Acta metallurgica et materialia, 1992, vol. 40, no. 9, pp. 2381-9.

6. Cao, H.C., et al., "Test procedure for characterizing the toughening of brittle intermetallics by ductile reinforcements", Acta Metall., 1989, vol. 37, no. 11, pp. 2969-2977.

7. Evans, A.G. and F.W. Zok, "Physics and mechanics of fibre-reinforced brittle matrix composites", Journal of Materials Science, 1994, vol. 29, no. 15, pp. 3857-3896.

8. Evans, A.G., F.W. Zok, and J. Davis, "Role of interfaces in fiber-reinforced brittle matrix composites", Compos. Sci. Technol., 1991, vol. 42, no. 1-3, pp. 3-24.

9. Sedlak, J., et al., "Production of Prototype Parts using Direct Metal Laser Sintering Technology", Acta Polytechnica, 2015, vol. 55, no. 4, pp. 260-6.

10. Promoppatum, P., R. Onler, and Y. Shi-Chune, "Numerical and experimental investigations of micro and macro characteristics of direct metal laser sintered Ti-6Al-4V products", J. Mater. Process. Technol., 2017, vol. 240, pp. 262-73.

11. Mendible, G.A., J.A. Rulander, and S.P. Johnston, "Comparative study of rapid and conventional tooling for plastics injection molding", Rapid Prototyping Journal, 2017, vol. 23, no. 2, pp. 344-352.

12. Lewandowski, J.J. and M. Seifi, "Metal additive manufacturing a review of mechanical properties", Annual Review of Materials Research, 2016, vol. 46, pp. 151-86.

13. Mierzejewska, Z.A., "Process Optimization Variables for Direct Metal Laser Sintering", Advances in Materials Science, 2015, vol. 15, no. 4, pp. 38-51.

14. Ciocca, L., et al., "Direct metal laser sintering (DMLS) of a customized titanium mesh for prosthetically guided bone regeneration of atrophic maxillary arches", Medical and Biological Engineering and Computing, 2011, vol. 49, no. 11, pp. 1347-1352.

15. Koyanagi, T., et al., "Microstructural evolution of pure tungsten neutron irradiated with a mixed energy spectrum", J. Nucl. Mater., 2017, vol. 490, pp. 66-74.

16. Xunxiang, H., et al., "Defect evolution in single crystalline tungsten following low temperature and low dose neutron irradiation", J. Nucl. Mater., 2016, vol. 470, pp. 278-89.

17. Hu, X., et al., "Irradiation hardening of pure tungsten exposed to neutron irradiation", J. Nucl. Mater., 2016, vol. 480, pp. 235-243.

18. Litnovsky, A., et al., "Smart tungsten alloys as a material for the first wall of a future fusion power plant", Nucl. Fusion, 2017, vol. 57, no. 6,

19. Lessmann, M.T., et al., "The effects of ion irradiation on the micromechanical fracture strength and hardness of a self-passivating tungsten alloy", J. Nucl. Mater., 2017, vol. 486, pp. 34-43.

20. Lessmann, M.T., et al., "Fracture strength testing of a self-passivating tungsten alloy at the micrometre scale", Philosophical Magazine, 2016, vol. 96, no. 32-34, pp. 3570-3585.

21. Zinkle, S.J., "Applicability of copper alloys for DEMO high heat flux components", Physica Scripta T, 2016, vol. 2016, no. T167, pp. 014004 (10 pp.).

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22. Barabash, V.R., et al., "Specification of CuCrZr alloy properties after various thermo-mechanical treatments and design allowables including neutron irradiation effects", J. Nucl. Mater., 2011, vol. 417, no. 1-3, pp. 904-7.

23. Jahan, S.A. and H. El-Mounayri, "Optimal Conformal Cooling Channels in 3D Printed Dies for Plastic Injection Molding", Procedia Manufacturing, 2016, vol. 5, pp. 888-900.

24. Pessard, E., et al., "Complex cast parts with rapid tooling: Rapid manufacturing point of view", International Journal of Advanced Manufacturing Technology, 2008, vol. 39, no. 9-10, pp. 898-904.

25. Coenen, J.W., et al., "Advanced materials for a damage resilient divertor concept for DEMO: Powder-metallurgical tungsten-fibre reinforced tungsten", Fusion Eng. Des., 2016, vol. 10.1016/j.fusengdes.2016.12.006

26. Calvo, A., et al., "Manufacturing of self-passivating tungsten based alloys by different powder metallurgical routes", Physica Scripta T, 2016, vol. 2016, no. T167, pp. 014041 (6 pp.).

27. Calvo, A., et al., "Manufacturing and testing of self-passivating tungsten alloys of different composition", Nuclear Materials and Energy, 2016, vol. 9, pp. 422-429.

28. Lopez-Ruiz, P., et al., "Powder metallurgical processing of self-passivating tungsten alloys for fusion first wall application", J. Nucl. Mater., 2013, vol. 442, no. 1-3 SUPPL.1, pp. S219-S224.

29. Innomia. https://3dprintingindustry.com/news/sliced-3d-printing-digest-klm-airlines-waseda-university-milacron-holdings-weta-workshop-108410/. 2016.

30. Huynh, L., J. Rotella, and M.D. Sangid, "Fatigue behavior of IN718 microtrusses produced via additive manufacturing", Materials & Design, 2016, vol. 105, pp. 278-89.