Influence of neutron irradiation on Al-6061 alloy produced via ultrasonic additive manufacturing

https://doi.org/10.1016/j.jnucmat.2021.152939Get rights and content

Highlights

Abstract

Samples of aluminum alloy 6061 produced via ultrasonic additive manufacturing (UAM) were irradiated in the High Flux Isotope Reactor (HFIR) up to 17.3 dpa at ~70°C while in contact with water using perforated rabbit capsules. The irradiation campaign included as-received (AR) material, specimens subjected to various post-weld heat treatments (PWHTs, including hot isostatic pressing [HIP]), and reference (wrought) alloy samples. Mechanical tensile tests, accompanied by digital image correlation (DIC) analysis, fractography, and metallography, were performed as a part of the post-irradiation evaluation. The X- and Y-specimens (i.e., oriented in the sonotrode moving and vibration directions, respectively) showed pronounced radiation hardening and ductility decrease. Specific serration flow behavior and propagation of deformation bands were observed under various material conditions up to 3.5 dpa but disappeared at 17.3 dpa. In all cases, the fracture mechanism of X- and Y-specimens was ductile; ductile dimples dominated the fracture surface. Irradiated X- and Y-specimens showed good performance, regardless of material conditions (AR or PWHT). The performance of Z-specimens oriented in the build direction was strongly dependent on the PWHT. The AR and aged specimens showed fracture stress decrease with dose, and they experienced fracture under irradiation after 3.5 dpa; specimen cross section analysis revealed specific interface degradation that was likely related to corrosion. Recrystallization significantly improved in-reactor performance. HIP suppressed interface degradation due to recrystallization and pore removal, which led to good in-reactor performance for Z-specimens.

Introduction

Nuclear energy is produced predominantly by approximately 400 light water reactors across the globe. These reactors are based on designs conceptualized and demonstrated in the 1940s and 1950s, and they are facing a challenging cost paradigm [1]. The motivations for adopting advanced materials and manufacturing methods to transform this cost paradigm are well known [2]. Modern manufacturing techniques like additive manufacturing (AM) offer advanced and flexible designs, extended part functionality, and the opportunity for a data-rich and accelerated qualification approach. Additionally, AM often allows for reduced cost and delivery time for critical nuclear components. One of the AM techniques, ultrasonic additive manufacturing (UAM), is an advanced solid-state joining process that uses ultrasonic vibrations to fabricate parts layer by layer [3,4]. Compared to traditional industrial technologies, UAM can produce complex geometry parts with internal channels, cavities [3], insertions of different material/components, or even with embedded fibers or sensors in composite or multifunctional materials [5,6]. For instance, UAM was recently used to produce a prototype of a control plate for the High-Flux Isotope Reactor (HFIR) [7]. Even though AM has made significant progress, there is limited literature on the performance of AM materials and components under an irradiation environment that induces displacement damage.

Additive manufacturing may require some post-manufacturing processing to tune properties and enhance performance. Components fabricated via AM are often produced by localized joining strategies that subject the material to sharp temperature, strain, and stress gradients locally, resulting in unique microstructures. Anisotropy in AM material properties is typical. In the case of UAM, one manifestation of anisotropy is in mechanical behavior, in which components have relatively low strength when loaded in the orientation perpendicular to the welding interfaces [8], which is often termed as Z. The decreased Z-direction strength is usually attributed to the presence of voids [9], lack of bonding between the layers [10], texturing, or a high strain level in the interface-adjacent area [11]. As demonstrated in the literature [11,12], post-weld heat treatments (PWHTs) may be used to improve the properties of welded parts. Advantages of PWHT for UAM components are discussed in [11], which demonstrated that PWHTs of UAM-produced material may improve the Z-strength level by a factor of approximately 3–3.5 via material aging and grain growth across the welding interfaces. Moreover, a combination of annealing, quenching, and aging has limited or no impact on the manufacturing-induced porosity structure. Hot isostatic pressing (HIP) decreases porosity by applying external pressure at elevated temperatures and has been shown to be beneficial for UAM material performance [13]. Using PWHTs, including HIP, the UAM material properties may be adjusted to fit specific applications; however, the in-reactor performance of the applied materials should be assessed.

Aluminum alloy 6061 is widely used in research nuclear reactors because of its well-known corrosion resistance under high heat flux in water [14], [15], [16], [17], its low degree of radiation-induced embrittlement and small neutron capture cross section, as well as its good workability and availability. Mild research reactor conditions such as temperatures below 100°C and low strength level requirements also favor the use of Al-based alloys [18]. Typical applications are reactor tanks/vessels [19], core grids and claddings [17,20,21], control plates [7], rods, and in-core devices. Many of these structures, particularly those related to the core, require complex, multi-step fabrication techniques, so these items may be produced more effectively via UAM.

The present work investigates the mechanical properties, fracture behavior, and microstructure of the UAM-produced aluminum alloy 6061 after neutron irradiation in HFIR. The study includes a post-irradiation examination of the as-received (as-printed) alloy, followed by examination of the same material after several different treatments, including the HIP. Within the present project, the specimens were irradiated up to 17.3 dpa; this range is a good estimation of the lifetime doses of many in-reactor components/parts.

Section snippets

UAM process details and composition of studied materials

Commercially available 150 μm tapes of Al-6061 H-18 alloy were used to produce builds with dimensions of 150 × 25 × 20 mm in X, Y, and Z-directions [11]. The UAM-builds were fabricated using the UAM machine at Fabrisonic LLC in Columbus, OH [3]. The UAM-process was conducted using the following parameters: a vibration amplitude of 35 μm, a normal force of 5,000 N, and a travel speed of 85 mm/s. The temperature was maintained at 75°C during the process [11,13]. The element composition of the

Mechanical behavior of bulk 6061 alloy

Figure 2 shows the engineering tensile curves for unirradiated and irradiated bulk 6061 alloy; mechanical properties calculated from the curves are provided in Table 4. As shown in the results, the bulk reference (0 dpa) material was soft, with sufficient ductility (total elongation over 30%). The reference bulk alloy specimen revealed serration in the flow curve, force drops, and fluctuations on the tensile curve, suggesting dynamic strain aging processes, which is expected for this class of

Summary, conclusions, and future work

The present work investigated the mechanical properties and fracture behavior of 6061 aluminum alloy produced via ultrasonic additive manufacturing (UAM). As-received tensile specimens, as well as those subjected to various post-weld heat treatments (PWHTs) and hot isostatic pressing (HIP), were irradiated in the High Flux Isotope Reactor (HFIR) up to 17.3 dpa at ~70°C in contact with water.

Post-irradiation mechanical tests, accompanied by digital image correlation (DIC) analysis, revealed that

Credit author statement

M.N. Gussev: planning and performing experiments, data analysis, manuscript writing. N. Sridharan: performing experiments, editing and reviewing the manuscript. S.S. Babu: conceptualization, data analysis and discussion, editing the manuscript. K.A. Terrani: planning the project, analyzing and discussing the results, writing the manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank Daniel Pinkston, Chris Bryan, T.S. Byun, and David McClintock at Oak Ridge National Laboratory (ORNL) for useful discussions and review of the work. The help and support of ORNL's Irradiated Materials Examination and Testing Facility (IMET) staff (manager: Mark Delph) and the Low-Activation Materials Design and Analysis Laboratory (LAMDA) staff (manager: J. Schmidlin) are gratefully acknowledged. The authors also would like to thank Mark Norfolk and Adam Hehr at

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    This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy(DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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