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Cold Spray AM of Oxide Dispersion Strengthened Alloys

Overview of ODS Alloys

Oxide Dispersion Strengthened (ODS) alloys are engineered materials designed for superior performance at elevated temperatures. These alloys are characterised by their high creep resistance, toughness, and oxidation resistance, which are achieved through the uniform dispersion of nanoscale oxide particles within a metal matrix. The dispersed oxide particles effectively absorb and trap radiation-induced defects, preventing their accumulation and subsequent material degradation. This makes ODS alloys particularly suitable for use in nuclear reactors, where components are exposed to high neutron flux and temperatures. In fusion reactors and advanced fission reactors, Reduced Activation Ferritic-Martensitic (RAFM) ODS alloys are employed for critical structural components. RAFM ODS alloys are a specific category of ODS alloys, designed to minimise activation and radiological hazard while maintaining excellent performance in high radiation environments.

Why Cold Spray for ODS Alloys?

Conventionally, the production process for ODS alloys involves two main stages: powder fabrication and consolidation. Consolidation methods include extrusion, forging, cold pressing, field-assisted sintering, or near-net-shape hot-isostatic pressing (HIP). These methods have significant limitations, including scalability to large and complex structures, consistency and homogeneity, reproducibility, and difficulty in achieving near-net-shape parts.

Cold Spray Additive Manufacturing (CSAM) is a solid-state material deposition technique that uses pressurised, pre-heated gas to accelerate powder particles to supersonic speeds. These particles are then directed onto a substrate, where they bond through high-velocity impacts, producing dense near-net-shape components directly from powder with high deposition rate and low wastage. CSAM can be scaled to produce large structures and incorporate internal features such as cooling channels, eliminating the need for joints and welds that are potential failure points.

Unlike conventional fabrication processes such as casting, welding, and fusion-based additive manufacturing methods, CSAM does not involve melting the powder, which allows for precise control over the microstructure of the deposited material. CSAM preserves the microstructure of the feedstock powder, which is crucial for maintaining the beneficial properties of ODS alloys. Fusion-based processes, on the other hand, may compromise these properties as melting tends to destroy the oxide particle distribution. CSAM of ODS alloys therefore addresses a major need for the future development of fusion power at a commercial scale.

Objectives

This case study aims to demonstrate the feasibility of Cold Spray Additive Manufacturing (CSAM) for producing large-scale, in-vessel components for fusion reactors, including diverters, first-walls, breeder-blankets, and centre-columns. Specifically, the study seeks to:

  • Investigate the suitability of ODS alloys for cold spray, including determining the optimal cold spray parameters and conditions for depositing these alloys
  • Develop and refine toolpaths for creating complex geometries, including features such as internal cooling channels. This involves designing strategies for producing intricate geometric features and fabricating medium- to large-scale demonstration components with minimal waste and high deposition rates
  • Examine post-processing techniques, including the development of suitable post-deposition thermal treatments to enhance the microstructural and mechanical properties of the ODS alloys

This case study is part of the UK Atomic Energy Authority (UKAEA) Fusion Innovation Programme (FIP) Cycle 2 projects. In Phase 1, a feasibility study was conducted using ODS PM2000. Following the successful completion of this phase, Phase 2 involves further research focused on RAFM ODS Eurofer97.

Open video

Video: Cold Spray Additive Manufacturing of Hollow Cylinder

Key Findings (Phase 1): Cold Spraying of ODS PM2000

In Phase 1, cold spray deposition trials were performed in the large-scale spraying facility at TWI Cambridge using the Impact Innovation 5/11 cold spray system with nitrogen process gas. Two ODS PM2000 powders were used: mechanically alloyed (MA) irregular morphology powder and gas atomised (GA) spherical morphology powder.

ODS PM2000 was deposited with measured density ~98% for MA powder, and >99% for GA powders (Figure 1 showing SEM microstructure). Deposition efficiency was found to be higher than 80% for both powders. The production CSAM plates have been successfully demonstrated for two ODS PM2000 powders, producing material that is sufficiently dense and free from defects such as cracks or delamination. This suggests that larger and thicker deposits could be achieved using this material, as will be required for CSAM of larger, freestanding components.

Heat treatment led to the disappearance of meso- and microstructural features in the CS-deposited material, with improved metallurgical bonding along particle boundaries and a slight reduction in porosity. This treatment significantly enhanced tensile strength, achieving 1039 MPa for MA powder deposits and 725 MPa for GA powder deposits (Figure 2 showing CSAM ODS PM2000 plates).

For the demonstrator parts, thick deposits of SS316 were successfully created with high density and no visible delamination, followed by machining of grooves for creating internal cooling channel features (Figure 3).

Key Findings (Phase 2): Cold Spraying of ODS Eurofer97

Upon the successful completion of Phase 1 studies on the cold sprayability of ODS alloys, Phase 2 involves investigating the RAFM ODS Eurofer97 alloy using TWI’s Titomic TKF-1000 system. The cold sprayability of ODS Eurofer97 has been successfully demonstrated using both MA and GA powders. With the optimal set of process parameters, ODS Eurofer97 deposits exhibited a measured density of >99% with no signs of delamination or cracks. The deposition efficiency was found to be >75%, with a deposition rate of 1.1 kg/hr. Figure 4 shows the SEM microstructure of cold spray-deposited ODS Eurofer97 using MA powder. Similar to ODS PM2000, post-deposition heat treatment of CSAM ODS Eurofer97 resulted in significant improvement in tensile strength, reaching 940-1000 MPa.

Furthermore, toolpaths have been developed to create high-fidelity, free-standing geometrical features of intermediate complexity. These build strategies are utilised to produce small-scale demonstrator components, including straight walls, cylindrical walls, and cones (Figure 5).

Figure 1. Microstructure of as-deposited cold spray ODS PM2000 using MA and GA powders
Figure 1. Microstructure of as-deposited cold spray ODS PM2000 using MA and GA powders
Figure 2. Cold spray additive manufactured ODS PM2000 plates
Figure 2. Cold spray additive manufactured ODS PM2000 plates
Figure 3. Cold spray deposited SS316 onto aluminium substrate, following machining of the cooling channel grooves
Figure 3. Cold spray deposited SS316 onto aluminium substrate, following machining of the cooling channel grooves
Figure 4. Microstructure of as-deposited cold spray ODS Eurofer97 using MA powder
Figure 4. Microstructure of as-deposited cold spray ODS Eurofer97 using MA powder

Future prospects and research direction

Further research and ongoing activities within the scope of Phase 2 will focus on:

  • Optimisation of post-deposition heat treatment parameters: Enhance the microstructural and mechanical properties of CSAM ODS Eurofer97 material
  • Development of a powder feed controller: Improve build quality, particularly at edges and corners, aiming for near-net-shape parts
  • Understanding the effect of various toolpath patterns: Mitigate/control residual stresses for building large-scale parts
  • Production of functional demonstrator components: Such as breeding blanket assemblies, fuel cladding tubes, and diverter cassette plates

 

Acknowledgement:

This project has been supported by UK Atomic Energy Authority through the Fusion Industry Programme. The Fusion Industry Programme is stimulating the growth of the UK fusion ecosystem and preparing it for future global fusion powerplant market. More information about the Fusion Industry Programme can be found online: https://ccfe.ukaea.uk/programmes/fusion-industry-programme/

Figure 5. Example demonstration components produced using cold spray additive manufacturing
Figure 5. Example demonstration components produced using cold spray additive manufacturing
Avatar Dr Dibakor Boruah Senior Project Leader - Surface, Corrosion, and Interface Engineering

Dr Dibakor Boruah is a Senior Project Leader at TWI Ltd, Cambridge, UK. Dibakor's journey with TWI began in 2017 through the NSIRC programme, where he pursued his PhD, focusing on the structural integrity of cold spray-deposited Ti6Al4V alloy for repair and additive manufacturing applications. After completing his PhD, Dibakor expanded his expertise at Ghent University in 2020, where he was a leading researcher on multiple collaborative projects related to wire+arc additive manufacturing, funded by the EU and local governments. Returning to TWI's surface engineering team in 2022, Dibakor brought with him over seven years of industry-led research experience in materials engineering and advanced manufacturing. 

At TWI, Dibakor served as the Technical Manager for the EU-funded collaborative project FORGE and currently leads the CSAM ODS projects funded by UKAEA FIP Cycle 2, and the Joint Industry Project: Industrialisation of Cold Spray Repair. In addition, he continues to spearhead and contribute to various projects related to cold spray, thermal spray processes, and materials characterisation.

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