Akisin, Cletus John
(2024)
Investigation of CoCrFeNiMn high entropy alloy depositions during cold spray additive manufacturing.
PhD thesis, University of Nottingham.
Abstract
Cold spray is steadily becoming a popular solid-state additive manufacturing (AM) and repair technique, referred to as cold spray additive manufacturing (CSAM). It is a material deposition technique that utilises the kinetic energy of microparticles to form dense deposits layer-by-layer on a target surface. Because of the solid-nature of the CSAM process, various components manufactured with CSAM have found applications in aero-engines as those components can withstand the extreme operating conditions of aero-engines. Since the CSAM process retains the initial microstructure of feedstock materials, being also an eco-friendly process, makes it a viable option for the deposition of several metals and alloys. Hence, substantially reducing environmental footprint and manufacturing cost.
The recent advancements in new materials development characterise the cutting-edge high performance of high entropy alloys (HEAs), attracting industrial attention for their end-use in critical parts of aero-engines. These alloys are composed of multi-principal elements with near or equal atomic proportions and striking mechanical properties that surpass those of conventional superalloys. The CoCrFeNiMn HEA has been found to possess properties like or that surpass alloys employed in manufacturing aero-engine components. The HEA's good strength-ductility combination, resistance to hydrogen embrittlement, and high fatigue resistance make it an alternative alloy for manufacturing components such as the integral parts of liquid hydrogen fuel turbopumps. However, very little has been done to explore the CSAM of the CoCrFeNiMn HEA. Therefore, this research project aims to develop and investigate deposits of the CoCrFeNiMn HEA using the CSAM technique, that can find applications for the repair and manufacture of aerospace components, such as restoring the structural integrity of integrally bladed rotors or turbine blisks, jet engine fan shaft, thrust chamber, impellers and nozzle guide vanes. This study focused on the deposition mechanism of the HEA during the CSAM process, the effect of subsequent post-deposition annealing on the microstructure and mechanical properties of the deposits, and the residual stress formed in the deposits.
To demonstrate the repair and manufacturing of components made of several materials with CSAM of the HEA, the deposition mechanisms of the HEA on austenitic stainless steel, titanium alloy, and aluminium and its alloy materials were investigated using experimental and numerical techniques. It was found that metallurgical bonding and mechanical interlocking mechanisms were responsible for the deposition of CoCrFeNiMn HEA particles on the substrates depending on the substrate material properties. When particles are sprayed with the CSAM process, they travel at supersonic speed, and the particles stick to the targeted surface at a particle impact velocity above a certain threshold called critical velocity. At the critical velocity, the sprayed particles severely deform at the impact interfaces and bond to the substrate and previously deposited particles. The particle bonding mechanism is attributed to adiabatic shear instability (ASI). With the concept of ASI, the critical velocities of the HEA on the substrates were determined. Consequently, the optimum process conditions for the repair and manufacturing of components with CSAM of the HEA can be obtained.
With the determined process conditions, the possible manufacture and or repair of aerospace components made of stainless steel with the HEA using the CSAM process was demonstrated by spraying thick deposits on the substrate. The deposition mechanism of the HEA during deposit build-up on the stainless steel substrate was investigated. The investigation involved the use of advanced materials characterisation techniques and numerical analysis. It was found that during the deposition of the HEA on the substrate, dynamic recrystallisation (DRX) produced by subgrain rotation and ASI are the dominant deposition mechanisms at the particle interfaces. These thermomechanical phenomena result in particle interfaces characterised by highly misoriented equiaxed ultrafine grains, whereas the particle interiors consist of coarse grains with limited deformation. The heterogeneous microstructure formed in the CSAM deposit can contribute to a good strength-ductility combination of the HEA. Nevertheless, porous microstructures were obtained in the deposits, hence post-deposition annealing treatments were performed to improve the microstructure of the deposits. The post-deposition annealing treatment resulted in the consolidation of the deposit and increased metallurgically bonded areas, leading to enhanced mechanical properties. Also, the annealing treatment thus changed the failure behaviour of the as-sprayed deposit from mostly particle-particle interface failure to ductile failure.
Interestingly, it was found that the deposit annealed at 600 °C exhibited partially recrystallised microstructure with a small volume fraction of the Cr-rich phase formed at grain boundaries of the sprayed particles. It was argued that this distinct microstructure can contribute to improved strength without loss of ductility of the deposited HEA.
For the repaired or manufactured parts, it is important to measure and understand the residual stress formed in the part during the CSAM of the HEA. The residual stress of the deposited HEA on the stainless steel 304 substrate sample was studied using the contour method of stress measurements and numerical analysis. Tensile and compressive residual stresses were formed in the HEA deposit and substrate sample. The formation of these residual stresses was based on two dominant mechanisms: temperature gradient and thermal mismatch after the cool-down stage. These mechanisms were dominant over the compressive peening effects typical of the CSAM process. Because of the thermal impacts that arose from the spraying parameters that were employed for the CSAM deposition of the HEA, tensile residual stress was formed in the sample. Consequently, the tensile stress formed in the deposit would compromise the structural integrity of the manufactured or repaired parts, which can result in early components failing in service. Hence there is a need for optimisation of the CSAM of the HEA process conditions for restoring the structural integrity of repaired parts and obtaining required structural properties of the manufactured parts. The results obtained in this study shed light on the effect of process conditions on the residual stress formation mechanisms during CSAM.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Hussain, Tanvir
Bennett, Christopher
Venturi, Federico |
| Keywords: |
Cold spray; Additive manufacturing; Residual stress formation; High entropy alloys; Aerospace components |
| Subjects: |
T Technology > TS Manufactures |
| Faculties/Schools: |
UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering |
| Item ID: |
79525 |
| Depositing User: |
AKISIN, CLETUS
|
| Date Deposited: |
10 Dec 2024 04:40 |
| Last Modified: |
10 Dec 2024 04:40 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/79525 |
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