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Title: Cold spray additive manufacturing of Ni alloy 718 and high entropy alloy CoCrFeMoNi
Author: Walker, Michael
Awarding Body: University of Leicester
Current Institution: University of Leicester
Date of Award: 2020
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Cold spray is a technique where metallic powder is deposited in solid state at high velocities using high pressure inert gas. High entropy alloys (HEAs) are a type of alloy in which there are five or more constituents that are in approximately equal atomic proportions. HEAs have unique thermodynamic properties which gives the alloys increased solid solution strengthening, high temperature stability and fracture resistance. A HEA composition was selected based on thermodynamic analysis of alloys reviewed in literature and those which contained desirable properties for cold spray. Ni alloy 718 and the HEA were sprayed to achieve optimised coatings based on deposition efficiency, porosity and ductility. Microstructures were studied via microscopy, X-ray diffraction, and X-ray fluorescence. Indentation, residual stress, bending and tensile loading measurements facilitated the study of mechanical properties. Particle impacts revealed the ultrafine grain structure and high defect density at the interface. Nano X-ray diffraction resulted in intense diffraction at the interface, confirming the existence of sub 100nm crystallites. The addition of laser assisted cold spray increased interfacial mixing and recrystallisation during deposition, however, the tensile residual stress was increased. 6mm thick Ni alloy 718 deposits successfully without delamination. Heat treatments of the thick Ni alloy deposits resulted in increased UTS and ductility. The HEA powder consisted of FCC and BCC structures. The small, rapidly cooling particles avoid Mo segregation, resulting in BCC structures to accommodate strain. In the as-sprayed condition the FCC structure dominated over the BCC structure due to microstructural changes during deposition and rebounding of BCC particles. Ageing of the HEA resulted in severe embrittlement and loss in strength because defect density was not reduced and hard precipitates formed. The annealing and annealing + ageing treatments increased strength but had little effect on ductility.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council (EPSRC) ; Lloyds Register Foundation (LRF)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Cold spray ; additive manufacturing technique ; Ni Alloy 718 ; High Entropy Alloy CoCrFeMoNi