Cold Spray Additive Manufacturing of Ni Alloy 718 and High Entropy Alloy CoCrFeMoNi

2020-01-17T16:14:58Z (GMT) by Michael Walker
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.