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

Abstract

Cold spray is a technique where metallic powder is deposited in solid state athigh velocities using high pressure inert gas. High entropy alloys (HEAs) area type of alloy in which there are five or more constituents that are in approximatelyequal atomic proportions. HEAs have unique thermodynamic propertieswhich gives the alloys increased solid solution strengthening, high temperaturestability and fracture resistance. A HEA composition was selected based on thermodynamicanalysis of alloys reviewed in literature and those which containeddesirable properties for cold spray. Ni alloy 718 and the HEA were sprayed toachieve 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 measurementsfacilitated the study of mechanical properties.Particle impacts revealed the ultrafine grain structure and high defect densityat the interface. Nano X-ray diffraction resulted in intense diffraction at theinterface, confirming the existence of sub 100nm crystallites. The addition oflaser assisted cold spray increased interfacial mixing and recrystallisation duringdeposition, however, the tensile residual stress was increased. 6mm thickNi alloy 718 deposits successfully without delamination. Heat treatments of thethick Ni alloy deposits resulted in increased UTS and ductility. The HEA powderconsisted of FCC and BCC structures. The small, rapidly cooling particlesavoid Mo segregation, resulting in BCC structures to accommodate strain. In theas-sprayed condition the FCC structure dominated over the BCC structure dueto microstructural changes during deposition and rebounding of BCC particles.Ageing of the HEA resulted in severe embrittlement and loss in strength becausedefect density was not reduced and hard precipitates formed. The annealing andannealing+ageing treatments increased strength but had little effect on ductility.