Formation mechanism and microstructure characterization of nickel-aluminum intertwining interface in cold spray

Experimental investigation was carried out to explore the formation mechanism of nickel-aluminum intertwining interface in cold spray, and to characterize the microstructure of deposited nickel particles at the intertwining interface. Shear stress was found to induce the intertwining interface through elongating and breaking of the nickel particles at the coating-substrate interface. The in-situ temperature measurement indicated that the temperature at the intertwining interface did not exceed the recrystallization temperature of nickel during the entire deposition process, suggesting that the nickel particles at the intertwining interface were in solid state rather than thermally softened viscous state. Electron channeling contrast (ECC) and electron backscatter diffraction (EBSD) imaging revealed a development of elongated subgrain (200 nm < D < 1 μm) and localized equiaxed ultrafine grain (D < 200 nm) microstructure within the highly deformed and fractured nickel particles at the intertwining interface. Such microstructures were induced by the dislocation accumulation due to the high strain/strain-rate plastic deformation and grain refinement caused by adiabatic temperature rise, respectively. Moreover, equiaxed ultrafine grains were also found to localize within a shear band near the center of the nickel particles, which experimentally confirms the existence of shear stress at the intertwining interface.