Characterization of microstructural evolution and bond formation in cold sprayed aluminum alloy splats and substrates

Abstract

Cold spray is an emerging coating process in which feedstock powder particles, accelerated in a gas stream to velocities up to 1200 m/s, are deposited on a substrate to form a coating at temperatures far below the melting point of the powder or the substrate. Although cold spray has been widely studied and even commercialized in the recent decades, the bonding mechanism at the splat-substrate and splat-splat interfaces is still not clearly understood. We conducted extensive electron microscopic studies on splats of Al6061 and Al2024 cold spray-deposited on a T6-Al6061 substrate at a super-critical velocity to characterize the microstructural evolution in the impacting powder particle and the substrate and understand the occurrence of metallurgical bonding at the splat-substrate interface in relation to the structural changes. It was confirmed that metallurgical bonding at the splat-substrate interface is possible only when oxide-free mating occurs between the splat and the substrate. Oxide-free surfaces are created by the sweeping action of localized material flow along the interface that is promoted by rapid dynamic recovery and recrystallization in the materials along the interface. Material flow along the interface occurs more readily toward the splat peripheries than at the polar region of the splatsubstrate interface where normal stresses dominate. Improved bonding may be achieved with heat treated (softened) powder particles that would deform more uniformly along the splat-substrate interface. With a feedstock Al6061 powder pre-heat treated (softened) to 69 HV, the range of metallurgical bonding was extended toward the polar region. However, with much harder (142 HV) as-received feedstock Al2024 powder, no metallurgical bonding was observed at the polar or equatorial interface. Nano-scale precipitation IV observed in the vicinity of the Al6061 splat-Al6061 substrate interface indicates a high excess vacancy concentration during the impact, which could also explain the occurrence of rapid dynamic recovery and recrystallization in the material along the interfacial. V