A Study of the Effects of Amorphous/Crystalline Oxide Films on the Cold Spray Behavior of 6061 Al Alloy Powders

This paper presents the effects of nanoscale surface oxide layers on the cold spray behavior of 6061 Al alloy powders. The surface oxide films are revealed via Transmission Electron Microscopy (TEM), and Energy Dispersive X-ray Spectroscopy (EDS). Phenomena associated with surface contacts, contact-induced elastic-plastic deformation, heating, and cracking are then simulated using a combination of analytical models, finite element analysis and Molecular Dynamics (MD) simulations. MD simulations are used to provide insights into the effects of powder impact on deformation and fracture at powder impact velocities that are consistent with previously reported critical velocities and predictions. MD simulations are also used to obtain estimates of oxide film moduli, toughness, strains to failure, and the fracture energies of aluminum oxide films with crystalline and amorphous structures. These are incorporated into finite element simulations of cold spray contact-induced deformation and cracking. The impact between the powder particles with nanoscale oxide layer and the substrate is modeled using a bi-linear Johnson-Cook model. The powder impacts are shown to result in localized splat deformation and heating, and the cracking of the oxide layers in ways that can expose fresh metallic surfaces to high temperature contacts (above the recrystallization temperature) that can give rise to bonding and mechanical interlocking.  The implications of the results are discussed for the design of cold spray processes for the fabrication and repair of 6061 Al structures.