High Velocity Impact Phenomena and Coating Quality in Cold Spraying

Abstract

Abstract In Cold Spraying, bonding occurs when the impact velocities of particles exceed a critical value. This critical velocity depends not only on the type of spray material, but also on the powder quality, particle size and the particle impact temperature. For metallic materials, the critical velocity is in the range of 200 – 1200 m/s. In analogy with explosive welding, bonding in Cold Spraying is associated with adiabatic shear instabilities caused by high strain rate deformation during impact. Numerical and experimental methods are developed to investigate the influence of impact conditions and related phenomena on the coating quality. For a deeper understanding of impact phenomena and coating formation, the particle impact was modelled by using the finite element software ABAQUS/Explicit. The numerical analyses indicate shear instabilities localized to the particle surfaces, and thus provide a basis for the calculation of critical velocity in terms of materials properties and process parameters. In addition, modelling is used to obtain information about the effect of process parameters on the bonding quality. For most materials, high-strain-rate data are not available. For a quantitative analysis, therefore, the respective materials behaviour was investigated through individual spraying experiments, which were complemented by additional relevant experiments such as impact tests or explosive powder compaction. In this way, impact dynamics, bonding mechanism and critical velocities could be linked. This type of analysis was proved as a powerful tool to reduce the number of experiments for the optimisation of coating quality in Cold Spraying and also to provide a broader overview of the process.