Abstract
In cold gas spraying, successful bonding occurs when particle impact velocities exceed the critical velocity. The critical velocity formula depends on material properties and temperature upon impact, relying mainly on tabulated data of bulk material. However, rapid solidification of powder particles during gas atomization can result in strengths up to twice that of bulk materials, causing an underestimation of the critical velocity. Thus, a re-adjustment of the semi-empirical calibration constants could supply a more accurate prediction of the requested spray conditions for bonding. Using copper and aluminum as examples, experimentally determined particle strengths for various particle sizes were 43% and 81% higher than those of the corresponding soft bulk materials. Cold gas spraying was performed over a wide range of parameter sets, achieving deposition efficiencies ranging from 2% to 98%. Deposition efficiencies were plotted as functions of particle impact velocities and temperatures, as calculated by a fluid dynamic approach. By using deposition efficiencies of 50%, the critical velocities of the different powders and the corresponding semi-empirical constants were determined. Based on particle strengths, the results reveal slight material-dependent differences in the mechanical pre-factor. This allows for a more precise description of individual influences by particle strengths on critical velocities and thus coating properties. Nevertheless, the general description of the critical velocity based on bulk data with generalized empirical constants still proves to be a good approximation for predicting required parameter sets or interpreting achieved coating properties.
