Calibration of the Critical Velocity Description in Cold Spray

In cold spray, successful bonding occurs when particle impact velocities exceed the critical velocity. The description of the critical velocity includes temperature upon impact and material properties, relying on tabulated data of bulk material. However, rapid solidification of powder particles during gas atomization results in higher strengths than reached by respective bulk materials, causing an underestimation of the critical velocity. Thus, a readjustment of the semiempirical calibration constants can 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 spraying was performed over a wide range of parameter sets, achieving deposition efficiencies (DE) ranging from 2% to 98%. DEs were plotted as a function of particle impact velocities and temperatures, as calculated by a fluid dynamic approach. By using DEs of 50%, the critical velocities of the different powders and the corresponding semiempirical constants were determined. The results reveal material-dependent differences in the mechanical pre-factor. This allows a more precise description of individual influences by particle strengths on critical velocities and enhances the understanding and prediction of coating properties.