The critical velocity is an important parameter in cold spraying, which determines the deposition efficiency under a given spray condition. The critical velocity depends not only on materials types, but also on particle temperature and oxidation conditions. In the present paper, three types of materials including copper, 316L stainless steel, Monel alloy were used to deposit coatings by cold spraying. The critical velocities of spray materials were determined using a novel measurement method. The oxygen content in the three powders was changed by isothermal oxidation at ambient atmosphere. The effect of oxygen content on the critical velocity was examined. It was found that the critical velocity in cold spray was significantly influenced by particle oxidation condition besides materials properties. The critical velocity of Cu particles changed from about 300 m/s to over 610 m/s with the change of oxygen content in powder. It is evident that the materials properties influence the critical velocity more remarkable at low oxygen content than at high oxygen content. The results suggest that with a severely oxidized powder the critical velocity tends to be dominated by oxide on the powder surface rather than materials properties.
A typical feature of cold spray process is that a deposit can be formed without change of the original structure and compositions of spray materials. Only particles which reach the velocity higher than the critical velocity can be deposited on a substrate in cold spraying. When the spray particle impacts on the substrate at an off-normal angle, the normal component of particle impact velocity will change with the approaching angle of spray particle to substrate. In the present study, copper and titanium powders are used to deposit coating using cold spray process at different impact angles with regard to substrate. The deposition characteristics of spray materials are examined. The results show that the impact angle has a significant influence on the deposition characteristics. The relative deposition efficiency changes with the spray angle. It has been found that there is a critical impact angle at certain particle conditions below which no deposition occurs. The relation between spray angle and relative deposition efficiency can be divided into three spray angle ranges: maximum deposition angle range, transient angle range and no deposition angle range. In the transient angle range, the relative deposition efficiency increases with an increase in spray angle from zero at the critical spray angle to 100%. The transient angle range depends on the particle velocity distribution. A model is proposed to explain the relation between the spray angle and the relative deposition efficiency.
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