No abstract

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 convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly the particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and the operating gas pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that the deposition can be realized under a lower gas pressure with a CB nozzle.

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using the compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post-annealing. In this study, a nanostructured Fe/Al alloy powder was prepared by ball-milling process. The cold sprayed Fe/Al alloy coating was evolved in-situ to intermetallic compound coating through a post-annealing treatment. The microstructural evolution of the Fe-40Al powder during mechanical alloying and the effect of the post-annealing on the microstructure of the cold sprayed Fe(Al) coatings were characterized by optical microscopy, scanning electron microscopy and X-ray diffraction analysis. The results showed that the milled Fe-40Al powder exhibits lamellar microstructure. The microstructure of the as-sprayed Fe(Al) coating depends significantly on that of the as-milled powder. The annealing temperature significantly influences the in-situ evolution of the intermetallic compound. The annealing treatment at a temperature of 500oC results in the complete transformation of Fe(Al) solid solution to FeAl intermetallic compound.

Nanostructured NiCrAlY coating was deposited by cold spray, using a milled powder for applications as a bond coat to thermal barrier coating. A shot-peening treatment was then applied to the as-sprayed coating to modify the coating surface morphology. The oxidation behavior of the coating with the shot-peened surface was investigated under isothermal oxidation at 900°C and 1,000°C for different times. The oxidation behavior of the coating was characterized through surface morphology and cross-sectional microstructure by scanning electron microscopy and X-ray diffraction analysis. It was found that a uniform oxide layer was formed on the surface of the shot-peened nanostructured NiCrAlY coating during oxidation at temperatures of 900°C and 1,000°C. The nanostructure of the initial coating possibly promoted rapid formation of α-Al2O3 oxide. It was clearly revealed that the surface morphology of the coating significantly impacted the morphology of the oxide. The surface geometry of the cold-sprayed MCrAlY coating must be modified to promote formation of a protective oxide film during oxidation, through application of a post-treatment process such as shot-peening.