| Cold spray is an emerging coating technology. In this process, a coating is deposited through plastic deformation of metallic spray particles in a completely solid state via high velocity impact.Compare with traditional thermal spray technology, this method has the advantage of deposition of chemical actively element coatings as well as minimizing or even eliminating the deleterious effects inherent to the conventional thermal spray. It can be used in repairing the crystallizer in order to prolong its service life, which is particularly important both in engineering and academic field.In order to improve the coating, response surface methodology experimental approach was employed to optimize the carrier gas parameters in cold spray process. Within the investigation of deposition efficiency, density, micro-hardness and friction coefficient, the best couple of gas temperature and pressure is selected by Minitab analyzing software. With the aid of OM, SEM, EPMA, micro-hardness and tensile tester, micro-friction tester, laser thermal conductivity instrument, microstructure and mechanical properties of as-sprayed CrZrCu alloy coating were studied. The bonding mechanism of the cold-sprayed metallic coatings and the influence of alloying elements were also discussed. The influence of coating thickness was also investigated. By using differential scanning calorimetry analyzer and high-temperature laser scanning microscope (confocal), the in situ observation of microstructure evolution and thermal changes in annealing process was carried out. The effects of annealing heat treatment were also discussed.Experimental results showed that the carrier gas temperature and pressure have great influence on deposition efficiency, density and other properties of CrZrCu alloy coating, affect the performance significantly. Although these parameters mainly contribute the speed of particle, temperature and pressure of carrier gas also affect as-sprayed coating properties in different ways. Using Minitab software analysis, it could be illuminated that the as-sprayed coating properties would achieve the best, when the carrier gas parameters is 370℃and 2MPa.There are two bonding mechanism in the deposition of CrZrCu alloy particles, while the dominant bonding mechanism is the mechanical interlocking. Adiabatic shear instability only happened in small partial. The rupture property of as-sprayed coating is brittle. Micro-forging influences significantly the properties of cold-sprayed coating, but it works only in the scope of 1~3 particles distance because of its short-range effect. The effect of Coating thickness is not obvious on the mechanical properties, compared to the CrZrCu alloy substrate, cold spray coating microhardness is about 21.2% of the CrZrCu alloy substrate, and its thermal conductivity is about 86.4% of bulk material.The experiment results indicate that during annealing treatment, thermal flow of the coating can be divided into three different stages, corresponding to recovering process, recrystallization nucleation and grain growth, and association of particles interface stage. The third stage is similar to the integration of particle boundary in sintering. So cold spray CrZrCu alloy's critical annealling temperatures were 350℃, 500℃, 600℃and 850℃, respectively, which were corresponding to different stages of microstructure change.The results showed that the coating was dense with a density of 98.02%. However, with the enhancement of annealing temperature, the porosity increased, which influenced the properties of the coating significantly. Considering various factors, it can be concluded that 500℃is the most optimal anealling temperature for CrZrCu alloy coating. Under such condition, the coating reached the hardness which was comparatively similar to the CrZrCu substrate, the highest tensile strength and relatively high elongation and thermal conductivity. |
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