The Research On Cold Spray Process And Mechanism On Magnesium Alloy

In this paper, a new method to prepare cold spray layer on Mg alloy is systematically studied. A kind of A1-Si-Fe alloy is designed to improve the corrosion resistance and the wear-resistance of Mg alloy surface. The rapid solidified A1-Si-Fe alloy powders are prepared with the supersonic gas atomization equipments. The experiments have been carried on to study the character of the powders and the distribution of the grain size. And the cold spray process of the rapid solidified A1-Si-Fe alloy powders on the surface of magnesium alloy has been optimized. The diffusion treating has been conducted on the coated samples with different heat treatment process and the diffusion law between the coat and the substrate has been studied. The bonding strength between coat and substrate has been tested. And the corrosion behavior of the coat has been studied. Meanwhile, in order to improve the spray effect, the supersonic nozzle has been designed on the basis of fluid mechanics and aerodynamics. And the flowing process of the gas-particle two phase in the supersonic nozzle has been simulated with the FLUENT software. With the consideration of the nozzle design theory and viscosity boundary layer theory, the nozzle has been optimized.The results show that the shape of the rapid solidified AI-12Si-3Fe-3Mn-2Ni alloy powders for cold spray is globular or near globular. The grain size distribution of the powders which is larger than 400 orders reaches 80%. In all, it can completely meet the requirement of powders for cold spray. The appropriate parameters of cold spray alloy powders on magnesium alloy surface are that: the pressure of spray gas to alloy powder is 2.1MPa; the temperature of spray gas to alloy powder is 550℃and the spray distance is 35mm. The coating prepared by cold spray technology becomes more compact and homogeneous after heat treatment, accompanied with the diffusion between coating and substrate alloy. Moreover, the diffusion quantities diffused into coating from substrate alloy are more than that of diffused into substrate alloy from coating. As temperature rising and time increasing, the diffusion degree between coating and substrate alloy improves. However, the diffusion layer varies little when the temperature rises to 300℃and the time increases to 3h. The bonding strength between the coat and the substrate meet the requirement, and there is a little improvement on the bonding strength after heat treatment. The coat performs better corrosion resistance than the substrate in both salt mist corrosion and electrochemical corrosion, which can protect the substrate from being corroded. The stimulation results of the gas-particle two phase flow in nozzle show that the velocity of the gas phase and the particle phase at the exit in the optimized nozzle both increase to a certain extent. And the gradient of the velocity curve in the divergent section increases in varying degrees, which illustrates a better acceleration effect of the nozzle. Both the temperature and the pressure curve are getting more closer to the results of the theoretical analysis. In a word, the desired results are achieved by the optimization of the nozzle. And the nozzle designed in this way can meet the requirement of the exit velocity in cold spray.